Patent Application
20080283469
The present invention relates to methods and devices for processing, refining, and/or treating liquid compositions. More specifically, the invention relates to membrane separation methods and devices employing a selective, semi-permeable, microporous, or other partitioning membrane for processing, refining, and/or treating liquid compositions, for example membrane waste-water purification processes and apparatus.
High efficiency wastewater treatment has become increasingly important as the world's population continues to grow. The quantity of water needed for human consumption and other uses has increased at a rapid pace, while the amount of naturally available water remains unchanged. The ever-increasing demand for usable, clean water has made reclamation of wastewater al essential component of growth and development of human populations.
In the United States and other developed nations, as existing metropolitan areas become overcrowded, developers are encouraged or required to construct new housing in previously undeveloped areas. Many of these undeveloped areas lack sufficient water for consumption, irrigation and similar purposes, necessitating reclamation and reuse of available water resources. For development in these areas to be successful, sewage from the residential use of water, commonly referred to as wastewater, is therefore a primary target for reclamation.
Residential wastewater has a high water content, but requires substantial processing before it can be reused because of the human waste and other contaminants mixed with it. To achieve reclamation of residential wastewater in many new development areas, isolated from existing sewage treatment facilities, on-site wastewater treatment and reclamation is highly advantageous or essential.
A wide variety of different wastewater treatment systems have been proposed for reclaiming residential sewage and other categories of wastewater. One such system disclosed in U.S. Pat. No. 2,528,649, incorporates a simple sedimentation tank for separating solid waste, or “sludge”, from wastewater. After sedimentation, the sludge is passed to a digestion system where it is allowed to settle so that clear aqueous liquid separates from the sludge. The clarified liquid is redirected back to the sedimentation tank. Unfortunately, this system suffers from a number of shortcomings that make it inefficient. In particular, the system incorporates a relatively crude sedimentation system that merely allows the influent sewage to separate and does not aerate or facilitate processing of the sewage in any other way.
A number of wastewater treatment processes comprise “biological” systems utilizing microorganisms contained in an activated biomass, or sludge for the removal of COD, phosphorous and/or nitrogen from wastewater. These treatment processes typically incorporate multiple treatment phases or “zones”, namely: (1) a preliminary treatment area; (2) a primary treatment area; and (3) a secondary treatment area. Preliminary treatment is primarily concerned with the removal of solid inorganics from untreated wastewater. Typically, this preliminary treatment encompasses a two-stage treatment process in which the debris is removed by screens and/or settling. Organic matter is carried out in the fluid stream for subsequent treatment, Primary treatment entails a physical process wherein a portion of the organics, including suspended solids such as feces, food particles, etc. is removed by flotation or sedimentation. Secondary treatment typically encompasses a biological treatment process where microorganisms are utilized to remove remaining organics, nitrogen and phosphorous from the wastewater fluid stream. Microorganism growth and metabolic activity are exploited and controlled through the use of controlled growth conditions.
In large scale municipal or industrial applications, biological treatment processes typically utilize a basin or other reservoir in which the wastewater is mixed with a suspension of biomass/sludge. Subsequent growth and metabolism of the microorganisms, and the resultant treatment of the wastewater, is carried out under aerobic and/or anaerobic/anoxic conditions. In most large scale municipal or industrial treatment systems, the various components of the treatment process are performed in discrete basins or reactors. As such, there is a continuous flow of the wastewater from one process step to the next. Biomass containing the active microorganisms may be recycled from one process step to another. The conditioning of such biomass to enhance growth of particularized subgroups of microorganisms possessing a proclivity for performing a specific type of metabolic process, e.g. phosphate removal, nitrogen removal has been the subject matter of numerous patents, including: U.S. Pat. No. 4,056,465; U.S. Pat. No. 4,487,697; U.S. Pat. No. 4,568,462; U.S. Pat. No. 5,344,562. The optimization of other components or aspects of biological wastewater treatment has also engendered a variety of patents, including: U.S. Pat. No. 2,788,127; U.S. Pat. No. 2,875,151; U.S. Pat. No. 3,440,669; U.S. Pat. No. 3,543,294; U.S. Pat. No. 4,522,722; U.S. Pat. No. 4,824,572; U.S. Pat. No. 5,290,435; U.S. Pat. No. 5,354,471; U.S. Pat. No. 5,395,527; U.S. Pat. No. 5,480,548; U.S. Pat. No. 4,259,182; U.S. Pat. No. 4,780,208; U.S. Pat. No. 5,252,214; U.S. Pat. No. 5,022,993; U.S. Pat. No. 5,342,522; U.S. Pat. No. 3,957,632; U.S. Pat. No. 5,098,572; U.S. Pat. No. 5,290,451; Canadian Patent # 1,064,169; Canadian Patent # 1,096,976; Canadian Patent # 1,198,837; Canadian Patent # 1,304,839; Canadian Patent # 1,307,059; Canadian Patent # 2,041,329.
Biological removal of organic carbon, nitrogen and phosphorus compounds from waste water requires attention to special environmental conditions within the processing equipment. For instance, for bacteria and other microbes to convert organic carbon compounds (measured as BOD) to carbon dioxide and water, a well mixed aerobic environment is required. Approximately one pound of oxygen is required for each pound of BOD removed. To convert nitrogen compounds to nitrogen gas and carbon dioxide, nitrosomas and nitrobacter operate in an aerobic environment consuming inorganic carbon. Approximately 4.6 pounds of oxygen is required for each pound of ammonia-N converted to nitrate-N (assuming alkalinity is sufficient). Subsequently, facultative bacteria operate in an anoxic environment consuming organic carbon and liberating nitrogen gas. Approximately 2.6 pounds of oxygen is recovered for each pound of nitrate-N converted to nitrogen gas. To biologically tie up phosphate in the cell mass, an anaerobic step to produce volatile fatty acids is required. This is followed by Poly P microbes consuming large amounts of phosphorus required to metabolize the volatile fatty acids in an aerobic environment thus concentrating the phosphate in the biomass (see, e.g., Abstract by Dr. W. Wilson Western Canada Water and Wastewater Conference, Calgary AB. January 2002).
The combination of these many biological processes ideally results in a Biological Nutrient Removal (BNR) process, sometimes called tertiary treatment. However, a well-designed tertiary treatment operation requires coordination and sequencing of a complex assemblage of components, processes and conditions. Each of the constituent biological processing steps proceeds at its own rate, with specific environmental parameters required. Efficient tertiary processing also requires the correct amounts of specialty microbes to sustain the microbial populations and perform specific processing functions.
Current wastewater treatment systems which attempt to provide tertiary treatment include Up flow Sludge Bed Filter (USBF), Sequencing Batch Reactor (SBR) and Membrane Separation Activated Sludge (MSAS) systems. The Sequencing Batch Reactor (SBR) process is a modification of the conventional activated sludge process. U.S. Pat. No. 5,503,748 discloses a long vertical shaft aerator applied to the SBR technology. The SBR process employs a number of discrete steps, typically comprising sequential fill, reaction, settlement and decantation of wastewater with biomass in an enclosed reactor. In the initial step of this process, wastewater is transferred into a reactor containing biomass, and combined to form a mixed liquor. In the reaction step of the treatment process the microorganisms of the biomass utilize and metabolize and/or take up the nitrogen, phosphorous and/or organic sources in the wastewater. These latter reactions may be performed under anaerobic conditions, anoxic conditions, aerobic conditions, or a combination thereof to manipulate organism growth, population dynamics and contaminant processing. The length of this stage will be dependent on the waste's characteristic, concentration of the biomass, and other factors. Following the reaction cycle, the biomass in the mixed liquor is allowed to settle out. A sludge blanket settles on the bottom of the reactor leaving a treated effluent supernatant. The treated and clarified wastewater (i.e. effluent) is subsequently decanted and discharged. The reactor vessel is then refilled and the treatment process cycle reinitiated. Thus, the sequencing batch reactor's process is based on discrete operation in time, whereas other wastewater treatment processes are based on distinct operations in space, e.g., by performance of different reactions in separate vessels.
A number of additional wastewater treatment designs feature an air-lift reactor, which is a mechanically simple, combined gas-liquid flow device characterized by fluid circulation in a defined cyclic pattern through a set of specifically designed channels. Fluid motion is due to the mean density difference in an upflow (riser) and downflow (downcomer) sections of the reactor. The air-lift reactor is ordinarily comprised of distinct zones with different flow patterns. The riser is typically the zone where the gas is injected creating a fluid density difference, resulting in upward flow of both liquid and gas phases. At the top of the reactor, there is a gas-liquid separator section, which is typically a region of horizontal fluid flow and flow reversal where gas bubbles disengage from the liquid phase. The downcomer is the zone where the gas-liquid dispersion or degassed liquid ordinarily recirculates to the riser. The downcomer zone exhibits either single-phase, two-phase cocurrent, or two-phase mixed cocurrent-countercurrent downward flow, depending on whether the liquid velocity is greater than the free-rise velocity of the bubbles. The base section at the lower end of the vessel communicates the exit of the downcomer to the entrance of the riser.
The air-lift reactor has predominantly been used for microorganism fermentation processes such as the ICI single cell protein production. Nonetheless, a number of systems are known which utilize air-lift reactors for wastewater treatment. Among these examples is the Betz reactor (Gasner, Biotech, Bioeng. 16:1179-1195, 1974), and “deep shaft” bioreactors for effluent treatment (see, e.g., Hines et al., Chem. Eng. Sym. Ser. U.K. 41:D1-D10, 1975).
Following the original development of deep shaft bioreactor technology, recent efforts have led to improvements in long vertical shaft bioreactor systems for wastewater treatment. Among these improvements, U.S. Pat. Nos. 4,279,754, 5,645,726, and 5,650,070 issued to Pollock each disclose a modified vertical shaft bioreactor system for the treatment of biodegradable wastewater and/or sludge. Generally, these vertical shaft bioreactor systems comprise a bioreactor, a solid/liquid separator and intervening apparatus in communication with the bioreactor and separator. The bioreactor comprises a circulatory system which includes two or more vertical, side-by-side or coaxial chambers, a downflow chamber (downcomer) and an upflow chamber (riser). These chambers are connected at their upper-ends through a surface basin and communicate at their lower ends via a common “mix zone” adjacent the lower end of the downcomer.
In addition to the mix zone, these reactors feature a “plug flow zone” located below the mix zone and communicating therewith. As previously described, the term “plug flow” has referred to a net downward migration of solid particles from the mix zone toward an effluent outlet located at the lower end of the reactor. In one application to sludge digestion the net downward migration has been reported by Guild et al. (Proceedings WEF conf., Atlanta Ga., October 2001), to include local back mixing only, but over extended periods of operation (e.g., about 16 hours), inter-zonal mixing occurs.
The waste-containing liquor (“mixed liquor”) is driven through the circulating system (i.e., between the downflow and upflow chambers, the surface basin and the mix zone) by injection of an oxygen-containing gas, usually air, near the bottom of the reactor (e.g., at the mix zone and plug flow zone). A portion of the circulating flow is directed to the plug flow zone and is removed at the lower end thereof as effluent. In wastewater treatment reactors, the air is typically injected 5-10 feet above the bottom of the reactor and, optionally, immediately below the lower end of the downcomer. The deepest air injection point divides the plug flow zone into a quasi plug flow zone with localized back mixing above the deepest point of air injection, and a strict plug flow zone with reportedly no mixing below the deepest point of air injection.
At start-up of the bioreactor, air is injected into the riser in the nature of an air lift pump, causing liquor circulation between and through the upflow and downflow chambers. Fluid in the downcomer has a higher density than the liquid-bubble mixture of the riser and thereby provides a sufficient lifting force to maintain circulation.
Once the bioreactor circulation is thus initiated, all of the air injection is diverted to the mix zone and/or plug flow zone. The air bubbles that rise out of these zones are trained into the upflow chamber and are excluded from the downflow chamber where the downward flow of liquor exceeds the rise rate of the bubbles. Dissolved oxygen in the circulating mixed liquor is the principal reactant in the biochemical degradation of the waste. As the liquor-ascends in the riser to regions of lower hydrostatic pressure, this and other dissolved gases separate and form bubbles. When the liquid/bubble mixture from the riser enters the basin, gas disengagement occurs. To facilitate this purpose, the surface basin is ordinarily fitted with a horizontal baffle at the top of the upflow chamber to force the mixed liquor to traverse a major part of the basin and release spent gas before re-entering the downflow chamber for further treatment.
U.S. Pat. No. 5,650,070 discloses a process where influent waste water is introduced at depth into the riser chamber through an upwardly directed outlet arm of an influent conduit. A zone of turbulence is created at the lower end of the downflow chamber by the turn-around velocity head as the circulating flow reverses from downward to upward flow. This mix zone is not well defined but typically is between 15-25 feet deep. A portion of the mixed liquor in the mix zone flows downwardly into the top of the plug flow zone in response to an equal amount of treated effluent being removed from the lower end of the plug flow zone into an effluent line, as discussed above. During operation of the bioreactor the flow of influent liquor to and effluent liquor from the bioreactor are controlled in response to changes in level of liquid in the connecting upper basin.
Reaction between waste, dissolved oxygen, nutrients and biomass (including an active microbial population), substantially takes place in an upper circulating zone of the bioreactor defined by the surface basin, the upflow and downflow chambers and the mix zone. The majority of the contents of the mix zone circulate upwardly into the upflow chamber. In this upflow chamber undissolved gas, mostly nitrogen, expands to help provide the gas lift necessary to drive circulation of the liquor in the upper part of the reactor. The spent gas is released from the liquor as it traverses the horizontal baffle in the surface basin. The plug flow zone located below the upper circulating zone provides a final treatment or “polish” to the mixed liquor flowing downward from the mix zone to effluent extraction at the lower end of the reactor.
The injected oxygen-containing gas dissolves readily under pressure in the liquor in the plug flow zone where there is localized back mixing resulting in a slow net downward movement of liquor. Undissolved gas (bubbles) migrate upward to the very turbulent mix zone under pressure. The gas to liquid transfer in this zone is very high, reaching overall reactor oxygen transfer efficiencies in excess of 65%. The products of the reaction are carbon dioxide and additional biomass which, in combination with unreacted solid material present in the influent wastewater, forms a sludge (or biosolids).
In addition to aerobic digestion of BOD, it is becoming more and more important to couple biological nutrient removal (BNR) of nitrogen and phosphorous compounds with conventional wastewater treatment. As the demand for higher quality liquid effluent discharges increase, the need for technologies as provided by the present invention has become increasingly more compelling. The old Secondary Biological treatment standard of 30 mg/L BOD and 30 mg/L TSS is no longer adequate in many jurisdictions and limits are now often placed on nitrogen and phosphorus as well. Effective removal of these nutrients is essential in view of existing and developing environmental laws aimed at preventing eutrophication of natural waters and the attendant ecosystem damages that result therefrom.
In basic terms, nitrogen removal is accomplished by converting ammonia contained in a mixed liquor stream to nitrites and nitrates, in the presence of oxygen, which is known as an aerobic nitrifying stage. Ammonia conversion to nitrite is carried out by microbes known as Nitrosomonas, while the conversion of nitrite to nitrate is accomplished by Nitrobacters. Nitrate conversion to nitrogen gas occurs in an anoxic denitrifying stage that takes place in a suspended growth environment devoid of dissolved oxygen. Nitrogen, carbon dioxide and water is produced, with the gas being vented from the system. Nitrification rates can be optimized by regulating interdependent waste stream parameters such as temperature, dissolved oxygen levels (D.O.), pH, solids retention time (SRT), ammonia concentration and BOD/TKN ratio (Total Kjeldahl Nitrogen, or TKN, is organic nitrogen plus the nitrogen from ammonia and ammonium). Higher temperatures and higher dissolved oxygen levels tend to promote increased nitrification rates, as does pH levels in 7.0 to 8.0 range. Sludge retention times of from 3.5 to 5, and preferably 5-8, days dramatically increase nitrification efficiency, after which time efficiencies tend to remain constant. Increases in ammonia concentration increases the nitrification rate but only to a maximum level attainable after which further ammonia concentration increases do less to increase the rate of nitrification. Rates have also been shown to be maximized at BOD/TKN ratios of less than 1.0 (see, e.g., Abstract by Dr. W. Wilson, Western Canada Water and Wastewater BNR conference, Calgary AB Canada Jan. 2002).
Physical/Bio-Chemical phosphorous removal typically requires an anaerobic suspended growth zone at the start of the system, and a sludge fermentation tank to supply volatile fatty acids (VEA's) for the energy needs of the phosphorous ingesting organisms (Acinetobacters). Recently it has been reported that anaerobic force mains can generate sufficient volatile acids to permit substantial biological phosphorus removal.
Refractory treatment and polishing stages may be added to the process, downstream of the final clarification stage. In many waste streams, the majority of organic compounds (80%-90%) are easily biodegraded. The remaining fraction biodegrade more slowly and are termed “refractory” compounds. Prior art biological nutrient removal designs incorporate a single sludge and a single clarifier, for example, U.S. Pat. No. 3,964,998 to Barnard, but in that case the overall oxidation rate of the system has to be reduced to satisfy the slowest compound to oxidize.
Biological nutrient removal (BNR) systems can take various process configurations. One such embodiment is the five stage Modified Bardenpho™ process, which is based upon U.S. Pat. No. 3,964,998 to Barnard. It provides anaerobic, anoxic and aerobic stages for removal of phosphorous, nitrogen and organic carbon. More than 24 Bardenpho™ treatment plants are operational, with most using the five stage process as opposed to the previously designed four stage process. Most of these facilities require supplemental chemical addition to meet effluent phosphorous limits of less than 1.0 mg/L. Plants using this process employ various aeration methods, tank configurations, pumping equipment and sludge handling methods. WEF Manual of Practice No. 8, “Design of Municipal Wastewater Treatment Plants”, Vol. 2, 1991.
In the context of vertical bioreactor technology, Pollock (U.S. Pat. No. 5,651,892, issued Jul. 29, 1997, incorporated herein by reference) discloses an innovative process utilizing a vertical bioreactor linked to a flooded filter via a flotation separator. According to this design, improved reaction rates are achieved by separating the biomass into a high rate aerobic organic carbon removal step, followed by an aerobic nitrification step using a separate nitrifying biomass. These steps are then followed by a high rate denitrification step in an anoxic environment created by feeding influent and return mixed liquor or effluent into that zone to provide a source of organic carbon and consume the oxygen.
Incorporation of an anaerobic processing step for phosphate removal is typically done in a separate reactor—due to the long fermentation time required for volatile fatty acid production. Furthermore, phosphorus removal in single mixed liquor systems is difficult: to implement because the phosphate rich biomass produced in the aerobic portion of the process should not contact the anaerobic fermentation reactor product due to the risk of re-solubilizing the entrapped phosphate. In other instances, biological phosphorus removal is augmented by addition of metal salts such as ferric chloride or alum. These can be added directly into the aerobic zone of the reactor to chemically bind the phosphate.
Thus, a variety of treatment systems, including coupled vertical shaft reactors and SBR's, have been successfully used to provide tertiary wastewater treatment. However, these tertiary treatment systems involve a single mixed liquor process wherein all of the specialty microbes involved in the process are mixed together. These include autotrophic organisms that utilize energy from inorganic material (e.g., the nitrifiers Nitrosomonas and Nitrobacters), and heterotrophs which utilize organic energy sources and include the aerobic BOD removers and the Acinetobacter biological phosphorous removers (Bio-P organisms). Therefore, in all of these types of systems, the rate of treatment is controlled by the slowest performing microbe, usually nitrosomas which converts ammonium to nitrite. Due to the slow overall rate of treatment, these single mixed liquor systems are called extended aeration systems and are quite energy intensive.
Despite the foregoing developments and advancements in wastewater treatment technologies, there remains an urgent need in the art for improved wastewater treatment systems that can satisfy a broadened range of uses and perform expanded and enhanced functions not satisfied by existing wastewater treatment systems. For example, there is a long unmet need in the art for a simplified wastewater treatment process and apparatus that provides enhanced biological nutrient removal (BNR) and which, in certain embodiments, can produce class A bio-solids required for unrestricted land applications. In addition, there remains an unfulfilled need for wastewater treatment systems and methods that satisfy these expanded functions while minimizing the costs and environmental impacts that attend conventional wastewater treatment plant installation and operation.
Membrane separation, which employs a selective, semi-permeable, or partitioning membrane is a rapidly evolving aspect of industrial separation technology for processing, refining, and/or treating liquid compositions, for example as employed in modern membrane waste-water purification processes and apparatus. In general membrane separation devices and processes, a first liquid composition, for example an influent liquid waste water stream or flow, contact one surface of the membrane, and one or more constituents of the first liquid composition typically pass through the membrane, often as a result of a driving force or forces, for form a second liquid composition, for example an effluent flow stream at a second surface of the membrane, whereby one or more separated or partitioned components of the first liquid composition are excluded or left behind (i.e., they are partitioned or retained at the first membrane Surface to remain in solution, suspension, or contact, with the first liquid composition.
Membrane separation technologies that can be employed within the methods and devices of the invention for processing, refining or treating liquid compositions include microfiltration, ultrafiltration, nanofiltration, reverse osmosis, electrodialysis, electrodeionization, pervaporation, membrane extraction, membrane distillation, membrane stripping, membrane aeration, and other membrane-based processes. Various driving forces may be used principally, or in combination with other driving forces disclosed herein, to effectuate or enhance membrane function, depending on the type of the membrane separation employed. Pressure-driven membrane filtration, also known as membrane filtration, includes microfiltration, ultrafiltration, nanofiltration and reverse osmosis, and uses pressure as a driving force, whereas electrical driving force is used in electrodialysis and electrodeionization.
Historically, membrane separation processes or systems have not been considered cost effective for water treatment due to the adverse impacts that membrane scaling, membrane fouling, membrane degradation and the like impose on the efficiency of removing solutes from aqueous water streams. More recently, however, advancements in technology have made membrane separation a more commercially viable technology for treating aqueous compositions suitable for use in industrial and residential water treatment processes.
The technology of solids-liquid separation using membranes has been rapidly developing the in the wastewater treatment industry and in other membrane separation fields of use. For early membrane wastewater treatment plants, the predicted useful lifespan of membranes was between about 5-7 years. Currently, useful membrane lifespan in waste water treatment applications is often as long as 8 years or greater.
In North America and other areas of the world, water rationing has become increasingly common, even in cities that normally having good water resources, such as Vancouver, Seattle, and Calgary; Water rationing has become critical in many parts of the prairie and desert states. The moisture content in the soil in some areas is already less than in the “dirty thirties.” The primary factor in progressive water rationing restrictions has been attributed to the inability of existing potable water treatment plaints to produce enough potable water to satisfy increasing domestic and commercial demands. Associated with this problem, there is a need for improved wastewater treatment capacity to increase production of mid-quality water for irrigation that is currently produced more expensively by potable water plants.
Presently, there are a number of membrane bio-reactor plants operating at over eight million gallons per day (8 MGD), and a 12 MGD plant is reportedly tinder construction in Europe. Newer hotels have been engineered to have two sets of plumbing, one for potable water and one for recycle water for Such uses as toilet flushing.
Most cities in North America that are growing have segregated surface drainage lines and sewer lines. Wherever there is an existing still-face water drain line, it is feasible to run a small diameter recycle water return line inside the much larger drain line, without the high costs associated with excavation and new line placement. In this development model, cross contamination is not a significant concern, because when it is raining the recycle water is not required. Furthermore, the recycle line is pressurized with a higher quality water than the runoff water. The less expensive recycle water can be delivered to most locations in the city for use in irrigation and/or maintenance of streets, golf courses, parks, sod farms, nurseries, lawns, etc.
Alternatively, small treatment plants, such as those using improved long vertical shaft bio-reactors that provide tertiary treatment, could be strategically placed throughout the urban areas and could be privately owned and operated without municipal involvement. In low demand periods, they could discharge directly into the Surface water drains, thereby substantially reducing loads on municipal plants.
The improved long vertical shaft bio-reactors accomplish BNR treatment in a single integrated bioreactor that uses sequential zones, each dedicated to a specific part of the total treatment. Therefore each zone may be optimized individually.
Technological advances in membrane separation, processing, and treatment technologies have been occurring at a rapid pace. The flux rate of membranes (flow rate per sq. feet of membrane surface) has been increasing while the cost per sq. feet has steadily decreased. In addition, costs of membranes used in modern treatment and processing plants has been decreasing, and will decrease even more significantly over the next decade. These factors, taken together, will further encourage the use of membranes in the treatment of recycle wastewater, among other processes. For example, recent membrane bio-reactor (MBR) pilot plant trials at San Diego indicate that recycle water will cost $3.05/1000 gal and $1.92/1000 gal when produced in plants of 1 and 5 MGD size respectively. This cost includes amortization of capital, operating and maintenance costs based on year-round operations. At least one golf course in Seattle pays $3.96/100 cu. feet of potable water ($5.29/1000 gal) on a seasonal demand basis.
Membrane bioreactors require periodic cleaning to maintain their performance. The cleaning frequency depends on the type of membranes and their operating environment, and is typically as frequent as every few months. The existing reactors typically are not operational during membrane cleaning, causing a temporary and reoccurring loss of wastewater treatment capacity. Further, cleaning often involves use of expensive, specialized chemicals requiring compliance with environmental regulations in use and disposal.
The improved long vertical shaft bio-reactors offers distinct process advantages over other bioreactors, and there is a need for a method and apparatus incorporating membranes in such reactors.
Aspects of the present invention satisfy these needs and fulfill additional objects and advantages that will become apparent from the following description and appended drawings.
The invention provides methods and apparatus having improved though-put and operating life of submerged membranes used in biological treatment of waste waters, and increased time between cleaning and maintenance of the membranes. More specifically, the invention relates to membrane separation methods and devices employing a selective, semi-permeable, microporous, or other partitioning membrane for processing, refining, and/or treating liquid compositions, for example membrane waste-water purification processes and apparatus. Other aspects of the invention improve diffusion of a gas in a liquid by creating a substantially uniform pressure differential between opposite sides of a membrane.
Within one aspect of the invention a submerged membrane assembly and associated methods and apparatus are provided. The submerged assembly typically includes a membrane having at least a first surface and a second surface, which most often comprise opposing faces of a planar membrane. In certain embodiments the opposing surfaces of the membrane are square or rectangular, and the membrane has a vertical axis (e.g., a vertical defined by one side of a square-configured membrane or an elongated side of a rectangular membrane). The membrane is permeable between the first and second surfaces by molecules of less than a predetermined size.
Within other aspects of the invention, the submerged membrane assembly includes a first fluid compartment that contains a first fluid having a first specific gravity in fluid communication with the first membrane surface. The assembly also includes a second fluid compartment that contains a second fluid having a second specific gravity in fluid communication with the second membrane surface.
Additionally, the membrane assembly typically includes means for imposing a differential hydraulic head between the first fluid contained in the first compartment and the second fluid contained in the second compartment, and means for changing the second specific gravity. The differential hydraulic head imposing means may include the first fluid compartment, wherein the first fluid compartment defines a first column height, and the second fluid compartment, wherein the second fluid compartment defines a second column height. The second column height may be selected relative to the first column height to produce a selected pressure differential across the membrane along the vertical axis of the membrane at the first specific gravity and a changed second specific gravity (i.e., the second specific gravity altered from an initial second specific gravity value to the changed second specific gravity value by operation of said means for changing the second specific gravity). The first column height and the second column height may each be established solely by gravity and construction and design of the first and second fluid compartments (typically by having an outflow or overflow port or opening in the second fluid compartment that is lower in correspondence to the membrane vertical axis than a fluid column height in the first fluid compartment). The differential hydraulic head imposing means may alternatively include a means for applying a pressure differential between the first and second fluid compartments. For example, a negative pressure generating means or vacuum may be applied to the second compartment or fluid to generate a reduced pressure in the second fluid compared to fluid pressure of the first fluid in the first compartment. Alternatively, a positive pressure generating means or pressurizing device may be applied to the first compartment or fluid to generate an elevated pressure in the first fluid compared to fluid pressure of the second fluid in the second compartment.
Within various embodiments of the invention, the second specific gravity changing means may include a means for directly or indirectly introducing gas into the second fluid in the second compartment. For example, gas can be directly dissolved in the second fluid or directly introduced into the second fluid in the from of bubbles, thereby reducing the second specific gravity to the desired, changed second specific gravity value. Typically, the first fluid contains a dissolved gas, and gas is introduced from the first fluid to the second fluid by passing through the membrane from the first side to the second side, either in solution or in the form of microbubbles or larger gas bubbles. In certain embodiments, dissolved gas (e.g., air or oxygen) in the first fluid passes between the first and second surfaces of the membrane and, at or near the second surface, nucleates to form gas bubbles that are incorporated in the second fluid. When the gas introducing means thus involves transfer of dissolved gas from the first fluid into the second fluid, the gas can nucleate at or near the second membrane surface, which may include nucleation between the first and second membrane surfaces, at the second membrane surface, within the second fluid compartment, and/or dissolution of the gas within the second fluid. The gas introducing means can alternately achieve dissolved gas introduction from the first to the second fluid without dissolution of the gas and formation of bubbles, which can alternatively take place after the gas introduction or not at all. In yet additional embodiments, the dissolved gas of the first fluid may nucleate in response to a mechanical action impaired by passing through the membrane, in response to a pressure differential across the membrane, or in the second fluid in response to a difference. In dissolved gas levels between the first fluid and the second fluid. In certain other embodiments, the means for changing the second specific gravity may include a gas introduction port coupled to the second fluid compartment for introduction of gas into the second fluid. Gas can be introduced into the second fluid via this gas introduction port in the form of pressurized gas or in other forms, for example by introducing a gas-saturated fluid that mixes with the second fluid.
Another aspect of the invention provides a submerged membrane assembly. The submerged membrane assembly includes a membrane having a first surface, a second surface, and a vertical axis, and which is permeable between the surfaces by molecules of less than a predetermined size. The assembly further includes a first fluid compartment in fluid communication with the first membrane surface that contains a first fluid having a first specific gravity at a first column height, a second fluid compartment in fluid communication with the second membrane surface that contains a second fluid having a second specific gravity at a second column height, and means for changing the second specific gravity. The second column height selected relative to the first column height to produce a selected pressure differential across the membrane along the vertical axis at the first specific gravity and the changed second specific gravity. The second specific gravity changing means may include a gas added to the second fluid, and the gas may be added by direct or indirect introduction of gas into the second fluid (typically in bubble form, but optionally in an initially dissolved form). In exemplary embodiments, the second specific gravity changing means includes a gas added to the second fluid by a dissolved gas of the first fluid permeating through the membrane and nucleating proximate to, or within, the second fluid. The gas may nucleate at or near at least a portion of the second surface of the membrane and optionally impart a desired scouring action on the membrane by nucleation (either between the first and second membrane surfaces in the event nucleation occurs within the membrane, or more typically at or near the second membrane surface) and/or by the mechanical effects of bubbles rising in the second fluid.
The membrane assembly may optionally include a gas inlet port coupled to the second fluid compartment for direct introduction of gas (e.g., dissolved in a fluid, or in pressurized gas form) into the second fluid.
The assembly may further include a fluid collector that collects fluid from the second compartment, for example through an overflow port at or near the second fluid column height. In certain embodiments, the first fluid compartment may be a head tank or a saddle tank of a vertical bioreactor or other wastewater treatment apparatus.
For use in wastewater treatment applications, the membrane assembly of the invention typically includes a semi-permeable membrane that excludes particle exchange between the first and second surfaces (permeation) by particles of a size greater than a selected size indicated for the processed (effluent) water. For most treated wastewater, the selected membrane pore size will be less than or equal to about 2 microns, more typically less than or equal to 0.5 microns, and often less than or equal to 0.1 micron. The membrane may include any of a variety of commercially available membranes for use in wastewater treatment applications, for example a flat plate membrane, or a hollow fiber membrane.
In related aspects of the invention, the submerged membrane assembly includes a membrane having a first surface, a second surface, and a vertical axis, and is permeable between the first and second surfaces by molecules of less than a predetermined size. The assembly includes a first fluid compartment in fluid communication with the first membrane su-face which contains a first fluid having a first specific gravity at a first column height. The assembly also includes a second fluid compartment in fluid communication with the second membrane surface which contains a second fluid having a second specific gravity at a second column height. The second fluid contains, or is altered to contain, a gas in an amount sufficient to adjust the second specific gravity to more closely approximate the first specific gravity. In exemplary embodiments, the gas contained in the second fluid is in the form of gas bubbles. A fluid collector is fluidly connected to the second compartment at the second fluid column height to collect fluid from the second compartment. The second column height is selected relative to the first column height to produce a selected pressure differential across the membrane along the vertical axis. The first fluid compartment further may include a first fluid outflow at the first column height. The first fluid may include dissolved gas. The gas in the second fluid may include bubbles formed by a dissolved gas of the first fluid that has permeated the membrane and nucleated (within or proximate to the second fluid, for example by nucleating at or near the second membrane surface). A gas bubble rising in the second fluid may impart a cleaning action on the second membrane surface. The second fluid compartment may include a gas inlet port to introduce gas directly into the second fluid (as an alternate, or complementary gas introduction means to gas that permeates between the first and second membrane surfaces from the first fluid. The first column height and the second column height may be established without a mechanical device, e.g., solely as determined by gravity, or by application of negative pressure to the second fluid or positive pressure to the first fluid.
The methods and devices of the invention are broadly applicable within fluid treatment methods and devices. In various treatment processes and devices where membranes are employed, where fluids containing solids tend to foul the membranes or where clean fluids have a slow permeate rate, the invention provides substantial advantages. In the case of drinking water, membrane run time can be extended by adding CO2 to the first and/or second fluids, which is also desirable for pH adjustment of the water. Industrial filters, for example filters to remove sediment and precipitated protein from chilled beer, this will also be advantageous for recarbonation prior to bottling. Inert gas filtration, such as gasoline purification using nitrogen gas, is also amenable to optimization using the methods and devices of the invention. In this case, a gas recovery system is provided downstream of the membrane, and a repressurization system may also be employed. Nitrous oxide may also be employed as an added gas (e.g., as a gas introduced into the second fluid) to yield desired fuels/additives.
In the case of viscous fluids, such as lubricants, processing of such fluids will also be facilitated by the methods and devices of the invention, particularly by using an inert gas within said methods and devices. Inert gases, such as nitrogen, argon, helium, carbon dioxide, are all candidates for such applications. Active gasses, such as methane, are only sparingly soluble in water, and therefore will have more limited uses within the invention. Some gasses are sensitive to pH changes. For instance, bicarbonate of soda dissolves in water without pressure but a shift in pH will release CO2 in the same fashion that pressure changes do.
Other fluid processing technologies to which the methods and devices of the invention can be applied include, for example, desalinization plants, biotechnical and biomedical separation procedures (e.g., dialysis of blood and other body fluids), and environmental decontamination processes (e.g., oil and other petroleum contaminant removal from marine and flesh water sites).
In more detailed aspect of the invention, methods for treating fluids by membrane separation are provided that employ a selective, semi-permeable, microporous, or other partitioning membrane for processing, refining, and/or treating liquid compositions, for example membrane waste-water purification processes and apparatus. These methods include containing a first fluid having a first specific gravity, containing a second fluid having a second specific gravity, separating the first fluid from the second fluid with a permeable membrane having a first surface in fluid communication with die first fluid, a second surface in fluid communication with the second fluid, the membrane further having a vertical axis and being permeable between the surfaces by molecules of less than a predetermined size. The method further includes imposing a differential hydraulic head (e.g., passively by gravity and differential chamber overflow levels, or actively by application of positive or negative pressure as described herein) between the first fluid and the second fluid, adjusting the second specific gravity (typically by introduction of gas), and collecting the second fluid. Imposing the differential hydraulic head may further include containing the first fluid at a first column height, and containing the second fluid at a second column height, wherein the second column height is selected relative to the first column height to produce a selected pressure differential across the membrane along the membrane vertical axis at the first specific gravity and the adjusted second specific gravity.
Another aspect of the invention provides a method of treating a fluid by membrane separation employing a selective, semi-permeable, microporous, or other partitioning membrane for processing, refining, and/or treating liquid compositions, for example membrane waste-water purification processes and apparatus. The method includes containing a first fluid having a first specific gravity at a first column height, and containing second fluid having a second specific gravity at a second column height. The method includes separating the first fluid from the second fluid with a permeable membrane having a first surface in fluid communication with the first fluid, and a second surface in fluid communication with the second fluid. The membrane has a vertical axis and is permeable between the Surfaces by molecules of less than a predetermined size. The method further includes adjusting the second specific gravity to more closely approximate the first specific gravity in value. Alternate normalization of specific gravities between the first and second fluids can be achieved in other ways, for example by introduction of non-gaseous solutes into the first fluid. In certain embodiments, the second specific gravity is adjusted to within approximately +/−5 percent of the first specific gravity (i.e., to a value that is 95% of the value of the first specific gravity). In another embodiment, the second specific gravity is adjusted to within approximately +/−2.5 percent of the first specific gravity. The method also includes production of a selected pressure differential across the membrane along its vertical axis at the adjusted second specific gravity, for example by providing or selecting a second column height that differs from the first column height. In more detailed embodiments, the method further includes collecting the second fluid, for example by overflowing or off-draining the second fluid as a processed effluent.
Another aspect of the invention provides an improved vertical shaft bioreactor and associated methods for treatment of wastewater. The vertical bioreactor and associated methods are as described herein, above. The bioreactor receives an influent of wastewater containing biodegradable matter for treatment and produces an effluent flow which is directed to a submerged membrane assembly of the invention. The improvement in the bioreactor includes a membrane-adapted head tank that functions as a normal vertical shaft bioreactor head tank but is modified to receive and contain the effluent flow and removably receive the submerged membrane. The submerged membrane includes a permeable membrane having a first surface, a second surface, and a vertical axis, and which is permeable between the surfaces by molecules of less than a predetermined size. The first membrane surface is in fluid communication with the effluent flow in the head tank, and the second membrane surface is in fluid communication with a second fluid having a second specific gravity and contained in a second fluid compartment. The improvement includes a means for imposing a differential hydraulic head between the effluent flow contained in the tank and the second fluid contained in the second fluid compartment, and a means for adjusting the second specific gravity. In more detailed embodiments, the improvement also includes a fluid collector that collects the second fluid.
In other detailed aspects the invention provides an improved bioreactor for treatment of wastewater, the bioreactor receiving an influent of wastewater containing biodegradable matter for treatment and producing effluent flow having a first specific gravity. The improvement includes a tank that receives and contains the effluent flow at a first column height, and that removably mounts a submerged membrane assembly, and a fluid collector that collects the second fluid. The submerged membrane assembly includes a permeable membrane having a first surface, a second surface, and a vertical axis, and which is permeable between the surfaces by molecules of less than a predetermined size. The first membrane surface is in fluid communication with the effluent flow. A second fluid compartment (separated by the membrane from the head tank) contains a second fluid having a second specific gravity at a second column height, and the second membrane surface is in fluid communication with the second fluid. The improvement further includes a means for adjusting the second specific gravity. The second column height is selected relative to the first column height to produce a selected pressure differential across the membrane along the vertical axis at the changed second specific gravity. A portion of the contained effluent flow may be exposed to a normal atmospheric pressure.
In yet additional detailed aspects the invention provides a submerged membrane gas diffusion apparatus. The apparatus includes a membrane having a first surface and a second surface, and a vertical axis, and which is permeable between the surfaces by molecules of less than a predetermined size. The apparatus includes a first containment member, typically a tubular containment member, having a bubble capture aperture, a first membrane mounting portion in fluid communications with the first surface of the membrane, and a first chamber in fluid communication with the first membrane mounting portion and the bubble capture aperture, the chamber including a rising gas bubble capture portion proximate to the bubble capture aperture and having a first vertical length. The apparatus further includes a second containment member, typically a tubular containment member, having a gas release aperture, a second membrane mounting portion in fluid communication with the first surface of the membrane, and a second chamber in fluid communication with the second membrane mounting portion and the gas release aperture, the chamber including a gas reservoir portion proximate to the gas release aperture and having a second vertical length that is less than the first vertical length. Notably, the first and second containment members can be constructed and dimensioned according to a variety of designs to function in the manner disclosed herein below, whereas the tubular design described herein is provided for exemplary purposes only.
Another aspect of the invention provides a submerged membrane gas diffusion assembly. The assembly includes a membrane having a first surface and a second surface, and a vertical axis, and which is permeable between the surfaces by molecules of less than a predetermined size. The assembly includes an aeration compartment that contains a first fluid and rising bubbles of a gas, a static fluid compartment that contains a second fluid, and a fluid treatment compartment that contains a fluid to be treated in fluid communication with the second membrane surface. The assembly also includes a first tubular member having a bubble capture aperture located in the aeration compartment, a first membrane mounting portion in fluid communication with the first surface of the membrane, and a first chamber in fluid communication with the first membrane mounting portion and the bubble capture aperture, the chamber including a rising gas bubble capture portion proximate to the bubble capture aperture and having a first vertical length. The assembly further includes a second tubular member having a gas release appetite located in the static fluid compartment, a second membrane mounting portion in fluid communication with the first surface of the membrane; and a second chamber in fluid communication with the second membrane mounting portion and the gas release aperture, the chamber including a gas reservoir portion proximate to the gas release aperture and having a second vertical length that is less than the first vertical length.
A further aspect of the invention provides a method for diffusing a gas into a target fluid. The method includes permeably separating the target fluid from the gas with a membrane, the membrane having a first surface in contact with the gas, a second surface in contact with the target fluid, and which is permeable between the surfaces by molecules of less than a predetermined size. The method also includes capturing the gas by receiving a first fluid that includes rising bubbles of the gas into a bubble capture aperture of a first chamber, the first chamber including a rising gas bubble capture portion proximate to the bubble capture aperture and having a first vertical length. The method further comprises imposing a hydraulic head on the gas in the first chamber using a buoyancy of the gas in the first fluid to displace the first fluid from the bubble capture portion. Imposition of the hydraulic head forces the gas to flow between the gas bubble capture portion of the first chamber and a first membrane mounting portion of the first chamber, which is in fluid communication with the first surface of the membrane. The method further includes permeation of at least a portion of the gas through the membrane and into the target liquid in response to imposition of the hydraulic head. In addition, the gas flows between a second membrane mounting portion, which is in fluid communication with the first surface of the membrane, and a second chamber. The second chamber has a gas reservoir portion proximate to a gas release aperture and a second vertical length that is less than the first vertical length. The method automatically releases the gas through the gas release aperture when the hydraulic head displaces a second fluid from the gas reservoir portion.
Additional aspects of the invention are set forth in detail in the following description and appended drawings.
As illustrated in the attached Figures, the instant invention provides a long vertical shaft bioreactor 10 for wastewater treatment. The bioreactor of the invention shares a number of structural and functional characteristics with previously described vertical shaft bioreactor systems (see, e.g., U.S. Pat. Nos. 4,279,754, 5,645,726, and 5,650,070 issued to Pollock, each incorporated herein by reference), but departs in several important and novel aspects therefrom.
In reference to
The downcomer 12 and riser 14 channels are typically defined by separate conduits, for example by separate, cylindrical-walled pipes. Alternatively, they may be defined as interconnected compartments or chapels sharing one or more walls, for example as parallel channels separated by partitioning structures (e.g., radial partitions or septa) within an elongate, compartmentalized reactor vessel or frame. The downcomer and riser channels are preferably oriented substantially parallel to one another, for example in a side-by-side or coaxial relative configuration.
Typically, the downcomer 12 and riser 14 channels are defined as separate conduits over at least a portion of their lengths. In one example, the downcomer channel is defined by a separate, cylindrical-walled downcomer conduit (e.g., a steel pipe) 22 nested coaxially within a larger diameter, cylindrical walled riser conduit 24 (which will often correspond to an outer wall or casing of the entire bioreactor assembly). As such, the attached Figures are generally to be interpreted as schematic illustrations, wherein for ease of illustration the drawings which show the downcomer conduit laterally displaced relative to the riser conduit are intended also to schematically illustrate an alternative, parallel or coaxially nested configuration of the downcomer conduit within the larger riser conduit.
In one embodiment of the invention adapted for residential use, the waste water treatment bioreactor 10 of the invention is constructed to service a small residential community of about 5,000 population. Typically, two parallel bioreactors are installed in accordance with EPA redundancy requirements, in vertical in-ground shafts bored using conventional drilling technology. In various embodiments, the bioreactor of the invention can be constructed, configured with secondary features, or adjusted to provide the secondary and/or tertiary levels of treatment, listed below.
a) Secondary treatment (BOD and TSS removal) only.
b) Secondary treatment with nitrification of ammonia (conversion of ammonia to nitrate).
c) Secondary treatment with nitrification and denitrification (removal of ammonia and nitrate).
d) Secondary treatment with nitrification, denitrification, and chemical phosphorus removed (tertiary treatment). Some biological phosphorus removal will occur at low loads.
e) Thermophilic aerobic digestion and pasteurization of sewage sludges to produce class A biosolids.
In brief reference to the following description, the secondary treatment of a) above may be completely aerobic both in the zone 1 head tank 16 and downcomer channel 12 of zone 1, and in the zone 2 upflow channel(s) 82 and head tank 15. This configuration requires a shaft of about 30 inches diameter and 250 feet deep, a zone 1 head tank of about 6 feet diameter×10 feet deep and a concentric zone 2 head tank of about 12 feet diameter×10 feet deep. The concentric clarifier is about 28 feet diameter×10 feet deep and is fitted with a race mechanism to assist in sludge removal. In more detailed embodiments, this reactor will treat residential sewage from at least a 2,500 member human population and produce<30 mg/L TBOD and <30 mg/L TSS.
The secondary treatment process of b) is also completely aerobic and of the same general dimensions as a) except the zone 2 head tank is about 16 feet in diameter. A larger portion of the air originating at the bottom of zone 1 is diverted into zone 2 using a diverter mechanism 84. The treatment system of c) above is designed for anoxic conditions in the head tank and downcomer of zone 1. In certain embodiments, this reactor will treat residential sewage from at least a 2,500 member human population and produce<1 mg/L ammonia-N, <15 mg/L TBOD, and <15 mg/L TSS.
Only a small fraction of air from the lower portion of zone 1 is diverted into the zone 1 upflow channel(s) 40. In addition to raw influent feed in the upper end of zone 1, recycled nitrified effluent or return activated sludge from the carrier or, alternatively from zone 2 head tank, is added to the raw influent to create the anoxic conditions.
In this treatment process the reactor is enlarged to approximately 36 inches in diameter, zone 1 head tank is increased to about 8 feet diameter, zone 2 head tank is increased to about 16 feet in diameter. The concentric clarifier has an outside diameter of about 30′ and is fitted with a rake mechanism. In more detailed embodiments, this process will treat residential sewage from a human population of 2,500 or greater to <5 mg/L TKN, <10 mg/L TBOD, and <10 mg/L TSS.
The treatment system of d) above is the same general dimension of c). Within the treatment process of d), alum of ferric chloride may be added into zone 2 for chemical precipitation of phosphorus. It is usually uneconomic to use only a biological phosphorus removal process alone to achieve a high degree of phosphorus removal (e.g., 2-3 mg/L residual) on small plants, since a pre-fermentation step to produce volatile fatty acids (VFA) may be required. Typical characteristics of effluent from this plant are: TBOD<10 mg/L; TSS<10 mg/L; TN<5 mg/L; PO4<1 mg/L.
In the case of sludge treatment e), the reactor is reconfigured such that zone 1 surrounds zone 2, or may be adjacent to zone 2 throughout the major portion of the reactor length and zone 2 head tank 15′ surrounds the zone 1 head tank 16′. Zone 1 and zone 2 are hydraulically connected at the bottom of zone 2 through a self batching air lock device which precludes zone 1 contents from entering zone 2 while processing each batch. The thermophilic aerobic digester volume of configuration e) is about one half the volume of the wastewater treatment reactor producing the biomass. Because sludge storage provision is more economic to build than redundancy in reactors, only one digester is required for two treatment reactors. Accordingly the small town of about 5000 people requires 2 treatment reactors and 1 sludge digester all of the same size. The foregoing example is a typical design for small communities of about 5000 people.
Since about 80% of the voidage (air lift) occurs in the top 80-100 feet of any air lift reactor, the superior channels can be effective between 150 and 50 feet deep, preferably 80-88 feet which is the standard length of two joints of double random length pipe. Off the shelf air compressors are readily available in 100, 125 and 150 psi models corresponds to shaft depth of 200, 250 and 300 feet. Although airlift bioreactors have been built between 60 feet and 500 feet depths, a more common range is 150 to 350 feet depth and a range of 200 feet to 300 feet is now most common.
Conventional water well rigs can drill holes up to about 48 inches and deep foundation equipment for pilings can drill up to about ten feet in diameter. Augers (where geology permits) can drill up to about 20-feet diameter but are limited to about 200-feet depth. Mined shafts can be tip to 30 feet diameter and of virtually any depth.
Small municipal plant reactors (5000 population) will typically be placed with conventional water well rigs and preferably be about 24 to 48 inches in diameter.
Larger communities (10,000-50,000 population) may require shafts of 5 to 10 feet diameter×200 feet depth placed by deep foundation piling machines and augers, whereas very large industrial plants (e.g. pulp mills) may require shafts placed by mining techniques.
The long vertical shaft bioreactor 10 of the invention receives influent, typically wastewater or sludge, through an influent conduit 30 which introduces the influent into an influent channel 32. The influent flows downward to the bottom of the influent channel, where it exits through a shielded influent port 34 and combines with upflow in a zone 1 upflow channel 40 delineated at its lower end by the influent poll. The influent port is uptunred or otherwise shielded to prevent admission of bubbles from below the zone 1 upflow channel from entering the influent channel.
In alternate embodiments of the invention, the influent channel 32 can optionally accept recycle flow of liquor from the head tank 16 portion of zone 1 of the bioreactor 10. This flow is regulated by a zone 1 recycle flow regulator 50, for example a manual or motor-actuated baffle, valve or other flow-regulating apparatus. In this context, the influent flow through the zone 1 recycle regulator 50 is ordinarily throttled via an influent flow throttling control mechanism. This can include, for example, a system control unit 51 (e.g., a system control microprocessor) operatively linked to a valve or baffle actuator 52 and an optional flow sensor 53 or 53′ for determining influent and/or zone 1 recycle flow or alternatively dissolved oxygen DO probe 49 to monitor oxygen levels. Control of influent flow through the regulator functions in part to adjust the air lift in zone 1 upflow channel 40 and facilitate gravity influent flow. The combined flow in the zone 1 upflow channel contains some anoxic air bubbles (see below) and is therefore lighter than the fluid in influent channel 32, and rises.
By anoxic air bubbles is meant bubbles predominately containing gasses other than useable oxygen. Flow in the zone 1 upflow channel 40 traverses a horizontal degas plate 54 and descends substantially free of entrained bubbles in the downcomer channel 12 under gravity and enters the main riser channel 14 in the vicinity of the mix zone 18, where it is intensively aerated.
The compressed air or other oxygen-containing gas or liquid serving as the oxygenation source for the bioreactor 10 is typically delivered through one or more dedicated oxygenating lines, typically compressed air lines 62. A dedicated compressed air line is connected to a compressed air supply at the surface and runs downward parallel to the riser channel (e.g., nested within the riser conduit 24) extending to an oxygenation port, typically an air deliver) port 64, that opens in fluid connection with the riser channel 14. The air delivery port 64 is generally positioned beneath the air distribution header 60 to release the compressed air for dispersal by the header, as described above. Within certain embodiments of the invention, compressed air (or other oxygen-containing gas or liquid) is optionally, or additionally, delivered within the bioreactor by a dual-service aeration/solids extraction line 66. Functioning of this line can be controlled, e.g., by a system control unit 51 as described above, to optionally deliver compressed air or other oxygen-containing gas or liquid and, in a second operation mode, serve as a waste solids extraction line 66 to purge waste solids from a sump 67 portion of the reactor located at (e.g., bottom of the riser channel. The waste solids extraction line extends from the surface (e.g., from a surface-located, waste-solids extraction/flotation reservoir) to a aeration/waste solids extraction port 68 opening in fluid connection with the stump. Solid particles that settle into the sump will accumulate over a period of hours of operation. For the majority of the bioreactor's operation time, the aeration/solids extraction line is continuously purged by flow of compressed air, and therefore the stump 67 is substantially mixed and aerated and forms a functional part of the mix zone 18. Periodically, the aeration/extraction line can be depressurized, whereby settled solids within the sump will rush to the top of the reactor to be purged therefrom. These solids are highly aerated, well stabilized (odor free) and because of the high gas content will spontaneously float to a thickened sludge.
In related embodiments of the invention, the improved vertical shaft bioreactor 10 features two simultaneously-operating aeration lines or ports to enhance the formation of small, dispersed bubbles to generate upflow currents and supply process air within the bioreactor. The use of two aeration lines is exemplified by the dedicated compressed air line 62 and dual-function aeration/solids extraction line 66, which each operate at least for a majority of the bioreactor process time in a compressed air delivery mode. In this mode, the two lines in concert provide a cooperative, multiple source compressed air injection mechanism of the invention, which serves to enhance the turbulence and small bubble-forming capacity within the mixing zone 18 of the reactor, which is in turn expanded by the cooperation of multiple compressed aeration lines or ports. In one aspect of this enhanced mixing/bubble forming mechanism, a first aeration line opening, exemplified by the air delivery port 64 of dedicated air line 62, is positioned below the air distribution header 60 and above a second aeration line opening, exemplified by aeration/extraction port 68 of the dual-function aeration/solids extraction line. Compressed air released from this lower aeration port stimulates fluid mixing and bubble formation near the bottom of the riser channel 14 to set up a first circulation path or vector. The resultant circulating fluid-bubble mixture impinges upwardly and/or transversely against mixed fluid and bubbles generated by the introduction of compressed air from the first, upper air line 62. This results in increased shear forces and the production of smaller air bubbles in an enlarged mixing zone, compared to the results achieved by operation of a single aeration line (see,
In conjunction with the above-described use of a cooperative, multiple source compressed air circulation regime, certain embodiments of the invention incorporate a modified (typically stepped, chambered, or baffled) header, or a multi-component header-complex, to augment the enhanced mixing/bubble forming mechanism provided by multiple, interactive aeration sources. In one aspect, a second, cooperating shear header 70 is mounted within the riser chamber 14 below the main bubble distribution header 60 and works in conjunction with two, vertically tiered aeration sources generally as described above. The shear header can be any flow diverting or channeling device that enhances an upward and/or transverse or radial flow component within the mixing zone generated by a second, lower-positioned aeration source (exemplified by the aeration/solids extraction port 68). In one exemplary embodiment, the shear header comprises an internally stepped draught tube (
Yet additional embodiments of the invention are distinguished by vii-tie of their novel features for channeling, circulating, and segregating fluid, air and/or biomass within the reactor 10. These features are in turn variable, combinable in alternative reactor configurations, and/or adjustable within additional aspects of the invention—allowing use or modification of the reactor for different wastewater treatment applications and results. In general aspects, the bioreactor of the invention features a first treatment or processing “zone” designated zone 1, wherein the majority (e.g., greater than 80%, up to 90-95% or greater) of the primary reaction between waste, dissolved oxygen, nutrients and biomass (including an active microbial population), takes place. Within certain embodiments, this zone is defined to include an upper circulating zone of the bioreactor comprising the surface basil or head tank 16, a primary reaction chamber 80 comprising a central volume of the riser channel 14, the downcomer channel 12, and the mix zone 18.
The majority of the contents of the mix zone 18 represent a fluid-bubble mixture that is less dense than the fluid in the downcomer channel 12 and therefore circulates upwardly from the mix zone into the primary reaction chamber 80. Undissolved gas, mostly nitrogen, expands to help provide the gas lift necessary to drive circulation of the liquor in the upper part of the reactor 10 in the patterns as shown by the arrows throughout the Figures. The products of this primary reaction are carbon dioxide and additional biomass which, in combination with unreacted solid material present in the influent wastewater, forms a sludge (or biosolids).
In certain embodiments of the invention, as illustrated in
The diversion plate 84 is configured and dimensioned to segregate the primary reactor channel 80 upflow into multiple superior channels. Typically, the diverter plate is configured and dimensioned to intercept and divert a larger fraction of total upflow volume of the fluid-bubble mixture from the primary reactor channel into a selected “aerobic” upflow channel, depending on the desired mode of operation of the bioreactor 10, as further explained below. In the exemplary embodiment shown in
Selection, positioning and adjustment of the flow diverter mechanism depends on the selected mode of operation of the bioreactor 10. In alternative embodiments, the diverter plate 84 can be positioned, shaped, dimensioned and/or adjusted to channel upflow of the fluid-bubble mixture from the primary reactor channel 80 into one or more superior channels to achieve higher aerobic environmental conditions in the selected channel(s), while limiting the upflow (particularly of high oxygen-containing fluid) into one or more superior channels selected for lower aerobic, even anoxic, environmental conditions. By way of example, the following steady state functionality of adjustable baffles 86 and 84 is described. In
Therefore the oxygen demand per bubble oxygen unit is higher in head tank 16 than in head tank 15 by 83/19=4.3 times. Consequently, if there is measurable dissolved oxygen in head tank 16 there will be surplus DO in head tank 15, and if there is surplus DO in head tank 16 there will substantially more DO at any level below baffle 86 down to the mix zone 18. Thus baffle 86 can be adjusted to accommodate a wide range of load and flow criteria.
Thus, in one aspect of the invention, the improved long vertical shaft bioreactor functions for multi-purpose waste treatment by providing aerobic digestion of BOD as well as single mixed liquor processing BNR treatment. Referring to
Within this embodiment and adjustment/operation mode of the bioreactor 10, the superior channel referred to as the zone 1 upflow channel 40, can be selected to provide an anoxic environment, achieved in part by the low relative influx of oxygen and the high oxygen demand of the raw influent stream. This anoxic zone continues throughout the circulation path between the zone 1 upflow channel and the downcomer channel 12, as approximately indicated by the arrows in
Within the foregoing operation mode of the bioreactor 10, a major portion of the uprising air flow in the primary reactor channel 80 flows upward into the other superior up now channel(s), exemplified by the zone 2 upflow channel 82. The relative lower liquid upflow fraction thus segregated includes the majority of bubbles originating at the lower end of zone 1 (e.g., bubbles generated by the dedicated air line 62 and optional multipurpose aeration/waste solid extraction line 66, functioning in conceit with the bubble distribution header 60 and optional shearing enhancer mechanism exemplified by the shear header 70). This active, fluid-bubble mixture segregated into zone 2 by operation of the diverter 84 enters the zone 2 upflow channel, then mixes with vigorous recirculating flow entering zone 2 through a zone 2 recirculation channel 110 (which recycles liquor lion the zone 2 head tank 15). This recirculation flow is optionally regulated by a zone 2 recirculation flow regulator 112, for example a manual or motor-actuated baffle, valve or other flow-regulating apparatus. This recycle flow regulator is also optionally controlled by the system control unit 51 (e.g., system control microprocessor) operatively linked to a valve or baffle actuator 52 and optional flow sensor 53 for determining zone 2 recycle flow).
When the bioreactor 10 is thus configured and/or adjusted for BNR removal, nitrification of mixed liquor can be efficiently conducted and controlled within zone 2 of the bioreactor, in accordance with the above-described construction and operation details. Some of the mixed liquor from zone 2 may be discharged to a detached 120 or integrated 120′ solids-liquid separator (clarifier) (see, e.g.,
In more detailed embodiments of the invention, influent, return clarified effluent (e.g., recycled from a separate clarifier 120 or integrated clarifier 120′), and return activated sludge are combined in a preselected ratio to facilitate operation of the bioreactor 10. This can be achieve using various flow control features of the invention, and is facilitated in part by incorporation and controlled operation of a zone 1 activated sludge return channel 122 and a zone 2 activated sludge return channel 124 which receive activated sludge (e.g., via a sludge extractor line 126 connected to the clarifier) and direct the sludge into the zone 1 influent channel 32 or zone 2 recycle channel 110, respectively (see, e.g.,
The selected mix ratio per volume of influent of typical municipal waste may be as high as 3 volumes of clarified effluent and 1 volume of return activated sludge to as low as 1 volume of clarified effluent and 1 volume of return activated sludge. Approximately 85% of total nitrogen will be converted to N2 with 1.75 volumes of either clarified effluent or mixed liquor per volume of influent (see, e.g., Naohiro Taniguchi et al. report on air lift recirculation for nitrification and denitrification, R&D Division, Japan sewage works agency 1987, incorporated herein by reference.) It should be noted, however, that some industrial wastes may require 100 or more recycled volumes per volume of influent.
With respect to the nitrification process functions of the bioreactor 10, this can be further modified or enhanced by selection or adjustment of the various reactor features and operation parameters described above. In addition, the system can readily incorporate, or be coupled with, additional system features or components to enhance BNR process functions. Because the BOD is low in zone 2, growth of BOD-removing organisms is generally minimized, which allows nitrifying bacteria to dominate the biomass. In addition to this advantage, a substantial improvement in the rate of conversion of ammonium to nitrite and nitrate can also be realized by increasing the concentration of nitrifying bacteria. Since nitrifiers are attachment organisms, the provision of attachment sites in a mixed liquor in the form of sponge balls, suspended media, bits of small diameter plastic or rubber (elastomeric) polyethylene tubing, hanging strings of porous fabric in the liquor, etc., can be used quite effectively within the devices and methods of the invention (see, e.g., Keith Ganze “Moving Bed Aerobic Treatment” Industrial Waste Water November/December 1998, incorporated herein by reference.) For example, referring to
Within additional aspects of the invention, a novel nitrification process is provided which relies substantially or entirely upon residual dissolved oxygen originating near the bottom of zone 1 as the source of oxygen to drive the process. Yet another important benefit and distinction that arises by using the unspent gases from zone 1 in this fashion is the high level of CO2 available, which is also required by nitrifying bacteria as a source of inorganic carbon. In other nitrification systems, the primary inorganic carbon source depends on alkalinity of the wastewater and is typically determined by the presence of CaCO3. The bioreactor process systems of the invention are therefore more compact and require less energy than current, extended aeration systems. Bioreactors constructed and operated according to the invention also produce a better quality biomass (including class A biosolids if desired) that is easier to separate from the mother liquor.
To further enhance the functions and operation of the bioreactor 10 of the invention, various coupled or integrated features can be incorporated with the bioreactor for enhanced processing of waste water. As illustrated in
Alternatively,
Alternate embodiments of the bioreactor 10 illustrated in
Typically, for long vertical shaft bio-reactors, the optimum biological air supply rate required for bio-oxidation process creates excessive “voidage” at the top of the reactor, comparable in the present case to the superior upflow channels exemplified by the zone 1 upflow channel 40 and zone 2 upflow channel 82. Excessive voidage produces undesirable slugging (water hammer), which can cause reactor damage attributed to vibration. The occurrence of slugging air voidage also signifies poor oxygen transfer characteristics within the circulating fluids. The invention addresses these problems in a number of ways, including by providing novel means for regulating circulation velocities and modulating gas content in selected parts or channels of the reactor.
Since oxygen transfer rate and oxygen utilization rates are relatively slower than upward hydraulic velocities in the reactor 10, increasing velocity only reduces the operating efficiency of the reactor. Increased flow decreases bubble contact time and slows oxygen transfer, thus more aeration is required to optimize the process. Similarly, reducing aeration reduces reactor capacity. One proposed method for resolving air voidage and related problems is presented in U.S. patent application Ser. No. 09/570,162, filed May 11, 2000 (incorporated herein by reference) describing the “VerTreat II” bioreactor. In this disclosure, flow velocity is beneficially reduced by incorporation of an orifice plate in the lower section of the riser channel. However, this solution does not substantially resolve the problem of slugging, and the orifice plate creates additional problems including risk of fouling and flow aberrations particularly in small municipal plants.
The bioreactor 10 of the present invention resolves these problems in part by incorporating a novel relative configuration of zone 1 and zone 2. Unlike the previously described “VerTreat I” bioreactor (see, e.g., U.S. Pat. No. 5,650,070, issued Jul. 22, 1997, incorporated herein by reference), where zone 2 is below zone 1 and therefore no voidage control in zone 2 is possible, the present invention can control flow and gas content in each zone, independently. Conventional prior art “Deep Shaft” reactors start slugging at a upflow velocity of about 2 feet per second. The above-noted VerTreat II reactors with orifice plates can operate down to about one and a quarter feet per second. Within the present bioreactor, this value can be dampened to as little as one quarter to one half feet per second in the lower part of the riser channel. At lower riser velocities, some heavier solid particles will settle into the sump 67. These solids are conveniently extracted, along with surplus biomass (e.g., circulating within the shear header 70 and surrounding mix zone 18) when desired, by purging of the dual-purpose aeration/solids extraction line 66.
The invention provides substantially more efficient new features and methods for slowing velocity over prior art methods, which includes the ability to dilute the air lift stream in one or more superior upflow channel(s) of the reactor with bubble free fluid, as described above. The advantage of these features and methods over the VerTreat II technology includes the elimination of potential plugging of the orifice plate in the lower and inaccessible section of the riser channel, which is particularly problematic in smaller diameter reactors.
In long vertical air lift reactors such as the bioreactor 10 of the invention, where fluid/gas mixtures are caused to circulate in vertical channels, the volume of gas in a defined volume of liquid changes with the pressure (gas laws). Consequently at the bottom of the reactor, the volume of gas in liquid (voidage) is small, whereas at the top of the reactor the same expanded gas volume to liquid volume ratio is many times larger. Since 34 feet of water is equivalent to about one atmosphere of pressure, it can be readily calculated that 1 cubic foot of air on the surface (1 scf) becomes 0.5 cubic feet at 34 feet depth and 0.33 cubic feet at 68 feet and 0.25 cu. feet at 102 feet. Therefore integrating the area tinder the volume vs. depth curve shows 78% of the gas volume voidage occurs in the top 102 feet of the reactor.
Many studies on air-lift pumps and other bubble/water columns show that slugging in water occurs at 11-14% voidage. Slugging is undesirable because the bubbles coalesce into large air pockets which set up vibrations in the reactor, and most importantly, large bubbles have very poor oxygen transfer characteristics. Proposed controls of voidage to ameliorate these effects have been attempted in at least two different ways. One proposed control is to increase the reactor cross section sufficiently to allow disengaging the gas from the gas/liquid mixture. Alternatively, efforts have been undertaken to maintain residual pressure on the gas/liquid mixture at the top of the reactor. Each of these proposed controls have attendant drawbacks making them undesirable for use within the bioreactor of the present invention. For example, head tank designs of some air-lift reactors are provided where liquid depths of ½ atmosphere (17 feet) are used. This reduces the maximum voidage by 17%, but head tank depths much deeper than 17 feet are difficult to construct. In addition, tall head tanks above ground require pumping influent against a significant hydraulic head, wasting substantial energy.
The invention provides novel features and method for controlling voidage and ameliorating the adverse effects of slugging. Briefly, these features and methods reduce the quantity of bubbles per unit of fluid in one or more selected channels or chambers of the reactor 10, either by adding more fluid or reducing the gas. In more detailed aspects, liquid flow in one or more superior upflow channels of the reactor is increased by recycling liquor from an upper segment (e.g., 60-90′) of the reactor, through a degas step, and back down to a lower, recycling influx point near the bottom of the upper segment (e.g., 60-90 feet below the surface). It is generally considered that total gas flow (air flow) is determined by biological optimization requirements, however this total gas flow can also be proportioned into selected, superior upflow channels in the upper part of the reactor using novel flow control mechanisms described herein.
Because approximately 75-80% of the voidage occurs in the top 60-90 feet of the reactor, the recycle channels (exemplified by the influent channel 32 which optionally nested receives zone 1 recycle input from zone 1 recycle port 140, and the zone 2 recycle channel 110), are only about 25-35% of the total depth of a typical bioreactor and occupy only a small fi-action of the reactor cross section area and volume. In practice, zone 1 and zone 2 of the reactor comprise approximately equal fluid volume, but in the case of BNR removal zone 2 is expanded in volume for nitrification by increasing the diameter of the zone 2 head tank 15. The voidage in the zone 2 recycle channel can be readily controlled under a wide range of operating conditions by designing for sufficient, adjustable recycle flow of degassed liquor from the zone 2 head tank 15 as regulated by the zone 2 recycle regulator 112. The bubble volume in the zone 1 upflow channel 40 can therefore be diluted by degassed liquor to the extent limited by the acceptable range of minimum and maximum values for influent flow, which is somewhat limited. To resolve this limitation, a regulated amount of liquor may be diverted through the zone 1 recycle port by adjustment of the zone 1 recycle flow regulator 50 (effectuated by operation of the system control unit 51). Controlling flow from the head tank in this coordinated manner is necessary to maintain gravity feed of the effluent.
The instant invention therefore provides a number of separate and optionally cooperative mechanisms and methods to alleviate the problems of slugging at low bioreactor 10 flow velocities. In another aspect, this problem is alleviated by providing a choice of adjustable diverter or baffle devices, exemplified by the fixed or adjustable diverter mechanism 84. The configuration (including size, shape, location and orientation) of this exemplary diverter plate can be fixed at the time of construction and installation of the reactor. Alternatively, these and other flow diverter parameters can be selectably altered, for example by employing a manual or motorized diverter plate adjustment mechanism optionally integrated for functional control (e.g., to control positional and orientation parameters) by the system controller 51. Operation of the flow diverter serves to direct a greater or lesser fraction of air bubbles entrained in the upflow from the primary reactor channel 80 into one or more selected superior channels, for example to divert a greater fraction of the fluid-bubble mixture toward the zone 2 upflow channel 82, allowing a lesser to pass upward into the zone 1 upflow channel 40.
Once the desired fraction of bubbles have been thus diverted into the zone 2 upflow channel 82, the voidage in this channel can be easily corrected by changing the amount of zone 2 recycle flow through adjustment of the zone 2 recycle flow regulator 1112. The circulatory loop (following arrows between zone 2 upflow channel 82, across zone 2 degas plate 150, through zone 2 recycle regulator 112, down zone 2 recycle channel 110, and through zone 2 shielded recirculation port 152), together with a surface basin or zone 2 head tank 15 at the top, comprise zone 2 and represent the polishing process and optional nitrification features of the bioreactor which are driven by waste gas from zone 1. The configuration of the diverter which segregates flow into the superior upflow channels prevents liquor transfer from zone 2 into zone 1, since both liquid and air flow in the zone 2 upflow channel 82 is unidirectionally upward. In this regard, as noted above, zone 2 circulation characteristics are ideal for the application of fixed media 132 (
Hydraulically, any influent flow into zone 1 of the bioreactor 10 (and any required external recycle streams from the clarifier 120 or zone 2 head tank 15) that enter zone 1 must leave zone 1 by entering the bottom of zone 2. Since zone 1 is a closed loop, namely zone 1 upflow channel 40, zone 1 head tank 16, downcomer 12 and primary reactor channel 80, the number of recycles in this loop and the liquid velocity depends directly on the volume of air bubbles diverted by diverter plate 84 into zone 1 upflow channel 40. For example, in a typical municipal effluent of 200 mg/L of BOD, the number of internal recycles is approximately the BOD in mg/L divided by the 0) potential in the reactor, divided by the oxygen transfer efficiency. In a 250 feet deep reactor, oxygen is injected at about 7.3 atmospheres of pressure. Solubility of O2 in water at 1 atmosphere and 20° C. is about 8 mg/L. This means the dissolved oxygen potential at 7.3 atmospheres is 7.3×8=59 mg/L or about 40 mg/L at an oxygen transfer efficiency of 70%. Therefore, the minimum number of recycles is 200 divided by 59×0.70=about 5. In practice 6 or 7 recycles might be used as a safety factor. A hydraulic loss calculation will determine the fraction of air required for 6 or 7 internal recycles; e.g., 30% of the air that is applied at the bottom of zone 1. As the organic load to the plant increases or decreases, the air rate is adjusted accordingly, causing the number of internal recycles to increase or decrease to satisfy the BOD requirement. However, 30% of the air applied remains consistent, constant as determined by diverter plate 84 placement. Field trimming is achieved, for example, by adjusting regulator valve 50, which changes recycle flow within the air lift section at zone 1 upflow channel 40, thus reducing or increasing its air lift capability.
Similarly, any flow from zone 1 that enters zone 2 must leave as effluent from zone 2. Since the lower portion of zone 2 comprising upflow channel 82 and adjacent downflow channel 110 typically has no internal recycle connection with zone 1, any air diverted from zone 1 into zone 2 will simply cause circulation in the superior channel(s) of zone 2 with no change in the circulation rate of zone 1 (change ii air rate in zone 1 does, however, affect the circulation rate in zone 2, but not vice versa).
Therefore, within certain aspects of the invention, diverting for example 70% of the air originating at the bottom of zone 1 into zone 2 only affects the circulation in zone 2 which can be easily controlled by the zone 2 recycle regulator 112. Hydraulically, influent flow into zone 1 upflow into zone 2 and effluent from zone 2 within the reactor 10 are equal in quantity, i.e., influent flow entering the reactor in zone 1 exits through zone 2. With reference to prior art vertical bioreactors treating municipal waste, the internal recycle flow is about ten to twelve times the influent flow, or effluent flow. The present process, which features novel air lift controls as described above, can reduce this flow by about a 2-3 fold reduction, often a 5-6 fold or even greater reduction.
By adjusting the configuration of the diverter (generally referring to any diverter device for segregating flow from the primary reactor channel 80 into a plurality of superior upflow channels), the selected bubble fraction only (not typically the same as the liquid flow fraction) in the primary reactor channel can be segregated among any desired number of channels (typically 2, 4 or 6, depending on reactor size and purpose) in any ratio selected to achieve optimum operation of zone 1 and zone 2 (note that each superior channel shown in
After diluting the zone 2 upflow, for example using 8 to 10 times the recycle flow from the zone 2 head tank 15 via the zone 2 recycle regulator 1112, the air lift effect in the zone 2 upflow channel can be readily controlled. This control depends on the novel mechanisms and methods set forth above for segregating flow in an aerated and flowing vertical column, providing for selectable channeling of flow in different proportions into two or more other superior vertical columns, while the air bubbles may be split in a completely different ratio among these vertical columns. This novel ability to control air lift allows a better biological match between oxygen supply (dependent on the time available at pressure to dissolve oxygen, which is in turn a function of flow velocity) and oxygen utilization which is a function of respiration rate, (dependent on dissolved oxygen—not primarily upon the amount of bubbles present).
Within yet another aspect of the invention, novel features and methods are provided for addressing the challenges involved in the disposal of by-product sludge and/or surplus bio-solids from the bioreactor 10 treatment processes. Recognizing the nutrient value of these biosolids, the EPA in the US adopted 40 CFR 503 in 1993, which proscribes various process criteria to achieve class A bio-solids for unrestricted use as a soil supplement. Whenever possible, beneficial reuse of bio-solids is encouraged. One set of criteria for Class A bio-solids requires a minimum volatile solids reduction, as well as a Time-Temperature relationship, for example a 38% volatile Solids reduction and a 60° C. temperature for 5 hours qualifies as a Class A product.
Within a modified embodiment of the invention, referring to
Published data demonstrate two areas of concern for existing vertical shaft bioreactors that seek to produce class A biosolids (see, e.g., Report on VerTad operations King County WA, project 30900 May 20001, incorporated herein by reference.) First, small vertical bioreactors (e.g., “VerTad reactors”, as described for example in U.S. patent application Ser. No. 09/570,162, filed May 11, 2000 (incorporated herein by reference), feature a relative disposition of zone 2 (polishing zone) below zone 1. These reactors have a comparatively large surface area to volume ratio, and excessive heat is lost to the surrounding geology. Small reactors therefore require supplemental heat to support class A biosolids production, which is available at additional cost by recapturing the waste heat from the compressor or from the hot effluent stream.
A second area of concern for previous vertical bioreactors directed to high quality biosolids production is that there is insufficient liquid to liquid separation between zones 1 and 2. Published data of tracer studies in VerTad reactors show that the zone 2 (polishing zone) behaves as a plug flow reactor, with a critical feature of localized back-mixing. Over a period of about 8 hours, zone 2 begins to mix with zone 1 and the whole system (zone 1 and zone 2) is mixed in 16-20 hrs. Accordingly, some solid particles, potentially containing salmonellae or other prohibited contaminants, can settle from zone 1 into zone 2 without being exposed to the required retention time at pasteurizing temperature to meet class A biosolids requirements.
The improved bioreactor/digester 10′ of the present invention is configured in a distinct manner with zone 1 surrounding zone 2 (
Within this aspect of the invention, it is considered critical that when the apparatus is being used as an aerobic thermophilic sludge digester, bubbles from zone 1 must not enter zone 2 because of the risk of re-inoculating the pasteurized product in zone 2. To prevent this from occurring, pressure in the air lock is maintained by fresh clean compressed air, and there is no liquid flow or contact between zone 1 and zone 2 or transmission of contaminated air from zone 1 to zone 2. The air lock is designed to prevent inter-zonal mixing of liquid between batches, ensuring that zone 1 does not re-inoculate the pasteurized biomass in zone 2 with pathogenic bacteria during batch processing. As an example, one batch of sludge may be processed every 5-8 hours, thus ensuring that the critical time temperature of 60° C. for five hours is always met within each batch.
In operation of this embodiment of the invention, waste biomass is fed continuously or intentionally into the reactor/digester 10′, e.g., into the zone 1 head tank 16′. As the head tank level in zone 1 rises above that of the zone 2 head tank 15′ level, a pressure differential develops across the center baffle 170 in the air lock. Eventually the zone 1 liquid level in the air lock exceeds the baffle height and fluid transfers from zone 1 to zone 2. Line 64 air supply is placed slightly below the liquid level of zone 2 within the airlock, whereby at the first onset of flow between zone 1 and zone 2, the bubbles are swept away into zone 2 and the air lock collapses. Flow stops when the head tank levels are again equal and the airlock re-establishes itself. A batch can also be initiated by draining the zone 2 head tank 15′.
When the bioreactor 10′ functions as a waste sludge digester (see, e.g.,
In addition to zone 1 and zone 2 being hydraulically separated by a diaphragm-less air valve (air lock 172), the lower portion of each zone functions as a pseudo plug flow zone while the top portion of each zone is circulated in the superior channels and is well mixed. As a result each of zone 1 and 2 is further divided into two additional smaller zones to double guard against reinocculation of the finished product with the raw influent. When the present invention is used as a sludge digester, baffle 86 extends to about 70-90% of the reactor depth and baffle 84 completely seals off the bottom of zone 2 from zone 1. For certainty that no cross contamination can occur, zone 2 may be further sealed with second outer wall 197 in close proximity to the outer casing 196 as shown in
The reactor/digester 10′ of
Once sludge enters the reactor/digester 10′ it has a mean residence time of approximately 2 to 3 days in zone 1, and 2 to 3 days in zone 2. The EPA criteria for the production of class A bio-solids dictates the time between batches, which varies with temperature—as an example the minimum residence time for a batch at 60° C. is 5 hours, or about 4.8 batches per day. Therefore, zone 1 and zone 2 theoretically contain between 9.6 and 14.4 batches each. In practice, however, each batch would be about 8 hours, and therefore zone 1 and zone 2 would contain between 6 to 9 batches each. The overall residence time is determined by the biodegradability of the sludge. For class A bio-solids, the process must achieve a minimum of 38% volatile solids reduction which typically takes 3.5-5 days. The batching time is determined by the temperature (see, e.g.,
As noted above, the air line 62 can be operated to maintain the air pressure in the air lock 172 of the reactor/digester 10′ to control batching. Stopping the air flow in line 62 will also trigger a batch discharge after the appropriate processing time has elapsed. A batch can also be triggered by lowering the liquid level in the zone 2 head tank 15′. Once the batch in zone 2 is discharged, the head tank level in zone 1 is automatically lowered an equal amount by the action of the automatic batching valve located between the bottoms of zone 1 and 2, and the cycle repeats. When a batch is processed through the reactor, it is reduced in solids content from approximately 4-5% down to about 2-3%. This product (class A biosolids) may then be de-watered.
Published research by The University of Washington (Guild et al., Proceedings of WEF Conference, Atlanta Ga., 2001, incorporated herein by reference) indicates that when thermophilic aerobic digested sludge from a vertical shaft reactor having certain features in common with the reactor of the present invention was fed to a mesophilic anaerobic digester, the retention time in the anaerobic digester was reduced, the overall volatile solids reduction was better, the dewaterability was better and required less polymer. The thermophilic aerobic digester is operated with a about a 2 day retention time and can generate enough heat to comply with Class A biosolids.
It is well documented that during the aerobic thermophilic digestion of biomass, there is minimal nitrification of ammonia at temperatures above 42° C. It is also well documented that in anaerobic digestion of biomass (where there is no air stripping), ammonia and carbon dioxide react to form ammonium bicarbonate. In a vertical aerobic thermophilic digester, it is reasonable to believe that ammonium bicarbonate also forms, due to large amounts of both ammonia and carbon dioxide remaining in solution due to pressure.
The selection of operating temperatures is very important in long, vertical thermophilic aerobic digesters because ammonium bicarbonate decomposes at about 60° C. Ammonium bicarbonate is very important in the efficiency of the solids liquid separation (dewatering) step of the process. For instance, when operating a deep vertical thermophilic aerobic digester at 55° C. to 58° C., the digested sludge samples were very granular before drying the sample but not after drying at about 104° C. On one occasion when the head tank was opened without cooling the reactor (for emergency repair of a float switch), the inside surface, particularly the uninsulated access cover, was coated with tiny white angular crystals much like white sugar or salt. These crystals subsequently disappeared and were not found again at the higher operating temperatures. Another observation that is common, is that when a batch of product is transferred into the soak zone at about 58° C. (where there is negligible biological activity), the temperature increases and holds constant for about 2 hours, then cools at the cool-down rate of the reactor when operating on hot water. The heat of crystallization of 10,000 mg/L of ammonium bicarbonate would account for the apparent heat generated in the soak zone. Empirically, these observations would suggest the formation of ammonium bicarbonate crystals below 60° C. This is contradicted by the fact that ammonium bicarbonate and very soluble in water, but less so in the presence of high levels of other dissolved solids, and perhaps the surface chemistry of the microbiology facilitate the crystallization process. For instance, Struvite (magnesium ammonium phosphate) is readily formed in anerobic digesters of plants using biological phosphorus removal but not in plants using chemical phosphorus removal. Controlling the reactor temperature to below 60° C. may allow ammonium bicarbonate crystals to form which would easily float separate with the sludge.
Table 1 compares the performance of floatation, nutrient fractionation, and dewaterability of thermophilic aerobic digested sludge that was taken from a deep vertical thermophilic aerobic digester similar to the present invention. It is known that thermophilically digested sludge will dewater better than anaerobically digested biosolids however at much higher polymer dose. Previous studies investigated the cause of the high polymer requirement and found that monovalent ions such as sodium, potassium, and particularly ammonium ions can interfere with the charge-bridging mechanisms in the floc. In conventional thermophilic aerobic digesters the nitrification of ammonia is inhibited over 42° C. and therefore the ammonia produced is in largely in solution, as evidenced by typically high pH. The carbon dioxide produced is substantially stripped out by the large air flows required in these digesters and less carbon dioxide remains in solution to form ammonium bicarbonate. Since the air bubble contact is in the order of seconds, and the rate of solution of ammonia is much faster than that of carbon dioxide, the environment does not favor the formation of ammonium bicarbonate.
It is believed that below 60° C. ammonium bicarbonate forms in a deep vertical bioreactor due to the high level of carbon dioxide and ammonia in contact and under pressure for long periods of time. Above 60° C. ammonium bicarbonate decomposes and the carbon dioxide and ammonia are stripped out with the air stream, very similarly to the conventional thermophilic aerobic processes. When the final product, processed below 60° C., is acidified with sulfuric acid, alum, or ferrous sulphate, etc, ammonium sulfate is formed and CO2 is released, thus floating the sludge. Unexpectedly, the floated product dewaters exceptionally well. In recent reports by Murthy et al. (Mesophilic Aeration of Auto Thermal Thermophilic Aerobically Digested Biosolids to Improve Plant Operations, Water Environment Research 72, 476, 2000; Aerobic Thermophilic Digestion in A Deep Vertical Reactor, Project 30900, Prepared for King County Department of Natural Resources, Mar. 28, 2001, each incorporated herein by reference) the concentration of biopolymer (proteins and polysaccharides) in thermophilically areobic digestion could be minimized by limiting the residence time of the thermophilic digestion. The present invention has ⅓ to ½ the residence times of conventional thermophilic aerobic digesters. The presence of biopolymer and monovalent ions, particularly ammonia, in solution correlates well to an increase of polymer consumption. The formation of ammonium bicarbonate would significantly reduce ammonium ions.
Lowering the pH with acid to about 5.0, causes the biosolids to float to about 10-12% concentration. Lowering the pH to 4.5-4.0 and lower yields a faster float separation but may require adjustment, e.g., to pH 5.5-6.0, which is the pH range of the sludge before digestion. Digestion below 60° C. controls the reactor pH to 7.8-8.0 while digestion over 60° C. results in an operating pH of 8.6-8.8, reflecting the effect of more free ammonia due to the decomposition of the ammonia bicarbonate. Flotation separating is better below 60° C. than above 60° C., in all categories, where the less acid used yields a thicker float blanket and better nutrient fractionation. These biosolids can be further centrifuged to 30-35% solids concentration using a low polymer dose of about 15 pounds polymer per ton dry weight biomass. The acidification process may cause some cell lysis, which will also help dewater the sludge.
These results are substantially better than conventional thermophilic aerobic digestion processes which require 30-50 pounds polymer per ton dry weight biosolids and centrifuge to only 20-25% solids. Acidifying the conventional thermophilic aerobic digester product does not float separate the solids, presumably due to the lack of ammonium bicarbonate.
Examination of the data in Table 1 shows the profound effect on flotation, dewatering, and nutrient fractionation, between operating the reactor under 60° C. and over 60° C.
Operation under 60° C. generates less free ammonia and more ammonium bicarbonate, therefore the pH is lower and there is less ammonia in the off-gas. In order to get a common base for a comparison between the two sets of data, a concentration factor is calculated. The concentration factor (CF) is the ratio of the final concentration to the starting concentration.
Looking at the “under 60° C.” set of data the float solids were 2.2 times more concentrated compared to the digested sludge solids; the total nitrogen in the float was 2.4 times as concentrated; the ammonia in the float was 1.6 times as concentrated; the organic nitrogen was 3.1 times as concentrated; and the total phosphorus was 2.8 times as concentrated. Except for ammonia the nutrient concentration factor ranged from 2.4 to 3.1 when the solids concentration factor was 2.2.
Looking at the “over 60° C.” set of data the float solids were 1.5 times more concentrated compared to the digested sludge solids; the total nitrogen in the float was 1.7 times as concentrated; the ammonia in the float was 1.2 times as concentrated; the organic nitrogen was 2.1 times as concentrated; and the total phosphorus was 1.3 times as concentrated. The nutrient concentration factor, including ammonia, ranged from 1.2 to 2.1 when the solids concentration factor was 1.5.
These data strongly suggest that the nutrient fractionates into the sludge solids in nearly the same ratio as the solids concentration factor (except for ammonia under 60° C. which is explained later). It is expected that the same fractionation will also occur during dewatering of the floated solids.
However, looking at the float solids concentration factor compared to the subnatent or recycle stream, a completely different and surprising discovery emerges.
The “under 60° C.” set of data shows the total nitrogen in the float was 7.1 times as concentrated as in the recycle; the ammonia in the float was 1.2 times as concentrated; the organic nitrogen was 500 times as concentrated; and the total phosphorus was 24 times as concentrated. Except for ammonia all the nutrients shifted dramatically from the clear recycle into the sludge solids. In other words, except for ammonia, the other nutrients are substantially removed from the recycle streams thus benefiting the operation of the treatment plant and improving the nutrient value of the bio-solids.
The “over 60° C.” set of data shows the total nitrogen in the float was 3.4 times as concentrated than in the recycle; the ammonia in the float was 1.8 times as concentrated; the organic nitrogen was 10 times as concentrated; and the total phosphorus was 1.6 times as concentrated. Except for ammonia and phosphorus, the nutrient shifted significantly, but less dramatically from the turbid recycle into the solids.
A possible explanation of the minimal shift of ammonia into the solids is that the acidification of ammonium bicarbonate results in ammonium sulphate which is very stable but very soluble. The shift in the organic nitrogen to the sludge solids is likely because organic nitrogen is present in the particulate matter of digested sludge and would likely float separate. The ammonium bicarbonate crystals, if any remain after acidification, might also float separate as particulate matter. The shift in phosphorus to the sludge solids by acidification of the sludge can be explained by the formation of insoluble precipitates in the presence of a high concentration of metals occurring naturally in the sludge. This effect is not so pronounced over 60° C., probably because the float separation was poor and the tiny particles formed in the precipitate are difficult to float.
In constructing and installing the improved vertical shaft bioreactor 10 of the invention, twin bioreactors (to satisfy EPA redundancy requirements) will often be placed in cased and grouted steel shafts approximately 36 inches in diameter and 250 feet deep. The exemplary scope and reactor design described here for illustration purposes is suited for a community of about 5000 people requiring a tertiary treatment plant with biological nutrient removal would proceed as follows. Also described here for illustration purposes is a novel, modular bioreactor assembly design, while it will be understood that the use of a modular assembly method is not necessary to practice the invention.
The inner head tank for this exemplary installation is about 8 feet in diameter and approximately 12 feet high. The shop fabricated reactor internals include 6 flanged tube bundles each about 40-feet long. The bottom 40-feet length (first length) is made up of the aeration distributor 60, the shear header 70, the airlines 62 and 66, attached to a short length of downcomer 12. The second, third and fourth tube bundles, include 40 feet, modular sections 190 typically including a central downcomer conduit 22 with airlines 62 and 66 attached (see, e.g.,
To facilitate modular construction of the bioreactor 10, the superior channel-forming radial partitions 194 are relaxed from the inner wall 196 of the reactor during insertion by expanding the diameter of the central (e.g., downcomer 22) conduit in a direction generally perpendicular to the radial partition (see, e.g.,
After assembly to this stage is complete, the zone 1 head tank 16 is bolted to the top of the last section. The zone 2 head tank 15 is field-erected from pre-fabricated sections. The modular reactor tube bundles can be delivered to a site for installation by a single truck, and the head tanks by a second truck. The clarifier 120 shell can be cast in place using concrete or made from prefabricated steel sections. The clarifier is fitted with a conventional skimmer, mechanism. Finally the compressors and other ancillary equipment are connected. Because of the small footprint these small plants can easily be housed in a building.
To further understand the distinct and diverse methods of waste water treatment employing the novel apparatus provided herein,
The upper left area of
Membrane separation, although expensive, eliminates many of the operational problems of clarifiers in BNR plants. In the present invention membrane separation allows much higher MLSS and a smaller reactor. Membrane separation provides a better quality recycle water than the present standards require.
The upper right of
The lower right of
The BNR process above has been examined in detail in
In addition, it is notable that downflow in channel 12 enters the bottom of zone 1 in the vicinity of the aeration distributor in an area of vigorous mixing. Channel 80 which is the major portion of zone 1 is highly aerobic, removes the BOD, rapidly oxidizes the VFA's consuming phosphorus and in some cases nitrifies a portion of the ammonia.
Further notable is the fact that rising liquor in channel 80 splits into the deoxygenation area and a portion passes upward into zone 2. Zone 2 substantially degrades the remainder of the BOD and converts the remainder of the ammonia to nitrate.
In additional aspects, waste gas from channel 80 circulates via deoxygenation channels 32 and 40 and also provides the oxygen for bio-oxidation of BOD and ammonia in zone 2.
Also noted, a portion of nitrified liquor can be returned to the denitrification step where the nitrate —N is converted to nitrogen gas while a second portion goes to a clarification step where the biomass is separated from the effluent. The biomass is returned to the denitrification step and the clarified effluent is discharged.
In related embodiments, anoxic gas is used for mixing anoxic liquor. Unit D deoxygenates not only the various liquid streams, but the gas stream passing through the unit. This deoxygenated gas can be used subsequently to mix the contents of the denitrification unit E. This eliminates the need for mechanical mixers saving energy, maintenance and capital.
Additional embodiments of the invention provided novel anaerobic processes. Unit F is a long vertical channel which may converted to an anaerobic chamber for the purpose of creating VFA's. In the present invention there are no airlines or extraction lines in unit F. This allows the use of media such as open weave rope or tubes to be suspended in the reactor without the fear of plugging the channel or becoming entwined with other pipes. The purpose of the fixed media is to accumulate attached growth anaerobic bacteria (acid formers). The amount of fixed media and anaerobic biomass can be adjusted from the surface by rolling up a portion of the rope or fabric tube. The amount of media can be monitored oil line by measuring the weight of the rope. The liquid velocity downward in channel 12 keeps excess biomass from forming and any excess will fall off. Since channel 12 is open at the bottom waste anaerobic biomass would collect in sump 67 and be removed trough the flotation tank Unit J.
In still additional embodiments, wasting sludge through an air line 66 or 69 provides instant spontaneous flotation upon depressurization. Wasting sludge (WAS) from a well aerated and mixed part of zone 1, a process not contemplated in previous designs, favors the capture of phosphate in the sludge. Float solids are suitable for digestion without any further thickening.
This description next addresses membrane separation systems, methods and devices employing a selective, semi-permeable, microporous, or other partitioning membrane for processing, refining, and/or treating liquid compositions, for example membrane waste-water purification processes and apparatus. These systems, methods, and devices provide improved throughput and/or improved operating life of submerged membranes, particularly membrane bioreactors providing biological treatment of wastewaters.
There are several technical considerations for incorporating membrane bioreactors in wastewater treatment facilities, including long vertical shaft bioreactors. A first consideration is a popular misconception that the membranes alone produce an exceptional quality effluent. This is not necessarily accurate because membranes, in themselves, do not produce recycle quality water. The treatment of wastewater to recycle quality is primarily the result of biological treatment, however a micro filtration membrane is responsible for physically separating substantially all the microorganisms from the water, down to about 0.1 micron in diameter. Viruses smaller than 0.1 micron are also typically removed because about 99% of viruses stick to host bacteria. The better the bioreactor, the better the quality of effluent.
In cases where inorganic dissolved solids must also be removed, the effluent from the biological treatment membrane reactor can be further treated by using ultrafiltration, nanofiltration, or reverse osmosis (RO). This quality of water is suitable for aquifer recharging etc.
A second consideration is that recycle quality water not only requires the removal of biological oxygen demand (BOD) and total suspended solids (TSS), but also requires the removal of the nutrients, nitrogen and phosphorus, (N & P) to low levels that will not support aquatic growth. This requires the use of a good biological nutrient removal (BNR) process. Typical existing membrane bioreactor processes operate on a single sludge back-mixed bioreactor, which is less efficient and more expensive to build and operate than the improved long vertical shaft bio-reactors.
For example, a presently proposed installation of twin 0.25 MGD (0.5 MGD total) conventional membrane biological reactors is estimated to cost about 1.2 million dollars, (reactor and membranes only), occupy about 8000 sq. feet, draw about 75 HP, and require 1000 standard cubic feet per minute (scfm) of air. By comparison, twin improved long vertical shaft 0.25 MGD reactors would cost about 1.0 million dollars including the price of the membranes estimated at $400,000. The improved long vertical shaft bio-reactors would occupy about 1000 sq. feet and draw about 30 HP. Only 100 scfm of air is required for the improved long vertical shaft bio-reactors, reducing the process off-gas flow to the equivalent of a household kitchen or bathroom fan. The improved long vertical shaft bio-reactors operate in a plug flow configuration with internal recycle streams. Plug flow reactors are known to produce a better quality effluent than back-mixed reactors. This is because in a plug flow reactor the effluent is at the lowest possible concentration achievable with that biomass. In a back-mix reactor, the effluent constituents are at the same concentration as the contents of the reactor. Indeed, in some cases in a back-mixed reactor, a portion of the influent may short circuit directly to the effluent. It is also known that with a single sludge bio-reactor, where specialty microbes such as nitrifiers must compete with more robust and faster growing BOD microbes, larger quantities of biomass are required (to prevent wash-out of the nitrifiers). This leads to larger reactors.
An additional consideration is that, aside from the biological advantages of the improved long vertical shaft bio-reactors, there are certain hydraulic advantages that are not possible with other reactors. Several unique hydraulic characteristics observed in existing long shaft vertical aeration reactors suggest that membrane separation systems will operate better in a vertical aeration reactor than in a surface back-mixed reactor because of substantial concentrations of supersaturated dissolved gases.
To confine this prediction regarding supersaturated dissolved gases, a membrane separator was adapted to an existing long vertical shaft aeration reactor. A principal hydraulic characteristic of vertical aerators is that the reactor circulates a mixture of bio-solids, liquid, dissolved gasses and dispersed gas (bubbles), in a very long vertical pathway. The pressure at the lower end of this pathway can be up to 150 psi. As a result of the pressure, there are substantial concentrations of supersaturated dissolved gasses in the liquid even when brought to the surface. These supersaturated gasses represent a significant resource of stored energy. For example, in a 0.25 MGD improved long vertical shaft bio-reactor, the surface area in contact with the moving fluid in the reactor changes from about 4000 sq. feet in the reactor to about 20,000 thousand sq. feet in a membrane cell. When liquids containing supersaturated dissolved gasses contact a large surface, the dissolved gas tends to come out of solution and create a scouring action. This is like using soda water to remove spots on clothing. Actually, there are many cleaners that use foaming agents to improve scouring action.
In the case of long vertical shaft bioreactors with submerged membrane bioreactors, it is predicted that the action of supersaturated dissolved gas in the mixed liquor will help keep a membrane surface proximate to the mixed liquor sufficiently clean, thus increasing the flux rates (rate of liquid flow through the membrane) and the time between cleaning. There are several observed factors of long vertical shaft bioreactors that provide support for this prediction. For example, a vertical bioreactor that had run 22 years was recently dismantled. The head tank was made of steel plate, sand blasted and coated with 6-mil (0.006″) epoxy. The remainder of the reactor was bare steel. The epoxy coated surfaces were exceptionally clean and even the bolts in the epoxy coated head tank could be easily undone. There was no evidence of any biomass buildup on the epoxy surface, even near the downcomer end of the head tank where the flow velocity would be very slow (perhaps 0.1-0.5 ft/sec). The dissolved gas content at that point would be about 25-35 Mg/L and the colloidal gas content would be about 40-50 ml/L (50-65 Mg/L). There were, a few locations where the epoxy coating had been damaged resulting in a localized accumulation of biomass attached to the bare steel. The bare steel surfaces in the rest of the reactor were coated with a gray slime layer, even in the areas of high turbulence and high dissolved gas content. This gray biomass slime, typically found on metal surfaces in these types of reactors, contains phospholipids and is useful in protecting the bare steel against corrosion, referred to as bio-passivation.
To further validate these findings, a rubber hose about 100 ft long, weighted at its lower end with 90 feet of steel pipe was used in a vertical shaft aerator for an air line in the downcomer. The liquid flow velocity in the upper end of the downcomer was in the order of 3-4 feet/sec. The hose could be reeled up to change the point of air injection in the downcomer. On the upper end of the hose there was no biomass build up in the zone where dissolved gasses were present, but there was a significant biomass build tip in the zone where these gasses were re-dissolved due to increasing pressure in the downcomer. The liquid velocity was the same for both the upper and lower zones in the downcomer. This hose was designed for air service and was not permeable to air from the inside. This observation also shows that in the absence of dissolved gas in the downcomer biomass will build up to provide an anoxic/anaerobic zone.
In additional studies, an early design vertical bio-reactor was equipped with a fiberglass downcomer. This plant is still in service with no report of any failures. Another plant built at the same time, also using a fiberglass downcomer, was shutdown and filled with clean water. Video inspection showed no build up of biomass on the wall of the fiberglass tube and no delamination of the resin and fibers. Fiberglass is typically not permeable to dissolved gas.
In a separate study, a small vertical aeration shaft was inspected after about 26 months of service. The ABS downcomer was in good condition with no biomass build up. A similar vertical aerator was fitted with a steel downcomer. Inspection revealed a phospholipid biomass coating commonly found in steel reactors.
Further validating the present findings, during the early development of vertical bioreactors rubber downcomer tubes were installed to reduce the suspended weight and to prevent flow reversal. Three of these downcomers failed due to de-lamination of the tube wall between the rubber surface and the reinforcing fabric. These tubes were designed for water service and were permeable to dissolved air. The maker of the tube claimed that dissolved air had become entrapped between the inner and outer rubber layers causing the failure. This phenomenon is seen in tubeless radial tires where the air in the tire leaks into the cord layer and causes delamination.
In a separate study, another vertical aeration reactor was examined for corrosion after 20 years of service. The only part of the reactor that had any significant wear was at the outlet of the air-lift influent pump that was located in the riser section of the reactor. It would appear in this extreme duty, the air/water velocity is sufficient to remove the protective phosphate coating allowing corrosion of the bare metal.
A frequent observation regarding surface condition of several head tanks examined after long periods of operation is that in locations featuring an abrupt change in fluid flow, such as immediately following a baffle, the epoxy coating is often deteriorated. These areas may be considered “hydraulic shadows.”
Releasing a dissolved gas and its stored energy provides a powerful scouring effect on the epoxy and/or metal surfaces. This energy level is sufficient to remove the epoxy but not enough to significantly damage metal surfaces. However the bacterial slime coating found elsewhere in a reactor, even adjacent to the shadow, is removed as evidenced by the formation of a light rust coating on the metal surface.
Similarly when the test membrane was installed in the Y branch of a vertical shaft reactor described below, there was an air line, which was in close proximity to one corner of the membrane. The air line did not touch the membrane but acted as a baffle and caused a downstream “hydraulic shadow” over about 10-15% of the surface area at one corner and on one side of the membrane. In this “hydraulic shadow”, the membrane had begun to delaminate slightly. The membrane is made of non-woven polyolefin strands, perhaps 10-20 microns in diameter, compressed and sintered together by some means, probably heat and pressure. Under the microscope there were hair-like whiskers, approximately ⅛ inch long, protruding perpendicularly to the surface. These whiskers were found on the membrane only on one side, and only in the proximity, of the air line. It is likely that the abrupt change in flow causes the dissolved gas to nucleate and to erode/wear the polymeric surface. Cavitation may be occurring because the whiskers are protruding outward and appears to have been lifted from the surface. The remainder of the membrane was unaffected by the high levels of dissolved gas and had no evidence of surface deterioration. When examined under the microscope.
The amount of dissolved gas is surprising. As an example, the solubility of air in water is about 21 mg/L at one atmosphere of pressure. A 500 ft. deep vertical shaft reactor could theoretically dissolve 287 mg/L of air. Assuming a dissolving efficiency of 70% and a recovery efficiency of 70%, there would be about 140 mg/L of air in the liquid in the head tank of the reactor. Since 1 ml of air weighs 1.29 mg, this translates to about 10% by volume of the liquid would be derived from dissolved gas. This represents substantially more dissolved (stored) bubble volume than the dispersed bubble volume used to circulate the contents of the vertical bioreactor. Surprisingly it is more dissolved bubble volume than the dispersed bubble volume (4-6%) required to circulate either a Kubota or Zenon membrane reactor. Furthermore this stored bubble volume represents considerable stored energy.
It is predicted that by releasing this stored energy at the critical time and controlled rate across the membrane surface a very powerful cleaning action can be created. In fact, there is enough energy stored in this manner to delaminate/cavitate the membrane if released in an uncontrolled way, such as can occur in the proximity of the air line. This phenomenon now explains observations of failed rubber down-corners.
The total dissolved air may be calculated quite accurately in the liquid in a vertical reactor by using a dissolved oxygen probe. Under no load conditions, i.e., no BOD load, the total dissolved air is about 2.61 times the dissolved oxygen reading. Under load, the oxygen readings are reduced but the oxygen consumption can be calculated from the BOD values. At the time of this study, the vertical bioreactor was operating under a typical diurnal organic load patterns. Note that when the riser air is maintained at substantially a constant value (55-65 scfm) the dissolved oxygen values increase by a factor of nearly two when only 43 scfm of down-corner air is applied. This indicates that the down-corner air is mainly responsible for dissolved gas while the riser air is mainly responsible for dispersed. More importantly, the dissolved oxygen level in the permeate (even though reduced 50-60% by the BOD reaction) reaches supersaturated values (nitrogen gas and carbon dioxide gas would therefore be even higher) proving that supersaturated gasses in the liquid easily pass easily through the membrane. As an example, if the residual dissolved oxygen in the permeate is 10 mg/L and 50% of the oxygen was consumed in the reaction, then the starting value would have been at least 20 mg/L. Therefore the starting dissolved air would then be 2.6×20=52 mg/L and the nitrogen fraction would be 32 mg/L. This is conservatively, the amount of dissolved gas going through the membrane. Remember that some of the dissolved gas, perhaps half, is also precipitating on the outside of the membrane.
In consideration of the magnitude of the observed scouring effect of uncontrolled gas nucleation on polymeric surfaces, the flow redistribution device located between each level of membranes has been redesigned within the present invention. The new design consists of a series of adjustable and/or removable baffles, which will create low level but controlled “hydraulic shadow” effect across the membrane. This controlled effect is similar to, but much less intense than, the one inadvertently created/discovered in the proximity of the air line.
The importance of this discovery is that, where it was thought this type of vertical aeration reactor could supply only a fraction of the air required to operate the membrane; there is actually more than enough air in the “stored energy” form (i.e., dissolved). It is now possible to get the stored gas out of solution in the right amount and at the critical location to achieve the novel objects and advantages disclosed herein.
Thus, the invention provides for the employment of supersaturated dissolved gasses in fluid processing methods and devices to clean surfaces that the subject fluids contact. Various observations that validate these results include:
a) Polymeric surfaces submerged in liquid flowing at wide range of velocities from about 0.1 to 4.0 feet/sec. do not experience a build up of biomass in the presence about 20-30 Mg/L of dissolved gas and/or about 30-50 Mg/L or colloidal gasses in the liquid.
b) Non polymeric surfaces, (bare metal exposed by damage to polymeric coating) submerged in liquid flowing at wide range of velocities from about 0.1 to 4.0 feet/sec. do experience a build up of biomass even when there is about 20-30 Mg/L of dissolved gas and/or about 30-50 Mg/L of colloidal gasses present in the liquid.
c) Biomass build up is experienced in the absence of dissolved gasses even at relatively high liquid velocities of 3-4 feet/sec.
d) Biomass build up can occur on metallic (steel) surfaces at flow velocities up to about 4 feet/sec. This biomass contains phospholipids that protect the metal by bio-passivation.
e) Flow velocities over about 10-feet/sec and in the presence of large amounts of air (over about 100 mg/L) prevent the build up of biomass and the build up of the corrosion inhibiting phospholipids. As a result, metal corrosion and metal erosion occur.
f) Non-permeable polymeric membranes can delaminate if the pores do not go right through the wall.
g) The high airflow rates suggested by membrane manufacturers are not necessary for efficient operation of submerged membranes in a presence of supersaturated dissolved gases. Kubota, a leading membrane manufacturer, states in its literature on membranes used for solid-liquid separation of mixed liquor that a thin film biomass is allowed to form on the surface of the membrane to increase its effectiveness in removing small particles. At a flux rate of about 0.5 gal/hr/sq. feet the time between cleaning membranes is about 6 months. A minimum air rate of about 40 scfm/1000 sq. feet is required and a minimum cross-flow liquid velocity of about 1 feet/sec is required.
Zenon membranes operate at a nearly double the flux rate of the Kubota membranes, but provision is made for pulse reverse flow cleaning. In one mode of operation a ten-second pulse is applied for every ten minutes of operation. Zenon also use a mechanically-applied vacuum to draw on the membrane. Overall, the Zenon technology requires a lower air rate to stimulate and clean the membrane than the Kubota membrane.
The airflow rates suggested by these two leading membrane manufacturers is 8 to 10 times higher than the airflow rate typically available in improved long vertical shaft bioreactors. Both Kubota and Zenon have designed their membranes to operate in relatively shallow basins. The improved long vertical shaft bio-reactor is configured on a vertical axis, and allows membranes to be stacked 2-5 units high and still maintain enough driving head in the reactor to circulate the system. In shallow tanks the driving head that causes air/liquid circulation through the membranes amounts to a few inches at best. In an improved long vertical shaft bio-reactor plant, the driving head might be 10-12 feet. A re-distribution header is located between each deck of membranes thus allowing the same air to be used 4-5 times. By stacking membranes, the superficial cross-flow (actually up-flow) liquid velocity across the membrane increases as the cross-sectional (footprint) area decreases. Although not optimized, a first trial design of a 0.25 MGD improved long vertical shaft bio-reactors plant incorporating membrane bioreactor technology indicates it would supply about half the air and about ⅓ liquid flow velocities recommended by the membrane manufacturers. For the reasons stated above and the evidence gathered, the dissolved air fraction in the liquid flow is a far more important factor in keeping the membranes clean than either the air rate or the liquid rate. Over-design is to be avoided because it is possible to clean the membranes too well and destroy the required thin bio-film. The scouring action can be adjusted by using fewer decks of membranes or less air. Conversely, one can always add air and more decks if more velocity and/or scouring are needed.
h) Dissolved salts and particles smaller than 0.04 microns pass through microfiltration membranes and therefore there is no reason to suspect that dissolved gasses will not pass through. The dissolved gasses that do pass through the membrane may help in keeping the inside of the membrane clean.
i) The membranes can be cleaned with bleach and therefore the material that is blocking the pores is probably mostly organic.
j) Typically membrane reactors need fine screening of the raw influent because the plant may be dealing with whole raw sewage.
The foregoing findings and conclusions were further validated by installing membrane bioreactors in a deep shaft vertical reactor at Virden, Manitoba, Canada, beginning in August of 2003. The Virden Reactor was the first commercial deep shaft vertical reactor installed in North America in 1978. The treatment plant was started up in 1980, and has been in continuous service since then. The plant is one of the older deep shaft designs where both downcomer and riser air is used in the circulation and aeration of the shaft contents. The reactor is 30″ in diameter and 500′ deep. The downcomer is 18″ in diameter and the riser is formed by the annular space between the casing wall and the downcomer. At the top of the reactor there is a Y branch to allow the mixed liquor to transfer from the riser to the downcomer via a head tank. The head tank is approximately 25 feet long, 6 feet wide and 4.5 feet deep. The configuration of this reactor is ideal for tests since it allows the ratio of dissolved to dispersed air to be selectively changed. Applying more downcomer air results in more dissolved air while applying more riser air results in more dispersed (bubbles) air. As shown later, the ratio of dissolved to dispersed air makes up to a nine-fold difference in membrane flow rates at the same hydraulic head.
In order to install membrane bioreactors in the Virden reactor, it was necessary to cut an access-way into the top of the head tank. The access-way is located directly over the 21″ ID Y branch. When the head tank was opened after 23 years of continuous operation, the same pattern of bio-fouling was discovered on the epoxy coating as found in another long vertical shaft bioreactor opened after 22 years of operation. The patterns were almost identical. In each case, the floor of the head tank, where both the liquid velocity and the bubble content is the lowest, had a minimum of attached biomass. This is contrary to the teaching of the membrane manufacturers, who recommend a much higher velocity and higher bubble content. However it should be noted that although there would be few, if any, bubbles on the floor, the fluid would be supersaturated with dissolved gas. The conclusion that it is the dissolved gas nucleating on the polymeric surface that reduces bio-fouling is further supported by this observation. The fact that the shaft of the other examined bioreactor was used for treating high strength warm industrial waste, while the Virden shaft was treating cold low strength municipal wastewater, appears to have little influence on this phenomenon.
A test membrane solid/liquid separator was installed in the Virden deep shaft reactor for one month to assess improvements in through-put and operating life of the submerged membrane assemblies of the invention. The test membrane was removed from the reactor and carefully examined. Notably, the membranes were clean, and any matter on the exterior surface easily washed off in water despite having been operated in a thick concentration of sticky mixed liquor for a month. Additionally, it was observed that different air rates in the reactor produced different effluent water flow rates from the membrane. When downcomer air was increased, (more dissolved air in circulation), the reactor circulation flow rate decreased, but the flow through the membrane increases. Conversely, when more riser air was applied (more dispersed air) the circulation velocity in the reactor increased but flow out of the membrane decreases. This is contrary to conventional understanding of membrane function and operation, as evinced by operation instructions of membrane manufacturers. In conventional membrane plants, a high aeration rate is required to maintain circulation velocity across the membrane. Typically a conventional plant would use (as a minimum) about 2 times the trans-membrane velocity and 8-10 times the airflow that is available in a deep shaft type reactor.
In additional studies to clarify the disclosure herein, the test bioreactor was fitted with a sample port in the head tank located close to the outlet of the membrane. Dissolved gas concentrations across the membrane were measured with a dissolved oxygen meter (DO meter) and reactor circulation velocities across the membrane are calculated from the time to circulate tracers such as soap. Permeate flow out of the membrane was measured in a calibrated flask, and the hydraulic head is maintained by an overflow to the flotation tanks. In this test case, the head over the membrane was maintained at 1 foot. A drop leg was provided to cause a siphon effect of one meter, a typical operating value for this type of membrane. The membrane support frame can hold a lower membrane submerged between, about 6-9 feet, and an upper membrane submerged between about 1-4 feet. The overflow heights are the same for both membrane locations.
The following Tables 2 and 3 shows the effect of various air rates on the membrane performance:
Table 3 is a plot of data points of Table 2. The data of Table 2 do not reflect the importance of the dissolved air fraction. However, the effect of varying the air rates was noted and the information provided in Tables 2 and 3. There are two data sets that illustrate the importance of the downcorner air (dissolved air). Point 1 and point 2 have the same total volume of air applied (108 scfm). Point 1 has 108 scfm in the riser (mostly dispersed) and no downcomer air (i.e. no dissolved air). Remember that the conventional teaching says that high velocity and high air rates yield highest flow rates in the membrane. However, in the trial run, (point 1), the highest air rate and the highest circulation rate yields the lowest flow rate out of the membrane.
At point 2, there is a total air rate of 108 scfm but this time, 43 scfm is applied to the downcomer. The effect of downcomer air is to slow the circulation velocity. Conventional teaching predicts that the flow out of the membrane would also slow but in the trial run the flow out of the membrane increased nine fold.
Further, at points 3 and 4, both points have 75 scfm of air in the riser but point 4 has 55 scfm in the downcomer that serves to slow the circulation velocity. The output from the membrane increases three fold.
Therefore, the dissolved air fraction in the wastewater has a dominant effect on the throughput of the membrane. Other factors will also influence the performance of the membrane. Among these factors are the concentration of biomass, the sludge age, the biological health of the sludge, the amount of exo-cellular polymer present, the condition of the membrane, etc. These factors are expected to have a minor impact on overall results since most of the results are from one-day's operation during which sludge conditions are not predicted to change much during the subject period.
One important cause of increased membrane through-put within the present invention relates to gas dynamics of vertical bioreactors. In particular, deep shaft reactor systems provide significant advantages over other bioreactors and fluid treatment apparatus by providing a high dissolved air fraction. In addition, they involve distinct biochemical and physicochemical processes, for example oxidation of organic carbon, and dissolution of oxygen and other gases that result in supersaturated levels of desired gases, e.g., carbon dioxide.
In fluid dynamics, the term “rheology” describes a complex, non-linear relationship between fluid deformation and stress occurring in fluid flow patterns. The increased throughput phenomena is believed related to a change in rheology on a membrane surface. Because of high amounts of dissolved gas in the fluid, the rheology of both the biomass (solids containing fluid and gas) and the fluid media change on contact with the membrane, perhaps making the membrane more permeable.
The test membrane was fitted with clear vinyl tubing, which allowed visual observation of the permeate stream. The permeate stream contained a significant amount of bubbles, perhaps 1/16 to ⅛ inch in diameter. It is estimated that as much as 10-15% of the permeate flow is made up of discrete bubbles. It is believed that the dissolved gas passes through the membrane unimpeded and then nucleates at or near the membrane surface, which may include nucleation between the surfaces, at the membrane surface, or within a fluid proximate to the membrane surface, and causes an air-lift effect proximate to the membrane surface. It is reasonably expected that discrete bubbles will not pass at high levels through a semi-permeable membrane. Unless there is dissolved air present in the water passing through the membrane, (or alternatively air bubbled into the clean water side of the membrane) no air-lift can be expected on the permeate side of the test membrane. This air-lift caused by the bubbles has a significant pumping effect because during the installation of the permeate line, permeate flow from the membrane can be raised almost to the surface of the liquid level in the head tank. Since the liquid being filtered located outside the membrane contains about 9% air voidage, the nucleating gas volume inside the membrane would be likewise be about 9% gas voidage in order for liquid to flow out of the membrane at similar interior and exterior hydraulic heads. It is apparent that the dissolved gas fraction helps keep the membrane outside surface clean and therefore, the dissolved gas fraction inside the membrane will also help keep the inside of the membrane clean.
In a conventional membrane application, the water inside the membrane (on the permeate side) contains very little air and is much more dense than the water outside the membrane which contains the air required for scouring. Therefore, in conventional systems the water will not flow out of the membrane unless a slight vacuum is applied to the effluent side (Zenon uses a vacuum pump) or the influent is pressurized (Kubota uses compressed air). When a vacuum is applied, water tends to flow preferentially through the pores closest to the top of the membrane. When pressure is applied with compressed air, the resulting head is the sum of the heads due to density difference between the water inside and outside the membrane plus the head required to cause flow through the pores plus any hydraulic losses due to fluid motion. In the improved long vertical shaft bio-reactors system, the head due to density differences is largely eliminated and potentially enough dissolved gas could enter the membrane to cause enough air-lift to overcome the head loss through the pores as well.
The membrane 410 schematically represented in
As further illustrated in
As illustrated in
As illustrated in
If the adjusted or changed second specific gravity does not closely equal the first specific gravity, the selected pressure differential across the membrane is expected to vary only a minor degree along the vertical axis of the membrane. For example, variation of the pressure differential along the vertical axis is expected to be generally uniform, i.e., not vary more than +/−30% per vertical linear foot, when the second specific gravity is adjusted to within approximately +/−5 percent of the first specific gravity.
In the embodiment illustrated in
In an alternative embodiment, the differential hydraulic head 452 can be imposed by enclosing the first fluid compartment 420 and applying a pressure, such as by compressed air generated by a mechanical compressor, thus increasing the first column height 422 without physically increasing the vertical dimension of the first fluid compartment. In another alternative embodiment, the differential hydraulic head 452 can imposed by applying a vacuum, such as generated by a mechanical vacuum pump, to the second fluid compartment 430, thus decreasing the second column height 432 without physically decreasing the vertical dimension of the second fluid compartment. Using gravity solely to impose the differential hydraulic head 452 may be considered preferable because gravity does not require any mechanical devices that consume power and require maintenance, such as pumps. In addition, using gravity solely eliminates any problems associated with maintaining an enclosed fluid compartment.
Additional features of the embodiment illustrated in
This description will next address embodiments for changing or adjusting the second specific gravity by including diffused gas or air bubbles 436 in the second fluid 434 as previously described in conjunction with
The Zenon and Kubota submerged membrane processes of
Each membrane bioreactor assembly 510 includes a plurality of flat plate semi-permeable membranes 511 coupled by a membrane output line 512 to an exterior manifold 514. The exterior manifold 514 is coupled by a collection line 516 to a collection trough 538. The plate membranes 511 are typically include a frame that supports two rectangular semi-permeable membranes having their second surfaces 412 facing each other and defining in cooperation with the frame an interior second fluid compartment 430 between. The first surfaces 411 of the semi-permeable membrane are exposed to a fluid surrounding the exterior of the membrane assembly 510. The collection line 516 may be made of any tubular member suitable for carrying permeate or fluid outputted by the plate membranes 511. The collection line 516 may be transparent or clear, allowing a user to visually inspect the bubble 436 content and clarity of the output from each individual plate membrane 511.
The collection lines 510A-C flow permeate upward into the collection trough 538 as illustrated in
In an embodiment of the invention, each membrane assembly 510 includes 75 flat plate membranes. Using eight separate saddle tanks 506A-D and 506E-H (not shown) around a head tank 75 provides a total number of flat plate membrane bioreactors 511 in this configuration of 75/tier×4 tiers/saddle tank×8 tanks=2400 flat plate membranes. Experience with the plate membrane indicates that this arrangement would process about 0.3 MGD on average and 0.6 MGD at peak flow. The head tank 502 diameter in this embodiment is approximately 9 feet, and with the saddle tanks 506 makes the reactor about 13 feet in diameter.
In operation, the first fluid 424 as inflow 526 of effluent from the long vertical shaft bioreactor flows into the bottom of the saddle tank 506A from a long vertical shaft bioreactor (not shown). The first fluid 424 has a first specific gravity, and includes bubbles 426 and supersaturated dissolved air. The first fluid 424 rises through the saddle tank 506A past the column of submerged membrane bioreactor assemblies 510A-D, and becomes outflow 528 as it overflows the saddle tank at a 12 foot elevation. The outflow 528 returns to the long vertical shaft bioreactor for further processing or removal from the reactor. The individual flat plate membranes 511 filter the first fluid 424 as described in conjunction with
Furthermore, by nucleating the dissolved gas of the first fluid 424 in the second fluid 434 as described in conjunction with
In conventional systems, there is no gas nucleation in the second fluid 434, and therefore the difference in pressure differential between the top and the bottom on a 1 meter (40″) high membrane 511 is 40″×10%=4 inches of water. This may not appear significant, but it is enough to cause unequal flow through any particular membrane.
For peak flows, the pressure differentials across all the membranes 511 can be raised equally by simply increasing the height of the dirty water column, the first column height 422. Note that the air bubbles 426 are used four times as they travel from the bottom membrane assembly 510A to the top tier 510D.
In an alternative embodiment, the membrane bioreactor assemblies may be arranged within the head tank 502, and the saddle tanks 506A-H eliminated.
The second saddle tank 556B is open to the atmosphere, but the first saddle tank 556A is not. The second column height 432 exists in two segments across the folded saddle tank system 550, a first portion 432A across the first saddle tank 556A, and a second portion 432B across the second saddle tank 556B. The first column height 422 is not shown in
In operation, the folded saddle tank system 550 functions substantially similarly to the system 500 of
Note that all the gage pressures on the discharge lines from the membranes are equal. The pressure (head) in the discharge lines of the membranes in saddle tank 556A is due to the 3.5 psi of off-gas pressure (equivalent to 8 ft. of water) superimposed on the liquid in tank 556A. The pressure at the collection trough 538A is reduced by the 1.75 psi (4 feet of liquid standing in the discharge line of membranes 510B), and 3.5 psi (8 ft. of water) standing in the discharge line of membrane 510A.
Similarly, the discharge line from membrane 510C is under a hydraulic lead of 1.75 psi (4 feet) and the discharge line from membrane 510D is under a siphon (vacuum) of 1.75 psi. Experience in the field shows that air bubbles are permitted in a siphon line provided the lines are sized properly to maintain adequate discharge flow velocities, generally of greater than 2 ft./sec.
To further elucidate various aspects of the invention, a bench test apparatus was constructed according to the teachings herein and was used to conduct a series of bench tests of membrane throughput under varying membrane conditions and levels of diffused gas in water.
Field observations show that the membrane permeability increases with an increase in dissolved gas. For example, see Table 3 above where adding dissolved air resulted in a significant increase in the permeate throughput. Other field observations demonstrate a scouring effect that the pressurized gasses in the reactor liquor exert on membrane and other surfaces. In vertical shaft bioreactors, a significant cleaning action occurs at strategic locations within the reactor, which typically are locations where dissolved gasses come out of solution.
As noted above, the bench test apparatus was devised using a Kubota membrane of the same type used in field tests described in Table 3. These tests demonstrated that increasing the dissolved gas content by adding downcomer air in the reactor liquor had a large effect on permeate flow.
The test apparatus was configured like an aeration shaft with an outer casing of 3.488″ ID with a downcomer of 1″ inside diameter. The liquid circulation was driven with a large aquarium air pump with two injection ports near the bottom of the downcomer. The membrane was located in a machined recess at the bottom of the 3.5″ diameter tube and a removable bottom cover supports the membrane from movement in the downward direction. The bottom cover plate included a series of machined grooves dimensioned similarly to the grooves in the Kubota membrane. A piece of coarse felt blotter membrane, taken from the field trial membrane unit, is installed between the membrane and the permeate collection system. Membrane discharge tubes were installed both vertically upward and downward from lower surface of the membrane. Additionally, the lower tube can be used as a siphon or drain to remove permeate from the lower side of the membrane.
Permeability tests were conducted to measure the effect of dissolved gas on flow rates through the membrane. In order to do this the influence of air-lift effect in the membrane discharge line must be separated from the effect of increased flow due to degassing. As a result the membrane is oriented horizontally at the bottom of a Plexiglas tube 24″ tall and 3.5″ in diameter. The first tests used a membrane glued to the bottom of the cylinder. This test simply determined that gas saturated liquid would pass through the membrane but there was no provision for the effect of vacuum, the effect of the felt wicking layer under the membrane skin, or the effect of the permeate channeled collection system.
The test apparatus used a porous felt layer under the membrane and a channeled permeate collection system similar to the Kubota design. The area of the test membrane is 9.5 sq. in. or 1/130 of the area of both sides of the field test Kubota membrane. The membrane used in the field was (½ meter×1 meter) and had a rated surface area of 8.6 sq.ft. or 1238 sq.in.
In the field trials, only 1 foot of positive head was available. In order to get flows over 150 U.S. gal per membrane per day, a vacuum of up to 28″ was successively applied to the permeate side. Actually, the field unit will self prime the siphon by using only the 1 ft. of positive head. However, in the test apparatus, the ⅛ inch diameter clear vinyl discharge line from the membrane worked quite well when used as a siphon on test runs 1, 3, and 4 when there was little or no gas in the permeate. In fact, on run 3 the number and size of bubbles in the siphon could be visually estimated.
In the saddle tank design described above in conjunction with
Air-lift circulation through a riser and downcomer. Initially, it was thought that tap water and/or soda water would permeate the membrane over long periods of time without loss of throughput. This was not the case. When tap water is left standing in the test apparatus, the flow slows over time. When soda water is left to stand in the test apparatus there is virtually no deterioration in flow. However when an air-lift circulation was employed with tap water there was no noticeable deterioration in flow perhaps due to surface scouring of the membrane. The soda water product likely uses reverse osmosis water while the city water is sand filtered. Initially it was thought that there might be a measurable difference in the quality of water but that turned out to not be the case on run 8. Later it is shown that the gas nucleation effect on the downstream side of the membrane has the dominate effect on permeate flow.
Flow calibrated in micro-litres/min: The method employed to measure flow involved measuring the change in liquid height in a small diameter cylindrical catch tube and a stopwatch. This method is quite accurate with a high degree of repeatability (+/−25 micro litters.) as demonstrated by the good fit of the curves to the data points.
A second step was to clean the membrane according to field observations. Soda water was air circulated across the face of the membrane overnight. This simulated the bubble nucleation concept seen in the field.
A third step was to establish permeate base line flows on clean membranes. Run 3 used the stale soda water that had been aerated overnight. Run 4 used tap water.
A fourth step was to determine the effect of gas content (function of soda water “out of the bottle” age) on permeate flow compared to tap water. Run 5 was on 1 hour-old soda water. Run 6 was on tap water. Run 7 was on 12 hr old soda water.
A fifth step was to approximate the gas content of the liquor in a typical bioreactor. Run 8 was on 50% tap water and 50% soda water. The soda water/tap water mixture was changed frequently to keep the age of the soda water to less than 30 minutes out “of the bottle.” In the field the CO2 in a long shaft vertical reactor is replenished every 6-10 minutes, so run 8 is conservative.
The plot of test run results in
1) Tap water with, air circulation, was run through a dirty membrane and at 24″ of head pressure and about 1050 micro liters per mill of permeate was produced. There were no bubbles visible in the siphon line and a vacuum was easily maintained.
2) Fresh soda water was then processed on the same dirty membrane and about 1875 micro liters of permeate was produced or about a 78% gain in flow. This is approximately the same gain in performance as in the field trial when the bio-reactor fluid was supersaturated with dissolved air, (i.e. downcomer air was added). It is interesting to note that the soda water used was fresh, between 1 and 5 hours old, yet the degree of nucleation on the membrane was sufficient to preclude the use of a siphon from 24 to 32 inches of head. This was interpreted as proof that the dissolved gas permeates the membrane easily.
3) Soda water was then air circulated across the membrane face for a further 12 Hours. The permeate lines were blocked off so that the dissolved gas impinged on the membrane surface and very little, if any, fluid or gas transferred though. This simulates the conditions in the reactor where it is alleged that a polymeric surface can be effectively cleaned by bubble effervescence. The permeate discharge lines were then unplugged and the permeate flow reached 2200 micro liters/min at 24″ of head. Stale soda water (run 3) achieved a 40% increase in flow over tap water (run 4) when both were processed on a clean membrane. Note that the test runs illustrate that it does not matter whether the tap water or the fresh soda water is run first, the fresh soda water always outperforms the tap water. Note also that the lack of dissolved gas allowed full siphon effect and no bubbles were observed in the discharge lines. Also remember that the differential transmembrane pressure effect must be ignored in all of these runs because the membrane is horizontal. When the membrane is clean, the improvement in permeate flow appears to be related only to the effect of dissolved gas nucleating in or on the membrane. It is predicted that these results are related to a substantial change in the partial pressure of the gas in the fluid. The dissolved gas is at super-saturation pressure in the liquid on the upstream side of the membrane, but is at atmospheric pressure on the down stream side of the membrane. Consequently, the gas is moving from high pressure to low pressure across the membrane and possibly taking the fluid with it.
The Zenon membrane produces about 50% more flow per sq. ft. than the Kubota membrane but the Zenon membrane uses a vacuum on the permeate discharge line. It may be that Zenon membranes are influenced by the drop in partial pressure across the membrane thus causing a nucleating gas effect. From a differential density across the membrane perspective, a 40″ tall Kubota membrane should perform better than a 60″ tall Zenon membrane.
4) To quantify the effect of the membrane cleaning process of step 3, tap water was re-run on the alleged cleaned membrane. This time the permeate flow increased from 1050 micro liters per min. in test 1) to 1575 micro liters in test 4. This represents a 50% increase in permeate flow due to impingement/nucleating gas cleaning.
5) Fresh soda water (1-6 hr old) was processed on the clean membrane and the permeate flow (2400 micro liters per minute) was marginally better (9%) than run 3 (2200 micro liters per minute) which used soda that had been air stripped for 12 hrs. Again it is seen that extremely high levels of dissolved gas are not needed to create an effect.
6) Run 6 was on tap water and the permeate flow rate increased (44%) to 2275 micro liters per min. from 1575 micro liters per min. over the earlier run 4 also on tap water, both using a clean membrane. Run 6 on tap water produced slightly less permeate flow (5%) than fresh soda water in Run 5.
7) Stale soda water (12 hrs old) was run on a clean membrane. The permeate flow (1950 micro liters per min.) was 23% less than the 1-6 hi old soda permeate rate of (2400 micro liters per min.). The permeate flow rate for 12 hr old soda (1950 micro liters per min.) was surprisingly (16%) lower than for the tap water run 6 (2275 micro liters per min.). It would appear that when the filters are clean the rheological properties of the stale soda water and the tap water behave similarly. The data indicates that the difference in permeate flow of the two soda water runs, is related to the age of the soda which in turn is a function of the amount of dissolved gas present. However, in this case the tap water permeate flow exceeded the stale soda water run indicating that there is really no difference in the rheology of the two fluids when processed on a clean membrane. This gives credence to the idea that the increase in permeate flow is indeed a function of the gas nucleation phenomena rather than a difference in the physical/chemical properties of the two fluids.
8) Having determined that the rheology and the physicochemical properties of the two water sources are similar (once the dissolved gases are equilibrated), a final run (8) of a 50% tap water and 50% fresh soda water was evaluated. In this case the mixture was replenished often, at less than 30-minute intervals, to more closely approximate the nature of a vertical shaft bioreactor. In this run 8, the permeate flow reached 2600 micro liters per min. for a 15% increase over tap water alone, and 8% over fresh soda water alone. Run 8 at 2600 micro liters per minute is equivalent to 130 U.S. gal per day per full size membrane. Also keep in mind that these figures are at only 24″ of head, while in the field up to 39 inches were run.
The above observations strongly indicate that dissolved gas nucleation does play a role in membrane flow rate. These data also strongly indicate that the dissolved gas is instrumental in the cleaning process. The amount of dissolved gas effects the flow rate but the amount of dissolved gas in the bench test apparatus is time dependent. Fortunately the dissolved gas content of the liquor in the field test is constant unless changed purposely.
Another test run was performed to correlate a relationship between dissolved gas content and time of exposure to the atmosphere. Fresh soda water was processed on the dirty membrane at three pressure heads. The flow rate changed as follows:
These tests indicate that soda water more than 1 hour old is fairly stable (less than 0.75 micro liter per minute) and therefore all the data points on the curves (except for run 8) are for fresh soda water at least 60 min old. On the clean membrane the difference in permeate flow between 1-5 ha old soda and 12+-hour-old soda is about 25% at 24″ of head. In the field, the fluid in circulation is always freshly saturated with CO2 every 6-10 minutes, and may therefore achieve a much larger throughput than these tests indicate.
These tests also indicate that soda water, either stale or fresh, outperforms tap water in all cases on dirty or semi clean membranes. Once the membrane is clean with soda water (which also contains a small amount of citric acid) there is not much difference between tap water and soda water. The membrane in the test apparatus was visibly cleaner, after 5 days of exposure to soda water and tap water, than at the start of the test.
The pressure differential variation from top to bottom of a Kubota membrane is 4-6″ of water and for a Zenon membrane it is about 6-8″. Run 8 using a mixture of 50% tap water and 50% soda water on a clean membrane shows a throughput of 2600 micro liters/min at a head of 24″. A 4, “6” and 8″ pressure differential accounts for 15%, 25%, and 35% of the total permeate flow respectively. The influence on flow due to the pressure differential variation from top to bottom of the vertically oriented membrane is in addition to the increase of flow due to the degassing phenomenon, cited above, occurring at the face of the membrane. Combined, these two effects could potentially double membrane throughput. Based in part on the above, it is contemplated that the increase in flow of permeate through the membrane is due to one or more factors selected from a change in partial pressure of the gas effect, a nucleating gas effect, or a release of stored energy effect.
The vertical long shaft bioreactors are installed in the ground, and develop over time a huge thermal flywheel effect. That is to sly, the effluent temperatures are much less variable than a conventional plant and therefore should have much less difficulty dealing with temperature variations than conventional treatment processes.
While the above description describes with respect to
The invention therefore includes treating an influent that includes removal of a targeted fluid from the influent with increased membrane throughput. The method typically involves a flowing influent stream that includes a fluid that includes a dissolved gas, and a flowing permeate stream that consists essentially of the fluid and the gas. The two streams are separated with a permeable membrane having a first surface in fluid communication with the influent stream, and a second surface in fluid communication with the permeate stream. The membrane is permeable between the surfaces by molecules of less than a predetermined size, the permeability size being selected to allow the targeted fluid to pass and reject unwanted components of the influent stream. The gas may be dissolved in the fluid by any manner or means, for example by injection and as a result of a chemical process occurring within the influent. The amount of the dissolved gas in the fluid of the influent stream is an amount that increases the permeate stream flow over the permeate stream flow when the fluid of the influent stream does not include the dissolved gas. This amount may vary depending on the nature of the fluid, the gas, and operating parameters of a system performing the membrane separation. The amount of dissolved gas in the fluid of the influent stream may be at least the saturation level of the gas, or may be a supersaturation level of the gas. The dissolved gas may include air, or a component of air such as carbon dioxide. The targeted fluid may be water, blood, or any other fluid.
Another aspect of the invention includes treating an influent that includes imparting a self-cleaning action on membrane surfaces. The method includes a flowing influent stream that includes a fluid that includes a dissolved gas, and a flowing permeate stream that consists essentially of the fluid and the gas. The two streams are separated with a permeable membrane having a first surface in fluid communication with the influent, and a second surface in fluid communication with the permeate. The membrane is permeable between the surfaces by molecules of less than a predetermined size, the permeability size being selected to allow the targeted fluid to pass and reject unwanted components of the influent stream. The fluid of the influent stream includes the dissolved gas in an amount that permeates the membrane and nucleates proximate to the second surface. The fluid of the influent stream may include the dissolved gas in an amount that imparts a scouring action on the first surface. The fluid of the influent stream may include the dissolved gas in an amount that nucleates on the second surface and impales a scouring action on the second surface. The nucleation of the gas proximate to membrane surface imparts a scouring action on the surface that helps clean the surface. This increases operating life of the membranes by increasing time between scheduled membrane cleaning cycles that remove the membrane from service. Previous
A common conventional technology uses low-pressure horizontally orientated membrane diffusers, typically flat plate membranes placed horizontally on a floor of an aeration tank. The floor area, even if completely covered with membranes, has a relatively small area compared to the tank volume to be aerated. In such horizontal applications, a liquid being aerated is contained above the membrane. This liquid subjects the entire membrane surface to a hydrostatic pressure. A disadvantage of this horizontal membrane design is that bubbles generated are quite large when they leave the surface of the membrane. This is because a bubble must grow in low-pressure horizontal membrane systems until buoyancy exceeds attraction force before the bubble is released. Low-pressure, horizontal membrane systems typically generate bubbles about 1-2 millimeters in diameter. Current practice is to force the bubble from the surface of the horizontal membrane by increasing the internal gas pressure to about twice the static liquid pressure. This makes small, fine bubbles, but requires substantially more energy in compressing the gas.
An emerging design places membranes in a vertical configuration, and allows the liquid being aerated to flow between the membranes. The membrane surface area in an aeration tank is greatly increased by arranging the membranes vertically, and the bubbles generated are smaller due to the shearing action of the liquid flow between membranes. Very low energy requirements that are 20-30% of conventional horizontal membrane systems have been reported. However, in the vertical layout, a top portion of the membrane sees a lower pressure from the liquid than a bottom portion of the membrane because the bottom portion is at a greater depth. This results in an unequal airflow along a vertical axis of the membrane surface. A common complaint in this design is that vertically orientated membranes “wet out” and cease air flow through the membrane. The “wet out” generally begins with a portion of the membrane at the greatest depth, and proceeds upward. The lack of airflow in the lower membranes allows water to enter the membrane, which restricts or stops gas diffusion by the membrane.
Elements of the submerged membrane gas diffusion apparatus 600 include a membrane bundle 610, a membrane-mounting member 612, a fluid treatment compartment 601, a bubbling fluid compartment 602, and a static fluid compartment 603. For clarity in viewing
Each membrane-mounting member 612, which is a tubular member with a right hand 612R and a left hand 612L portion in a preferred embodiment, mounts or carries a respective end of the membrane bundle 610 at a membrane-mounting portion. Each membrane-mounting member 612 includes a chamber 614 that provides the fluid communication FC between the bubbling fluid compartment 602, the first surface 411 of each membrane of the membrane bundle 410 mounted to the mounting member, and the static water compartment 603. The chamber 614L of left-hand portion 612L of the membrane mounting member 612 includes a substantially vertically orientated bubble capture chamber 617 and a bubble capture aperture 619, which are illustrated in
For purposes of describing an embodiment of the invention, a fluid to be diffused 620 is described as air 620. In other embodiments, the fluid 620 to be diffused may be any type of gas, or may be a liquid. Diffusion will be described herein as aeration, but the invention is not so limited. Further, a liquid 634 to be treated into which the diffusion occurs will be described as wastewater or water. In other embodiments, the fluid 634 to be treated may be any type of liquid or gas.
The fluid treatment compartment 601 includes a configuration that contains the wastewater 634, such as a reactor basin tank that contains high concentrations of suspended solids or mixed liquor for aeration in conjunction with treatment. Typically, the wastewater 634 flows into the fluid treatment compartment 601 for aeration, receives aeration, and flows out, usually for further processing or disposal.
The bubbling fluid compartment 602 includes a configuration that contains a first fluid 632 and the rising bubbles 626 of the air 620. The first fluid 632 will be described as clean water 632, but may be any fluid having a specific gravity greater than the air 620. The compartment 602 optionally includes a source for the bubbles 626, which may include a gas inlet port 622 that receives the air 620 to be formed into air bubbles 626 in the water 632. The port may receive the air 620 from an external source that, upon entry into the bubbling fluid compartment 602 and the clean water 632, forms the bubbles 626. Alternatively, the port 622 may receive the clean water 632 including the bubbles 626 into the compartment 602. The gas inlet 622 may include any apparatus that forms the air bubbles 626 in the water 632.
The static fluid compartment 603 includes a configuration that contains a static fluid 636, described as clean water 636, but which may be any fluid, but may be any fluid having a specific gravity greater than the air 620. Optionally, the compartment 603 includes a configuration allowing a user to visually observe whether any bubbles of the gas 620 are being discharged from the gas release aperture 611 of the gas release member 616, or are otherwise present.
As illustrated in
In use, the bubbling fluid compartment 602 is filed with aerated clean water 632, and the static water compartment 603 is filled with static clean water 636. The fluid treatment compartment 601 is filled with the wastewater 634 to be aerated to a level sufficient to submerge the membranes 610A-C. The wastewater 634 optimally is flowed through the compartment 601 from a low elevation to a high elevation proximate to the second surfaces of the membranes in a manner that facilitates aeration, and then flowed from the compartment.
The vertical position of the aerated water 632/air 620 interface 658 within the capture member chamber 617 with respect to a lowest elevation of the membranes of the membrane assembly defines a gas column 652 having a vertical length, which can also be described as a hydraulic head or differential hydraulic head. The gas column 652 imposes a hydraulic head on the air 620, which is a function of the buoyancy of the air 620 in the aerated water 632.
That imposed hydraulic head is transmitted to the portion of the air 620 in fluid communication with the first surface of the membrane of the tube membrane bundle 610. If the specific gravities of the aerated water 632 and the wastewater 634 are substantially similar, the hydraulic head between the first membrane surfaces 411 exposed to the chamber 614 and the second membrane surfaces 412 of the membranes of the membrane bundle 610 exposed to the fluid 634 in the fluid treatment compartment 601 will approximate the hydraulic head created by the gas column 652.
The gas column 652 vertical length and resulting differential hydraulic head are established by the amount of the bubbles 626 in the bubbling fluid compartment 602 that enter the bubble capture aperture 619. Increasing the number of air bubbles 626 formed in the aerated water 632 increases the number of air bubbles rising into the bubble capture aperture 619, thus increasing the flow of air into the membrane-mounting member chamber 614. This increased air flow will exceed that which can permeate the membranes 610 at the existing imposed hydraulic head. The air 620 will accumulate in the chambers 614, 617, and 618, and the vertical elevation of the aerated water 632/air 620 interface 658 will decrease. This increases the gas column length 652, and increases the imposed hydraulic head on the released air 620, thus increasing the air flow through the membranes until an equilibrium is reached in response to the amount of bubbles 626 in the bubbling fluid compartment 602. The internal air pressure of the membrane bundles 610 self adjusts to the air flow provided by the bubbles 626. The higher the air flow provided by the bubbles 626, the lower the water 632 level in the rising bubble capture member 615, and the greater the differential hydraulic head 652.
If a hollow tube of the membrane bundle 610 becomes blocked, or if the captured bubbles 626 produce more air 620 than the membranes of the membrane bundle 610 can diffuse, the air will build up in the tube membrane bundle 610 until the air fills and overflows the air release member chamber 618 from the gas release aperture 611, transferring the air to the static water compartment 603. This release occurs because the air release member chamber 618 has a smaller vertical length 656 than the rising bubble capture member chamber 617 vertical length 654, and will vent the air 620 before the air 620 fills and overflows the rising bubble capture member chamber. An appearance of air bubbles in the clean water 636 of the compartment 603 indicates that excessive air 620 is being supplied to the membrane bundle 610, or that the membrane bundle needs cleaning. Because the membrane bundle 610 is connected to clean water compartments 602 and 603, no internal fouling of the membranes should occur.
On startup, the membrane surfaces of the membranes of the tubular membrane bundle 510 have differing vertical elevations. Using the membrane bundle 610C as an example, a top hollow tube membrane of the bundle is at elevation 9.0 feet and a bottom hollow tube is at 8.5 feet. Initially, the top membrane in the tube membrane 610C bundle will see a little greater pressure differential than the bottom membrane because it is at a lesser depth, and will therefore produce a little more air bubbles 628 until its maximum flow rate is achieved, thus increasing the internal pressure on the air 620 and causing the bottom membrane to approach maximum transfer as well.
The hydraulic head created by the gas column 652 can be calculated as follows: Since the water 634 in the fluid treatment compartment 601 is aerated as a result of its processing, there is a voidage of between about 2-10%. For purposes of describing the system 600, a voidage of 5% will be assumed. The dynamic water levels in both the fluid treatment compartment 601 and the bubbling fluid compartment 602 are established at 12 feet×105%=12.6 feet. The hydraulic head across the membrane surfaces of the top bundle tubes of the membrane bundle 610C is the pressure of the water 634 outside the second membrane surface 412 minus the pressure of the air 620 inside at the first membrane surface 411. The outside water 634 pressure is (12.6−9.5)/2.31×0.95=1.27 psig while the inside air 620 pressure is (12.6−8)/2.31×0.95=1.89 psig. The hydraulic head is 0.62 psig. Similarly the outside water 634 pressure on the bottom membrane bundle 610A is (12.6−4.5)12.31×0.95=3.33 psig and the inside air 620 pressure is (12.6−3)/2.31×0.95=3.94. Again, the hydraulic head is 0.62 psig. These calculations illustrate an aspect of the invention providing a selected hydraulic head or pressure differential across all the membranes of the assembly 600.
Occasionally it will be necessary to shut down the gas diffusion apparatus 600, and clean water 632 and 636 will enter the membranes 610. When the air 620 is restarted, the water will be forced out of the air release members 616 and into the static water compartment 603, thus self-purging the airways of the tubular membranes of the membrane bundles 610. In an alternative embodiment, the compartment 603 could be filled with a cleaning fluid for periodic cleaning of the membranes by stopping the air bubbles 626.
It should be noted that there are many applications where the apparatus 600 could be used. Some examples are ozonation (O3), chlorination (Cl2), or recarbonation (CO2) of drinking water, disinfection of wastewater or re-oxygenation of effluent using pure O2, or biochemical nutrient addition or feedstock, such as NH3, CH4, SO2, etc.
Although the foregoing invention has been described in detail by way of example for purposes of clarity of understanding, it will be apparent to the artisan that certain changes and modifications are comprehended by the disclosure and may be practiced without undue experimentation within the scope of the invention that is described herein by way of illustration not limitation, All publications, patents, and patent applications cited herein are hereby incorporated by reference in their entirety for all purposes.
| Number | Date | Country | |
|---|---|---|---|
| 60271201 | Feb 2001 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10895432 | Apr 2004 | US |
| Child | 12099033 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10083995 | Feb 2002 | US |
| Child | 10895432 | US |
