Patent Application
20220315500
Biosolids comprise solid, semi-solid, or liquid residue generated during biological wastewater treatment process. Biosolids for beneficial use (i.e. land application or distribution) must be treated to reduce pathogens and vector attraction reduction (“VAR”). Insects, birds, rodents, and domestic animals may transport sewage sludge and pathogens to humans. Vectors are attracted to sewage sludge as a food source. VAR is a focus of federal regulation, e.g., 40 C.F.R. Part 503. VAR can be accomplished in two ways: by treating the sewage sludge to the point at which vectors will no longer be attracted to the sewage sludge or by placing a barrier between the sewage sludge and vectors.
After wastewater treatment, biosolids may be beneficially reused or disposed. Generally, biosolids are transported from wastewater treatment plants and beneficially reused in rural areas for application to farm fields. Alternately, biosolids are transported to a landfill for disposal. Large volumes biosolids create significant transportation expense. Wastewater sludge contains a relatively high percentage of water, and large volumes are created for reuse or disposal. The sludge also can create environmental and or health problems.
Federal, state, and local governments regulate the distribution and marketing of Class A biosolids. Class A biosolids represent the highest quality biosolids produced and may be used as fertilizer through commercial distribution. To achieve a Class A status, the biosolids must be treated to substantially eliminate pathogens and must meet stringent maximum concentration limits for heavy metals. Class A biosolids may be distributed in bulk or bagged for sale at retail centers. Class A biosolids may be marketed in different physical forms, and, like traditional commercial fertilizer, are not subject to site management restrictions if the product is registered as a fertilizer or distributed and marketed to a person or entity that will sell or give-away the biosolids (wholesale) or market biosolids products as a fertilizer (retail).
The United States Environmental Protection Agency's (EPA) Regulations recognize at least two classes as explained 40 C.F.R. Part 503. Class B pathogen reduction standards require a fecal coliform level of less than two million most-probable-numbers (MPN) per gram of total solids. Class A pathogen standards require fecal coliform densities less than 1,000 MPN per gram total solids and/or; Salmonella densities less than 3 MPN per four grams total solids. Enteric virus must be less than 1 plaque-forming unit per four grams of total solids. Helminths ova must be less than one viable helminths ova per four grams of total solids.
Traditionally, biosolids disposal involves transportation to rural areas and applying the sludge onto fields. This process increases health and environmental concerns. Other methods of disposal may include incineration, adding chemicals, or disposal into landfills. Concerns about contaminants, runoff, air pollution, tipping fees, and rising transportation costs have resulted in cities and municipalities seeking alternate and more efficient methods to remove of biosolids.
Double drum dryers have been used to dry biosolids. Historically, limitations regarding the moisture content and throughput (volume of biosolids to be processed into fertilizer measured gallons per day or dry tons per day) of the biosolids negatively impacted efficient processing using a double drum dryer. The biggest limitation has been the ability of the biosolids to enter the nip between the two rotating drums. Double drum dryers have been limited in the percent (%) solids or thickness that they can process by the ability of the biosolids to flow through the nip area between the drums. If the biosolids were too dry, they would bridge between drums and not enter the nip area. Consequently, double drum dryers could only process biosolids that would flow, e.g., biosolids with no more than about 15% solids.
Typically, biosolids are sprayed with a pendulum slurry feed system or pumped into the nip as a liquid. If the biosolids are too stiff or dry, they will bridge over the nip and not effectively pass through the nip. Because the double drums rotate without throughput falling into the nip, the drying process becomes uneconomical. The prior art operational limits are restricted to product that is fluid enough to flow. If the product was not fluid enough to slip into the nip, water would be added first to liquefy the product so that it could be processed. This defeats the purpose of drying the biosolids and reduces throughput.
The systems and processes of the claimed invention combine pre-screening, chemical flocculation, dewatering, double drum steam impulse drying, and flash evaporation to dry and shrink the volume of biosolids, pasteurizing the biosolids, pelletizing, and cooling for distribution as fertilizer. Optionally, additional fertilizer, water absorbent granular polymer, or both, may be added to the pellets to promote drought durability for the crop that is fertilized. The disclosed systems and processes solve the problems in the field of endeavor related to the decreased flowability of drier biosolids and allows it to pass efficiently through the nip. This increases efficiency and productivity of the double drum drying process. Thus, the disclosed inventions make it more economical to operate in terms of time, money, energy, and other resources.
A feeding chamber comprises a novel nip feeder to solve the problems in the art relating to feeding the biosolids into the nip. In the feeding chamber, positioned above the double drums in the dryer component, a conveyor transports biosolids into the nip feeding chamber. A novel nip feeder, positioned at the bottom of the feeding chamber, feeds biosolids into the nip between two rotating drums thereby eliminating or reducing the bridging of biosolids in the feeding chamber. Although this description focuses on one double drum dryer comprising two rotating drums, a person of skill in the art will appreciate that more than one set of dryer drums could be used within the scope of this invention.
Further embodiments, forms, objects, features, advantages, aspects, and benefits shall become apparent from the following description and drawings.
This description provides contemplated modes of carrying out embodiments of the invention. The description illustrates the general principles of the inventions without limiting their scope.
Most biosolids from wastewater treatment plants comprise a slurry. The concentration of solids typically ranges from 1-8% solids (92-99% moisture). Wastewater will typically comprise a large volume of water and a small volume of solids. Typically, 99% of what comes into a wastewater plant is water. This water is usually treated biologically and discharged into a nearby stream or water body. The remaining 1% that consists of solids is separated and treated independently. Commonly, the treatment process for these solids includes either aerobic or anaerobic digestion. In anaerobic digestion, a tank is used which is heated to about 95°-98° F. This causes the solids to start digesting without the introduction of additional air. Aerobic digestion involves adding air to the solids. Aerobic digestion comprises using a tank that has continuous air injected in it to digest the biosolids. Both options are expensive and require energy and maintenance. The inventions disclosed and claimed herein eliminate the need for aerobic or anaerobic digestion.
Referring to
Referring to
Centrifuge 90 removes water from the biosolids. The screened biosolids 15 are pumped to centrifuge 90. Alternately, the dewatering may be accomplished with a belt press (not shown), a screw press (not shown), or other similar devices known in the art. While being pumped from the screener 40 to centrifuge 90, a polymer 50 may be injected into the biosolids 15. The polymer 50 causes the Biosolids to “flock”. This flocking process aids in the separation of the solids from the water. In the case of the centrifuge 90, the specific gravity is magnified, which causes the water to separate from the solids more efficiently and exit the centrifuge 90 at 20-30% solids (70-80% moisture). The centrate, or water that is separated from the centrifuge, is discharged back into the wastewater plant and after further treatment is discharged.
Suitable polymers include for example, those available from Polydyne, BASF, and Solenis. The polymer 50 may be delivered to the polymer tanks 70 using polymer tote 60. Polymer pump 80 delivers the polymer 50 to the biosolids 15 at a desired volumetric rate over a desired time of delivery. In one preferred embodiment, the volume of polymer added to the biosolids is within the range of about 2 lbs. per dry ton to about 90 lbs. per dry ton. The precise volumetric amount depends on the type of biosolids being processed. The dewatering process thickens the biosolids 15 to approximately 20-30% solids (70-80% moisture) by weight. The biosolids are then delivered to a first conveyor 110.
Referring to
Conveyer 130 may be a self-leveling ribbon screw conveyor or other similar system. The nip feeder 150, positioned at the bottom of the feeding chamber 140, delivers biosolids 15 through the nip 155 to the rotating dryer drums 160, 170 and eliminates bridging in the feeding chamber 140 on the upward stroke. The gap of the nip 155 is selectively sized depending on the product being processed and the desired throughput. In one embodiment, the nip 155 is sized from about 0.1 mm to about 0.4 mm.
This invention overcomes that problem and can process a higher throughput of Biosolids than other systems known in the field. This is accomplished by doing two things. Referring to
Feeding chamber 140 stores biosolids 15 ready to process above dryer drums 160, 170. Second conveyor 125 transports the biosolids generally horizontally from the first hopper 100 to the vertical conveyor 120 to horizontal leveling conveyor 130 to feeding chamber 140. The biosolids are unloaded into a self-leveling ribbon screw conveyor 130 that drops the biosolids into the feeding chamber 140. The self-leveling ribbon screw conveyor 130 operates at a speed that will transport more biosolids than can be processed. The excess biosolids 15 exit the opposite end of the dryer and discharge back into second storage hopper 100.
Referring to
The main bar 153 on the nip feeder 150 provides stability and minimum horizontal “flexing” across the drums. The nip feeder 150 promotes rolling of the biosolids on the upward motion which encourages the biosolids to roll each direction and fall into the nip 155 thereby refilling the nip 155 for the next downward stroke. The up and down motion of the nip feeder 150, combined with the head pressure, feeds the biosolids down into the “grab” zone. The grab zone is the point in the nip 155 where the pulling action of the rotating drums grabs the biosolids and pulls it in. The nip feeder 150 may provide a maximum stroke of up to about 5 inches, which may be selectively adjusted as desired. The nip feeder 150 pivots up and down and its speed is adjustable by a variable frequency drive controller.
Referring to
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Boiled and evaporated flash water is selectively removed through a venturi 250 to limit condensation. The biosolids 15 may adhere to outer surfaces 163, 173 of dryer drums 160, 170 and continue to dry as dryer drums 160, 170 rotate about axes 162, 172 respectively. First and second scrapers 180, 190 remove biosolids 15 off outer surfaces 163, 173. Dryer drums 160, 170 continue to rotate to collect and dry additional biosolids 15 introduced through nip feeder 150.
If a higher level of throughput is desired, the nip 155 may be widened, the rotational speed of dryer drums 160, 170 may be increased, or both. The exiting biosolids 15 will not be as dry but the throughput volume will increase. The % solids can be as low as 50%. The biosolids 15 may then be sent to a fueled pasteurization drying system such as a belt dryer 300 as disclosed in U.S. Pat. No. 9,751,813 and 10,259,755 that are incorporated herein by reference.
At the bottom of the second storage hopper 100, a second conveyor 125 transports biosolids 15 to a vertical conveyor 120 that transports the biosolids 15 to a fourth horizontal self-leveling conveyor 130 which transports the biosolids to a feeding chamber 140. Conveyors 120, 125 may be screw conveyor, shaftless screw conveyors, or other variations known in the art. The feeding chamber 140 is positioned above the dryer drums 160, 170 and spreads biosolids 15 over the outer surfaces 163, 173. Preferably, the biosolids are delivered as evenly as possible over the available surface of the rotating drums.
Conveyors 120, 125, 130 may transport more biosolids 15 than the feeding chamber 140 can hold. The excess biosolids 15 exit the opposite end of the feeding chamber 140 and are discharged back to storage hopper 100. This always allows the system 10 to operate at about 100% capacity. It also creates an ease of operation for the operator. A level sensor 145 operatively connected with feeding chamber 140 may continuously read the level of biosolids 15 in storage hopper 100. This signal may be operatively connected to a programmable logic control (“PLC”) that regulates the volume at rate of biosolids 15 coming from the centrifuge 90 depending on the level of biosolids 15 in the hopper 100 or feeding chamber 140. Thus, the system becomes self-calibrating.
Once the biosolids 15 have been pulled into the nip feeder 150, pressure applied to biosolids 15 may be increased while the temperature from dryer drums 160, 170 transfers to and heats biosolids 15. Under pressure greater than atmospheric pressure, the boiling point of the biosolids 15 increases as a basic law of physics. This process is known as Impulse Drying. As biosolids 15 exit the nip 155 and the pressure drops to atmospheric, the biosolids 15 boil and flash evaporate. This reaction causes some of the water in biosolids 15 to “Flash” before it has time to boil. This phenomenon reduces the amount of thermal energy that would normally be needed.
Once biosolids 15 pass through the nip 155, a majority adheres to the surfaces 163, 173 of dryer drums 160, 170 and continue drying as dryer drums 160, 170 rotate. Scrapers 180, 190 remove biosolids 15. Some biosolids 15 may separate from dryer drums 160, 170 due to centrifugal force. A live bottom screw auger 200 (
Positioning biosolids 15 above dryer drums 160, 170 allows heat to dissipate from the outer surfaces 163, 173 to biosolids 15. This process causes the steam inside dryer drums 160, 170 to cool and condense. The resultant hot water inside dryer drums 160, 170 needs to be removed. Referring to
Referring to
The dryer drums 160, 170 are substantially enclosed and operated under a vapor hood 400. Vapor hood 400 pulls steam and flashed water out of dryer drums 160, 170 before cooling to condensation. Vapor hood 400 may comprise a variable frequency drive controlled 3,000 cfm fan or another similar device. The air flow is then processed through a venturi 250 and/or chemical scrubber 320 to capture contaminants and odor before discharge.
Once the biosolids 15 leave dryer drums 160, 170, a fourth conveyor 335 transports biosolids 15 to fourth mixing hopper 345. Hopper 345 is positioned above shafted reversing screw auger 420. When auger 420 is running in reverse it may optionally transport biosolids 15 to a belt dryer 300 for further drying. When running in forward position, auger 420 transports biosolids to pelletizer 310 where biosolids 15 are formed into pellets 315.
Optionally, pellets 315 may be spiked with a desired recipe in the mixing chamber 345 Fertilizer chemicals such as Nitrogen, Potash, Phosphorus, or Lime may be added to the pellets 315 at desired concentrations to provide the fertilizer with a desired material composition. Alternatively, water absorbent polymer 50 may be added. This allows selected customization of the chemical content of pellets 315 depending on the desired application. The addition of the water absorbent polymer is unique in that this polymer can hold up to 200 times its weight in water. The spiked pellets 315 can be planted underneath agricultural seeds to provide the desired supply of nutrients and a reservoir of water to help during dry times of the year. This ability is allowed by using naturally occurring lignin in biosolids 15 to help bind or pelletize the polymer and additional fertilizer. Lignin is a class of complex organic polymers that form key structural biosolids in the support tissues of most plants. Lignans are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity and do not rot easily.
This invention seeks to eliminate the need for either type of digestion. This invention takes the raw waste and within minutes transforms the solids into a Class A biosolids without any digestion required, thereby saving the user money and other resources.
The dried biosolids 15 removed from dryer drums 160, 170 fall into a third conveyor 335 that conveys the biosolids 15 to a holding/mixing chamber 345. The third conveyor 335 may comprise a screw auger system. Inside the holding/mixing chamber 345, the biosolids 15 fall into reversable auger 420. In one direction, the reversible auger 420 moves the biosolids 15 to a belt driven dryer 300. The reversible auger 420 may be selectively reversed to convey biosolids 15 to pelletizer 310 where the biosolids are formed into pellets 315.
In one embodiment, prior to pelletizing, additional fertilizer may be added to the biosolids 15. A water absorbent polymer may optionally be added to form a custom pellet 315. The pellets 315 are conveyed into pellet cooler 350 where atmospheric air is drawn through the pellets 315. Once through the cooling chamber 350, a fourth conveyor 360 moves pellets 315 to a hopper 370. Hopper 370 may comprise a truck, sacks, or other bagging equipment.
Once the pellets come out of the pelletizer, they will be 175° F. or hotter. To avoid internal combustion of the pellets 315 during storage they may need to be cooled. From the pelletizer 310, fourth conveyor 340 transports pellets 315 to pellet cooler 350 where 2,600 cubic feet per minute (CFM) of fresh room air is pulled through the cooling chamber 350 lowering the temperature of the pellets 315. Once cooled, a fifth conveyor 360 transports pellets 315 to a fourth hopper 370 where they can be stored or unloaded into super sacks, trucks, or bags. Fourth hopper 370 may comprise or be operatively connected to scales 380 to weigh the biosolids for potential sale to the market.
When the biosolids 15 complete the foregoing process, they are at least 90% solid, which qualifies as a Class A status as determined by the Federal EPA Part 503 regulations. The biosolids 15 may then be sold as a Class A fertilizer. Another use would be as an alternative energy source for incineration processes.
