Patent Application
20120264198
(1) Field of the Invention
This invention concerns in-situ dry anaerobic composters as well as methods for their construction and operation.
(2) Description of the Art
The European community has been using anaerobic digesters to remediate food and yardwaste for many years. Manufacturers like Becon, Drainco, and Kompogas have been successfully building and operating these units in Europe and Asia for a number of years. An example of a prior art composter/digester is shown in
Disposal and recycling fees in countries where anerobic digesters are used are supported by a tax base that makes their construction and operation affordable. Capital cost for these dry anaerobic digesters are typically $300 to $500 per ton of capacity. For example a 24,000 tons per year facility costs between $8,000,000 and $13,000,000. This capital cost leads to an amortization cost per ton for a 20 year life of site plant of about $20 to $40 per ton in today's market which is too high to be economically feasible in the United States. There is a need, therefore, for reusable anaerobic digesters that have been improved in a manner that causes them to be economically feasible in the United States and more profitable when used outside of the United States.
The present inventions demonstrate at least one of the following advantages. The present invention is directed to in-situ and reusable anaerobic digesters (composters) with capital costs that are up to 60% to 80% lower than prior art anaerobic digesters while providing similar or better gas yields per ton. It is believed that the digesters of the present invention are economically feasible in the U.S. and Canada.
Another aspect of the present invention is a flexible anaerobic digester complex that allows for the construction of different sized digester cells depending upon the anticipated dispersion of heat that will be generated during the fermentation process. The complex will include many small digester cells in warmer weather locations where fermentation heat is not easily dispersed and larger digester cells in cooler weather locations.
Still another aspect of the present invention are anaerobic digesters that allow for a decrease the parasitic heating load by placing it in-situ and by providing for indirect heating or warming of the fermenting mass.
In a further aspect, the present invention includes an in-situ dry anaerobic composter comprising a section of ground including a pit having side walls and a bottom; an essentially impervious liner located in the pit such that the liner abuts the pit side walls and bottom to form a lined pit; a compostable material located in the lined pit; a gas management system for extracting a gaseous anaerobic decomposition product from the compostable material; at least one pipe for injecting an aqueous stream into the compostable material; and at least one pipe for removing aqueous materials that collect on the bottom of the lined pit from the composter.
Yet, another aspect of the present invention is a method for composting material in a in-situ reusable dry anaerobic composter cell, the method including the steps of; preparing compostable material for fermentation; preparing a cell for holding the compostable material the cell including a pit constructed in a section of ground, the pit including side walls, a bottom, an essentially impervious liner located in the pit such that the liner abuts the pit side walls and bottom to form a lined pit; placing the prepared compostable material in the cell; covering the cell with a cover to form an essentially gas tight anaerobic composter cell; bringing the cell to fermentation conditions and operating the cell at anaerobic fermentation conditions sufficient to form digestate and anaerobic fermentation gasses; collecting the anaerobic fermentation gasses using gas extraction piping located in the cell; halting the anaerobic fermentation when a defined anaerobic fermentation end point is reached; and opening the cell and removing the digestate to form an emptied cell.
The present invention relates to an improved organics diversion system that includes one or more batch in-situ reusable anaerobic composter cells—RAC cells 10. The RAC cells 10 of this invention use flexible membrane liners as construction materials and accept and remediate shredded compostable materials. The RAC cells 10 can be used to compost any type of compostable material know in the art including, but not limited to, yard waste, manure, sludges, wood, pallets, brush, food waste, cellulosic materials like cardboard, construction waste, and combinations there of. RAC cells 10 are typically operated in a manner that produces both methane for energy and useful solid. The solids that are not fermented to form methane gas are reclaimable as digestate or compost solids. The resultant solids are useful as soil amendment, as a peat moss substitute or as compost.
In one embodiment, the RAC cells 10 of this invention are used to compost a mixture of yard waste and food waste in a dry fermentation (50% to 70% solids) process. The RAC cells 10 of this invention may be arranged in an array of two or more RAC cells to form a composting complex. Each individual RAC cell 10 is generally operated as a discrete batch. Cycle time is variable and is dependent on feedstock methane potential and weather. Anerobic cycle time can vary from about 30 days to several months or more.
Further details of this invention are presented below, in part by reference to the accompanying Figures. Referring now to
Pit 20 can be constructed by any conventional methods such as by using a bulldozer or an excavator. The walls 22 and/or bottom 23 of pit 20 will typically be formed of soil. However, the walls can, if desired, be formed of structural materials such as concrete or pilings driven into the ground.
RAC cell 10 will have a width of about 50 feet but can be from about 30 inches to 70 feet wide. The cell will have a depth of from about 6 inches up to a depth of about 20 feet. The RAC cell length will generally be between 40 feet and 300 feet with a more typical length ranging from about 80 feet to about 120 feet in length. The apex of RAC cell 10—which typically lies above grade—allows for a 2% to 10% slope (preferably about 4%) on the top of the cell. Pit wall slopes are typically 1.5/1 or steeper, up 0/1 (or vertical). In some cases the end wall 22′ associated with leachate recirculation piping can be constructed with a gentler angle of from about 3/1 to 4/1 to allow the digestate (the RAC cell product) to be removed by a loader or dozer during the removing step.
RAC cell 10 shown in
A unit operation that is typically shaped by two or more RAC cells 10 is a biofilter 35. Biofilter 35 can be any type of structure or device that is able to safe fully and effectively remove unwanted materials such as volatile organic compounds, methane, and sulfur compounds from gases collected in the fermentation mass and headspace in RAC cell 10 that would otherwise cause unwanted odors and/or emissions. An example of a useful biofilter is a trench including wood chips that have been seeded with or that includes microorganisms that remediate the odor compounds and other organic compounds in gases withdrawn from the RAC cell. The RAC cell gases are directed to the bottom of the biofilter and allowed to percolate through the biofilter into the atmosphere.
The gas extracted from RAC cell 10 by gas extraction piping 14 is directed to gas management system 19. The anaerobic fermentation gases will typically be rich in methane and carbon dioxide and will include smaller amounts of other gases such as ethane, nitrogen, oxygen, and so forth. The gas management system extracts valuable biofuel as methane from the anaerobically fermenting mass which is typically food waste and yard waste. The extracted gases typically include methane in an amount ranging from 50% to 74% by volume. The fermentation gas is preferably extracted by vacuum and is preferably directed to an energy processing facility. In one embodiment, the methane rich gas recovered by gas management system 19 is directed to internal combustion engines for electricity production. Alternatively, the extracted methane rich gas can be used for any purposes that methane is used such as for heating, steam generation or in chemical processes
Note that in
As RAC cell 10 is being filled with compostable material, the material can sit in an anoxic state while additional materials are added. This series of additions can take weeks. During that time a vacuum can be intermittently or continuously applied to the partially filled pit that has a temporary cover using aeration piping system 13. The malodors, volatile organic carbon and odor causing sulfur compounds in the extracted gases are directed to biofilter 35 where they are removed form the extracted air biologically.
Other details of significance shown in
RAC cell 10 of
Liner 24 and liner cover 25 can be selected from any geomembrane material that is commonly used in landfills. Such geomembrane materials are essentially water and gas impervious. The liners will preferably be selected from a polymer material such as high density polyethylene (HDPE), polyvinyl chloride (PVC) or linear low density polyethylene (LLDPE). The liner thickness will range from about 20 mil to 100 mil or more. In addition, liners 24 and cover 25 can be formed from a combination of layers—both permeable and impermeable so long as at least one layer is essentially gas and liquid impermeable.
In the embodiment shown in
In order to further seal RAC cell 10 in the region of piping offset, liner 24 preferably covers the rim 62 and angled wall 64 of piping offset 37. Piping offset 37 may be associated with an edge of RAC cell 10 only where piping is entering and exiting the landfill. Alternatively, piping offset 37 may be formed around part to all of the top perimeter of pit 20 to form an anchor trench around the perimeter of cell 10 that, in combination with a soil plug or other seal material anchor liner 24 and cover 25 in place in RAC cell 10.
Liner 24 and cover 25 are each include a perimeter edge 57 and 58 respectively. Liner 24 and cover 25 are sealed in anchor trench 38 by locating perimeter edges 57 and 58 in anchor trench 38 such that edges 57 and 58 lie entirely in anchor trench 38 or such that edges 57 and 58 lie beyond anchor trench 38 in relation to cell 10 as shown in
The piping used in and around the RAC cell and composter complexes of this invention may be any type of piping useful in landfill or composter applications. While the piping can be metal piping, it is preferred that the piping is plastic piping because of its price and ease of installation. Examples of useful plastic piping include, but are not limited to, PVC piping and HDPE piping. The piping used in RAC cell 10 will generally have diameter ranging from 2 inches to about 8 inches with diameters of 3 to 4 inches being preferred.
The piping that lies outside of RAC cell 10 will be solid piping. The piping installed inside RAC cell 10 can be solid piping or it can be perforated piping depending upon the piping application. For example, the gas removal piping will typically include many perforations or perforated sections to remove fermentation gasses from cell 10 in a manner that minimizes the pressure drop across the piping during vacuum gas recovery. The type of piping used and locations of perforations or pipe openings within the composter is well within the knowledge of one skilled in the art.
During normal operations, the quality of gas from each RAC cell is monitored—preferably automatically using sensors and a system that uploads readings to a monitoring location remote to the cells and activates alarms as necessary. Typical monitoring includes off gas methane level, balance gas, pH, gas flow, pressure and temperature. Additionally, hydrogen sulfide is sometimes monitored. Note the system can be monitored manually in the case of automation failure or in special circumstances.
The fermentation end of life is reached based on gas recovery and the gas curve. Once the gas curve has diminishing returns or looses temperature necessary for anaerobic digestion, the anaerobic fermentation is terminated by aeration and off gassing to the biofilter. Once the amount of methane in the off gas is reduced to a safe level, RAC cell dewatering also takes place through the sump. When the off gas shows greater than 5% oxygen in concentration and the odors are reduced, the cover can be removed. The RAC cell product—called digestate, is processed as noted below.
The composter embodiments of this invention may be prepared in accordance with one or more of the steps discussed below. An initial step can be a shredding and mixing stage. In the shredding and mixing stage, selected compostable material such as food and organics materials are source separated, sized by shredding if necessary, and then optionally mixed with other compostable materials such as an equal volume of shredded yard waste or woodchips to form a compostable mixture. If not loaded immediately into the RAC cell, the compostable material or compostable mixture is staged and odors and vectors are minimized by placing a layer of yard waste or compost over the pile until loading into the RAC cell is complete. The staged material may also be covered with a tarp. In some cases an alkaline material such as lime is added to the mixture.
Next, the compostable material or mixture is charged into the RAC. When operations are ready to charge a new RAC cell or to recharge a previously used RAC cell or pod, a seed material (digestate) from a recently finished cell is preferably mixed in a ratio of above 0% to 50% by volume with the compostable material or compostable mixture previously described to form a seeded compostable mixture. This seeding step decreases lag time in the anaerobic step and prevents a prolonged acid stage in the digestion process. In some cases leachate from an earlier digested cell is added instead or in combination with digestate to form the seeded compostable mixture. The use of leachate as a seed material is especially effective during warm weather periods and when the incoming waste materials include significant amounts of organisms that promote fermentation. This might include various manures, primary sludges and grease pit waste.
The seeded compostable mixture is loaded into the next open RAC cell which has its temporary plastic cover (such as 20 mil scrim) removed for loading. As loading of the RAC cell with the seeded compostable mixture continues, the cover is alternatively removed and replaced until the cell is full. Moreover, during RAC cell loading, a light vacuum may optionally be applied to the material in the partially filled cell using vacuum piping located at the cell bottom in order to prevent odors and VOC's from emanating from the partially filled cell. The gasses and odor bodies removed by vacuum are directed to a compost bio-filter adjacent to the cell. The seeded compostable material in the partially constructed cell is typically anoxic at this stage and is not producing significant methane.
The different piping systems discussed above will be added to RAC cell either before, during or after the compostable mixture is added to the cell. Generally, aeration system piping and leachate removal piping will be placed at or near the bottom of the cell pit before compostable material is added to the pit. The leachate injection piping can be added to the cell as vertically spaced planar piping manifolds as the compostable material is added to the pit. The gas extraction piping can be added to the cell as discussed above, as a plurality of vertically spaced planar piping manifolds or in any other manner known in the art including as vertical gas extraction wells.
Table 1 illustrates the impact of varying ratios of virgin compostable materials to recycled compostable materials in the seeded compostable material on fermentation cycle time;
Once filled with seeded compostable material, the RAC cell is ready to be sealed. Before the RAC cell is sealed piping is placed on the top of the mixed feed and the RAC is sealed with a cover (typically 40 mil LLDPE) that is secured either by plastic welding or by securing in an adjacent anchor trench backfilled with soil, clay or some similar seal material. The anaerobic (without air) phase of fermentation soon begins. Alternatively the cell is made airtight with a prefabricated cover. Each RAC cell is intended to be air tight and vacuum aids in removing anaerobic fermentation product gases.
Once an RAC cell is filled with compostable material and the cover is attached and sealed, the individual RAC cell reaches anaerobic fermentation condition quickly. Once sealed, the vacuum system to the biofilter is turned off and RAC cell off gas pressure and gas quality is monitored. As soon as the gas is oxygen free, vacuum can be applied to the methane removal system. Converting the RAC cell to anaerobic conditions can be accelerated by several methods including by using an optional air blow (aeration) step. The aeration step allows for transition of the compostable material out of the acid phase quickly thus preserving the fermentables for energy producing gas. In order to raise the internal waste temperature to an operating range between 40° C. and 75° C., short term air injection may sometimes be useful in certain circumstances where the feedstock may be particularly acidic in nature (citrus, tomato, or fruit dominated) and where ambient temperatures are below 70° F. This aeration step rapidly digests volatile organic acids and raises the pH to above 6.5. The air can be injected into the seeded compostable material in the RAC cell using any piping that is in place such as the aeration/vacuum piping located at the bottom of the cell or by using the leachate injection piping that is optionally placed throughout the seeded compostable material. During this optional step, the gas extraction system withdraws the exhaust gas products from the RAC cell and preferably directs them to a biofilter for treatment.
Once anaerobic fermentation conditions are reached, the gas extraction piping and gas extraction system begins removing the gaseous anaerobic fermentation products from the RAC cell, preferably using a vacuum pump to extract the useful gases. Moisture, in the form of liquid removed from other anaerobic RAC cell cells can be added to a newly operational RAC cell to increase the availability of methaneogenic seed. Additionally, the moisture content and pH of the new RAC cell is monitored at start-up and the cell pH adjusted to prevent undesirable acid phase conditions. Methane is expected to be present in the extracted gas at levels of approximately 40-75%. The extracted gases can be used for many purposes including for transportation fuel or for energy production. Because the RAC cell is completely sealed, no methane emissions from the fermentation process is anticipated. Estimated total fermentation time (residence time) of a single RAC cell is expected to as short as 25 days and as long as 270 days or more. Variance in residence time will be based on the digestion rate of variable feedstock and climate influence (colder, slower) on the rate of gas production.
Once the selected anaerobic fermentation end point is reached the RAC cell can be opened and the solid digestate removed or the RAC cell is operated in a maturation step. For example, in one embodiment, the anaerobic end point is reached when the gas generation rate is diminished significantly—e.g. to below 50% of original at which point the anaerobic phase is terminated by adding air to the system. However, because the RAC cells of this invention are so economical to install and operate, the anaerobic fermentation step can be allowed to continue until the methane product rate is significantly below the start-up methane product rate. It is expected that the RAC cells of this invention will be able to be operated at methane product rates as low as 25% or less of the start-up methane product rates. The anaerobic fermentation end point can alternatively be identified when the cell temperature reaches a certain point or by any other means known on the art for measuring anaerobic fermentation progress.
At the selected fermentation end point, air can be added to the RAC cell by blowing air through the vacuum piping installed at the bottom of the RAC cell. In addition to ending methane generation, stopping anaerobic fermentation begins the digestate maturation step, which will typically last 2-4 weeks. During digestate maturation, most free liquids are removed from the cell by leachate removal piping and sent to a storage tank and/or used as seed in another developing RAC cell. Upon completion of digestate maturation or once the RAC cell becomes aerobic, the cover is removed and the digestate is recovered for reuse as charge material or mixed and amended for a compost product. In one embodiment, the digestate is removed and at least part of the digestate is mixed with new incoming compostable material that is rough shredded. The shredded compostable material may include, for example, yard waste, manure, sludges, wood, pallets, brush, food waste, and cellulosic materials like cardboard. Mix ratios may vary based on the amount of moisture and particle size of the compostable material components. In the dry season more food waste is added to the reactor and conversely in the wet season or when yard waste is readily available, the amounts of green grass and wood chip ratio is changed. The amount of digestate mixed with the new material also varies. More previously treated material is mixed if a shorter cycle is needed, <60 days, and this ratio is modified up to a 12 month residence time. In some cases waste heat in the form of steam is added to the pit or lechate tank in order to maintain or increase fermentation rates.
During the entire process, sensors can be used to monitor and control the process. Temperature monitoring of all the RAC cells is preferably continuous. In the event out of range temperatures are observed in an operating RAC cell, liquid is added through leachate injection piping or any other available piping in order to quench and cool the fermentation reaction. The RAC cell design—which preferably includes berms—allows for flooding of the RAC cell up to the height of the side walls and direct recirculation of liquids. Liquid levels are controlled by the sump collection system and recirculation piping.
The RAC cell cells useful in the present invention can vary from a 400 ton capacity to 4,000 ton capacity at a placement density of 1400 to 1600 lbs/cubic yard. This variable capacity requires that a vacuum is applied to the material in the digester after it is partially filled. This action removes odors and other volatile gases for treatment in a compost based biofilter. Furthermore, a temporary cover is provided for daily covering of the digester to further aid in odor and volatiles capture. The application of the vacuum to the shredded material causes the material to start to aerobically compost. This action raises the temperature to temperatures >120° F. and as much as 160° F. As an alternative, aeration is initially supplied to the mass and excess air is treated in the biofilter. This activity also induces heterotrophic degradation of the mass yielding heat.
The in-situ RAC cells of the invention are useful for generating methane gas that is useful for producing energy from food waste and yard waste previously landfilled or aerobically composted. The in-situ RAC cell can be located on a landfill, a landfill buffer area, a transfer station, a composting yard, a closed landfill or at a food manufacturing facility. Residual solids in the digester also produce a product called digestate, this has various horticultural uses. The energy component is the result of anaerobic fermentation producing high quality methane. The in-situ design allows for fire suppression by complete aqueous filling of 80% to 90% of the reactor if needed. The invention allows for easy sourcing of commercial wastes (like grocery wastes) that includes large amounts of cardboard and wax covered cardboard. It reduces fuel use by hauling companies by not changing the delivery location at existing solid waste facilities in many instances.
This application is a divisional of U.S. application Ser. No. 12/706,384 filed on Feb. 16, 2010, which claims priority to provisional application Ser. No. 61/152,867, filed on Feb. 16, 2009, the specifications of which are incorporated by reference in their entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61152867 | Feb 2009 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12706385 | Feb 2010 | US |
| Child | 13399302 | US |
