Many landfills have been closed or are schedule for closure; yet, because of their small size, they have no gas management plans or gas collection systems. Cumulatively, these landfills can be significant source of greenhouse gases. Gas extraction is not required for such landfills and tends to be expensive--out of the reach of most small communities managing their own solid waste facilities. An attractive alternative is to incorporate biocovers on existing landfill covers. Biocovers can also be designed as part of a daily or an interim cover to reduce CH4 emissions.
Bio Filter Applications Experience:
Four Biofilter designs were developed and tested. Many landfills also passively vent CH4 to prevent its horizontal migration. These passive vent pipes serve as direct conduits for CH4 to pass from deep within the landfill to the atmosphere. Passive biofilters that uses CH4-oxidizing bacteria to reduce passive vent CH4 release can be incorporated into LFG management plans. Such biofilters can be contained in cylinders open at the top to allow air to diffuse into it from above. The cylinder should contain a porous substrate for the bacteria to grow upon. As LFG passes through this scrubber, entering at the base, CH4 and non-CH4 organic compounds will be oxidized. A large scale biofilter system could also be used in later stages on larger landfills when active recovery is no longer economically feasible. As landfills age, they produce less CH4 and at some point it will no longer be economical to power the pumps that extract the CH4 and direct it to engines and/or flares. The engineering challenge for the design of biofilters is to keep the gas permeability of the filter media high enough so the presence of the biofilter does not hinder flow. This can be performed by using coarse grained compost and by keeping the compost at target water content.
· A biofilter study was performed at the Leon County Landfill Tallahassee, Florida. The objective of this study was to investigate the feasibility of using methane oxidation to reduce emissions from passive landfill vents. Two designs of biofilters that vary in construction (vertical and radial biofilters) were developed and assessed in the field.
· The vertical biofilter design is named after the direction of gas flow within the filter, since landfill gas enters the bottom of the biofilter and migrates vertically up through the compost media mixture and out the top. A new concept of a “radial” biofilter was developed to increase oxidation of methane by biofilters. The radial design increases the surface area of flow. The surface area for the radial filters 460% of that of the vertical filters.
· The increased surface area results in a smaller influx, increased detention time, and better oxygen penetration. All these factors combined make the radial design a much better design for oxidizing passive methane emissions from landfills.
· Water-spreading biofilters were designed to use the capillarity of coarse sand overlain by a finer sand to increase the active depth for methane oxidation.
· Compost biofilters consisted of 238-L barrels containing a 1 : 1 mixture (by volume) of compost to expanded polystyrene pellets.
· Both designs were able to oxidize 69%, 63% of the injected methane. The water-spreading biofilter was shown to generally perform as well as the compost biofilter, and it may be easier to implement at a landfill and require less maintenance.
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