EPA began working with Waste Management Inc. under a cooperative research and development agreement in 2000 to examine various methods for improving landfill efficiencies. Related studies underway at the Outer Loop Recycling and Disposal Facility in Louisville, KY, involve two types of bioreactor-based technology. The Agency anticipates that bioreactors will enhance waste containment in unlined landfills and could accelerate microbial degradation of hazardous and solid waste contaminants by 50%. Accordingly, in March 2004 EPA began allowing states to issue research, development, and demonstration permits to large-scale landfill operations for innovative methods such as bioreactors.
By recirculating landfill leachate through existing waste material and trenches constructed outside the waste perimeter, bioreactors are designed to increase landfill moisture. This approach contrasts significantly with previously recommended and commonly used “dry tomb” methods relying on reduction of landfill moisture content.
One of the two bioreactors under evaluation involves only anaerobic mechanisms, while the second operates under both anaerobic and aerobic conditions. Although anaerobic conditions naturally occur in most landfills, optimum degradation of solid waste or CERCLA contaminants requires the addition of moisture. This type of bioreactor conceptually lends itself to retrofitting of existing landfills (Figure 4). In contrast, an aerobic-anaerobic “hybrid” bioreactor landfill is designed to cause rapid biodegradation of easily degradable waste in the aerobic stage, thus reducing the production of organic acids in the anaerobic stage and generating earlier onset of methanogenesis.