G. Makripodis, F. Simantiraki, M. Somara, E. Gidarakos
Laboratory of Toxic and Hazardous Waste Management, Department of Environmental Engineering
Technical University of Crete, Polytechneioupolis, Chania 73100, Greece ABSTRACT
The complexity of landfills, as well as the practical and health-related issues generated, demands special laboratory and modelling approaches as the first line of in situ landfill investigations. The approaches to date include literature reviews, design and construction of full scale landfill bioreactors and modelling of biodegradation processes for numerical simulation. Currently, landfills require specific liners at the bottom to capture leachate and impermeable caps at the top to limit infiltration. As a result, landfills are considered "dry tombs," (i.e., anaerobic and dry), with relatively slow biodegradation rates of the organic fraction of the waste and significant production of methane. An alternative approach in accelerating biodegradation and eliminating methane is aerobic bioremediation of landfills. In aerobic bioremediation, air and leachate are injected into the solid waste, resulting in relatively fast aerobic biodegradation and no methane production. At the same time, the compaction rate is enhanced (U.S. EPA, 2002).
The purpose of this research is to determine the critical physical, chemical and biological processes that control aerobic landfill bioremediation. The research is directed at understanding the process of aerobic landfill bioremediation so that optimal engineering designs can be developed. In a 340 L tank filled with 70 kg of fresh solid waste, parameters like BOD, COD, pH, conductivity, anions and heavy metals have been measured for 14 months.
The waste is subjected to various combinations of leachate recirculation along with air injection. The bioreactor is instrumented for temperature, moisture content and density, along with gas and leachate composition and flow rates. Moreover, the visual inspection through the clear plexiglas walls of the bioreactor is possible. So far, COD and BOD concentration marked a reduction of 94% and 99.5% respectively. Over the test period, methane production is insignificant and the settlement of the waste approaches 22%. The bioreactor operation demonstrates accelerated settling and improvement in leachate quality as indicated by decreased heavy metals, BOD and COD. Both airflow rates and leachate recirculation rates are key operating parameters for those who wish to design a bioreactor.
The increasing cost and the difficulties in planning new landfill sites, create a need to save some space in the existing landfill sites. Current environmental regulations require capping of landfills to isolate waste from water infiltration and collection of leachate for treatment before release to the environment. Moreover, landfill air emissions are required to be monitored to limit the release of methane (CH4) and other volatile organic compounds. While this technique reduces the potential for contaminating the environment, it intents to restrict exposure of air and water to the MSW. This slows biodegradation rates and increases the time required for landfill stabilization. The waste is considered stabilized when leachate is no longer a pollution hazard, gas production is negligible and the majority of settlement has occurred (Borglin et al., 2004).
In an effort to increase the biodegradation rates, landfills are being managed either as anaerobic or aerobic bioreactors. Bioreactors optimize the conditions for microbial decomposition and accelerate stabilization and settling, thus allowing for additional MSW disposal or faster land reuse (Kelly, 2002). In both aerobic and anaerobic bioreactors, leachate is recirculated, returning nutrients and bacteria to the MSW mass. In anaerobic bioreactors, the increased water content increases the rate of CH4...