Seepage Control Strategies at Bioreactor Landfills

The occurrence of leachate seepage at side slopes of municipal solid waste landfills represents a major design and operational concern at bioreactor landfills where leachate is recirculated into the waste mass to enhance degradation of organic materials. Full-scale bioreactor landfills typically add liquids to the waste under pressure to optimize moisture distribution. When liquids are added at a high pressure or at a flow rate higher than the absorption capacity of the waste mass, there is a possibility of seeps. Two general approaches to controlling landfill leachate seepage are described: a less aggressive strategy where liquid volumes are limited to avoid seeps and a more aggressive strategy where seeps are more likely to occur but with design and operation considerations in place to mitigate seepage. Liquids addition into waste using buried horizontal trenches was modeled to determine moisture distribution under a variety of conditions most likely to lead to leachate seepage at bioreactor landfills. Based on the modeling results, leachate seepage can occur in the presence of both high- and low-permeability soil layers. In addition, the injection pressure and setback distance of recirculation devices play important roles in seepage prevention and control. Several practical seepage control strategies are illustrated, including the selection and placement of daily or intermediate cover materials, pressure control, and the application of a clay plug as part of recirculation device design and construction. Finally, the setback distance for a horizontal injection trench was calculated based on the saturated zone equation; the setback distance provides a way to estimate the appropriate recirculation device design distance to prevent seepage at bioreactor landfills.