Leachate recirculation using horizontal trenches in bioreactor landfills

Leachate recirculation in municipal solid waste (MSW) landfills operated as bioreactors offers significant economical and environmental benefits. Subsurface leachate recirculation in MSW landfills is commonly achieved by using horizontal trenches or vertical wells. Currently, there are no design guidelines available for leachate recirculation using a subsurface leachate recirculation system (LRS). The key objective of this study is to prepare design guidelines for LRS consisting of horizontal trenches. This paper presents a numerical study of LRS consisting of horizontal trenches. The design parameters evaluated in this study include: (1) Leachate injection pressure head; (2) hydraulic conductivity of trench backfill and MSW; (3) dimensions of trench; and (4) spacing and geometric formation of trenches. The finite-element saturated/unsaturated flow model HYDRUS-2D was used for the numerical study. The hydraulic performance of the LRS was evaluated primarily using the simulated recirculated leachate flux and distribution of flow under steady-state flow condition. The key findings of this numerical study are: (1) Logarithm of leachate flux and leachate injection pressure head have a curvilinear relationship and leachate flux is directly proportional to the hydraulic conductivity of MSW; (2) if hydraulic conductivity of trench backfill is equal to or greater than that of MSW, any further increase in the hydraulic conductivity of the trench backfill has negligible impact on leachate flux; (3) for a given cross-sectional area, horizontal trenches having width greater than depth can recirculate greater leachate flux and can wet more area of the waste; and (4) reduction in the horizontal spacing between trenches and vertically staggering the trenches reduces "dry zones" between trenches where otherwise recirculated leachate may not reach.