Optimization of Leachate Recirculation Systems in Bioreactor Landfills

Several scenarios were assumed and numerical simulations were performed to study the response of horizontal trenches (HTs), vertical wells (VWs) and drainage blankets (DBs) when used as leachate recirculation systems (LRS) on moisture distribution and the pore water and gas pressures accounting for the effects of saturated and unsaturated hydraulic properties of municipal solid waste (MSW), inhomogeneity and anisotropic property of waste, LRS system geometric configurations, leachate injection rates and mode of leachate injection. Results indicated that the unsaturated hydraulic properties of MSW govern the moisture distribution in the waste when the gravity drainage exists. Further, the time required to reach steady-state will significantly differ for different sets of unsaturated hydraulic properties, though there is no considerable difference in the wetted area, wetted width and pore water pressure. Results on effects of inhomogeneity and anisotropy of waste indicated that the leachate injected will migrate in the lateral direction, which may reach the slope and endanger the stability of the slope, due to the anisotropy of MSW. Geometric configurations of different LRS depend greatly on the leachate injection rate and mode of leachate injection. Lesser the spacing of LRSs higher will be the area influenced; however, excess pore water pressures will be developed when leachate is injected continuously. Besides, injecting the leachate intermittently will result in developing the pore gas pressures due to unsaturated condition. Therefore, pore water and gas pressures are important key factors to be considered to evaluate the stability of landfill slope and the cover system.;Comparative evaluation of different LRS was then performed to examine the effectiveness of each LRS to distribute the moisture and pore water and gas pressures developed within the landfill. Results indicated that the DB when used as LRS will be more effective than the HT and VW to influence more area in the landfill when an equal amount of leachate is injected in all the three LRSs. Further, the pore pressures in case of DB was observed to be controllable than in case of HT. On the other hand, though VW did not indicate excess pore water and gas pressures, this system was not effective to produce better influence area in the landfill. Intermittent leachate injection will result pore gas pressure to dominate in case of all the three LRS considered.;Based on the system response of different LRS, a detailed parametric study by varying one design factor at a time was performed to develop design charts that can estimate the wetted width, wetted area and maximum pore water pressure build up in the landfill due to leachate injected until steady-state condition, considering the waste as inhomogeneous and anisotropic. The design charts developed are non dimensional and simple to use in the field to estimate the spacing of different LRS, and the volume of leachate to be injected based on the hydraulic properties of MSW and the location of LRS with respect to the LCRS.;Effect of spatial variation of hydraulic conductivity of MSW within the landfill to account for highly heterogeneous waste was evaluated on Orchard Hills Landfill, Davis Junction, Illinois, USA. Based on the literature review, large coefficient of variation (CoV) of saturated hydraulic conductivity was selected and the Monte Carlo simulations were performed to evaluate the wetted area, pore water pressure developed and the outflow collected at LCRS. Reliability analysis was then performed to examine the effective of the bioreactor landfill to serve as an excellent performing landfill. Reliability analysis indicated that the existing LRS and the leachate injection rates practiced in the field are not proficient. Therefore, it is highly recommended that that % area of influence of MSW should not be less than 60% and the ratio of PP/Total stress of 0.52 can be considered as safe. Therefore, it is suggested that landfill designers should install different configuration for the LRS and higher leachate injection rates to increase the wetted area. This will improve the biodegradation process of MSW and the overall efficiency of the functioning of the landfill and the increased outflow rate of leachate. (Abstract shortened by UMI.).