Optimization of Leachate Recirculation Systems in Bioreactor Landfills

Municipal solid waste (MSW) management is one of the important issues that the United States of America (USA) and the other parts of the world are facing in the present days. Though well documented and established waste management procedures exist, waste disposal in the landfills will continue into foreseeable future throughout the world. As per the recent study presented by the United States Environmental Protection Agency (USEPA), more than 180% increase in the MSW generation in past few decades has been documented with an average of about 4.3 pounds of waste being generated per person per day. Therefore, proper disposal of municipal solid waste (MSW) has become requirement and challenging among the waste management professionals focusing the landfills to accept more volume of MSW. In a conventional or engineered landfills, because of low moisture content in the MSW, prolonged settlement due to slow waste biodegradation exists which makes it difficult to accept the increased volume of the waste. Therefore, bioreactor landfill concept has been recently introduced to contain the MSW, wherein, the leachate generated is collected at leachate collection and removal system (LCRS), stored in storage facilities, and then recirculated back into the landfill to increase the moisture content of the MSW. Increase in moisture content helps distribute the nutrients and microbes required for the waste to degrade; as a result, rapid waste biodegradation and settlement will occur (within 15-30 years) and thus helps to receive the extra volume of waste generated. Prewetting, surface ponds, horizontal trenches (HTs), vertical wells (VWs), and drainage blankets (DBs) are the different method of recirculating the leachate in the landfill. Among these, HTs, VWs and DBs are the most common leachate recirculating techniques implemented in the field. Though leachate injection expedites the waste degradation, conversely will increase the pore water and gas pressures within the pores between the MSW solids. This will affect the physical stability of landfill containment system and the stability of landfill slope. Therefore, to understand the evolution of the injected leachate and the developed pore water and gas pressures, two-phase flow modeling was carried out in this study assuming the waste as unsaturated porous media. When leachate is injected in an unsaturated MSW, pore gas and pore water pressures will develop until all the pores between the MSW solids are fully saturated. Once the voids are fully saturated, pore water pressure will prevail. Several scenarios were assumed and numerical simulations were performed to study the response of HTs, VWs and 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 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.