Impacts of concentrated leachate recirculation on methane production suppression, odorous gas emissions, and salinity accumulation in municipal solid waste.

The recirculation of concentrated leachate (CL) in municipal solid waste landfills has been shown to effectively reduce organic biochemical indicators in the effluent; however, it may also increase salinity within the landfill, raising concerns about its long-term sustainability. In this study, a laboratory-scale bioreactor was established to periodically inject nanofiltration-treated CL into waste with high and low organic matter content. Variations in waste composition, leachate characteristics, and landfill gas production were monitored. Results indicated that CL recirculation inhibited waste degradation and methanogenesis, leading to a 47.5% and 77.0% reduction in methane potential compared to the control groups. At the end of the experiment, 34.0% and 33.7% of the biodegradable material remained in the solid phase of the high and low organic waste, respectively. CL recirculation facilitated the release of NH3 and H2S, with total emissions being one to two times higher than those of the control group. It also promoted salinity accumulation; influenced by the temperature of the waste core, the final leachate salinity exceeded that of the recirculated CL, reaching 33,800-36,000 mg/L. Low organic matter waste was more vulnerable to the adverse effects of CL recirculation, with degradation and methanogenesis processes nearly halted. These observations were consistent with salinity-related inhibition and may also have been influenced by free ammonia toxicity and substrate competition involving sulfate-reducing bacteria. Considering methane's economic value, the microbial environment in the waste, and the emission of odorous gases, the sustainability of CL recirculation appears to be relatively low.