Advancing High-Solids Anaerobic Digestion of the Organic Fraction of Municipal Solid Wastes

Abstract

In the first phase, the major focus was on high-solids anaerobic digestion (HSAD) of the organic fraction of municipal solid waste (OFMSW) with percolate recirculation supported by conductive materials for enhancing biomethane recovery to address the kinetic limitation identified in previous studies. In this study, two identical lab-scale mesophilic (36oC) HSAD systems were operated with OFMSW under similar operating conditions, including feedstock to inoculum ratio and daily percolate recirculation rate. The percolate tank of the test HSAD system was amended with 15 g/L of PAC. After 30 days of operation, the cumulative biomethane yield for the test reactor was 17% higher than the control reactor (109 vs. 93 L CH4/kg VS). Furthermore, PAC addition showed additional benefits by lowering free ammonia nitrogen (FAN) and volatile fatty acids (VFAs) concentrations that can inhibit anaerobic digestion. In the second phase, the effects of feedstock to inoculum (F:I) ratio and percolate recirculation time (PRT) were studied for the HSAD of OFMSW. Six mesophilic HSAD systems were operated at different F:I ratios (1 to 3 kg VS/kg VS; PRT=2.5 h/d) and PRTs (1.5 to 3.5 h/d; F:I=2). The F:I ratio of 1 provided up to 86% of the theoretical methane potential of OFMSW. In contrast, F:I ratio of 3 provided only 34% methane recovery due to VFAs accumulation and pH drop. Despite F:I ratio of 2 could provide 70% methane recovery, it could enable almost 45% higher organics processing capacity (VS basis) and lower solids washout during percolate recirculation, as compared to the F:I ratio of 1. However, different examined PRTs showed marginal impacts on methane yields with comparable changes in profiles of percolate characteristics.