BIOMETHANATION OF THERMAL PRE-TREATED ORGANIC FRACTION OF MUNICIPAL SOLID WASTE AND SEWAGE SLUDGE

Abstract

Anaerobic digestion is a clean and environmentally friendly technology that can be used to convert organic fraction of municipal solid waste (OFMSW) to valuable energy. The energy released is in the form of biogas that contains a mixture of methane (CH4) and carbon dioxide (CO2), which can be used for domestic and industrial purposes. In anaerobic digestion, hydrolysis is the rate-determining step. Pre-treatment helps to fasten the hydrolysis, thus speeding up the whole process. Pre-treatment of the substrate helps break down complex organic matter into simpler ones. It also leads to higher organic matter solubilization, acidogenesis, and methanogenesis activity and, in turn, higher methane production. This study investigates the effects of thermal (125 °C, 30 min) pre-treatment on the substrate solubilization and, ultimately, on the biomethanation of OFMSW and SS at an 80:20 ratio and 10% TS content. The pH, TS and VS (Total and Volatile solids), Ammonia, TKN (Total Kjeldahl Nitrogen), Alkalinity, soluble chemical oxygen demand (sCOD), Volatile Fatty Acids (VFAs) and C/N ratio were measured. The substrate's soluble COD is increased by almost 2.2 times over control after thermal pre-treatment. The soluble COD increased from 12989mg/l to 28689 mg/l. There was also an increase in ammonia and VFAs concentration in the substrate after the pre-treatment while the alkalinity of the substrate dropped from 1080 to 840 mg/l. Since the actual increase in biogas production cannot be accurately determined by the rise in solubilization. Therefore, a pilot scale anaerobic digestion was conducted in a floating drum-type anaerobic digester at different HRTs (30 days, 20 days, and 15 days) and corresponding OLR of (2.5 kg VS/m3.d, 3.8 kg VS/m3.d and 5.1 kg VS/m3.d). The highest biogas yield of 510 ml/gVSadded is observed at 15 days HRT over 30 and 20 days HRTs. The biogas yield at 30 days and 20 days HRT was observed to be 250 ml/gVSadded and 408 ml/gVSadded, respectively. The methane composition and methane yield under steady state conditions of 15 days HRT was 72% and 367 ml/gVSadded, respectively. The methane yield observed at 15 days HRT was almost 29% and 136% higher than that obtained at 20 days and 30 HRT respectively. The steady state VFAs concentration in the digester increased from 1500mg/l to 2550 mg/l as the HRT was reduced from 30 days to 15 days. The alkalinity in the digester is maintained accordingly to maintain a VFAs/Alkalinity ratio of around 0.35-0.40. No process inhibition occurred during the study period owing to good buffering conditions in the digester. A higher VS and sCOD reduction were achieved at 15 days HRT. The best VS reduction is around 50%, while a TS reduction of approximately 39% is achieved under steady-state conditions.