Advanced steam explosion as thermal hydrolysis process for high solids anaerobic digestion and enhanced methane yield: Proof of concept

Sewage sludge management is one of the major issues faced by wastewater treatment plant (WWTP) operators. Though anaerobic digestion (AD) is one of the most preferred sludge management techniques, the complex microbial floc structure of the sludge creates physical and chemical barriers that limit the rate of microbial degradation resulting in a longer hydraulic retention time (HRT) of 25-30 days. The thermal hydrolysis process (THP) can be used as a pretreatment tech-nique to enhance the rate of biodegradation which results in reduced retention time and in-creased methane yields. This work investigated the effects of the THP on the semi-continuous anaerobic digestion of high solids sludge (15% total solids, TS) obtained from a municipal waste-water treatment plant (WWTP) operating at higher solids retention time (SRT) of 40 days. The thermal hydrolysis process achieved the effective release of organics into the liquid phase resulting in 37% chemical oxygen demand (COD) solubilization. Two semi-continuous digesters, one fed with untreated sludge (control digester, 5% TS) to mimic the conventional anaerobic digester and the other fed with THP sludge (160 degrees C, 30 min, 6 bar, 10% TS) were operated under a mesophilic regime and a hydraulic retention time (HRT) of 15 days. The methane yield from the THP digester (0.366 +/- 0.010 m3/kg VSadded, 70 +/- 1.13% CH4) was 47% higher than the control digester (0.248 +/- 0.013 m3/kg VSadded). The volatile solids removal (VSR) was 51 +/- 0.6% in the THP digester, i.e., 43% higher over control digester (35.7 +/- 1.3%). The fecal coliforms concen-tration in the digestate of control and THP digesters were 9200 and 200 MPN/g, respectively. Thus, the THP sludge meets the Class A biosolids criteria (<1000MPN/g). THP integrated anaer-obic digestion achieved higher solids reduction with enhanced energy recovery, high solids load-ing with shorter retention time, and generated Class A biosolids.