Impacts of Temperature and Solids Retention Time, and Possible Mechanisms of Biological Hydrolysis Pretreatment on Anaerobic Digestion

Anaerobic digestion (AD) has benefits in sludge management, energy recovery, and pathogen reduction. In order to better understand the mechanisms of biological hydrolysis (BH) pretreatment on AD, biochemical methane potential (BMP) and continuous stirred-tank reactor (CSTR) tests were utilized to compare untreated municipal combined sludge with pilot-scale BH pretreated sludge. During the BH process, there was 15%, 30%, and 33% (w/w) volatile solids (VS) reduction after BH at 42 °C (BH42) for 24, 48, and 72 h, respectively; under BH61 (42 °C for 36 h and 61 °C for 6 h), and there was 10% and 30% (w/w) overall VS reduction after 36-h and 42-h hydrolysis, respectively. BMP results showed that BH42-pretreated sludge had 22.6% enhancement of methane yield compared to untreated sludge, and BH61 pretreated sludge had 29.4% enhancement of methane yield. Both temperature and solids’ retention time (SRT) contributed to the enhanced AD performance within 36 h, while temperature played more important roles after 36-h BH pretreatment. CSTR tests confirmed the acceleration of anaerobic digestion by BH pretreatment, and higher enhancement was observed when SRT of anaerobic digestion was shorter than 16 days. Through a literature review of BH-related studies, the possible mechanisms were highlighted for further optimization on the scale-up systems in order to reduce carbon footprint and operating expenditure for wastewater treatment plants.