Temperature-phased anaerobic co-digestion of sewage sludge and micro algae for enhanced biomethane production

Abstract

Temperature-phased anaerobic digestion (TPAD) has recently gained attention for its capability to improve the hydrolysis stage, considered to be the rate-liming step of anaerobic digestion (AD). The TPAD system involves thermophilic pretreatment with a short retention time (1-3 d) prior to mesophilic AD with its longer retention time (10-20 d). In this study, effects of varying pretreatment conditions affecting the degree of solubilization during the thermophilic stage on methane yields of the mesophilic stage were investigated to improve TPAD performance. Three different conditions of the thermophilic stage were simulated in laboratory scale using sewage sludge (mixed primary and secondary sludge) and microalgae (Chlorella vulgaris) as substrates: 2 d hydraulic retention time (HRT) at 65 degrees C, 2 d HRT at 75 degrees C and 3 d HRT at 75 degrees C. Biochemical methane potential (BMP) tests during the mesophilic stage were performed after the thermophilic stages under each pretreatment condition. Cumulative methane yield ranked by decreasing yield: 215.73 mL CH4/g VSadded after pretreatment at 75 degrees C with 2 d HRT; 147.55 mL CH4/g VSadded after pretreatment at 65 degrees C; 139.82 mL CH4/g VSadded after pretreatment at 75 degrees C with 3 d HRT. Increase in solubilization under thermophilic treatment resulted in enhanced performance of co-digestion.