High-rate anaerobic digestion of sewage sludge by membrane separation solubilization coupled with UASB process

Conventional anaerobic digestion (AD) process was often slow and thus required a large digestion tank. Upflow anaerobic sludge blanket (UASB) reactor is an AD technology intended mainly to treat wastewater with a short hydraulic retention time (HRT). However, the potential of this reactor was often limited when treating wet solid biomass. In this study, the performance of anaerobic submerged membrane bioreactor (AnMBR) was investigated under acidic condition by varying the hydraulic retention time (HRT), from 1.5 d to 2.5 d, and sludge retention time (SRT), from 6 to 12 d, of a mixture of primary and excess sludge. The acidic permeate was thereafter utilized in a UASB reactor at an HRT of 0.5 d. The results showed a COD reduction between 26 and 36% in the AnMBR, while the average COD removal rate was 83% in the UASB reactor. The average pH in the AnMBR was 5.40 and the transmembrane pressure was below 45 mbar throughout the experimental period. The average methane yield in the UASB reactor was 0.25 L/gCOD. These results implied that operating AnMBR under acidic condition is an alternative process to minimize membrane fouling and increase the energy footprint of an AD system.