A pilot-scale study of granule-based anaerobic reactors for biogas recovery from municipal wastewater under sub-mesophilic conditions.

The influence of hydraulic retention time (HRT of 3-5 h) and temperature (20-25 °C) on performance and microbial dynamics of two pilot-scale upflow anaerobic sludge blanket (UASB) reactors with different granule size distribution (UASB1 = 3-4 mm and UASB2 = 1-2 mm) were investigated for 217 days. Increasing the HRT to 5 h even at a lower temperature of 20 °C enhanced COD removal and biogas production with average of 59 ± 16% (up to 85%) and 73 ± 9 L/(m·d) (up to 102 L/(m·d)) for UASB1; 63 ± 16% (up to 85%) and 75 ± 9 L/(m·d) (up to 90 L/(m·d)) for UASB2, respectively. This is explained by sufficient contact time between microorganisms and substrate. Acetoclastic methanogenic activity was higher in UASB1 because Methanosaetaceae (produces methane from acetate) dominated (64 ± 4%). However, Methanoregulaceae (29 ± 3%) and Methanomicrobiales_unassigned (20 ± 6%) which produce methane from H/CO and formate were significant in UASB2. The extent of change in the microbial dynamics with HRT and temperature was more obvious in the smaller granule reactor.