High-solid anaerobic digestion (HSAD) of sludge has several advantages like smaller reactor, lower energy consumption and less digestate. However, the understanding about the mechanism especially the microbial mechanism is still limited. In this study, microbial communities of a pilot-scale sludge HSAD system at steady state were investigated with 16S rRNA clone library technology. The system employed an enhanced two-phase anaerobic digestion process, i. e. 'hyperthermophilic acidogenesis (70℃, 3 d)-thermophilic methanogenesis (55℃, 12.5 d)' to treat waste activated sludge with a solid content of about 9%. The volatile solid (VS) removal rate was 35.7% and methane yield (CH4/VSremoved) was 0.648 m3·kg-1. The bacterial compositions of the two phases were significantly different:there were plenty of proteolytic bacteria in hyperthermophilic acidogenesis phase; and the bacteria degrading polysaccharides like cellulose and the bacteria utilizing long-chain fatty acids were found in thermophilic methanogenesis phase; some bacteria degrading simple saccharides existed in both phases. In both phases, the dominant archaea were Methanothermobacter. Especially, 100% of the retrieved archaea in the thermophilic methanogenesis phase belonged to genus Methanothermobacter. This indicated that hydrogenotrophic methanogenesis was the predominant methanogenesis pathway in this system since methane was only detected in the methanogenesis phase.
Keywords: high-solid anaerobic digestion; hydrogenotrophic methanogenesis; microbial community; two-phase anaerobic digestion; waste activated sludge.