Cascading anaerobic digesters with different sludge retention times accelerates enzymatic hydrolysis and enhances reduction of waste activated sludge. Cascading anaerobic digesters with different sludge retention times accelerates enzymatic hydrolysis and enhances reduction of waste activated sludge

Hydrolysis, driven by hydrolytic enzymatic reactions, is considered to be the rate-limiting step in anaerobic digestion of waste activated sludge. In this study, a novel cascade anaerobic digestion system was developed in order to enhance in-situ enzymatic hydrolysis of waste activated sludge at mesophilic digestion (35°C). The cascade system consisted of three small and one larger digester, and was compared to a conventional digester in terms of process performance, hydrolytic enzymatic activity and microbial community dynamics. The results showed that the cascade system achieved a high and stable total chemical oxygen demand (tCOD) reduction efficiency of 40-42%, even at 12 days solid retention time (SRT). The conventional digester removed only 31 % tCOD and encountered significant deterioration at low SRTs researched. Hydrolytic enzymes including protease, cellulase, most of the tested amino peptidases, as well as glycosyl-hydrolases displayed significantly higher activities in first three small digesters of the cascade system, compared to the conventional digester. In agreement with this observation a high relative abundance of hydrolysing bacteria was found in those reactors. Furthermore, in the cascade system the dominance of hydrogenotrophic methanogens and existence of syntrophic bacteria was observed in the first three reactors. This indicates complete hydrolysis to hydrogen and carbon dioxide is followed their conversion to methane. However, in the fourth reactor acetoclastic methanogens dominated and their dominance was also observed in the conventional digester. The results illustrated the importance of using multiple reactors in the cascade to improve the hydrolysis in anaerobic digestion of waste activated sludge.