Effect of SRT and digester staging on the anaerobic digestion of municipal sludge

Abstract

The effect of solids retention time (SRT) on the mesophilic anaerobic digestion of a mixture of primary and waste activate sludge (WAS) was investigated. Twelve laboratory-scale (8 L) digesters were operated in triplicate at four different SRTs (5, 10, 20 and 40 days). The performance of the digesters was evaluated through measuring gas production and composition, total and volatile suspended solids (TSS & VSS) and other effluent quality parameters. VSS removal ranged between 37.6 and 53.6%. The rate of solids disintegration was modeled as a first-order decay reaction taking place in a plug-flow reactor with recycle. The calculated biodegradable fraction of the VSS and the first-order reaction rate constant were 55.3% and 0.38 d−1, respectively. During the second phase of the study, a comparison between multistage digesters in series versus a single-stage digester of equivalent total SRT was performed. Staging was found to always affect greater VSS destruction than single-stage operation. A mathematical model describing VSS destruction in a multistage reactor was developed.