Anaerobic digestion in a multi-stage plug flow bioreactor: Revisiting an age-old process with modern molecular tools

To address knowledge gaps in the complex interacting microbial associations that underpin anaerobic digestion, a mesophilic (25°C) continuous-flow four-stage reactor was constructed to separate both spatially and temporally the component microbial groups. The reactor influent consisted of primary settled sewage sludge (PSSS) and the organic fraction of municipal solid waste (OFMSW). Chemical (volatile fatty acids, sulphate, sulphide, chemical oxygen demand, gas) and molecular analyses were made during an operation period of 15 months. Spatial separation of the microbial groups resulted in process instability where acidogenesis/acetogenesis produced an effluent with a pH between 2 and 4 that inhibited the subsequent catabolic steps. An organic loading rate of 6.5 g COD d-1 prevented reactor acidification but resulted in low biogas production (0.04-0.12 l biogas l-1 hydraulic load d-1). Fluctuations in chemical and molecular profiles/characteristics, which may have been due to the inherently heterogeneous PSSS and OFMSW, were recorded and these were countered by the development of a model medium. The medium was then used to: explore reactor efficacy; and study pertinent microbial diversity and functional interactions