Comparison of single-stage and two-phase anaerobic sludge digestion systems – Performance and microbial community dynamics

This study compared reactor performance and the respective microbial community dynamics in the conventional single-stage and 2-phase anaerobic digestion (AD) systems, treating municipal sludge to generate methane. The 2-phase system’s COD and VS reduction, and methane production could be maintained throughout the three HRTs tested (p = 0.05), which was associated with an increase in organic loading (30 d = 1.5 g COD L−1 d−1, 20 d = 2.2 g COD L−1 d−1 and 10 d = 3.5 g COD L−1 d−1); but this was not so in the single-stage system where it deteriorated at HRT of 10 d (p = 0.05) due to impairment of particulate COD reduction. qPCR, DGGE and the subsequent phylogenetic analysis revealed that microbial adaptation occurred as the seed sludge formed a different community in each reactor at 30 d HRT; however, no further significant microbial shift occurred at lower HRTs. The presence of specific hydrolytic and acidogenic Flavobacteriales and Clostriales in the acidogenic reactor may have allowed for enhanced hydrolysis and acidogenesis, leading to higher organic loading tolerance at 10 d HRT. Methanogenic activity in the acidogenic reactor may have been performed by Methanobacteriales and Methanosarcinaceae. Operation of the acidogenic reactor at neutral pH may have to be considered to ensure the cultivation of propionate oxidising bacteria, which could in turn, prevent reactor “souring” during high load conditions.