Anaerobic digestion (AD) of secondary sludge is a rate-limiting step due to the bacterial cell wall. In this study, experiments were performed to characterize secondary sludges from three wastewater treatment plants (WWTPs), and to investigate the feasibility of using bacteriophage lysozymes to speed up AD by accelerating the degradation of bacterial cell walls. Protein was the main organic material (67.7% of volatile solids in the sludge). The bacteriophage T4 lysozyme (T4L) was tested for hydrolysis and biochemical methane potential. Variations in the volatile suspended solid (VSS) concentration and biogas production were monitored. The VSS reduction efficiencies by hydrolysis using T4L for 72 h increased and ranged from 17.8% to 26.4%. Biogas production using T4L treated sludges increased and biogas production was increased by as much as 82.4%. Biogas production rate also increased, and the average reaction rate coefficient of first-order kinetics was 0.56 ± 0.02/d, which was up to 47.5% higher compared to the untreated samples at the maximum. Alphaproteobacteria, Betaproteobacteria, Flavobacteriia, Gammaproteobacteria, and Sphingobacteriia were major microbial classes in all sludges. The interpretation of the microbial community structure indicated that T4L treatment is likely to increase the rate of cell wall digestion.

The influence of introducing fluctuations in acetate feeding concentrations on the process stability of a lab-scale horizontal anaerobic reactor (HAR) was investigated to ascertain its effects on acetoclastic methanogenesis. Acetate concentrations were randomized at 85 g COD/L ± 20% and discontinuously fed in the duplicate HARs for five days per week (giving the system time to rest and equilibrate for two days). The reactors were monitored daily with respect to performance indicators (physicochemical variables). The residual acetate concentration was observed to fluctuate at the initial stages, followed by a converging trend (decrease in variance) until the end of operation. Furthermore, letting the system self-neutralize and equilibrate during rest days resulted in improved process performance. The daily acetate degradation efficiency at ~90% and methane concentration at ~60% were attained after operating the reactors for 80 days. The results divulged that introducing fluctuations in acetate feeding concentrations does not affect the stability of biogas production and methane concentration. The acclimatization of the methanogenic population (predominantly Methanosaeta, then shifting to Methanosarcina) was also observed.