Applicability of temperature-phased anaerobic digestion in enhancing methanation of high-solid sludge: Process performance, microbial community analysis and energy balance assessment

High-solid anaerobic digestion has been paid more attention, expected to solve the increasing amount of sewage sludge. In order to cope with the new issues of high-solid sludge digestion, recently emerging thermophilic (stage I)-mesophilic (stage II) temperature-phased anaerobic digestion (TPAD) process was employed to probe into its applicability in enhancing methanation. High-solid sludge at a total solid (TS) of above 15 % was fed to a TPAD process and a single-stage mesophilic digestion (MD) process continuously. The increasing loadings from 3.96 g chemical oxygen demand (COD)/L/d to 8.05 g COD/L/d were set by gradually shortening hydraulic retention time from 20 d to 10 d. Methane yield could be increased from 0.11 L/g COD_added to 0.15 L/g COD_added, with 10 % higher TS removal achieved in the TPAD. The reason could be attributed to improved hydrolysis of the main fraction protein. Despite acetic acid accumulation in stage I, surplus alkalinity supply rendered acid/alkalinity ratios much lower. The interaction between the 2 stages offered more diverse microbial community, which led to intensive adaptive ability to external shocking. The density of archaea for stage II /stage I increased nearly linearly with higher organic loading. As high as around 60 % Methanosarcina became the main mesophilic archaea. The dominant functional bacteria Firmicutes in stage II was also promoted. On the premise of enhanced conversion efficiency, additional energy input from heat requirement of thermophilic stage in the TPAD was proven to be compensated by improved methane production, leading to similar or even higher net energy production with the MD.