Performance and microbial community analysis in alkaline two-stage enhanced anaerobic sludge digestion system

• An alkaline enhanced stage-1 was studied in two-stage configuration.
• Higher COD, VS reduction, and methane yield occurred in the two-stage system.
• No substantial system performance improvement as stage-1 was moved from 35 to 55 °C.
• 454 pyrosequencing confirmed differences in archaeal and bacterial populations.

This study investigated an alkaline two-stage enhanced anaerobic sludge digestion system, which simultaneously combined biological and chemical mode of degradation. The alkaline enhanced mesophilic stage-1 was operated at pH 8 with 3 days hydraulic retention time (HRT), while the mesophilic stage-2 was without pH adjustment at 17 days HRT. The system achieved higher chemical oxygen demand (COD) removal, volatile solids (VS) reduction, and methane yield than the conventional 20 days HRT single-stage system. Further enhancement was obtained by moving the stage-1 from 35 to 55 °C, but it did not yield better energy balance with the 3 + 17 days HRT configuration implemented in this study. 454 pyrosequencing revealed the acclimation of specialized communities in the alkaline two-stage system. Methanosarcina, Methanobrevibacter and Methanothermobacter could survive at pH 8 in the alkaline enhanced stage-1 and contributed to regulating the potentially inhibitory volatile fatty acids (VFA) or hydrogen levels under the enhanced sludge solubilization and acidogenesis condition. Various fermentative populations, distinct to those in the single-stage system, were also enriched in the stage-1s. These populations could grow at pH 8, were transferred into the stage-2, and ensured continuity of the biochemical reactions under mild alkaline condition, leading to the enhanced sludge digestion process.