Abstract
When a bench-scale digester fed thickened mixed sludge was operated over an SRT range of 4–20days, removal efficiencies for total chemical oxygen demand and volatile suspended solids declined with decreasing SRT (especially <10days), but methanogenesis was stable for SRT as low as 5days. Quantitative PCR analyses showed that methanogens declined steadily for SRT <10days, with the acetate-cleaving Methanosaetaceae becoming more dominant. Clone-library analyses indicated significant shifts in bacterial population from 20 to 4day SRT: declining Chloroflexi (28 to 4.5%) and Syntrophomonas (9 to 0%), but increasing Bacteroidetes (12.5 to 20%) and two acetogenic genera belonging to the phyla Firmicutes and Spirochaetales (6.3 to 12%). Thus, the decrease in the apparent hydrolysis constant (khyd-app) with higher SRT and the process limiting size of Methanosaetaceae with the lower SRT are proactive signs for defining rate limitation in anaerobic digestion.
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