Effects of solids retention time on methanogenesis in anaerobic digestion of thickened mixed sludge

► Hydrolysis of organic solids was the main limiting process for 5-20 days SRT. ► Washout of methanogens became the rate-limiting factor for an SRT of 4 days. ► Homo-acetogenic Eubacterium and Spirochaetales increased with decreasing SRT. ► All methanogens declined for SRT below 5 day, but Methanosaetaceae became more important.