The impact of solids retention time (SRT) and hydraulic retention time (HRT) on anaerobic digestion of thickened waste activated sludge (TWAS) in a pilot‐scale anaerobic membrane bioreactor (AnMBR) was compared with that achieved in conventional anaerobic digestions (CD). The AnMBR was able to successfully digest municipal TWAS at HRTs ranging from 7 to 15 days and SRTs ranging from 15 to 30 days. Increasing SRT in the AnMBR resulted in a significant improvement in COD and VS removal efficiency when compared against CD operating at the same HRT. The VS and COD destructions (35%–50%) observed in the AnMBR were similar to those observed in CD operating at the same SRT but longer HRTs. Operation at elevated ratios of SRT/HRT resulted in the production of a thickened biosolid (2%–3% TS). Specific methane production values for AnMBR operating at HRT‐SRT ratios of 15–30, 7–30, and 7–15 were 0.19, 0.19, and 0.14 m³ CH₄/kg of COD fed, respectively, showing a 25% increase in methane production with SRT. A model based upon describing hydrolysis of biodegradable solids using first‐order kinetics was able to describe VS destruction as a function of SRT.

Practitioner points

• The AnMBR process was able to successfully digest waste activated sludge at a shorter seven‐day HRTs
• Operation at elevated ratios of SRT/HRT resulted in enhanced biogas and thickened biosolid (2%–3% TS) production requiring reduced downstream processing
• The AnMBR process produces a particle‐free permeate that might be suitable for side stream nutrient recovery
• A model developed by considering hydrolysis as a limiting process can be used to determine design SRTs.