Impact of AnMBR operating conditions on anaerobic digestion of waste activated sludge

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 m(3) 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.

Automated summary

The AnMBR process achieved higher COD and VS removal efficiency compared to conventional anaerobic digestion (CD), with increased SRT resulting in higher methane production. Operating at elevated SRT/HRT ratios can lead to the production of thickened biosolids.