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.
Keywords: anaerobic membrane bioreactor; biosolid; hydrolysis kinetics; resource recovery; waste activated sludge digestion.
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