Optimization of the Acid Stage of a Two-Stage Anaerobic Digestion Process to Treat Urban Wastewater Sludge

Abstract

The efficiency of a two-stage mesophilic anaerobic sludge digestion process was studied in comparison with a conventional process to treat urban sludge in relation to the hydraulic retention time (HRT) of the acid stage. This comparison, performed to optimize methane production in relation to the acid stage, was carried out in a pilot plant operating a two-stage process in parallel with conventional anaerobic digestion. The conditions of the conventional process were 22 days of HRT at 33.6°C; while the methanogenic stage was not changed (HRT 8.5 days at 33.5°C), the HRT of the acid stage ranged from 1.6±0.3 days to 3.2±0.6 days under a constant temperature of approximately 21.5°C. The results showed that the total anaerobic digestion process HRT can be reduced by at least 10.8 days using the two-stage technology without reducing the quality of the sludge effluent, operating at a HRT minimum of 1.6 days. Moreover, in the two-stage process, the optimum acid stage HRT to maximize methane production is 2.18 days.