A thermophilic/mesophilic dual digestion system for treating waste activated sludge

Five configurations for a dual digestion were examined. The units were based on three 5 dm3 completely stirred tank reactors (CSTR). A first-stage thermophilic digester was used to provided the feed to each of the two second-stage mesophilic (35 °C) digesters. The thermophilic digester was operated initially at 55 °C with a hydraulic retention time of 4 h and then, later, at 60 °C with a hydraulic retention time of 3 h. The mesophilic digesters were operated at hydraulic retention times of 9, 12 and 15 days. In terms of volatile solids’ (VS) reduction, the systems with a first stage at 60 °C gave the better performance. However, based on the specific methane yield (m3 CH4 per kg VS removed), the configuration with a first stage operating at 55 °C and a secondary digester with a hydraulic retention time of 12 days was the most effective. © 1999 Society of Chemical Industry