Dual anaerobic co-digestion of sewage sludge and confectionery waste

Three configurations for a dual digestion system were examined. The units w ere based on three 5 1 completely stirred tank reactors (CSTR). A first-sta ge thermophilic digester was used to provide the feed to each of the two se cond-stage mesophilic (35 degrees C) digesters. Using a mixture of sewage s ludge and strong confectionery waste, the thermophilic digester was operate d at 55 degrees C with a hydraulic retention time of 4 h. The mesophilic di gesters were operated at hydraulic retention times of 8, 12 and 15 days. In terms of the reduction of volatile solids (VS), the three dual digestion c onfigurations were similar but were more effective than the single-stage re actor which was used as a control. However, based on the specific methane y ield (m(3) CH4/kg VS removed), the configuration with a first stage operati ng at 55 degrees C and a secondary digester at 35 degrees C with a hydrauli c retention time of 12 days was the most effective. This configuration also maintained a more stable pH, irrespective of the quality of the feed sludg e. (C) 1999 Elsevier Science Ltd. All rights reserved.