Two-stage anaerobic co-digestion of waste activated sludge and fruit/vegetable waste using inclined tubular digesters

The anaerobic co-digestion of a 10% total solids (7.4% VS) waste activated sludge/fruit and vegetable mixture with approximately 25% of the VS arising from the fruit and vegetable waste was studied in duplicate two-stage syst ems. Acidogenic CSTRs and methanogenic inclined tubular digesters operated at 30 degrees C achieved stable anaerobic digestion at an overall system lo ading rate of 5.7 kg VS m(-3) d(-1), overall HRT of 13 days (3 day acidogen ic HRT, 10 day methanogenic HRT), with 40% VS destruction and a system biog as yield of 0.37 m(3) kg VS-1 added. The biogas methane content was 68% and bicarbonate alkalinity in the methanogenic stage was over 4000 mg CaCO3 l( -1), although TVFA levels were relatively high at 1300 mg l(-1). By increas ing the overall system HRT to 17 days (system OLR 4.3 kg VS m(-3) d(-1)) wi th the methanogenic HRT increased to 13 days, the average TVFA in the metha nogenic stage was reduced to 300 mg l(-1) and the overall VS destruction wa s 44%. Using these results an embodiment design was developed for a full-sc ale plant. The duty cycle was such that HRTs could vary from 4 to 26 days i n the case of the acidogenic stage and 10 to 65 days in the case of the met hanogenic stage. (C) 1999 Elsevier Science Ltd. All rights reserved.