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 systems. Acidogenic CSTRs and methanogenic inclined tubular digesters operated at 30°C achieved stable anaerobic digestion at an overall system loading rate of 5.7 kg VS m^-3 d^-1, overall HRT of 13 days (3 day acidogenic HRT, 10 day methanogenic HRT), with 40% VS destruction and a system biogas yield of 0.37 m³ 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 increasing the overall system HRT to 17 days (system OLR 4.3 kg VS m^-3 d^-1) with the methanogenic HRT increased to 13 days, the average TVFA in the methanogenic stage was reduced to 300 mg l^-1 and the overall VS destruction was 44%. Using these results an embodiment design was developed for a full- scale plant. The duty cycle was such that HRTs could vary from 4 to 26 days in the case of the acidogenic stage and 10 to 65 days in the case of the methanogenic stage.