Effect of hydraulic retention time on anaerobic co-digestion of cattle manure and food waste

Abstract

Anaerobic digestion of cattle manure has a low efficiency due to the high hydraulic retention time (HRT) required to degrade the abundant degradation-resistant compositions, co-digestion with food waste is effective at improving the methane production. Lowering the HRT can therefore increase the methanogenic efficiency during co-digestion. This study considered the effects of different HRTs (25, 20, 15, 10, 7, 5, and 4 days) on cattle manure and food waste co-digestion. The highest methane production was achieved at 1.48 L/L/d with an HRT of 5 days. The maximum methane yields (236–257 mL/g-VS) were attained at HRT ≥15 days and decreasing the HRT to 10-5 days resulted in low methane yields and complete process failure at HRT 4 days, due to volatile fatty acids accumulated and microorganisms washed out. From a high HRT of 20 days to a low HRTs of 5 days, Bacteroidetes and Firmicutes were the dominant bacteria and the percentage of syntrophic acetate oxidizing bacteria (mainly Pelotomaculum and Pseudothermotoga) clearly increased. The dominant methanogen changed from the acetotrophic Methanosaeta to the hydrogenophilic Methanobrevibacter. These results enable biogas plants to utilize surplus amounts of cow manure and food waste in a sustainable manner with high process capacity and methane recovery.