Biogas production from food waste by two-stage semi-dry-thermophilic anaerobic digestion

A new two-stage semi-dry-thermophilic anaerobic digestion system to produce biogas was studied. The system was composed of one hydrolysis stage and one methane producing stage under 50℃. The hydrolysis stage, running in batch, was inoculated with seed sludge from the methane producing stage and fed with food waste. The slurry product of the hydrolysis stage was fed to the methane producing stage after 1～2-d digestion. Food waste used in this study had TS and VS contents of 19.3% and 16.1% respectively. First, the methane producing stage with a working volume of 4 L was singly run under a VS loading rate of 4 g/d. At its stable status, the daily biogas production rate was (4.09±0.03)L, with a methane content of 69.0%; pH value was 7.50±0.02; and the TS and VS contents of the liquid were (0.05±0.02)% and (0.64±0.02)% respectively. After the stable methane producing stage was combined with the hydrolysis stage, the latter stage worked well under a substrate loading of 60～700 g while hydrogen content in the biogas was low. The methane producing stage reached the highest production performance(29.7 L/d) at the substrate loading of 600 g but was inhibited at higher loadings. The highest TS and VS contents of the liquid, 3.50% and 2.32% respectively, were obtained at almost the same time point. The methane contents before inhibition were 72.4%～74.0%. In the future, the volume of seed sludge from the methane producing stage to the hydrolysis stage should be decreased in order to further suppress methanogens to improve hydrogen production in the hydrolysis stage, and at the same time to keep the methane producing stage more stable under even higher loadings.