Improving thermophilic anaerobic digestion of swine manure

Thermophilic (55°C) anaerobic degradation of swine manure was found possible even at an ammonia content of 6 g-N/l, with a low methane yield of only 67 ml CH4/g-VS and a high concentration of volatile fatty acids (11.5 g acetate/l). Several methods were tested in order to increase the methane yield. Addition of 1.5% (w/w) activated carbon, 10% (w/w) glauconite or 1.5% (w/w) activated carbon and 10% (w/w) glauconite resulted in an increase of the methane yield to 126 ml CH4/g VS, 90 ml CH4/g VS and 195 ml CH4/g-VS respectively. Batch experiments showed that at an ammonia concentration of 4.6 g-N/l even small amounts of sulphide (23 mg S2−/l) inhibited biogas production. However, addition of activated carbon (2.5% (w/w)) or FeCl2 (4.4 mM) could counteract the inhibition which was mainly explained by a reduction of the sulphide content by adsorption to the activated carbon or precipitation as ferrous sulphide. The methane yield could be increased to 102 ml CH4/g-VS by switching off the stirrer half an hour before and after substrate addition, which was attributed to increased biomass retention due to improved sedimentation. Increasing the hydraulic retention time (HRT) from 15 to 30 days resulted in an increase of the methane yield to 182 ml CH4/g-VS. Addition of granules from a thermophilic upflow anaerobic sludge blanket reactor treating volatile fatty acids gave only a temporary increase in the methane yield.