Co-digestion of waste organic solids: batch studies

Abstract

Anaerobic digestion is a well established process for treating many types of organic waste, both solid and liquid. As such, the digestion of cattle slurries and of the organic fraction of municipal solid waste has been evaluated and has been successful. Attempts to apply the process to other types of solid organic waste, for example, to chicken manure, have met with limited success. In many areas of the UK, intensive chicken farming is leading to solids disposal problems. It has been proposed that co-digestion with cattle slurry could be a possible disposal route for chicken manure. In this study, cattle slurries were mixed with a range of solid wastes and allowed to digest in 1-1 batch digesters. The criteria for judging the success of a co-digestion were volatile solids (VS) reduction, total methane production and methane yield. In terms of the VS reductions (%), there was little difference between the various digestions. In terms of the cumulative methane production (1) the co-digestions with fruit and vegetable waste, the fish offal and the dissolved air flotation sludge were more effective than the digestion with cattle slurry alone. In terms of the specific methane yield (m 3 CH 4 kg −1 VS removed), the co-digestions containing fish offal and the brewery sludge gave higher values than the control digestion with cattle slurry alone. Compared with their control (cattle slurry alone), both co-digestions with poultry manure (7.5 and 15% TS) gave higher cumulative productions of methane and the system with the lower concentration of poultry manure gave a higher specific methane yield. However, there was some evidence of ammonia inhibition.