Semi-continuous anaerobic co-digestion of agro-wastes

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 a range of agricultural wastes has been evaluated and has been successful. Previous batch studies have shown that multi-component mixtures of agro-wastes can be digested successfully, although with some mixtures a degree of both synergism and antagonism occurred. A continuously stirred tank reactor (15 l) was used as a mesophilic (35 degrees C) anaerobic reactor to examine the effect of treating mixtures of fruit and vegetable wastes, cattle slurry, poultry waste and waste activated sludge in which the proportions of the latter two components only were varied. The retention time was kept at 20 days and the loading rate maintained in the range 1.51-1.89 kg VS m(-3) d(-1). Increasing the proportion of poultry waste from 0% to 100% improved the methane yield from 0.164 to 0.244 m CH4 kg(-1) VS added, and caused the VS reduction to decrease slightly. There did not appear to be any synergism or antagonism in the mixtures examined.