Anaerobic digestion of solid poultry slaughterhouse by-products and wastes

Abstract

The main objective of this work was to evaluate the viability of anaerobic digestion of solid poultry slaughterhouse by-products and wastes. In the biochemical methane potential batch assays blood, offal, and feather, showed methane yields of 0.6-0.7, 0.5, 0.7-0.9, and 0.2 m³/kg volatile solids (VS)added/ respectively, and the mixture produced methane 0.6-0.7 m³/kg VSadded. Combined thermal and enzymatic pre-treatments increased the methane yield of feather by 37 to 51 %, whereas thermal, chemical, and enzymatic treatments were less effective with methane yield increasing 5 to 32 %. The anaerobic degradation patterns of the mixture in batch assays indicated rapid hydrolysis / acidogenesis, accumulation of long-chain fatty acids (LCFAs) and volatile fatty acids (VFAs), removal of LCFAs and subsequently that of VFAs, and methane production. The dynamic modelling of the results from the assays suggested that inhibited propionate degradation by LCFAs and inhibited hydrolysis by a high propionate concentration constituted the rate-limiting steps in the degradation. The anaerobic digestion of the mixture in semi-continuously fed laboratory-scale digesters appeared sustainable with a loading of up to 0.8 kg VS/m³ d and a hydraulic retention time (HRT) of 50 days, showing a methane yield of up to 0.55 m³ of methane/kg VSadded. At loadings from 1.0 to 2.1 kg VS/m³ d, and HRTs from 12.5 to 25 days, the digester performance was inhibited. In the batch assays the accumulated LCFAs appeared the main factor affecting the slow recovery of the digester from inhibition. LCFAs floated on top of the digester, which could have affected their bioavailability and toxicity. The digested material was found to be rich in nitrogen, with ea. 20 % N of total solids (TS), mostly in the form of ammonia. Vascular plant growth assays showed the digested material to be potentially phytotoxic, apparently mainly because of VFAs and LCFAs present in it. Furthermore, aerobic post-treatment reduced the phytotoxicity. The review of the potential of and experiences with anaerobic digestion of solid poultry slaughterhouse by-products and wastes suggested that anaerobic digestion of these materials can be viable when operation conditions are carefully optimised.