Anaerobic co-digestion of poultry slaughterhouse wastes with sewage sludge in batch-mode bioreactors (effect of inoculum-substrate ratio and total solids)

High amounts of organic matter and protein in slaughterhouse wastes make them a viable choice for anaerobic digestion and biogas production. The purpose of this study was to investigate potentials of biogas production from poultry slaughterhouse residues (waste and sludge) in a batch-fed anaerobic co-digestion process. In the first phase of the study, laboratory-scale experiments were conducted using 1-liter reactors at 34 °C with a retention time of 50 days in order to optimize digestion parameters; i.e. inoculum-substrate ratio (ISR) and total solids (TS). Based on these experiments, the ISR of 4 and the TS of 5% were determined as optimal settings for digestion over a retention time of 50 days. The highest biogas and methane yields in these experiments were respectively reported by 0.631 and 0.462 m3/kg-VSadded, achieved with 66% removal of volatile solids (VS). The optimized parameters obtained from the first phase were also used for verification at a larger scale (20 liters), wherein biogas and methane yields were found to be 0.574 and 0.402 m3/kg-VSadded; respectively. Moreover, results showed that optimal anaerobic digestion of slaughterhouse wastes in 20-liter digester could lead to 63% removal of VS and 88% reduction of chemical oxygen demand (COD) over a retention period of 42 days. The findings also indicated that decreasing ISR and increasing TS could bring about reduced pH as well as accumulation of fatty acids and ammonia, which could severely undermine yields of methanogenesis process.