Semi-continuous co-digestion of solid slaughterhouse waste, manure, and fruit and vegetable waste

The potential of semi-continuous mesophilic anaerobic digestion (AD) for the treatment of solid slaughterhouse waste, fruit-vegetable wastes, and manure in a co-digestion process has been experimentally evaluated. A study was made at laboratory scale using four 2 L reactors working semi-continuously at 35 °C. The effect of the organic loading rate (OLR) was initially examined (using equal proportion of the three components on a volatile solids, VS, basis). Anaerobic co-digestion with OLRs in the range 0.3–1.3 kg VS m⁻³ d⁻¹ resulted in methane yields of 0.3 m³ kg⁻¹ VS added, with a methane content in the biogas of 54–56%. However, at a further increased loading, the biogas production decreased and there was a reduction in the methane yield indicating organic overload or insufficient buffering capacity in the digester.In the second part of the investigation, co-digestion was studied in a mixture experiment using 10 different feed compositions. The digestion of mixed substrates was in all cases better than that of the pure substrates, with the exception of the mixture of equal amounts of (VS/VS) solid cattle–swine slaughterhouse waste (SCSSW) with fruit and vegetable waste (FVW). For all other mixtures, the steady-state biogas production for the mixture was in the range 1.1–1.6 L d⁻¹, with a methane content of 50–57% after 60 days of operation. The methane yields were in the range 0.27–0.35 m³ kg⁻¹ VS added and VS reductions of more than 50% and up to 67% were obtained.