Semi-continuous anaerobic digestion of solid slaughterhouse waste

• AD of solid slaughterhouse waste following slow acclimatization was studied.
• A feed rate of 1.3 gVS L−1 d−1 and an HRT of 125 days yielded 2 L biogas L−1 d−1.
• Finally, with a loading rate of 1.6 gVS L−1 d−1 the system was destabilized.
• Anaerobic digestion is a suitable, although complex, treatment for this waste.
• It is necessary to study an operational alternative to avoid the system instability.

The disposal of solid slaughterhouse waste can lead to contamination of the environment with organic compounds and pathogens. The principal aim of this study was to evaluate a semi-continuous anaerobic digestion procedure as a treatment for solid slaughterhouse by-products (low risk material) following a slow acclimatization stage and prior pasteurization of the substrate. The material was digested in duplicate glass tank reactors (of volume 8 L), under mesophilic conditions (35 °C) and with continuous stirring. The procedure consisted of the gradual acclimatization of the inoculum (fed at a loading rate of 0.3–1.6 gVS L−1 d−1 and with a hydraulic retention time [HRT] of 105–160 days). The loading rate was increased and the HRT was decreased as the system reached stability. Gradual, slow acclimatization of the sludge and prior pasteurization of the substrate yielded biogas (methane content 65–70%) at a rate of 2 L L−1 d−1 and 1.07 L gVS−1 added, for an organic loading rate of 1.3 gVS L−1 d−1, an HRT of 125 days and a waste:water dilution rate of 1:2.7. However, at the maximum loading rate of 1.6 gVS L−1 d−1 and an HRT of 105 days, the biogas production decreased, indicating microbial inhibition that was probably caused by accumulation of volatile fatty acids (VFAs) and long chain fatty acids (LCFAs), due to the high lipid content of the substrate. In addition, the high protein content of the substrate led to the accumulation of NH4-N in the reactors. An operational alternative must be developed to prevent system instability.