Bioavailability of essential trace elements and their impact on anaerobic digestion of slaughterhouse waste

•Analysis of the bioavailability of trace elements in an AD process.•Sequential extraction data linked with classical AD-process monitoring results.•Process performance directly linked to the amount of bioavailable trace elements.•Methane yield decreased to 35% when trace elements were not supplemented.•Threshold concentrations of trace elements for optimal AD performance determined.

Slaughterhouse waste is an energy rich feedstock suitable for anaerobic digestion processes. However, chemical characterization showed a deficiency in essential trace elements which are critical for optimal performance of the process. Hence this study investigated the degree of bioavailability of trace elements in four semi-continuous lab-scale AD tests accepting slaughterhouse waste under mesophilic conditions (38 °C) and a moderate organic loading rate of 2.2 kg/m3 d. Parameters, such as volatile fatty acid (VFA) concentration, COD removal rate and specific methane yield were compared to the results of sequential extraction analysis. The highest methane yield (250–275 Nm3/t COD), lowest accumulation of VFA (<500 mg/l) and high COD removal rate (75–80%) was obtained when the total concentration of 11.4 mg/l Ni, 25.4 mg/l Co and 4.8 mg/l Mo was present in the reactor, of which 62% of Ni and Co, and 68% of Mo were bioavailable for microbial uptake. Based on these results it can be recommended that a supply of 2.5 g/t Ni, 3.5 g/t Co, 0.6 g/t Mo and 0.05 g/t Se provide optimal conditions for anaerobic digestion of slaughterhouse waste.