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.