Combined digestion of insect frass and cow manure for biogas production

In an experiment described by Elissen et al. (2019) on anaerobic digestion of black soldierfly (BSF, Hermetia illucens) larvae frass, the frass had been previously hygienised due to Dutch law. However, when the frass would stay on the same premises this hygienisation step may not be necessary. A synergistic effect of mixing insect frass with cattle slurry would provide a more positive business case for the insect farmer, since the frass becomes an interesting co-substrate for digesters, resulting in lower disposal costs or even benefits. To determine this synergistic effect, three substrates were tested in lab-scale mesophilic anaerobic digestion tests: Insect frass of BSF larvae grown on agricultural side-streams, either hygienised at 60 °C for 24h or non hygienised and cattle slurry. Also combinations of cattle slurry with insect frass were tested. Biogas production was determined by measuring gas pressure and analysing gas composition. No synergistic effect of adding insect frass to cattle slurry was found in this experiment, and neither differences between hygienised and non hygienised insect frass were found. In other words, adding insect frass to a cattle slurry digester will not lead to an increase in biogas production different from that expected based on adding up biogas productions of the single substrates. However, the partial replacement of cattle slurry with insect frass will increase the amount of biogas produced per digester volume with a factor 3.5 to 9 for respectively 10 % and 30 % replacement. Therefore, it is still interesting to add insect frass to a cattle slurry digester.