Process efficiency in relation to enzyme pre-treatment duration in black soldier fly larvae composting

Black soldier fly larvae (BSFL) composting is a treatment in which biodegradable food waste is converted into animal-feed protein and organic fertiliser. BSFL composting has greatest potential for mixed food waste, but under European Union regulations only plant-based waste is permitted as feed for larvae. Biomass conversion efficiency (BCE) in BSFL composting is lower for plant-based waste than for mixed food waste. One way of improving BCE for plant-based waste is to add enzymes to make the waste more available to the larvae, but enzyme pre-treatment is not commonly applied prior to BSFL composting. Therefore this study examined the impact of enzyme pre-treatment duration on process efficiency in BSFL composting of lettuce-cabbage waste pre-treated with enzymes for 0-4 days. The results showed that total solids (TS) in larvae decreased with longer enzyme pre-treatment. Direct addition of enzymes at the start of BSFL treatment (0 day pre-treatment) resulted in 22% higher BCE on a volatile solids (VS) basis compared with the control, while longer pre-treatment did not improve BCE further. Much of the VS was respired in the 0-day pre-treatment, resulting in lower mass of residues at the end of treatment. Longer pre-treatment increased microbial respiration, suggesting that the microbial community consumed more easily available carbohydrates during the pre-treatment step, which counteracted the purpose of enzyme pre-treatment, i.e. increasing BCE during BSFL composting.