Physical properties of food waste influence the efficiency of black soldier fly larvae bioconversion via microbial activity.

Black soldier fly larvae bioconversion is an emerging industrial technology for more sustainable waste management and feed production. Recycling heterogenous food waste using the larvae can present challenges, including excessive heat production, variable CO2 emissions, and low bioconversion efficiency. This study investigated how ascending levels of food waste bulk density affect microbial activity, temperature, CO2 emissions, and bioconversion efficiency. A 7-day feeding trial was conducted, using industrial rearing crates (10,800 larvae, 25 mg dry food waste larva-1 day-1) in respiration chambers. Three food waste bulk densities were created with cocopeat (0.8 g cm-3, 0.6 g cm-3, 0.5 g cm-3), while keeping food waste quantity and moisture constant. Substrate temperature, larval and microbial CO2 production, dry larval mass, and microbial abundance (quantitative PCR) were monitored. Food waste bulk density impacted the bioconversion strongly. With the lowest bulk density, the bioconversion rate dropped by 19 percentage-points, the average temperature rose by 8.8 °C, and microbial CO2 increased by up to 229 %. An initial spike in microbial CO2 coincided with fungal growth that was replaced by bacterial growth after day 2. The results suggested that food waste with a lower bulk density is better aerated, promoting CO2 emissions and heat generation through microbial activity. These findings demonstrate how modifications of the bulk density can regulate CO2 emissions, heat production and the bioconversion efficiency of black soldier fly larvae systems. We discuss the resulting potential solutions towards a more sustainable and efficient industrial waste management.