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.