Effects of salvaged cyanobacteria content on larval development and feedstock humification during black soldier fly larvae (Hermetia illucens) composting

Cyanobacteria salvage is widely used to deal with massive cyanobacterial blooms. Improper disposal of salvaged cyanobacteria would cause secondary pollution. Black soldier fly (Hermetia illucens) larvae (BSFL) can bio-convert organic wastes into larval biomass, which is rich in protein and lipid. This study evaluated the possibility of using BSFL composting for salvaged cyanobacteria treatment. Results showed that increasing salvaged cyanobacteria waste (CW) content (from 0 to 50%, dry weight basis) extended BSFL development time, e.g., BSFL fed with 50% CW needed 14 days more to finish development than Control (0% CW). The CW content (0–20%) in feeding substrates had no significant effect on BSFL body length and weight. Whereas further increase of CW content (from 20 to 50%) led to significant reductions in substrate-to-BSFL biomass conversion ratio, body size, body weight, and crude protein content of BSFL. Meanwhile, the presence of salvaged cyanobacteria in the feeding substrate reduced the degradation efficiency of feeding substrate. The dissolved organic matter (DOM) results demonstrated that the increased salvaged cyanobacteria content made it more difficult for BSFL to degrade the feeding substrate into simple organic matter and further into humic-like substances. Furthermore, salvaged cyanobacteria in feeding substrates affected the intestinal microbial community significantly. With 20% CW content in the feeding substrate, the relative abundance of Firmicutes decreased from 92.43 to 81.24%, while the relative abundance of Proteobacteria and Bacteroidetes increased from 4.10 to 2.93–8.75% and 7.51%, respectively. BSFL composting is feasible to convert salvaged cyanobacteria into insect biomass. However, the salvaged cyanobacteria content in the feeding substrate should be carefully controlled (e.g., less than 30%).