Greenhouse gas emissions from black soldier fly composting of silver and silver sulfide nanoparticle-enriched sludge

Silver nanoparticles (Ag-NPs) are widely used as antibacterial materials and accumulate in sewage sludge as silver sulfide nanoparticles (Ag₂S-NPs) after wastewater treatment. Composting using black soldier fly (BSF) is an effective method for treating sewage sludge. This study investigated the impact of Ag/Ag₂S-NP-containing sludge on greenhouse gas emissions during BSF composting. The results indicated that BSF significantly increased N₂O and CH₄ emissions during composting. However, the addition of 5 mg/kg or 100 mg/kg of Ag/Ag₂S-NPs significantly reduced N₂O emissions by 51.7-86.1 %, and CH₄ emissions by 44.3-92.9 %. Quantitative analysis of genes in the sludge revealed that the inhibition of hao and norB genes, and the enhancement of nosZ genes were critical factors reducing N₂O emissions. For CH₄, inhibition of methanogenic genes (mcrA) and the enhancement of methane oxidation genes (pmoA) were the primary mechanisms by which Ag₂S-NPs reduced CH₄ emissions. Based on X-ray absorption spectroscopy (XAS) and single-particle ICP-MS (spICP-MS), Ag was retained predominantly as Ag₂S throughout, with small fractions converting to AgCl in the larval gut, and particles exhibited modest size reduction. Further metagenomic analysis revealed Ag-driven alterations in BSF gut microbiota, including decreased microbial diversity, and suppressed denitrification and methanogenesis pathways. This study offers an economical and effective method to reduce greenhouse gas emissions during sewage sludge treatment using BSF composting when Ag₂S-NPs are present.