Composting and Vermicomposting of Fish Sludge with Egg Boxes and Lettuce Wastes with the Addition of Eggshells: Impacts on Chemical Properties, Nutrient Availability, and Safety

This study compared the composting and vermicomposting of fish sludge amended with egg boxes, lettuce residues, and eggshells, over a five-month period. Eight treatments (T1–T8) differing in fish sludge content and the presence or absence of earthworms (Eisenia andrei) were evaluated. Monitored parameters included pH, electrical conductivity, earthworm biomass and abundance, concentrations of available elements (P, K, Mg, S, Fe, Cu, Zn and Mn), volatile solids and C/N ratio. Final total levels of potentially toxic elements (PTEs), such as Cr, Ni, Pb and As were also measured. The results demonstrated that fish sludge, egg boxes, and lettuce at a 4:5:1 ratio plus eggshells with earthworms (T8) enhanced nutrient transformation and earthworm activity. Fish sludge and egg boxes at 1:3 plus eggshells (T2) and the same mixture with earthworms (T6) produced compost with PTEs concentrations within safe limits. Final concentrations of Cu, Zn, Cr, Ni, and Pb in T2, T6, and T8 remained below European regulatory thresholds. T8 showed significantly higher concentrations of available K and Mg compared to T2 and T6. T8 was identified as the most effective treatment for processing fish sludge while producing a safe, nutrient-rich product suitable for use as a high-quality organic fertilizer in sustainable agriculture. These findings support vermicomposting as an efficient and environmental strategy for fish sludge utilization.