Vermicomposting of sludge from recirculating aquaculture system using Eisenia andrei: Technological feasibility and quality assessment of end-products

Intensive aquaculture is an important and fast-growing food production industry generating significant amounts of nutrient-rich sludge, which represents a potential environmental threat. Vermicomposting aquacultural sludge has been suggested, but remained poorly understood - only survival and growth of initial earthworm stocks have been assessed so far. The present study provides a comprehensive evaluation of the production system, examining vermicomposting of three types of sludge each at four inclusion levels and the possibility of further utilising end-products (vermicomposts and earthworms). Through an 18-week experiment, high survival of initial earthworm stocks, exceeding 90% among treatments up to week 6, was documented. Higher inclusion levels and sludge types richer in nutrients positively influenced individual weight of initial stocks and their reproduction indices (cocoon and juvenile production). The most progressive treatments sustained >300 juveniles in experimental incubators containing 200 g dw of initial substrates. Original sludge and final vermicomposts were found suitable for use in agriculture, complying with limits for heavy metals given in the most usually applied regulations. In relation to the heavy metals, earthworms were found to be a generally safe feed for fish. Only arsenic concentrations may occasionally exceed given limits. Still, observed concentrations are considered safe, presuming arsenic presence primarily in organic forms having largely reduced toxicity. Vermicomposting is recommended as a clean and sustainable technology transforming aquaculture sludge into highly valuable vermicompost and earthworm biomass.