Nutrient recovery from African catfish sludge via aerobic mineralization and assessment of its fertilization potential in hydroponic basil cultivation

Abstract

The increasing demand for sustainable fertilizer sources highlights the potential of aquaculture by-products as alternatives to conventional mineral solutions in hydroponics. This study evaluated the fertilization potential of a liquid solution produced by aerobic digestion of fish sludge from a recirculating aquaculture system with African catfish (Clarias gariepinus) for indoor cultivation of basil (Ocimum basilicum L.) in a deep water culture system. Three nutrient solutions were compared: a conventional mineral solution (HYDRO), fish excrement mineralizate obtained by aerobic digestion (FEM), and a 1:1 (v/v) mixture of both (MIX). Cultivation with FEM alone resulted in a 17.5 % reduction in fresh shoot biomass and an overall reduction in N and P content in plant biomass compared to the hydroponic control, likely due to deficient potassium and iron levels in the FEM solution. However, plants grown in the FEM solution exhibited high potassium use efficiency and lower nitrate accumulation in plant tissues. The MIX variant achieved fresh shoot biomass comparable to the mineral control, enhanced root growth (+40.3 %), and reduced nitrate accumulation (MINUS SIGN 9.4 %) while maintaining vitamin C content and total antioxidant capacity. Fluorescence in situ hybridization confirmed the presence of Bacillus spp. and Azospirillum spp. in the FEM and MIX solutions, suggesting a potential biostimulant effect that needs further investigation. These findings indicate that FEM cannot serve as a full replacement for mineral nutrient solutions; however, appropriate supplementation with mineral fertilizers can achieve growth performance comparable to or exceeding that in mineral solutions, offering a sustainable and balanced cultivation strategy.