Utilization of Sludge from African Catfish (Clarias gariepinus) Recirculating Aquaculture Systems for Vermifiltration

Vermifiltration is a low-energy and low-cost option to reduce the environmental impact of aquaculture. A comparative study was performed for two different stocking densities of the epigeic worm Dendrobaena veneta (Michaelsen, 1890, Annelida: Oligochaeta), which were fed with sediment sludge from African catfish, Clarias gariepinus (Burchell, 1822), recirculation aquaculture systems (RAS). The intensive (I) and extensive (E) systems were stocked with 15 and 10 g of worm/L filter substrate, respectively, and were compared with a control (C) for four weeks. The total weight gain was 9.4–13.5% for (I) and 13.8–19.5% for (E), with low mortality rates of 3.46–5.84% (I) and 3.57–5.19% (E). The temperature inside the vermifilters was slightly higher than that in the control, indicating a favorable milieu for microbial activity. The worms supported the pH buffering capacity in the systems, with the effluent reaching 7.10 ± 0.02 (I) and 7.26 ± 0.04 (E) at the end of the experiment while the pH in the (C) was significantly higher (7.51 ± 0.05). The removal rates were 68.02–98.84% (I), 71.85–98.67% (E), and 72.80–98.68% (C) for the total nitrogen bound (TNb); 82.77–96.64% (I), 81.65–94.84% (E), and 77.79–94.74% (C) for the total organic carbon (TOC); and 50.43–97.51% (I), 50.89–96.84% (E), and 48.23–96.34% (C) for the chemical oxygen demand (COD). By utilizing the sludge as feed, the worms and associated microbiota significantly altered the African catfish sediments, removing organic loads, upgrading the composition, and reducing the possible environmental impacts.