A Novel Recirculating Aquaculture System for Sustainable Aquaculture: Enabling Wastewater Reuse and Conversion of Waste-to-Immune-Stimulating Fish Feed

Recirculation aquaculture system (RAS) is a unique indoor fish farming method. Yet, the RAS system needs to address challenges such as effective wastewater/waste management, reduction of feed cost, and usage of antibiotics during farming to become a sustainable aquaculture system. Here, we report a novel RAS-polyhydroxybutyrate (PHB) system to overcome these challenges simultaneously. The RAS-PHB enables effective aquaculture wastewater (AW) treatment for reuse while producing and harvesting PHB-rich biomass as a protein-rich, immune-stimulating fish feed without the need for antibiotics. The feasibility of nitrogen recovery from AW for nonsterile cultivation of a salt-tolerant PHB-producing strain (Zobellella denitrificans ZD1) with or without supplementing agro-industrial wastes/wastewaters (e.g., glycerol, cheese whey wastewater, and aquaculture solid waste) was examined. Glycerol-grown Z. denitrificans ZD1 showed high contents of PHB (48%) and proteins (45.5%). High harvest efficiency of PHB-rich Z. denitrificans ZD1 (97%) was achieved by using small amounts of medium-molecular-weight chitosan. A simple economic analysis showed that the production costs were 0.6-0.7 $/kg of fish produced for the RAS-PHB and 1.2-1.6 $/kg for the conventional RAS, suggesting a reduction of 56% of the total cost by using a RAS-PHB. Overall, this proof-of-concept study showed that the RAS-PHB system is promising for future sustainable aquaculture practice.