Evaluating the effects of organic carbon on biological filtration performance in a large scale recirculating aquaculture system

▶ Recirculating aquaculture conditions inhibit nitrification compared to lab studies. ▶ Increased organic carbon reduces biofilter nitrification performance. ▶ Increased organic carbon under normal conditions further reduced nitrification by 50%. ▶ Improving mechanical filtration and feed quality will minimize biofilter impacts. ▶ Future studies should be more indicative of biologically available organic carbon. Studies evaluating the impact of organic carbon on biological filters at the large scale for aquaculture production are lacking. Understanding the performance characteristics of different biofilters under actual production conditions will be the only means by which recirculating system designers may properly select and size biological filters for commercial production use. Previous studies have determined the impact of organic carbon on biofilter performance at the small lab scale often using artificial waste nutrients in the evaluation. Evaluations under actual production conditions using real wastewater produce vastly different results than previous lab scale studies using artificial nutrients. As such, this study is a preliminary step in evaluating the impact of organic carbon on three different commercially available biological filters at the large scale under actual recirculating aquaculture production conditions. The study was conducted at the North Carolina State University Fish Barn—a commercial scale research and demonstration recirculating aquaculture facility operated by the Biological and Agricultural Engineering department. The study was based on a 60m3 Tilapia system with average daily feed rates of 45kg using a 40% protein feed and an average biomass of 6750kg. The system was dosed with sucrose (C12H22O11) to increase the concentration of biodegradable organic carbon in the system. The effect of elevated organic carbon concentrations on total ammoniacal nitrogen (TAN) removal rates was evaluated and determined based on biofilter media volume. Variability increased substantially in the volumetric TAN removal rate (VTR) for all three filters. VTR for all three filter types was reduced by approximately 50% as compared to normal production conditions. The results demonstrate the importance of controlling the concentration of biologically available organic carbon in a recirculating aquaculture system.