Biological nitrate removal for marine recirculating aquaculture system

Abstract. The application of biological nitrate removal by denitrification and anaerobic ammonia oxidation (anammox) was tested to eliminate accumulated nitrate in land-based recirculating aquaculture system (RAS). Anaerobic solid digestion was incorporated at the upstream of nitrate removing bioreactor to create anoxic condition by organic solid decomposition depleting oxygen. Within the 4 hr of hydraulic retention time (HRT) for solid digestion, dissolved oxygen (DO) in wastewater was lowered from approx. 5 mg/L down to 0 mg/L. Conditioned wastewater was fed to an up-flow moving bed reactor for nitrate removal. Soluble carbon compounds (i.e., carbohydrates, organic alcohols, and fatty acids) produced by polymer degradation resulted in maintaining the optimal C/N ratio (3/1) for heterotrophic denitrification. Therefore, application of solid digestion reduced the operational costs of nitrate removal by diminishing the requirement of external carbon supply. Marine anammox bacteria, the group of Planctomycetes spp., were also identified in the nitrate removing bioreactor converting ammonia and nitrite directly to nitrogen gas autotrophically. Redox potential of denitrification-anammox (denammox) reactor was monitored and maintained between +50 mV to -50 mV by supplemental flow of oxic system water to provide denitrifyers and anammox the optimal condition that nitrate is selectively used as an electron acceptor. Sequential anaerobic reactors side loop resulted in effective removal of nitrate that reduced system discharge down to