Discharge management in fresh and brackish water RAS: combined phosphorus removal by organic flocculants and nitrogen removal in woodchip reactors

Abstract The current study combined P and N removal using organic flocculant chemicals and woodchip bioreactors in both freshwater and brackish water (7 ppm) recirculating aquaculture systems (RAS). The use of carbon (C) containing flocculant chemicals in the process was hypothesized to further stimulate C-demanding N removal (denitrification) in bioreactors. The trial of combined P and N removal consisted of four treatments: freshwater and brackish water RAS with and without the addition of supernatant from flocculation process to the woodchip reactor. Duplicate woodchip reactors were used per treatment and the trial was run for six weeks. 56 % and 49 % of P was removed from fresh and brackish sludge water, respectively. The nitrate-N (NO3-N) removal rate was improved in the treatment when supernatant from flocculation process was used together with RAS discharge water when compared against the control. In brackish water RAS, the improvement was more pronounced (from 6.6 to 16.5 g NO3-N m-3 d-1) than in freshwater RAS (from 5.1 to 6.5 NO3-N m-3 d-1). In the freshwater bioreactors using supernatant, N was largely discharged as a nitrite-N (NO2-N). High NO2-N concentrations in freshwater reactors allude to incomplete denitrification reactions taking place. The results suggest that the organic flocculants did provide an additional C source for denitrification, which improved the N-removal process. However, in freshwater RAS this might have been partly due to untargeted processes such as DNRA (dissimilatory nitrate reduction to ammonium), and/or insufficient denitrification reactions taking place (excessive NO2-N production).