End-of-pipe single-sludge denitrification in pilot-scale recirculating aquaculture systems

A step toward environmental sustainability of recirculat aquaculture systems (RAS) is implementation of single-sludge denitrification, a process eliminating nitrate from the aqueous environment while reducing the organic matter discharge simultaneously. Two 1700 L pilot-scale RAS systems each with a 85 L denitrification (DN) reactor treating discharged water and hydrolyzed solid waste were setup to test the kinetics of nitrate and COD removal. Nitrate removal and COD reduction efficiency was measured at two different DN-reactor sludge ages (high θX: 33-42 days and low θX: 17-23 days). Nitrate and total N (NO3− + NO2− + NH4+) removal of the treated effluent water ranged from 73-99% and 60-95% during the periods, respectively, corresponding to an overall maximum RAS nitrate removal of approximately 75%. The specific nitrate removal rate increased from 17 to 23 mg NO3−-N (g TVS d)−1 and the maximal potential DN rate (measured at laboratory ideal conditions) increased correspondingly from 64-68 mg NO3−-N (g TVS d)−1 to 247-294 mg NO3−-N (g TVS d)−1 at high and low θX, respectively. Quantification of denitrifiers in the DN-reactors by qPCR showed only minor differences upon the altered sludge removal practice. The hydrolysis unit improved the biodegradability of the solid waste by increasing volatile fatty acid COD content 74-76%. COD reductions in the DN-reactors were 64-70%. In conclusion, this study showed that single-sludge denitrification was a feasible way to reduce nitrate discharge from RAS, and higher DN rates were induced at lower sludge age/increased sludge removal regime. Improved control and optimization of reactor DN-activity may be achieved by further modifying reactor design and management scheme as indicated by the variation in and between the two DN-reactors.