Encapsulation of denitrifying bacteria for potential recirculating aquaculture system denitrification

To solve the nitrate accumulation phenomenon in recirculating aquaculture system (RAS). This study aimed to provide a sufficient organic carbon source, denitrifying bacteria, and a suitable microenvironment for denitrification in RAS. Denitrifying capsules were prepared using the encapsulation method, the morphology, microbial community and influencing factors for nitrogen removal efficiency of the denitrifying capsules in synthetic RAS wastewater were investigated. The denitrifying capsules had a porous, rough, and loose structure, with a diameter of approximately 6 mm and a density of approximately 1.05 g per cubic centimeter. The effect of a low ratio of solid-to-liquid (10:1) and temperature (15 °C) on nitrate removal was significant during the initial acclimation period (1-10d). After the enclosed denitrifying bacteria activity was recovered, the influence was not significant. At the end of the test, the nitrate removal rate could reach more than 80% even at a lower ratio of solid-to-liquid (10:1) or at 15 °C. The capsule exhibited a short adaptation time for diverse types of substrates and adapted well to the substrate concentration. Under low influent nitrate or nitrite loading, the removal rate reached 80% on the 6th day, which was earlier than with high influent nitrate loading. The denitrification rate experiment showed that the nitrate removal rate could reach 80.14% at 14 h and finally stabilize at approximately 81.89%. The capsule-supported denitrification process was a zero-order reaction and the denitrification rate of the capsule was calculated to be 109.0 mg N/L-capsule·h. The longtime evaluation indicated that until the semen litchi powder was exhausted, the capsule could retain a high nitrate removal rate. Functional enzyme analysis reveals the nitrogen conversion pathway in the capsule included denitrification, dissimilatory nitrate reduction and nitrogenfixation. The porous structure and good adaptability to environmental conditions of the denitrifying capsule would help to achieve denitrification and reduce the nitrate content on the present RAS, without increasing infrastructure investment.