Enhanced Nitrogen Removal from a Recirculating Aquaculture System Using a Calcined FeS _x -Packed Denitrification...

In this study, a recirculating aquaculture system (RAS) was constructed, and a denitrification bioreactor was installed to enhance nitrogen removal. In addition, the nitrogen removal performance of the system was investigated. FeS _x was prepared by calcining iron (Fe) and S⁰ powder, which was used as an electron donor for denitrification. In the phase using simulating aquaculture wastewater, the concentrations of NO₂ ^--N and NH₄ ⁺-N in the RAS were lower than 0.20 and 0.50 mg/L, respectively, and NO₃ ^--N gradually accumulated without the operation of the FeS _x -packed denitrification bioreactor. After introducing cultured fish and operating the denitrification bioreactor, NO₂ ^--N and NH₄ ⁺-N in the fish tank were lower than 0.01 mg/L and lower detection limit, respectively, and the NO₃ ^--N removal efficiency was 79.04%. After 24 days of operation, the SO₄ ^2- concentration was lower than 200 mg/L, and the pH was stable at around 7. The survival rate of fish was 95%, and they grew 6 to 7 cm at the end of the experiment. The average weight gain of fish was 5.31 g, and the culture density increased from the initial 10 to 26.54 kg/m³. Microbial community structure analysis showed that the diversity in the denitrification bioreactor operated in the RAS (RAS_Sludge) was higher than that in the reactor operated using synthetic wastewater (Synthetic_Sludge) due to the introduction of organic matter. Thermomonas, Longilina, Arenimonas, and Thiobacillus were dominant in RAS_Sludge, while unclassified genera were dominant in Synthetic_Sludge. Functional genes in RAS_Sludge and Synthetic_Sludge were predicted based on Functional Annotation of Prokaryotic Taxa, revealing differences in genes related to denitrification as well as sulfur and iron oxidation. This study provides a theoretical basis for the application of FeS _x -based autotrophic denitrification technology in RASs, promoting it from theoretical research to engineering practice.