Enhanced Nitrogen Removal from a Recirculating Aquaculture System Using a Calcined FeSx-Packed Denitrification Bioreactor

Abstract

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. FeSx 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 FeSx-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 SO42- 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 FeSx-based autotrophic denitrification technology in RASs, promoting it from theoretical research to engineering practice.