Heterotrophic nitrification-aerobic denitrification (HNAD) in marine aquaculture wastewater treatment: Nitrogen removal performance, mechanism and microbial community characteristics

Nitrogen pollution resulting from the rapid development of mariculture poses a significant threat to marine ecosystems. Heterotrophic nitrification - aerobic denitrification microorganisms (HNADMs), which can achieve nitrification and denitrification simultaneously under aerobic conditions, have shown great potential in treating nitrogen pollution. In this study, a laboratory-scale heterotrophic nitrification - aerobic denitrification (HNAD) biofilm reactor was continuously operated for 72 days to evaluate its performance in treating marine recirculating aquaculture system wastewater (RAS wastewater) and traditional extensive pond aquaculture tailwater in marine farm (Pond tailwater). The results indicated that with ceramsite as the filter media and solid polyhydroxyalkanoates (PHA) as the carbon source, yielded the most effective and stable nitrogen removal. Under these optimal conditions, the system achieved simultaneous nitrification and denitrification by the 7th day, with total inorganic nitrogen removal efficiencies of 87.37 ± 4.23 % for RAS wastewater and 65.00 ± 8.35 % for pond tailwater. Dominant HNADMs were consistently found on both ceramsite and PHA surfaces across different water sources. A more complex microbial ecological network when treating pond tailwater compared to RAS wastewater was found, with key microorganisms predominantly from HNADMs. The species composition and abundance of nitrogen-transforming microorganisms vary with water quality and carrier type, with traditional nitrifying and denitrifying bacteria being relatively less prevalent and HNADMs more dominant. Overall, the HNAD biofilm reactor system effectively removed nitrogen from both RAS wastewater and pond tailwater, providing valuable insights for simultaneous nitrification and denitrification processes in mariculture wastewater treatment.