Heterotrophic nitrification-aerobic denitrification (HNAD) in the treatment of marine aquaculture wastewater: nitrogen removal performance, mechanism and microbial characteristics
Nitrogen pollution brought by the rapid development of mariculture poses a great risk to the marine ecosystem. Microorganisms with heterotrophic nitrification- aerobic denitrification ability have received widespread attention for their efficient removal of nitrogen sources under aerobic conditions. However, there is limited research on the practical application of HNAD for nitrogen removal from seawater in the environment of actual mariculture wastewater. In this study, a laboratory-scale HNAD carrier biofilm reactor was operated continuously for 72 days based on the optimal C/N ratio, filler, and carbon source to investigate the performance of nitrogen removal in seawater recirculating aquaculture system wastewater and the wastewater from the pond of the marine aquaculture farm. It was found that at C/N 35, the biofilm reactor system had the most ideal denitrification effect. Ceramsite has certain advantages in the actual denitrification effect and economic benefits. Compared with liquid carbon source sodium acetate, solid carbon source PHA showed better denitrification stability. Under the optimal conditions, on the 7th day of operation, the system successfully realized the synchronous nitrification and denitrification system, and the TIN removal rates of two different influent water reached 87.37 +- 4.23% and 65.00 +- 8.35% respectively, and the effluent quality could reach up to Class II seawater.Under different water quality conditions, dominant HNAD-capable genera such as Bacillus, Pseudomonas, Marinobacter, Halomonas and Acinetobacter were found on both ceramsite and PHA surfaces. Microbial network analysis showed that the microbial ecological network of the system was more complex when the influent water was traditional aquaculture tailwater than RAS wastewater. Most of the key microorganisms in the microbial ecological network contained HNAD genera. It was mainly dominated by HNAD. Species composition and abundance in the nitrogen-transforming microorganisms varied with water quality and carrier. Overall, the HNAD biofilm reactor system loaded with ceramic granules and PHA could achieve good nitrogen removal for RAS and traditional mariculture wastewater. It provides a reference for the process of simultaneous nitrification and denitrification, which is utilized for the nitrogen removal from mariculture wastewater.