Simultaneous removal of ammonia, copper ions and sulfamethoxazole from aquaculture wastewater with low carbon to nitrogen ratio enhanced by manganese redox driven by a two-stage synergistic bioreactor: Optimization and potential mechanism

The problem of low carbon-nitrogen ratio (C/N) in wastewater is a major challenge for biological treatment, especially the complex pollution of ammonia nitrogen (NH₄⁺-N), sulfamethoxazole (SMX), and copper ions (Cu(II)). Herein, a strain of Pseudoxanthomonas sp. MA23 with manganese (Mn) reduction-coupled ammonia oxidation properties was isolated. Subsequently, kaolin and bentonite were used as the main raw materials, and a mixture of coconut shell biochar (CSBC) and different Mn ores were added to make ceramsite carriers to load the target strain MA23. To achieve complete N removal and Mn redox process, Dechloromonas sp. YZ8 with Mn redox and denitrification performance was introduced, and a second-stage bioreactor was constructed with volcanic rock as the biocarrier. The results showed that the bioreactor was most effective when the hydraulic retention time (HRT) was 20.0 and 2.0 h, C/N was 1.5, and pH was 6.5. The response of the bioreactors was investigated by inflowing different concentrations of Cu(II) and SMX. Appropriate Cu(II) concentrations promoted the electron transfer in the system, and Cu(II) and SMX were together removed by biological action and chemisorption. Furthermore, genes involved in N metabolism were enriched in the bioreactors and the microorganisms responded to environmental changes by up or down-regulating relevant metabolic genes. The synergistic system proposed in this study provided a promising attempt to simultaneously address NH₄⁺-N-Cu(II)-SMX pollution in low C/N wastewater.