Denitrification performance, kinetics, and microbial community of methanol‐driven heterotrophic denitrification processes for treating nitrate‐rich water in recirculating mariculture system

Abstract Large‐scale application of methanol‐driven heterotrophic denitrification (MHD) biofilms to recirculating mariculture systems (RMSs) is constrained by the requirement of precise methanol dosage. The emergence of intelligent aquaculture offers an opportunity to address this challenge, necessitating a comprehensive understanding of the denitrification performance, kinetics, and microbial community of MHD biofilm reactors during the treatment of nitrate (NO 3 − ‐N)‐rich RMS water. Therefore, this study conducted a systematic investigation on an MHD biofilm reactor for water purification in RMSs. Results show that the MHD biofilm reactor effectively eliminated NO 3 − ‐N with removal efficiencies of 53.0%–98.4% and rates of 66.77–643.73 mg L −1 day −1 . Complete denitrification was the dominant process of NO 3 − ‐N removal in the MHD bioreactor; however, partial denitrification (PD) and dissimilatory nitrate reduction to ammonium (DNRA) were also observed. Higher carbon‐to‐nitrogen (C/N) ratio (0.5–2.5), hydraulic retention times (HRT) (0.5–4.0 h), and temperatures (10–30°C) and lower influent NO 3 − ‐N concentrations (25–100 mg L −1 ) facilitated complete denitrification, whereas higher influent dissolved oxygen (DO) (4.0–6.5 mg L −1 ) slightly enhanced PD. These parameters had little impact on DNRA. The NO 3 − ‐N and nitrite distributions along packing height followed a first‐order model and a first‐order consecutive reaction model, respectively, with their rate constants depending on the operational parameters. Enrichment cultivation, reactor start‐up, and reactor operation promoted the proliferation of Methylophaga , rendering it the dominant bacterial genus in the biofilm and the primary functional group for MHD. Lesser genera (e.g., Methanolobus , Methylomicrobium , Sulfurimonas , and Marinicella ) also contributed to niche differentiation and elemental cycling in the MHD bioreactor.