Mariculture solid waste application for marine recirculating aquaculture system wastewater treatment: The role of neglected sulfide electron donor

Marine recirculating aquaculture systems (RAS) are acknowledged as sustainable fish farming models, and their discharge of high nitrate-rich wastewater presents potential environmental concerns. Additionally, the low carbon-to-nitrogen ratio of RAS wastewater poses a significant challenge for nitrate removal. In this study, a novel heterotrophic coupled with sulfide-based autotrophic denitrification (HSD) driven by mariculture solid waste (MSW) was developed for marine RAS wastewater treatment. MSW acidogenic fermentation involved both acidogenesis and sulfidogenesis processes, producing an acidogenic liquid rich in VFAs and sulfide, which was added to the HSD system to drive denitrification. Robust N-removal performance was achieved in the MSW-driven HSD system with 97.8% removal efficiency of nitrate and low nitrite residual (1.2 mg/L) despite high salinity conditions. Kinetic analysis demonstrated a two-stage nitrogen removal process, attributed to a two-step sulfide oxidation model involving the conversion of sulfide to biologically produced elemental sulfur (BPS⁰) and subsequent oxidation of BPS⁰ to sulfate. Combined organic matter and sulfide improved the denitrification rate and decreased nitrite accumulation by mitigating the competition between nitrate and nitrite for electrons. The corporation of heterotrophic (Thauera) and autotrophic (Thiobacillus) denitrifiers guaranteed stable nitrogen removal in the HSD system. Moreover, the complete metabolic pathways of C/N/S were further verified by metagenomic analysis. This study demonstrated the potential and capability of utilizing MSW to achieve efficient and cost-effective RAS wastewater denitrification.