Nitrogen removal via solid carbon source-driven heterotrophic nitrification and aerobic denitrification in marine aquaculture wastewater

Heterotrophic nitrification and aerobic denitrification (HNAD) enable the simultaneous removal of NH4+-N and NO3--N under aerobic conditions. This process is highly suitable for treating nitrogen-containing wastewater in oxygen-rich marine recirculating aquaculture system (MRAS). Natural cellulose materials, such as corncob (CC) and peanut shell (PS), are promising solid carbon sources (SCSs) for wastewater treatment. They are widely available, cost-effective, and environmentally friendly. This study developed an HNAD process using CC and PS as SCSs for MRAS wastewater treatment. The nitrogen removal performance and conversion pathways were investigated. CC and PS had dissolved organic yields of 0.44 g-COD/g-CC and 0.17 g-COD/g-PS, respectively. The HNAD process using CC demonstrated stable and efficient aerobic denitrification over a 6-day dosing cycle. Notably, NO3--N and total inorganic nitrogen (TIN) removal efficiencies peaked at 99.71 % and 96.72 %, respectively. The NO2--N accumulation was negligible (< 0.3 mg/L). Compared to PS, CC showed more significant surface changes and higher microbial utilization. Its surface exhibited increased porosity, depressions, and greater microbial adhesion. NH4+-N removal was driven by autotrophic nitrification, heterotrophic nitrification, and cell assimilation. Heterotrophic nitrification was the dominant pathway. NO3--N removal primarily occurred through heterotrophic denitrification. The efficiency of these processes depended heavily on the type of carbon source used. Microbial community analysis revealed that the coexistence of aerobic denitrifying bacteria and cellulose-degrading bacteria was critical for sustained denitrification efficiency. This study confirmed the underlying mechanisms of the HNAD denitrification process and provided a reference for its practical application in MRAS.