Enhanced nitrogen removal from aquaculture wastewater by biochar-assisted algal-bacterial bioaugmentation using direct ammonium-oxidizing and heterotrophic nitrification-aerobic denitrifying bacterial consortium.

Efficient nitrogen removal from low-C/N aquaculture wastewater remains challenging because carbon availability can constrain nitrogen conversion. A biochar-assisted algal-bacterial system was developed using a consortium capable of direct ammonium oxidation (Dirammox) and heterotrophic nitrification-aerobic denitrification. Four sequencing batch reactors were operated to compare the individual and combined effects of bioaugmentation and biochar. The combined system achieved a maximum total nitrogen (TN) removal efficiency of 88 % and a mean stable effluent TN concentration of 2.8 mg·L-1, compared with 7.2 mg·L-1 in activated sludge (AS). Bioaugmentation was associated with greater extracellular polymeric substance accumulation and intracellular metabolic activity, whereas biochar was associated with changes in floc-carrier characteristics and interfacial electrochemical properties. The combined system exhibited a charge-transfer resistance of 18.7 kΩ, only 7 % of that in AS, and an electron-exchange capacity of 3.7 μmol e-·g-1. Functional profiles indicated the coexistence of conventional nitrification with Dirammox-associated and aerobic denitrification potential, with dnfA reaching 6.6 × 108 copies·mg-1 suspended solids, 5.3-fold that in AS. These findings suggest that combining this bacterial consortium with biochar can improve nitrogen removal through complementary nitrogen-transformation routes and enhanced interfacial electron exchange, providing design guidance for biochar-assisted algal-bacterial treatment of aquaculture wastewater.