Advanced low carbon-to-nitrogen ratio wastewater treatment by electrochemical and biological coupling process

Nitrogen pollution in ground and surface water significantly affects the environment and its organisms, thereby leading to an increasingly serious environmental problem. Such pollution is difficult to degrade because of the lack of carbon sources. Therefore, an electrochemical and biological coupling process (EBCP) was developed with a composite catalytic biological carrier (CCBC) and applied in a pilot-scale cylindrical reactor to treat wastewater with a carbon-to-nitrogen (C/N) ratio of 2. The startup process, coupling principle, and dynamic feature of the EBCP were examined along with the effects of hydraulic retention time (HRT), dissolved oxygen (DO), and initial pH on nitrogen removal. A stable coupling system was obtained after 51 days when plenty of biofilms were cultivated on the CCBC without inoculation sludge. Autotrophic denitrification, with [Fe²⁺] and [H] produced by iron–carbon galvanic cells in CCBC as electron donors, was confirmed by equity calculation of COD_Cr and nitrogen removal. Nitrogen removal efficiency was significantly influenced by HRT, DO, and initial pH with optimal values of 3.5 h, 3.5 ± 0.1 mg L⁻¹, and 7.5 ± 0.1, respectively. The ammonia, nitrate, and total nitrogen (TN) removal efficiencies of 90.1 to 95.3 %, 90.5 to 99.0 %, and 90.3 to 96.5 % were maintained with corresponding initial concentrations of 40 ± 2 mg L⁻¹ (NH₃–N load of 0.27 ± 0.01 kg NH₃–N m⁻³ d⁻¹), 20 ± 1 mg L⁻¹, and 60 ± 2 mg L⁻¹ (TN load of 0.41 ± 0.02 kg TN m⁻³ d⁻¹). Based on the Eckenfelder model, the kinetics equation of the nitrogen transformation along the reactor was N_e = N₀exp (−0.04368 h/L^1.8438). Hence, EBCP is a viable method for advanced low C/N ratio wastewater treatment.