Electro-oxidation of ammonia nitrogen using W, Ti-doped IrO₂ DSA as a treatment method for mariculture and livestock wastewater

Animal farming wastewater is one of the most important sources of ammonia nitrogen (NH₄⁺-N) emissions. Electro-oxidation can be a viable solution for removing NH₄⁺-N in wastewater. Compared with other treatment methods, electro-oxidation has the advantages of i) high removal efficiency, ii) smaller size of treatment facilities, and iii) complete removal of contaminant. In this study, a previously prepared DSA (W, Ti-doped IrO₂) was used for electro-oxidation of synthetic mariculture and livestock wastewater. The DSA was tested for chlorine evolution reaction (CER) activity, and the reaction kinetics was investigated. CER current efficiency reaches 60–80% in mariculture wastewater and less than 20% in livestock wastewater. In the absence of NH₄⁺-N, the generation of active chlorine follows zero-order kinetics and its consumption follows first-order kinetics, with cathodic reduction being its main consumption pathway, rather than escape or conversion to ClO₃⁻. Cyclic voltammetry experiments show that NH₄⁺-N in the form of NH₃ can be oxidized directly on the anode surface. In addition, the generated active chlorine combines with NH₄⁺-N at a fast rate near the anode, rather than in the bulk solution. In electrolysis experiments, the NH₄⁺-N removal rate in synthetic mariculture wastewater (30–40 mg/L NH₄⁺-N) and livestock wastewater (~ 450 mg/L NH₄⁺-N) is 112.9 g NH₄⁺-N/(m²·d) and 186.5 g NH₄⁺-N/(m²·d), respectively, which is much more efficient than biological treatment. The specific energy consumption (SEC) in synthetic mariculture wastewater is 31.5 kWh/kg NH₄⁺-N, comparable to other modified electro-catalysts reported in the literature. However, in synthetic livestock wastewater, the SEC is as high as 260 kWh/kg NH₄⁺-N, mainly due to the suppression of active chlorine generation by HCO₃⁻ and the generation of NO₃⁻ as a by-product. Therefore, we conclude that electro-oxidation is suitable for mariculture wastewater treatment, but is not recommended for livestock wastewater. Electrolysis prior to urea hydrolysis may enhance the treatment efficiency in livestock wastewater.