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Nitrogen oxide (NOx) emissions cause significant impacts on the environment and must therefore be controlled even more stringently. This requires the development of cost-effective removal strategies which simultaneously create value-added by-products or energy from the waste. This study aims to treat gaseous nitric oxide (NO) by hollow-fibre membrane biofilm reactor (HFMBfR) in the presence of nitrite ( NO 2 - ) and evaluate nitrous oxide (N 2 O) emissions formed as an intermediate product during the denitrification process. Accumulated N 2 O can be utilised in methane oxidation as an oxidant to produce energy. In the first stage of the study, the HFMBfR was operated by feeding only gaseous NO as the nitrogen source. During this period, the best performance was achieved with 92% NO removal efficiency (RE). In the second stage, both NO gas and NO 2 - were supplied to the system, and 91% NO and 99% NO 2 - reduction were achieved simultaneously with the maximum N 2 O generation of 386 ± 31 ppm. Lower influent carbon to nitrogen (C/N) ratios, such as 4.5 and 2.0, and higher NO 2 - -N loading rate of 158 mg N day -1 favoured N 2 O generation. An improved NO removal rate and N 2 O accumulation were seen with the increasing amount of PO 4 3 - in the medium. The 16S rDNA sequencing analysis revealed that Alicycliphilus denitrificans and Pseudomonas putida were the dominant species. The study shows that an HFMBfR can be successfully used to eliminate both NO 2 - and gaseous NO and simultaneously generate N 2 O by adjusting the system parameters such as C/N ratio, NO 2 - and PO 4 3 - loading.