Removal of nitrogen from low pollution water by long-term operation of an integrated vertical-flow constructed wetland: Performance and mechanism

Highlights: • The system provides an efficient way for treating low pollution water. • Still better average COD and NH4⁺-N removal were observed in IVFCW after running for 8 years. • Ammoxidation and the adsorption and ion exchange of zeolite play a key role in NH₄⁺-N removal. • The denitrification capacity in this system was lower than that of nitrification capacity. • The drained reoxygenation in this study was the best choice for Zeolite regeneration. -- Abstract: The efficiency of nitrogen removal and its mechanism, aquatic organism distribution characteristics and regeneration capability of zeolite from an integrated vertical-flow constructed wetland (IVFCW) for low pollution water treatment were evaluated after steady and continuous operation for eight years. After running for eight years, better than average COD and NH₄⁺-N removal were observed in the IVFCW. The NH₄⁺-N removal rate in this system was controlled by ammoxidation and adsorption and ion exchange of zeolite. The low total nitrogen (TN) removal efficiency was due to NO₃⁻-N accumulation and zeolite desorption. In addition, this phenomenon indicated that because of poor organic carbon sources, nitrification was stronger than denitrification, consistent with the distribution of the functional genes for nitrification and denitrification. The biological activity in this system was abundant, especially that of spirogyra and navicula. The saturated adsorption capacity of zeolite was as high as 1.35 mg g⁻¹ with a desorption rate of < 20%. There were no obvious differences among the effects of aeration, water cleaning, drained reoxygenation and steam stripping for zeolite regeneration (adsorption capacity of >50%). However, the drained reoxygenation performance of was better due to zero energy consumption and regeneration in situ.