Nitrogen removal from summer to winter in a field pilot-scale multistage constructed wetland-pond system

A pilot-scale multistage constructed wetland-pond(MCWP) system with a "pre-ecological oxidation pond, two-stage horizontal subsurface flow constructed wetland(HSCW) and surface flow constructed wetland(SFCW) as the core and postsubmerged plant pond" as the process was used to treat actual polluted river water in the field, and the variation in nitrogen removal from summer to winter was investigated. The results showed that the average total nitrogen(TN) removal efficiency in the MCWP was approximately 40.74%. The significant positive correlation between the daily highest temperature and the TN removal efficiency of the whole system was fitted with a nonlinear curve(R~2= 0.7192). The TN removal load rate in the HSCWs was 2.7–3.7 times that in the SFCW. The SFCW, which had high-density plants(35 plants/m2), increased the proportion of nitrogen removed by plant harvesting and microbial function. The TN transformed by Iris pseudacorus L. accounted for 54.53% in the SFCW. Furthermore, bacteria completed the nitrogen cycle in the SFCW through a variety of nitrogen removal pathways. This research not only investigated the TN removal performance in an MCWP system but also made it possible to predict the TN removal efficiency according to the daily highest temperature from summer to winter in the field.