Abstract
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 (R2 = 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.
Highlights
In recent years, with the development of the national social economy, the contradiction between the construction of material civilization and the development of the water environment has become increasingly prominent
The concentration of total nitrogen (TN) in the multistage constructed wetland-pond system (MCWP) gradually decreased from 3.46 mg/L to 2.04 mg/L, and the average removal efficiency of TN was approximately 40.74%
When the surface flow constructed wetland (SFCW) was used for advanced treatment, a suitable high-density planting method could effectively increase the proportion of nutrients removed by plant harvesting and enhance the level of pollutant removal
Summary
With the development of the national social economy, the contradiction between the construction of material civilization and the development of the water environment has become increasingly prominent. According to monitoring data from recent decades, nitrogen concentrations in rivers throughout the world are increasing annually (Domangue and Mortazavi, 2018). The problem of nitrogen pollution in water, whether from the perspective of ecological balance or the impact on human health, merits more attention (Zhang et al, 2015; Smith et al, 2015). Single-stage constructed wetlands (CWs) has a single ecological service function and is greatly affected by temperature, which are general in removal of total nitrogen. Multistage hybrid CWs were proven to capable of enhancing removal of nitrogen. This study aimed to explore the variation in nitrogen removal in the combined CWs-pond process from summer to winter and the contribution of plant harvesting and the functions of bacteria to nitrogen removal
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