Recommended parameter combination of free water surface flow constructed wetlands based on seasonal fluctuation of hydraulic performance and purification effect

Free water surface flow constructed wetlands (FWS CWs) are widely utilized for nonpoint source pollution control and are greatly affected by seasonal variation. One of the key challenges is optimizing wetland design to ensure consistent performance throughout the year. To date, the impact of hydraulic performance on pollutant purification remains debated, and studies examining the correlation between their seasonal fluctuations are limited. This study conducted orthogonal experiments involving three categories of wetland parameters: geometric, hydraulic, and water quality parameters, to analyze the seasonal variability of the two wetland performance metrics. The results indicated that hydraulic performance presented high consistency across seasons and was more influenced by geometric and hydraulic parameters. Generally, FWS CWs with lower water depths and larger aspect ratios exhibited superior hydraulic performance. The purification effect for nitrogen showed obvious seasonal fluctuations, whereas phosphorus purification remained relatively stable. The concentration reduction efficiencies (CREs) of total nitrogen (TN) in Jun 2023, Sep 2023, and Dec 2023 were 16 % (±10 %), 19 % (±8 %), and 1 % (±9 %), respectively; for total phosphorus (TP), the CREs were 27 % (±11 %), 28 % (±16 %), and 16 % (±11 %). Both TN and TP CREs exhibited strong negative correlations with flow rate and influent nitrogen concentration, while load removal rates demonstrated strong negative correlations with water depth. There were few significant correlations between hydraulic performance and purification effect. It is recommended to use a parameter combination of a small aspect ratio, low water depth, and low flow rate to achieve stable purification effects year-round.