Impact of seasonal climate variability on constructed wetland treatment efficiency

Free-water surface constructed wetlands (CWs) are sustainable, low emission, nature-based solutions for water and wastewater treatment. However, the discharge of nutrient-rich effluents from CWs treating wastewater can adversely impact freshwater ecosystems and exacerbate eutrophication. Despite their ecological benefits, limited research exists on the treatment efficiency and pollutant dynamics of CWs under varying seasonal and environmental pressures. This study investigates the treatment efficiency of an integrated CW (ICW) serving as a nature-based solution for treating partially treated wastewater before release into the environment. Located in Ingoldisthorpe, Norfolk, near the East coast of the UK, the ICW receives 1014 ± 538 m 3 /day of effluent from a wastewater treatment plant (WWTP). The system comprises four interconnected vegetated ponds (i.e. Cell) with a mean effective volume of 2697 m 3 , operating at an average depth of 0.19 m. Seasonal variations in vegetation density and coverage range from sparse in winter and spring to dense in summer and autumn. Bi-monthly field investigations were conducted over one year (August 2022–June 2023) to examine the impacts of inter-seasonal climate variability on the ICW's treatment performance. Removal rates of solute and solid pollutants, including nitrate (NH 3 − ), nitrate‑nitrogen (NH 3 − N), ammonium (NH 4 + ), total nitrogen (TN) orthophosphate (PO 4 3− ), sulphate (SO 4 2− ), non-purgeable total organic carbon (NPOC), total inorganic carbon (TIC), and total solids (TS), were quantified. Significant seasonal variations were observed in Concentration Removal Rates (CRR) and Mass Removal Rates (MRR) for all nutrients. Nitrate CRRs ranged from −39.1 % to +51.64 %, corresponding to reductions of up to 14.57 mg/L and increases of 26.71 mg/L in effluent concentrations, while MMRs varied between −77.13 % to +84.25 %, reflecting changes of −38.93 kg/day to +26.69 kg/day. For phosphate, CRRs ranged from −22.79 % to +2.57 %, and MMRs ranged from −71 % to +93.22 %, equivalent to −0.57 kg/day to +0.26 kg/day. These findings highlight the dynamic and sensitive mechanisms influencing nutrient removal in CWs, driven by seasonal hydraulic conditions, vegetation phenology, and climatic factors. The study provides critical insights for optimizing CW design and management under fluctuating environmental conditions to enhance their resilience, ensure regulatory compliance, and maintain long-term treatment efficiency. This understanding is essential for guiding future regulatory policies and ensuring that CWs meet water quality standards in response to climate pressures. Effects of inter-seasonal climate variability on pollution treatment efficiency in integrated free surface water constructed wetland. • Seasonal variations of nutrient load and concentration of a constructed wetland is obtained. • Nutrient retention is influenced by vegetation growth and short-term climatic events. • Diurnal environmental and vegetation fluctuations impact nutrient treatment. • Environmental extremes disrupt treatment efficiency of CWs. • Vegetation density and spatial distribution are pivotal in shaping treatment performance of CWs.