Hydraulic efficiency and mixing dynamics in surface flow constructed wetlands: Influence of design, vegetation phenology, and climate variability

Surface flow constructed wetlands (CWs) are a nature-based wastewater treatment technology designed to serve as a buffer between wastewater treatment plants (WWTPs) and the receiving environment. While the treatment efficacy of CWs has been investigated, surface flow systems are susceptible to hydraulic inefficiencies, and a comprehensive understanding of the factors influencing pollutant transport remains limited, hindering their optimisation and predictability. The hydraulic performance of a CW, determined by the efficiency of flow hydrodynamics, dictates the residence time and spatial interactions between pollutant-laden water and purification mechanisms, such as those provided by vegetation and substrate. However, unstandardised designs and a limited understanding of water-sediment-plant interactions often result in sub-optimal hydraulic conditions, such as short-circuiting and dead zones, which impair treatment efficiency. This study investigates the influence of inter-seasonal climate variability, vegetation growth cycles, and operational conditions on the interplay between hydraulic performance, and subsequent pollution removal efficacy, in a full-scale integrated surface flow CW located in Norfolk, UK. Five tracer test campaigns were conducted during 2022–2023 using Rhodamine WT dye and fluorometric sensors to evaluate seasonal variations in hydraulic behaviour across four interconnected vegetated Cells. Hydraulic performance was characterised using indices such as mean residence time, tank-in-series model, hydraulic efficiency, effective volume ratio, short-circuiting, mixing, and dispersion coefficients. To understand the roles of CW design, operation, vegetation, and climate on hydraulic performance, high-resolution LiDAR vegetation scans, nutrient concentrations, and climate monitoring data were collected concurrently with the tracer tests. The combined mean residence time ranged from 30.03 h in autumn to 47.67 h in summer. Individual Cell hydraulic indexes revealed significant non-uniform flow patterns, with 80 % of tracer tests indicating poor ($\lambda$ < 0.5) hydraulic efficiency and 55 % exhibiting dead zones occupying >50 % of the Cell volume. These inefficiencies were predominantly driven by smaller Cell geometries, sub-optimal inlet-outlet configurations, high influent hydraulic loading rates (0.47 to 0.66 m³/day/m²), and high emergent vegetation cover. Despite these hydraulic deviations, Cell nutrient removal performance was more strongly influenced by vegetation growth stage and seasonal water physicochemical conditions. These findings provide novel field-scale data into how design, seasonal, and operational factors influence CW performance, highlighting the critical need for enhanced design and management strategies to optimise hydraulic and treatment performance in CWs, particularly in climate variability.