Dynamics of pollution transport in constructed wetlands

The failure of wastewater treatment plants (WWTPs) to protect ecosystems through their continued discharge of excess pollutants into the environment necessitates a fundamental shift toward more sustainable technologies. Constructed wetlands (CWs) are a nature-based solution that reduce the environmental footprint of WWTPs and polish their effluent before being discharged into the environment. However, the treatment efficiency of full-scale CWs is currently unreliable due to a limited understanding of pollution transport dynamics, which has resulted in uninformed design and management. This thesis systematically addresses this challenge by investigating the physical mechanisms governing pollution transport and treatment of an integrated constructed wetland (CW) treating WWTP effluent. A two-year, inter-seasonal experimental campaign quantifies CW treatment efficiency, hydraulic performance, and optimisation effectiveness. Removal efficiencies of nitrate (NH3−), ammonium (NH4+), total nitrogen (TN), phosphate (PO43−), sulphate (SO42−), biological oxygen demand (BOD5), total organic carbon (TOC), total inorganic carbon (TIC), and total solids (TS) were quantified alongside climate, inflow, and physicochemical conditions. Suboptimal treatment efficiency is identified as a function of low temperatures, vegetation senescence, and precipitation-driven inflows, reflecting the fundamental and dynamic link between CW treatment mechanisms and the ambient environment. During this study, 44 fluorometric tracer tests and 27 hydraulic performance metrics were analysed alongside water quality, climate, and high-resolution vegetation LiDAR scanning to identify the factors influencing CW hydraulic behaviour. Pond geometry, inlet-outlet configuration, hydraulic loading, and vegetation placement were dominant mechanisms shaping flow patterns and pollution transport. While Cell hydraulic behaviour showed no strong relationship with treatment variability, the presence of design- and plant-driven suboptimal hydraulic phenomena, including short-circuits and dead zones, defined and limited the overall treatment capacity. Given these inefficiencies, this thesis, for the first time, quantified the effectiveness of three maintenance and optimisation methods on field-scale hydraulic and treatment performance. Baffle installation improved volume usage by 31%, vegetation scything temporarily increased hydraulic efficiency by 85%, while new planting was more effective for improving nutrient removal rates. This long-term investigation provides a new understanding of the dynamic efficacy of CWs, which are essential for management and future regulatory policies. CW microplastic transport dynamics are an important yet understudied factor despite their influx from WWTPs. For the first time, this thesis investigates the role of CW biofilm growth on the density-driven retention of positively buoyant microplastics, a mechanism observed in other environments. In-situ incubation of polyethylene, polypropylene, and polystyrene particles over 2–12 months revealed clear seasonally-dependent growth rates, being 217 to 1972% higher during spring and summer than in winter. However, there was no evidence of biofilm-induced microplastic retention, challenging previous assumptions. This thesis also examines CW sustainability from a greenhouse gas (GHG) perspective, and the first to quantify methane and carbon dioxide fluxes of a full-scale CW in England under varying climate conditions. While GHG fluxes at the water-air interface were comparable to a nearby natural pond, the study revealed significant temporal and spatial variation, with methane efflux 1642% higher in spring and summer, and the identification of flow velocity as a driver of diffusive flux highlights the potential for CW GHG mitigation through hydrodynamic optimisation. Overall, this thesis provides new empirical insights into CW efficacy, pollution transport dynamics, and sustainability, enabling a step change in environmental protection and integrated catchment management by identifying key mechanisms influencing treatment efficiency and proposing evidence-based optimisation strategies.