Comparative analysis of conventional and modified constructed wetlands for the removal of trace organic compounds from municipal wastewater effluent

In response to the revised European Urban Wastewater Treatment Directive (UWWTD), which mandates advanced treatment of trace organic compounds (TrOCs), this study evaluates the performance of vertical subsurface flow constructed wetlands (VSSF-CWs) enhanced with various adsorbents. Five configurations comprising of a conventional sand-based and four modified systems containing biochar, granular activated carbon (GAC) and natural zeolite were assessed for their ability to remove 13 priority TrOCs from municipal wastewater effluent. The highest average concentrations were found for diclofenac (1.97 ± 2.11 μg/L), candesartan (1.57 ± 0.45 μg/L), metoprolol (0.90 ± 0.19 μg/L) and 1H-benzotriazole (0.47 ± 0.22 μg/L) accounting for 79 % of all TrOCs measured in the effluent of the investigated wastewater treatment plant (WWTP). The conventional constructed wetland (CW) showed variable performance with maximum removals of metoprolol (86 %), clarithromycin (62 %), and diclofenac (48 %) while candesartan, carbamazepine and hydrochlorothiazide were not removed. The CWs modified with adsorbents consistently reduced all target TrOCs to concentrations below the limit of quantification, even under elevated hydraulic loading rates (330 L/m²·d), achieving removal efficiencies of above 99 % for diclofenac, 98 % for candesartan, and 88 % for sulfamethoxazole. The investigation of the vertical behaviour of bulk organics and nitrogen species, as well as the chemical properties of the selected TrOCs identified adsorption, biodegradation, and plant uptake as the primary removal mechanisms. Ecotoxicological risk assessment revealed that only the modified CWs reduced the hazard quotients for all target compounds to <0.1. These findings demonstrate that adsorbent-enhanced VSSF-CWs provide a robust, low-energy post-treatment option for mitigating TrOC emissions and reducing the environmental risk in compliance with the revised UWWTD.