Removal Efficiency of Emerging Contaminants in Wastewater Treatment Plant Effluent by Gravel-Based Constructed Wetlands
Wastewater treatment plant (WWTP) effluent is a significant pathway for emerging contaminants (ECs) to enter natural water bodies. This study investigated the removal of ECs by two field-scale gravel-based constructed wetlands: a horizontal subsurface flow constructed wetland (QL-CW) and a surface flow constructed wetland (BL-CW), both treating actual WWTP effluent. The influence of operation mode and wetland plant type on EC removal was examined. Using liquid chromatography-mass spectrometry, principal component analysis, and ecological risk assessment, the removal efficiencies and mechanisms for various ECs were explored. In QL-CW, biodegradation was more pronounced, particularly via ammonia-oxidizing bacteria co-metabolism, favoring ECs with benzyl, secondary amine, secondary amide, tertiary amide, halogenated, and carboxyl functional groups. In BL-CW, electrostatic attraction and hydrophobic interactions were more significant, with plant and root-microorganism uptake and adsorption playing key roles. Surface flow mode achieved significantly higher removal of antibiotics (45.3% vs. 34.1%) compared to horizontal subsurface flow, while no significant differences were observed for non-antibiotic pharmaceuticals (66.6% vs. 64.4%) and pesticides (49.8% vs. 34.2%). Planting Cyperus alternifolius (windmill grass) was more beneficial for antibiotic removal (43.6% vs. 30.1%) than planting Ipomoea aquatica (water spinach). The wetlands effectively reduced the ecological risks of most ECs to marginal levels. This study provides insights into the deep treatment of ECs in WWTP effluent by constructed wetlands.