Enhancing urban wastewater treatment through nature-based solutions: The role of biofilm–plant development in subsurface retention of multiple solutes

Effluents from wastewater treatment plants (WWTPs) deliver a complex cocktail of nutrients and contaminants of emerging concern (CECs) to freshwater systems. These impacts are especially pronounced in water-scarce regions like the Mediterranean, where pollutants can persist due to a lack of dilution. Nature-based solutions (NBS) aim to reduce pollutant concentrations by exposing water to biofilms and plant roots along subsurface flow paths. We conducted an outdoor flume experiment at the Urban River Lab (NE Spain) using treated WWTP effluent in replicated 12-m flumes with fine-grained sediments during the growing season (May–July). Nutrient and CEC retention was assessed under four settings: i) unplanted sediments, ii) sediments planted with Iris pseudacorus , iii) with Cornus sanguinea , and iv) with Alnus glutinosa . Nitrate was consistently retained, likely through denitrification under hypoxic conditions with ample dissolved organic carbon. In contrast, ammonium and soluble reactive phosphorus retention declined over time, with net release by the end of the experiment, likely due to mineralization and clogging. Among the studied CECs, eight were retained (e.g., PEG n5, MEHP, dimethylbenzotriazole), likely via microbial degradation or sorption, whereas five (e.g., diuron, sulfamethoxazole, DEET) were released, possibly due to recalcitrance or formation as degradation byproducts. Water travel time increased over the season but was not positively related to net nutrient retention. Vegetation effects were limited, indicating a dominant role of microbial activity. Our findings highlight the potential of bioengineered systems to retain a broad range of solutes, while also revealing that NBS are not universally effective for all compounds. • In flume experiments, NO 3 − was consistently retained, likely via denitrification. • NH₄ + and SRP tended to be released, likely due to mineralization and clogging. • 8 of 36 contaminants of emerging concern were partially retained in the system • Microbial activity, not vegetation, mainly drove solute retention • Nature-based solutions reduce some but not all contaminants in treated wastewater