An experiment conducted by the Urban River Lab team has demonstrated an exceptional effectiveness in removing pollutants from water. Specifically, the microbial communities that developed in the sediments have shown a great ability to retain nitrates, a nutrient found in excess in Mediterranean rivers due, among other reasons, to wastewater effluents. This finding is of interest for designing nature-based solutions that complement the water treatment work performed at treatment plants. With additional purification, water reaches ecosystems of higher quality, thus improving their ecological status.
The study, led by scientific-technical staff from the Blanes Centre for Advanced Studies (CEAB-CSIC) in collaboration with the University of Barcelona, Naturalea Conservación, and the Besòs-Tordera Consortium, was carried out over three months.
The research was conducted using the facilities of the Urban River Lab, an open-air laboratory located at the Montornès del Vallès Wastewater Treatment Plant (EDAR). In this scientific infrastructure—with channels and artificial ponds fed by treated wastewater from the EDAR—various river conditions, particularly those of urban and Mediterranean rivers, are simulated to obtain scientific evidence that can guide conservation and restoration initiatives.
This study focused on testing the filtering capacity of three native riparian plant species—Iris pseudacorus, Cornus sanguinea, and Alnus glutinosa—to remove pollutants, primarily dissolved organic carbon, phosphorus, ammonium, and nitrate. The trials were conducted under varying water conditions with intermittent flow, simulating the real dynamics of Mediterranean rivers, where periods of drought—during which most of the input comes from wastewater effluents—alternate with significant and sudden flows. Each channel in the Urban River Lab was planted with one of the three species (three channels per species). Additionally, three channels were left without vegetation, containing only sediment.
Encouraging results for nutrient retention
Unlike previous experiments conducted in the same channels, which showed a strong ability to remove ammonium, this time the results revealed a significant capacity to eliminate nitrate. This ability is mainly attributed to the processes of the microbial communities living on the stones rather than the activity of the selected plants. This phenomenon can be explained by the exceptional anoxic conditions created in the sediment areas, which means an almost total absence of oxygen. Thus, the results obtained in recent years at the URL suggest that combining planted areas that promote sediment oxygenation with unplanted zones to stimulate anoxic microbial processes could be excellent candidates for creating/designing nature-based solutions aimed at treating wastewater.
Sara Castelar, a technician at CEAB-CSIC and the Urban River Lab, explained: “We were surprised by the exceptional capacity of microbial communities to retain nitrates, close to 90%. In previous experiments, similar nitrate removal percentages were achieved using waste from the brewing industry, but this time it was done naturally thanks to using a much smaller and more heterogeneous substrate. This allowed for the creation of exceptional physicochemical conditions in the sediments that stimulated the activity of the microbial communities that developed. Once again, the results point to the idea of combining different natural elements, including microbial communities, as each is more efficient at retaining different nutrients found in excess in wastewater inputs.”
One innovative aspect of this study was the simulation of intermittent flow, which reflects the natural seasonal patterns of Mediterranean rivers and their vulnerability to various forms of pollution. The results revealed that nutrient removal decreased over time, suggesting that hydrological fluctuations could play a role in maintaining retention capacity.
The research provides valuable information for the design and implementation of nature-based solutions that complement wastewater treatment by EDAR. These solutions are necessary in a context where traditional treatment systems, while meeting legal standards, still have a significant impact on receiving rivers because these typically carry little water due to the Mediterranean climate’s characteristics. This issue is particularly serious in the context of climate change, as all models indicate that it will exacerbate the duration and intensity of drought periods in the medium and long term.
Additional Information:
*The experiment was conducted as part of the Riparian EcoRest project, promoted by the University of Barcelona in collaboration with Naturalea Conservación, the Besòs-Tordera Consortium, and the Blanes Centre for Advanced Studies (CEAB-CSIC). It is also part of the Master’s Final Project of Gabino José Carballo Pérez. Image credits accompanying the news: M. Ribot.
