Improving small urban wastewater treatment plants with a nature-based reactor for tertiary treatment

Urban wastewater treatment has significantly improved in recent decades, reducing the environmental impacts of their effluents and improving the chemical and ecological status of receiving water bodies. However, specific treatments, focused on nitrogen and phosphorus removal, have been implemented principally in large urban wastewater treatment plants (UWWTPs) serving over 10,000 population equivalents (P.E). In contrast, small UWWTPs (<10,000 P.E.) are generally not required to meet nutrient discharge limits despite the revised Urban Wastewater Treatment Directive. Nature-Based Solutions (NBS) offer cost-effective alternatives for small facilities as potential tertiary treatments. This research evaluated a pond-stream system, based on the biological activity of benthic (biofilms) and planktonic microbial communities (biofilm-plankton reactor, BPR), as an additional treatment step for activated sludge UWWTPs. The BPR achieved removal efficiencies for nitrogen (67.4 ± 11.1 %) and Escherichia coli (75.4 ± 37.3 %), while phosphorus, carbon, and targeted contaminants of emerging concern were highly variable and were not consistently removed. Microbial communities' structure and functions were assessed through algal biomass, stoichiometry, and extracellular enzymatic activities, providing a distinctive perspective into the BPR's microbial ecological dynamics related to removal efficiencies. Shotgun metagenomics identified a broad range of nitrogen functional genes, mainly involved in biodegradation and biosynthesis processes. This next-generation sequencing approach complemented conventional E. coli count methods, offering a deeper understanding of potential pathogen hotspots in treated effluents. Overall, the BPR system demonstrated a promising NBS for nitrogen and microbiological contaminant removal in small UWWTPs, whereas further investigation is needed to optimise the removal of other important water quality parameters.