Improving urban wastewaters using natural microbial communities in bench-scale reactors.

Discharges from small urban wastewater facilities serving fewer than 10,000 population equivalents require improved effluent quality. Under the revised Urban Wastewater Treatment Directive (2024/3019), these facilities are not required to control nutrients, faecal contamination, or certain contaminants of emerging concern (CECs). This study evaluated the performance of benthic and planktonic microbial communities, operating independently and in combination, under controlled laboratory conditions to improve secondary-treated effluent. The use of microbial inoculum sourced from wastewater-impacted ecosystems was also assessed to reduce potential operational costs. Benthic compartments alone and combined benthic-planktonic systems achieved significant reductions in Escherichia coli (> 2 log), nitrogen (83-97%), and phosphorus (70-75%). Fluoxetine and citalopram showed greater removal than other CECs, although variability among treatments was high. The system performance was associated with algal biomass, total organic matter content, and microalgal abundances. These results highlight the experimental approach as a scalable pilot for tertiary treatment in small facilities.