Assessing the effects of salinity on microbial communities and pollutant removal in urban wastewater treatment plants

This study assessed the impact of electrical conductivity (EC) on the microeukaryotic community and pollutant removal efficiency in full-scale wastewater treatment plants (WWTPs) in Catalonia, Spain. Monthly samples from seven WWTPs (2010−2021) were collected, with microbial communities identified by microscopy and effluent quality parameters measured. Linear mixed-effects models (LMER), indicator species analysis (IndVal), and structural equation modeling (SEM) were used to evaluate the effects of EC on WWTP operation and microbial communities. EC levels varied widely among WWTPs (800 to 15,000 μS cm −1 ), with mean removal rates generally low, not exceeding 26 %. EC removal was inversely correlated with organic matter removal ( p < 0.05). IndVal analysis revealed distinct microbial communities associated with each WWTPs along the EC gradient (e.g., Beggiatoa spp. and Epistylis sp. were associated with high and low EC, respectively). High EC and peaks negatively affected the abundance of certain microorganisms (e.g., Acineria uncinata ) ( p < 0.05). Ciliate genera were most affected by EC peaks, with different species showing different salinity tolerance ranges (e.g. Holophyra discolor was affected by EC > 3000 μS cm- 1 ). Seasonal variations did not significantly alter community sensitivity to salinity ( p > 0.05). LMER and SEM analyses revealed strong adverse effects of high EC and EC peaks on microbial diversity, richness, and the efficiency of organic matter and TKN removal ( p < 0.05). This highlights the importance of monitoring and controlling EC levels in WWTP influent to maintain optimal treatment performance and the need for effective technologies or biological processes to mitigate saline discharges into rivers. • This is the first full-scale study on the impact of salinity on WWTPs operation. • EC varied strongly among WWTPs, with distinct taxa associated with EC levels. • WWTPs showed low EC removal rates, averaging below 26 %. • EC was negatively correlated with microbial diversity and pollutant removal. • EC peaks had very strong negative effects on microeukaryotes and pollutant removal.