The ecological gain of using bioactivated carbon and ozone sewage treatments assessed using freshwater microcosms

Advanced wastewater treatment technologies such as bioactivated carbon (BAC) filtration and ozonation are increasingly implemented to remove micropollutants from effluents. Yet their ecological impacts remain insufficiently understood. In this study, we evaluated the chemical and ecological effects of six wastewater treatment scenarios in outdoor freshwater microcosms over 42 days within the BO₃ treatment framework involving BAC filtration and ozonation, including BAC alone, two ozone doses (0.2 and 0.4 g O₃/g TOC), their combinations, and an untreated control. HPLC-MS/MS analysis confirmed the high persistence of micropollutants in untreated effluents, which showed the highest predicted mixture toxicity throughout the exposure. Treatments combining BAC with ozone resulted in the lowest residual micropollutant concentrations, which continued to decline over time. Ecological responses differed across trophic levels. Algal biomass was reduced in ozone-treated effluents, reflecting combined effects of altered nutrient supply, resource availability and residual chemical stress, whereas zooplankton and macroinvertebrate communities remained structurally robust. Microbial communities in the water column showed pronounced treatment-dependent restructuring, and functional prediction indicated reduced representation of taxa involved in pollutant degradation and nitrogen cycling under high ozone conditions. Structural equation modeling supported these patterns by showing that early changes at the base of the food web were linked to shifts in micropollutant levels and nutrient availability, while pathways to higher trophic levels remained weak and did not propagate beyond microbes and primary producers. Overall, combining BAC with moderate ozonation offered the most favourable balance between micropollutant removal and ecological compatibility, whereas high ozone intensity introduced discernible functional impact to microbial communities. These findings highlight the importance of jointly considering chemical removal performance and ecological responses when selecting advanced wastewater treatment strategies.