Ecotoxicological assessment of advanced treatment processes in German municipal wastewater treatment plants

Abstract

During the last years, surface water bodies have been under pressure worldwide. In Europe just 42% of surface water bodies achieved a good ecological potential and chemical status and specifically in Germany these percentage is lower with just 9% of its streams achieving the threshold. Numerous studies have shown that chemical pollution, mainly resulting from human activities, is a leading cause of poor ecological and chemical status in aquatic systems, posing risks not only to ecosystems but also to human health. This chemical pollution can enter the aquatic ecosystem through point and non-point sources such as municipal wastewater treatment plants (WWTPs), effluents from industries, agricultural runoff and rain events (REs). Among these sources of contamination, there is growing concern about certain compounds that were not previously considered pollutants but are now known to have significant environmental and human health impacts. These substances, known as micropollutants (MPs), can cause harmful effects even at very low concentrations (µg/L or ng/L). For this reason, products such as pharmaceuticals, personal care products, and pesticides are under increasing scrutiny and ongoing scientific investigation, as they have been linked to toxic, mutagenic, and endocrine-disrupting effects in various species. Although some industrial sectors have made efforts to reduce the load of MPs in their production processes, and certain measures have been taken at the societal level to decrease their consumption, the most effective way to address these compounds remains through treatment at WWTPs. However, conventional treatment processes alone are not sufficient to mitigate the impact of these contaminants, making the implementation of advanced treatment systems essential to ensure their effective removal. In this context, advanced systems using ozone and activated carbon have gained considerable relevance, as they have proven to be effective at removing MPs at both laboratory and full-scale levels. Nevertheless, when using ozonation, it is important to consider that it can generate transformation products (TPs) that are potentially more toxic than the parent compounds. Therefore, it is recommended to use combined systems with activated carbon, such as granular activated carbon (GAC) or powdered activated carbon (PAC) filters, to ensure that, if such by-products are formed, they can be adsorbed onto the surface of the carbon and prevented from entering natural water bodies. For that reason, analyzing the effectiveness of these treatments is crucial to ensure that the systems are operating properly and effectively reducing the contaminant load in the effluent. In this study, a comprehensive approach was developed to evaluate the performance of the advanced treatment process using effect-based methods (EBMs), which can integrate the effects of chemical mixtures, including substances not directly analyzed, identifying specific toxicity drivers, and linking chemical exposure to biological effects. Two full-scale WWTPs located in Mörfelden-Walldorf (MW) and Bickenbach (B) were evaluated across different seasons under conventional treatment. In the following year, MW was further investigated after implementation of an advanced treatment process with ozonation, followed by PAC. To assess the removal of biological activities, a battery of in vitro and in vivo bioassays was applied, targeting several endpoints: baseline (non-specific) toxicity using the Microtox assay with Aliivibrio fischeri, mutagenicity with the Ames fluctuation assay (Salmonella typhimurium), endocrine and dioxin-like activities using recombinant yeast reporter gene assays (Saccharomyces cerevisiae), and survival and reproduction through active biomonitoring with the freshwater amphipod Gammarus fossarum and the mudsnail Potamopyrgus antipodarum. Additionally, a screening of 17 selected MPs was conducted during the advanced treatment phase, alongside a macroinvertebrate community analysis in the receiving water body downstream of the WWTP. Following the field campaigns, we conducted a laboratory-scale experiment using our in-house ozone reactor for an advanced treatment of the conventionally treated effluents from both WWTPs, applying a single ozone dose higher than the full-scale dose in MW. During this project, we could demonstrate that conventional treatment processes were insufficient to reduce the biological effects observed in the receiving water body, both in laboratory-based assays and in the biomonitoring study. Additionally, it was clear that external sources of contamination must be considered to fully understand the situation around the WWTPs. This was particularly noticeable in MW, where a WWTP located 10 km upstream in Langen contributed significantly to the biological activities detected downstream. However, after implementing the combined advanced treatment process at MW, a clear reduction in MP concentrations and biological activities was observed in the effluent compared to the previous conventional process. In fact, the macroinvertebrate community analysis, conducted just eight months after implementation, already showed signs of ecological improvement. Nonetheless, some endpoints in the EBM battery indicated that the applied ozone dose (0.1 mg O₃/mg DOC) was sub-optimal, and further optimization was necessary.