Abstract
Micropollutants present in the effluent of wastewater treatment plants (WWTPs) after biological treatment are largely eliminated by effective advanced technologies such as ozonation. Discharge of contaminants into freshwater ecosystems can thus be minimized, while simultaneously protecting drinking water resources. However, ozonation can lead to reactive and potentially toxic transformation products. To remove these, the Swiss Federal Office for the Environment recommends additional "post-treatment" of ozonated WWTP effluent using sand filtration, but other treatments may be similarly effective. In this study, 48 h composite wastewater samples were collected before and after full-scale ozonation, and after post-treatments (full-scale sand filtration, pilot-scale fresh and pre-loaded granular activated carbon, and fixed and moving beds). Ecotoxicological tests were performed to quantify the changes in water quality following different treatment steps. These included standard in vitro bioassays for the detection of endocrine, genotoxic and mutagenic effects, as well as toxicity to green algae and bacteria, and flow-through in vivo bioassays using oligochaetes and early life stages of rainbow trout.Results show that ozonation reduced a number of ecotoxicological effects of biologically treated wastewater by 66 - 93%: It improved growth and photosynthesis of green algae, decreased toxicity to luminescent bacteria, reduced concentrations of hormonally active contaminants and significantly changed expression of biomarker genes in rainbow trout liver. Bioassay results showed that ozonation did not produce problematic levels of reaction products overall. Small increases in toxicity observed in a few samples were reduced or eliminated by post-treatments. However, only relatively fresh granular activated carbon (analyzed at 13,000 - 20,000 bed volumes) significantly reduced effects additionally (by up to 66%) compared to ozonation alone. Inhibition of algal photosynthesis, rainbow trout liver histopathology and biomarker gene expression proved to be sufficiently sensitive endpoints to detect the change in water quality achieved by post-treatment.
Highlights
Conventional wastewater treatment plants (WWTPs) discharge a wide range of organic and inorganic micropollutants into surface waters (Loos et al, 2013; Schwarzenbach et al, 2006)
When tested at higher relative enrichment factor (REF), several samples showed mutagenicity: The Ames fluctua tion assay with strain TA100 without S9 showed mutagenicity at REF 10 and 20 and probable mutagenicity at REF 20 in two of 12 samples tested (OZ and Sand filters (SF) (Campaign 1)) (SI Table S10, Fig. S3), while no mutagenicity was detected with strain TA100 with S9 (SI Table S11)
These results suggest that ozonation at WWTP Neugut occasionally produced mutagenic compounds, which were removed by fixed bed (FB), moving bed (MB), GACfresh and GACloaded, but not completely by SF
Summary
Conventional wastewater treatment plants (WWTPs) discharge a wide range of organic and inorganic micropollutants into surface waters (Loos et al, 2013; Schwarzenbach et al, 2006). The largest WWTPs and those discharging to lakes or watercourses with a high proportion of wastewater must be upgraded using advanced technologies such as activated carbon treat ment or ozonation Both advanced treatment methods are highly effec tive in removing micropollutants and their ecotoxicological effects still present in wastewater after conventional biological treatment (Escher et al, 2008b; Margot et al, 2013; Prasse et al, 2015; Stalter et al, 2013; Volker et al, 2019). Full-scale ozonation is implemented internationally as an advanced wastewater treatment technique, e.g. in Germany (Dopp et al, 2021; Wolf et al, 2022) and Canada (Maya et al, 2018; Nasu hoglu et al, 2018)
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