Abstract
Ozonation is increasingly applied in water and wastewater treatment for the abatement of micropollutants (MPs). However, the transformation products formed during ozonation (OTPs) and their fate in biological or sorptive post-treatments is largely unknown. In this project, a high-throughput approach, combining laboratory ozonation experiments and detection by liquid chromatography high-resolution mass spectrometry (LC-HR-MS/MS), was developed and applied to identify OTPs formed during ozonation of wastewater effluent for a large number of relevant MPs (total 87). For the laboratory ozonation experiments, a simplified experimental solution, consisting of surrogate organic matter (methanol and acetate), was created, which produced ozonation conditions similar to realistic conditions in terms of ozone and hydroxyl radical exposures. The 87 selected parent MPs were divided into 19 mixtures, which enabled the identification of OTPs with an optimized number of experiments. The following two approaches were considered to identify OTPs. (1) A screening of LC-HR-MS signal formation in these experiments was performed and revealed a list of 1749 potential OTP candidate signals associated to 70 parent MPs. This list can be used in future suspect screening studies. (2) A screening was performed for signals that were formed in both batch experiments and in samples of wastewater treatment plants (WWTPs). This second approach was ultimately more time-efficient and was applied to four different WWTPs with ozonation (specific ozone doses in the range 0.23-0.55 gO3/gDOC), leading to the identification of 84 relevant OTPs of 40 parent MPs in wastewater effluent. Chemical structures could be proposed for 83 OTPs through the interpretation of MS/MS spectra and expert knowledge in ozone chemistry. Forty-eight OTPs (58%) have not been reported previously. The fate of the verified OTPs was studied in different post-treatment steps. During sand filtration, 87-89% of the OTPs were stable. In granular activated carbon (GAC) filters, OTPs were abated with decreasing efficiency with increasing run times of the filters. For example, in a GAC filter with 16,000 bed volumes, 53% of the OTPs were abated, while in a GAC filter with 35,000 bed volumes, 40% of the OTPs were abated. The highest abatement (87% of OTPs) was observed when 13 mg/L powdered activated carbon (PAC) was dosed onto a sand filter.
Highlights
Oxidative treatment such as ozonation has been applied in water treatment for decades
Since assimilable organic carbon (AOC) or biodegradable dissolved organic carbon (BDOC), which is formed during ozonation, is usually well-degraded in the biological post-treatment, it can be assumed that a significant portion of the induced toxicity is related to compounds contributing to these bulk parameters (Zimmermann et al 2011)
OTP identification is simplified if experimental solutions are free of DOM
Summary
Oxidative treatment such as ozonation has been applied in water treatment for decades. Some studies showed an increased toxicity after ozonation (Magdeburg et al 2014, Stalter et al 2010), and it is unknown to what extent transformation products from MPs or by-products formed during oxidation of matrix components (i.e., dissolved organic matter, DOM) are involved in these observations (Lee and von Gunten 2016, Mestankova et al 2011, Mestankova et al 2014, Mestankova et al 2012, von Gunten 2018). Since assimilable organic carbon (AOC) or biodegradable dissolved organic carbon (BDOC), which is formed during ozonation, is usually well-degraded in the biological post-treatment, it can be assumed that a significant portion of the induced toxicity is related to compounds contributing to these bulk parameters (Zimmermann et al 2011). It is recommended to test the feasibility of ozonation at a specific WWTP, before its installation, with a test procedure including chemical and ecotoxicological parameters (Schindler Wildhaber et al 2015)
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