To protect the ecosystem and drinking water resources in Switzerland and in the countries of the downstream catchments, a new Swiss water protection act entered into force in 2016 aiming to reduce the discharge of micropollutants from wastewater treatment plants (WWTPs). As a consequence, selected WWTPs must be upgraded by an advanced treatment for micropollutant abatement with suitable and economic options such as (powdered) activated carbon treatment or ozonation. WWTP Neugut (105'000 people equivalent) was the first WWTP in Switzerland to implement a long-term full-scale ozonation. Differing specific ozone doses in the range of 0.35-0.97 g O3/g DOC were applied to determine the adequate ozone dose to fulfill the requirements of the Swiss water protection act. Based on this assessment, a specific ozone dose of 0.55 g O3/g DOC is recommended at this plant to ensure an average abatement of the twelve selected indicator substances by ≥80% over the whole treatment. A monitoring of 550 substances confirmed that this dose was very efficient to abate a broad range of micropollutants by >79% on average. After ozonation, an additional biological post-treatment is required to eliminate possible negative ecotoxicological effects generated during ozonation caused by biodegradable ozonation transformation products (OTPs) and oxidation by-products (OBPs). Three biological treatments (sand filtration, moving bed, fixed bed) and granular activated carbon (GAC, fresh and pre-loaded) filtration were evaluated as post-treatments after ozonation. In parallel, a fresh GAC filter directly connected to the effluent of the secondary clarifier was assessed. Among the three purely biological post-treatments, the sand filtration performed best in terms of removal of dissolved organic carbon (DOC), assimilable organic carbon (AOC) and total suspended solids (TSS). The fresh activated carbon filtration ensured a significant additional micropollutants abatement after ozonation due to sorption. The relative abatement of the indicator substances ranged between 20 and 89% after 27'000 bed volumes (BV) and was still substantial at 50'000 BV. In an identical GAC filter running in parallel and being fed with the effluent of the secondary clarifier, the elimination was less efficient. Seven primary OTPs (chlorothiazide and six N-oxides) formed during ozonation could be quantified thanks to available reference standards. Their concentration decreased with increasing specific ozone doses with the concomitant formation of other OTPs. The seven OTPs were found to be stable compounds and were not abated in the biological post-treatments. They were sorbed in the fresh GAC filter, but less efficiently than the corresponding parent compounds. Two OBPs, bromate (BrO3-) and N-nitrosodimethylamine (NDMA), were formed during ozonation but did not exceeded 5 μg/L for bromate and 30 ng/L for NDMA at the recommended specific ozone dose of 0.55 g O3/g DOC. NDMA was well abated in all post-treatments (minimum 41% during fixed bed filtration, maximum 83% during fresh GAC filtration), while bromate was very stable as expected.

The efficiency of ozone-based processes under various conditions was studied for the treatment of a surface water (Lake Zürich water, Switzerland) spiked with 19 micropollutants (pharmaceuticals, pesticides, industrial chemical, X-ray contrast medium, sweetener) each at 1 μg L-1. Two pilot-scale ozonation reactors (4-5 m3 h-1), a 4-chamber reactor and a tubular reactor, were investigated by either conventional ozonation and/or the advanced oxidation process (AOP) O3/H2O2. The effects of selected operational parameters, such as ozone dose (0.5-3 mg L-1) and H2O2 dose (O3:H2O2 = 1:3-3:1 (mass ratio)), and selected water quality parameters, such as pH (6.5-8.5) and initial bromide concentration (15-200 μg L-1), on micropollutant abatement and bromate formation were investigated. Under the studied conditions, compounds with high second-order rate constants kO3>104 M-1 s-1 for their reaction with ozone were well abated (>90%) even for the lowest ozone dose of 0.5 mg L-1. Conversely, the abatement efficiency of sucralose, which only reacts with hydroxyl radicals (OH), varied between 19 and 90%. Generally, the abatement efficiency increased with higher ozone doses and higher pH and lower bromide concentrations. H2O2 addition accelerated the ozone conversion to OH, which enables a faster abatement of ozone-resistant micropollutants. Interestingly, the abatement of micropollutants decreased with higher bromide concentrations during conventional ozonation due to competitive ozone-consuming reactions, except for lamotrigine, due to the suspected reaction of HOBr/OBr- with the primary amine moieties. In addition to the abatement of micropollutants, the evolution of the two main transformation products (TPs) of hydrochlorothiazide (HCTZ) and tramadol (TRA), chlorothiazide (CTZ) and tramadol N-oxide (TRA-NOX), respectively, was assessed by chemical analysis and kinetic modeling. Both selected TPs were quickly formed initially to reach a maximum concentration followed by a decrease of their concentrations for longer contact times. For the studied conditions, the TP's concentrations at the outlet of the reactors ranged from 0 to 61% of the initial parent compound concentration, CTZ being a more persistent TP against further oxidation than TRA-NOX. Finally, it was demonstrated in both reactors that the formation of bromate (BrO3-), a potentially carcinogenic oxidation by-product, could be controlled by H2O2 addition with a general improvement on micropollutant abatement. Post-treatment by granular activated carbon (GAC) filtration enabled the reduction of micropollutants and TPs concentrations but no changes in bromate were observed. The combined algae assay showed that water quality was significantly improved after oxidation and GAC post-treatment, driven by the abatement of the spiked pesticides (diuron and atrazine).

The efficiency of wastewater ozonation for the abatement of three nitrogen-containing pharmaceuticals, two antihistamine drugs, cetirizine (CTR) and fexofenadine (FXF), and the diuretic drug, hydrochlorothiazide (HCTZ), was investigated. Species-specific second-order rate constants for the reactions of the molecular, protonated (CTR, FXF) or deprotonated (HCTZ) forms of these compounds with ozone were determined. All three compounds are very reactive with ozone (apparent second order rate constants at pH 7: kO3,pH7 = 1.7·10(5) M(-1)s(-1), 8.5·10(4) M(-1)s(-1) and 9.0·10(3) M(-1)s(-1) for CTR, HCTZ and FXF, respectively). Transformation product (TP) structures were elucidated using liquid chromatography coupled with high-resolution tandem mass spectrometry, including isotope-labeled standards. For cetirizine and hydrochlorothiazide 8 TPs each and for fexofenadine 7 TPs were identified. The main TPs of cetirizine and fexofenadine are their respective N-oxides, whereas chlorothiazide forms to almost 100% from hydrochlorothiazide. In the bacteria bioluminescence assay the toxicity was slightly increased only during the ozonation of cetirizine at very high cetirizine concentrations. The main TPs detected in bench-scale experiments were also detected in full-scale ozonation of a municipal wastewater, for >90% elimination of the parent compounds.