The efficiency of a two-step ozonation with regard to disinfection and oxidation of micropollutants was assessed for a river-water treatment plant (Limmat River, Zürich, Switzerland). The assessment was based on laboratory experiments to estimate transient ozone and OH radical concentrations coupled with hydraulic modelling of the ozone reactors. The laboratory experiments were performed for various temperatures, ozone dosages and pretreatments to mimic the full-scale treatment. The hydraulics were determined by a conservative tracer test. The kinetic data for disinfection and oxidation were taken from the literature. The inactivation of microorganisms (Cryptosporidium parvum oocysts, Bacillus subtilis spores, Giardia lamblia cysts and polio virus) was more critical at low temperature (5°C) than at high temperature (20°C). For Cryptosporidium parvum oocysts it was calculated to be less than 1 log for the two ozonation steps, and an overall ozone dose of 1.5 mg/l (5°C). For the same microorganism it was >3.5-log inactivation at 20°C. For other microorganisms, the calculated inactivation was ≥2 log (5°C) with B. subtilis spores being the most resistant. The oxidation of micropollutants was much less affected by variations in the temperature. The fraction of oxidation increased in the following order: between 15 and 25% of tetrachloroethene, 15 and 25% of methyl tertiary butyl ether, 30 and 40% of atrazine, 80 and 90% of p-xylene and between 97 and 100% of trimethylbenzene were oxidized. For the oxidation of methyl tertiary butyl ether, the formation of degradation products was modelled in the pre-ozonation step. For all investigated treatment conditions the overall bromate formation was ≤2.5 μg/l.

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