If drinking water or wastewater containing bromide is treated with ozone to remove micropollutants, bromate – a potentially carcinogenic substance – is formed. Eawag scientists have developed a new process which makes it possible to minimize bromate formation during ozonation. This is achieved by transferring ozone to the water in small doses through the pores of PTFE membranes. By Andres Jordi
Because a wide range of micropollutants are broken down by ozonation, this is often the method of choice for the treatment of drinking water or wastewater (see Box). Over the next 25 years, around a hundred wastewater treatment plants (WWTPs) across Switzerland are to be upgraded with an additional treatment step for micropollutant removal. Here, ozone is a suitable option, and an ozonation system can generally be integrated into existing treatment operations.
However, if water is contaminated with bromide, ozonation leads to the formation of (potentially carcinogenic) bromate, which then enters drinking water or is released into rivers and lakes with the treated wastewater. The tolerance value specified for bromate in drinking water in Switzerland is 10 micrograms per litre. While levels of bromide in drinking water are usually low, significant bromide concentrations may occur in wastewater if chemical plants or waste incinerators are present in the WWTP catchment area.
Ozone dosing methods
On the basis of the so-called peroxone (ozone/hydrogen peroxide) process, scientists at Eawag have now developed an ozonation method which allows bromate formation to be minimized. “Bromate formation can be reduced by keeping ozone concentrations in the water at a low level,” says project leader Urs von Gunten of the Water Resources & Drinking Water department. This can be achieved, he explains, by dosing ozone in very small portions, so that it is rapidly transformed into hydroxyl radicals. The addition of hydrogen peroxide accelerates this transformation process and suppresses the formation of bromate precursors (Fig. 2).