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An important aspect of drinking water treatment is the balance between disinfection and disinfection byproducts (DBP) control. DBP such as trihalomethanes (THM), haloacetic acids (HAA) and bromate ion are potential carcinogens. The formation of these compounds with the use of drinking water disinfectants such as chlorine and ozone must be controlled. On the other hand, dangerous pathogens in drinking water can rapidly sicken or kill thousands of people if not controlled as well. So, finding the proper balance is indeed critical.
The US EPA has set limits for the levels of these compounds in drinking water as well as the as levels of certain pathogens typically found in water. Operators as a result try to use the minimum amount of disinfectant necessary to kill the pathogens while staying below the DBP limits.
Recently, there has been increased attention regarding the formation of bromate during ozone water treatment. The limit for bromate in drinking water is 10 ppb. Bromate formation during ozonation is affecting by the amount of ozone applied, pH, dissolved organic matter, temperature, bromide ion content of the water, and alkalinity. Out of these factors, pH and ozone dose are within the control of the operator. The other factors are function of water quality and not easily changed. The most easily applied rule is to minimize ozone dose while still achieving the disinfection objective.
It is fairly obvious that the more difficult to kill a pathogen is the more ozone would be required. One of the most difficult to kill pathogens is cryptosporidium. It goes to reason that if this compound can be killed in water containing bromide ion while staying below the 10 ppb limit for bromate, that it should be possible to use ozone to kill less difficult organisms at low bromate levels.
A study of 14 large drinking water plants using ozone with bromide ion in the water in 2001, showed that the majority could meet the disinfection objective for cryptosporidium while staying below the 10 ppb limit for bromate without any additional treatment steps in the process other than control of ozone dose and pH.
In cases where bromate formation will occur during ozonation, it is possible to control the levels below the 10 ppb target by the addition of chlorine or ammonia to the water. The presence of these compounds, often used in drinking water treatment prevents bromate formation in the presence of ozone. As a result, very few municipal WTP or bottled water producers should have issues with bromate formation during ozonation with proper control techniques.