Question

What is the effect of high bromide concentrations in source water during ozonation, and what disinfection byproducts are of concern?

Accepted Answer

High bromide concentrations in source water during ozonation can lead to the formation of bromate, a regulated disinfection byproduct, as well as other brominated DBPs. Ozone (O3) is a powerful disinfectant used in water treatment. When ozone reacts with bromide ions (Br-) present in the water, it oxidizes them to form hypobromous acid (HOBr) and hypobromite ions (OBr-). These compounds can then react with natural organic matter (NOM) to form brominated DBPs, including bromoform, dibromochloromethane, and bromodichloromethane. However, the primary DBP of concern is bromate (BrO3-), which is formed through further oxidation of hypobromite ions by ozone. Bromate is a suspected carcinogen, and its concentration in drinking water is regulated by the EPA. The rate of bromate formation is influenced by several factors, including the bromide concentration, ozone dose, pH, temperature, and the presence of hydroxyl radicals. Higher bromide concentrations and ozone doses increase the rate of bromate formation. High pH also favors bromate formation. To minimize bromate formation during ozonation, several strategies can be employed: 1) Bromide Removal: Removing bromide from the source water before ozonation can prevent bromate formation. This can be achieved through ion exchange or reverse osmosis. 2) Ozone Dose Optimization: Optimizing the ozone dose to achieve the desired level of disinfection while minimizing the residual ozone concentration can reduce bromate formation. 3) pH Control: Maintaining a lower pH can help to reduce bromate formation. 4) Hydroxyl Radical Scavengers: Adding hydroxyl radical scavengers, such as ammonia or hydrogen peroxide, can reduce bromate formation by reacting with hydroxyl radicals that promote the oxidation of hypobromite ions to bromate. 5) Advanced Oxidation Processes (AOPs): Using AOPs, such as ozone with hydrogen peroxide or UV light, can reduce bromate formation by promoting the formation of less harmful DBPs. 6) Granular Activated Carbon (GAC) Filtration: GAC filtration can remove brominated DBPs that are formed during ozonation. For example, if a water treatment plant using ozonation detects elevated bromate levels, the operators should first check the source water for bromide concentration and then consider optimizing the ozone dose, controlling the pH, or adding a hydroxyl radical scavenger.

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What is the effect of high bromide concentrations in source water during ozonation, and what disinfection byproducts are of concern?