A common type of oxidant that is used at most SW treatment plants to kill bacteria and to prevent trihalomethane formation is:

The use of oxidants in water treatment is crucial for both disinfection and controlling the formation of harmful byproducts. In this context, the correct choice points to combined chlorine.

Combined chlorine refers to the chlorine that has already reacted with ammonia and organic compounds in the water, forming chloramines. These chloramines are effective in controlling bacteria and can provide a residual disinfecting effect throughout the distribution system. However, while they do help with disinfection, they are not as effective as free chlorine.

Total chlorine, which encompasses both free and combined chlorine, does not specifically target bacteria or the formation of trihalomethane (THM) as effectively as free chlorine does. Free chlorine is often preferred in initial disinfection processes due to its stronger oxidizing properties, but it can lead to the formation of undesirable THMs when reacting with certain organic materials in the water.

Chlorine dioxide is also a powerful oxidant and can be used for disinfection without creating THMs, but it’s not as commonly employed as combined chlorine in many surface water (SW) treatment facilities.

Understanding the nuances of chlorine usage in the treatment process, especially regarding trihalomethane formation, is key for operators to maintain both water safety and quality.