What is the most common disinfectant used in water treatment?

• A. Ozone
• B. Ultraviolet
• C. Chlorine and derivatives
• D. Hydrogen peroxide

Answer: (C)

The key idea is that a disinfectant that leaves a lasting residual in the treated water is favored for municipal systems. Chlorine and its derivatives are the most common because they not only inactivate microbes at the treatment plant, but also remain in the water as a residual disinfectant as it travels through the distribution system. That ongoing residual protects against recontamination and provides continuous safety all the way to consumers. Chlorine is versatile and cost-effective, able to be added as gas or hypochlorite solution, and it effectively tackles bacteria and many viruses.

Other options have drawbacks as the sole disinfectant in distribution: ozone is powerful but expensive and does not leave a lasting residual; ultraviolet disinfects effectively but provides no residual once the water leaves the treatment facility; hydrogen peroxide isn’t typically used as the primary disinfectant in drinking water treatment. This is why chlorine and its derivatives are the standard choice.