Which form of chlorine is typically used for primary disinfection in drinking water treatment?

• A. Chloramines
• B. Free chlorine
• C. Combined chlorine
• D. Chlorine dioxide

Answer: (B)

Free chlorine provides rapid inactivation of microorganisms and leaves a measurable residual to protect the water as it travels through the distribution system. The active species, hypochlorous acid and hypochlorite, kill pathogens quickly, which is essential for primary disinfection, and the residual chlorine helps prevent recontamination downstream. Other chlorine forms either disinfect more slowly or are kept in the system to maintain a longer-lasting residual rather than deliver the initial kill, so they’re not as suitable for the primary step.