The most common means of disinfecting water mains is the use of

• A. Calcium Hypochlorite
• B. Chlorine gas
• C. Sodium hypochlorite
• D. Ozone

Answer: (A)

Maintaining a residual free chlorine in the distribution system is essential for ongoing disinfection of water mains. A chlorine source that is stable, easy to store, and simple to dose makes this practical on a wide scale. Calcium hypochlorite fits well because it is a stable solid that can be fed with straightforward equipment, stored safely, and dissolved quickly to release chlorine into the water. This combination allows utilities to achieve the needed initial disinfection and then maintain protection as water travels through the system. Chlorine gas, while effective, poses significant safety and handling challenges; sodium hypochlorite is a liquid with storage and dosing variability; and ozone does not provide a lasting residual in the distribution system, so it isn’t used for ongoing mains disinfection.