procedure sometimes follows the calcium hypochlorite method described above. Fill the main with water and then flush it to remove particulates. Apply a liquid chlorine solution while water is flowing through the system. Water flow during this process must be carefully regulated according to AWWA, state, or local standards.
Liquid chlorine solutions can be made by dissolving sodium hypochlorite or calcium hypochlorite in water; check the pesticide label for amounts to use. In addi- tion, you can inject pure liquid chlorine from a pressurized cylinder into the flowing water. However, you must use special injection equipment to prevent water from back flowing into the liquid chlorine cylinder. Water intrusion could cause severe cylinder corrosion and the escape of chlorine gas. This happens if the pressure in the water main exceeds the gas pressure in the chlorine cylinder. Therefore, injection equipment for compressed chlorine gas must consist of a vacuum device that pulls gas from the cylinder and mixes it with water in a separate container. The device then injects this mixture into the water main using a booster pump.
Chemical-feed pumps are also used to inject hypochlorite solutions into water mains. These pumps are powered by either gasoline engines or electric motors.
Continue injecting until monitoring indicates the system has the prescribed concentration of chlorine. At this point, stop the water and hold the solution in the water main for a period, usually 24 hours. State or local regulations usually specify the concentration and the amount of holding time. If regulations do not specify these criteria, follow AWWA specifications.
Slug Method The slug method of chlorination involves:
placing calcium hypochlorite granules into the main during construction
completely filling the main to eliminate air pockets
flushing the main to remove particulates
allowing a slug of water dosed with chlorine of a specific concentration to
slowly flow through the main
All parts of the main must be exposed to the required concentration of chlorine solution for a minimum period based on AWWA and state and local standards. Pre- pare the slug by dissolving sodium hypochlorite or calcium hypochlorite in water to a prescribed concentration. Inject sufficient quantity of this solution, based on the volume of the water system, into the main to develop a solid column of chlorinated water that will then pass slowly through the system. You need to periodically moni- tor the concentration of chlorine in the slug to assure that adequate disinfection takes place. If the concentration drops below the required level, stop the water flow and add additional chlorine solution.
As chlorinated water flows past fittings and valves, operate these devices to allow the solution to disinfect them and flow into pipe branches.
Monitoring Chlorine Concentration
Simple test kits and electronic devices are available to monitor the chlorine ion level in solutions injected into a water main as well as the concentration of chlorine ion in the water flowing through the main. Use these testing methods to determine if the concentration must be adjusted to keep it within the prescribed ranges needed for safe and effective disinfecting. Also, perform tests after final flushing to determine if there are any unsafe levels of chlorine in the system that would require additional flushing.
DISINFECTING POTABLE WATER LINES