Chlorine demand is the amount of chlorine required to kill bacteria, oxidize iron or other elements in the water, and oxidize any organic matter that may be present. Free available chlorine residual is the amount of chlorine remaining in the water after the chlorine demand has been met. If the chlorine demand is greater than the amount of chlorine introduced, there will be no free available chlorine residual. Unless a chlorine residual is present, adequate amounts of chlorine have not been added to the water. Swimming pool owners use these kits to test the chlorine level of their pool water.
Kits are available from local plumbing or water supply equipment dealers.
Contact time is the amount of time that the chlorine is present in the water. The combination of chlorine residual and contact time determines the effectiveness of the chlorination treatment. Thus the greater the chlorine residual the shorter the required contact time for bacterial kill. Neither shock nor continuous chlorination should be used as a substitute for a safe water supply. Wells should be located and constructed in accordance with existing state well codes and accepted sanitary standards.