Text Box: Definition of Terms Related to Chlorination 
Chlorine concentration—The concentration (amount) of chlorine in a volume of water is measured in parts per million (ppm). In 1 million gallons of water, a chlorine concentration of 1 ppm would require 8.34 pounds (3.8 kilograms) of 100% chlorine.
Contact time—The time, after chlorine addition and before use, given for disinfection to occur. For groundwater systems, contact time is minimal. However, in surface water systems, a contact time of 20 to 30 minutes is common.
Dosage—The total amount of chlorine added to water. Given in parts per million (ppm) or milligrams per liter (mg/L).
Demand—Chlorine solution used by reacting with particles of organic matter such as slimes or other chemicals and minerals that may be present. The difference between the amount of chlorine applied to water and total available chlorine remaining at the end of a specified contact period.
Parts per million—A weight-to-weight comparison; 1 ppm equals 1 pound per million pounds. Because water weighs 8.34 pounds per gallon, it takes 8.34 pounds of any substance per million gallons to equal 1 ppm. In water chemistry, 1 ppm equals 1 mg/L.
Residual—The amount of chlorine left after the demand is met; available (free) chlorine. This portion provides a ready reserve for bactericidal action. Both combined and free chlorine make up chlorine residual and are involved in disinfection. Total available chlorine = free chlorine + combined chlorine.
Breakpoint chlorination—A process sometimes used to ensure the presence of free chlorine in public water supplies by adding enough chlorine to the water to satisfy the chlorine demand and to react with all dissolved ammonia that might be present. The concentrations of chlorine needed to treat a variety of water conditions are listed in Table 7.4.