Several terms are used to identify the various stages and reactions that occur when chlorine is used as a disinfectant. The basic unit of measurement for chlorination, or any other chemical treatment is milligrams per liter (mg/l) or parts per million (ppm). These are very small units reflecting concentrations that are essentially one part chemical for every million parts of water. To get some idea of how small a concentration this really is it should be pointed out that 1% is equal to 10,000 mg/l or ppm.
The chlorine dosage is the amount of chlorine that is added to the water. The dosage can be determined from the number of pounds of chlorine used and the number of millions of pounds of water treated.
Chlorine is a very reactive oxidizing agent. It will react with certain substances that may be found in water. This list includes; iron, manganese, hydrogen sulfide, organic compounds and ammonia. When chlorine reacts with these substances, it loses it disinfecting properties. This is referred to as the chlorine demand. For chlorine to be effective as a disinfectant, the dosage must always exceed the demand that is present in the water. The chlorine demand may vary from day to day in a surface water supply. It is usually fairly constant in a groundwater supply.
The chlorine that remains in the water, after it has finished reacting with those substances that represent the demand, is known as the chlorine residual. The concentration of the residual is determined by subtracting the demand from the dosage.
EXAMPLE: A 4.0 mg/l dosage is added to water that has a demand of 2.5 mg/l. What is the residual?
Dosage – Demand = Residual
4.0 mg/l – 2.5 mg/l = 1.5 mg/l Residual
There are two types of residuals that result from the chlorination of water. They are free chlorine residual and combined chlorine residual.
FREE CHLORINE RESIDUAL
After the demand has been satisfied, any chlorine that is left will react with water to from hydrochloric acid and hypochlorous acid.
Cl2 + H2O > HCl + HOCl
The hypochlorous acid is the disinfecting agent and the presence of the hypochlorous ion (OC1-) is measured to obtain the free chlorine residual.
COMBINED CHLORINE RESIDUAL
Chlorine reacts with water to form hypochlorous acid. If ammonia is present, the hypochlorous acid will react with it to form compounds known as chloramines.
HOC1 + NH3 > NH2Cl + H2O
Chloramines are found in three forms. They may contain from one (NH2Cl) up to three (NCl3) atoms of chlorine. The chemistry of the water and concentration of chlorine will dictate which of the chloramines are formed. Chloramines are weak disinfectants. They require longer contact times and higher concentrations to achieve disinfection than free chlorine residual. However, they do not breakdown as quickly as free chlorine and remain in the system longer.
Changes in temperature and pH of the water can reduce the effectiveness of chlorine. Colder temperatures slow down reaction times requiring higher concentrations and longer contact times to achieve proper disinfection. A high pH impedes the formation of the hypochlorous acid and requires a higher dosage to obtain the proper residual.