Domestic Water
Chlorination is the process of adding chlorine to drinking water to kill parasites, bacteria, and viruses. Different processes can be used to achieve safe levels of chlorine in drinking water. Using or drinking water with small amounts of chlorine does not cause harmful health effects and provides protection against waterborne disease outbreaks.
Chlorine inactivates a microorganism by damaging its cell membrane. Once the cell membrane is weakened, the chlorine can enter the cell and disrupt cell respiration and DNA activity (two processes that are necessary for cell survival).
Any type of chlorine that is added to water during the treatment process will result in the formation of hypochlorous acid (HOCl) and hypochlorite ions (OCl-), which are the main disinfecting compounds in chlorinated water. More detail is provided later on in this fact sheet.
A Form of Chlorine + H2O -> HOCl + OCl-
Of the two, hypochlorous acid is the most effective. The amount of each compound present in the water is dependent on the pH level of the water prior to addition of chlorine. At lower pH levels, the hypochlorous acid will dominate. The combination of hypochlorous acid and hypochlorite ions makes up what is called ‘free chlorine.’ Free chlorine has a high oxidation potential and is a more effective disinfectant than other forms of chlorine, such as chloramines. Oxidation potential is a measure of how readily a compound will react with another. A high oxidation potential means many different compounds are able to react with the compound. It also means that the compound will be readily available to react with others.
Combined chlorine is the combination of organic nitrogen compounds and chloramines, which are produced as a result of the reaction between chlorine and ammonia. Chloramines are not as effective at disinfecting water as free chlorine due to a lower oxidation potential. Due to the creation of chloramines instead of free chlorine, ammonia is not desired product in the water treatment process in the beginning, but may be added at the end of treatment to create chloramines as a secondary disinfectant, which remains in the system longer than chlorine, ensuring clean drinking water throughout the distribution system.
The amount of chlorine that is required to disinfect water is dependent on the impurities in the water that needs to be treated. Many impurities in the water require a large amount of chlorine to react with all the impurities present. The chlorine added must first react with all the impurities in the water before a chlorine residual is present. The amount of chlorine that is required to satisfy all the impurities is termed the ‘chlorine demand.’ This can also be thought of as the amount of chlorine needed before free chlorine can be produced. Once the chlorine demand has been met, breakpoint chlorination (the addition of chlorine to water until the chlorine demand has been satisfied) has occurred. After the breakpoint, any additional chlorine added will result in a free chlorine residual proportional to the amount of chlorine added. Residual chlorine is the difference between the amount of chlorine added and the chlorine demand. Most water treatment plants will add chlorine beyond the breakpoint.
The chlorination process involves adding chlorine to water, but the chlorinating product does not necessarily have to be pure chlorine. Chlorination can also be carried out using chlorine-containing substances. Depending on the pH conditions required and the available storage options, different chlorine-containing substances can be used. The three most common types of chlorine used in water treatment are: chlorine gas, sodium hypochlorite, and calcium hypochlorite.