Chlorine is the most common disinfection method for drinking water in North America. It comes as chlorine gas, sodium hypochlorite or calcium hypochlorite.

A small community approached Okefenokee Technical College for information on sodium hypochlorite disinfection. The community had been using chlorine gas and ammonia as primary and secondary disinfectants.

Sodium hypochlorite (NaClO) is the active ingredient in commercial liquid bleach, which is commonly available in 6, 12 and 15 percent solutions. Sodium hypochlorite has a relatively short shelf life that depends on sunlight, temperature, vibration and the starting concentration. Increases in any of these shorten life.

Sodium hypochlorite should be stored in a cool room in an opaque container. The higher the concentration, the faster the chemical breaks down. However, the stronger the solution, the less room is needed to store it. As the chemical breaks down, its reaction in water slows.

The use of chlorine-based disinfectants in domestic drinking water has led to controversy because the process leads to formation of small quantities of harmful byproducts. In addition, transport and handling safety concerns around the use of chlorine gas have directed public opinion toward sodium hypochlorite.

Unstable compound

Sodium hypochlorite is a clear, slightly yellowish solution with a characteristic odor. As a bleaching agent it is usually a 5 percent sodium hypochlorite with a pH of about 11. More concentrated solutions (10 to 15 percent) have a pH of about 13.

Sodium hypochlorite is unstable. Chlorine evaporates at a rate of 0.75 gram per day of active chlorine from solution. Sodium hypochlorite disintegrates when heated or if it contacts acids, sunlight, certain metals, and poisonous and corrosive gases, including chlorine gas. It is a strong oxidant that reacts with flammable compounds and reducing agents, and it is flammable. These characteristics must be kept in mind during transport, storage and use.

The presence of caustic soda in sodium hypochlorite means the pH of water increases when the chemical is added. When sodium hypochlorite dissolves in water, two substances are formed that play a role in oxidation and the disinfection processes. These are hypochlorous acid and the less active hypochlorite ion. The pH of the water determines how much hypochlorous acid is formed. When sodium hypochlorite is used, hydrochloric acid is used to lower the pH and increase the disinfection ability.

Two means of production

Sodium hypochlorite can be produced in two ways. One is by dissolving salt in softened water, resulting in a concentrated brine. This brine is then electrolyzed to form a sodium hypochlorite solution containing 150 grams of active chlorine per liter. During this reaction hydrogen gas is also formed. The chemical also can be produced by adding chlorine gas to caustic soda, producing sodium hypochlorite, water and salt.

The advantage of salt electrolysis system in formation of sodium hypochlorite is that no transport or storage of chemical is required. Another advantage of the on-site process is that chlorine lowers the pH, and no other acid is required to lower pH. The hydrogen gas produced is explosive, and as a result ventilation is required. This process is slow, and a buffer of extra hypochlorous acid needs to be used. The maintenance and purchase of the electrolysis system is much more expensive than sodium hypochlorite.

When sodium hypochlorite is used, acetic or sulfuric acid is added to the water. An excessive dose can produce poisonous gases. If the dosage is too low, the pH becomes too high, and it can irritate the eyes.

The required concentration of sodium hypochlorite depends on the concentrations of pollutants, primarily organic pollutants. If the water is filtered before disinfection, less sodium hypochlorite is needed.

Sodium hypochlorite is a strong oxidizer. Oxidation reactions are corrosive, and solutions burn skin. In addition, chlorination of drinking water with sodium hypochlorite can oxidize organic contaminants, producing trihalomethanes, which are considered carcinogenic and are subject to regulation.

Right after the sodium hypochlorite is added to water, chlorine levels decline because the chlorine is reacting with organic matter and microbes. After those reactions are complete, chlorine will slowly escape into the air as a gas. For this reason, free and total chlorine levels slowly degrade over time in a container. The pH of hypochlorite solutions should be raised to over 11 in order to extend the shelf life before it is used.

Considering shelf life

In drinking water facilities, 12 percent sodium hypochlorite is a primary disinfectant. In the time that it takes to ship the chemical, it typically degrades to 11 percent. In the calculations of dosages or concentrations, 10 percent should be used as the starting point. The good thing is that the decline is predictable if environmental factors are controlled. Even at 1 percent, the solution can disinfect water.

Chlorine removes electrons from the outer shell of the atoms of living organisms, destabilizing the structure until the organism is dead. When sodium hypochlorite is added to water, it forms an OCl- (hypochlorite) ion that is called free chlorine. This ion should not be confused with chlorine gas. Free chlorine is what most municipal water treatment facilities use for disinfection.

If large amounts of water are stored, it may be useful to change from free chlorine to combined chlorine. Free available residual chlorine can be converted to monochloramine. The conversion is completed by adding an ammonia solution to the chlorinated water at a ratio of 4:1, free chlorine to ammonia.

Monochloramine has a much longer shelf life and has no known negative health effects. It is a much weaker disinfectant and usually it is not used for primary disinfection. It is, however, perfect for secondary disinfection (the prevention of recontamination of drinking water in the distribution system).

It is always best to use the highest-grade chemicals when adding them to drinking water. ANSI/NSF 60 is the standard for drinking water chemicals.

About the author

John Rowe, Ph.D., is a professor at Okefenokee Technical College in Waycross, Ga.