Water chlorination is a method of adding chlorine or other chlorine-based chemicals, such as sodium hypochlorite, to water. This process is used to kill harmful bacteria, viruses, and other microbes in water. This helps prevent the spread of waterborne diseases such as cholera, dysentery, and typhoid.

History

In 1894, a paper suggested adding chlorine to water to make it free of germs. Two other experts supported this idea and shared it in other papers in 1895. Early tests to use chlorine in water treatment began in 1893 in Hamburg, Germany. In 1897, Maidstone, England, became the first town to treat all its water with chlorine.

Permanent use of chlorine in water treatment started in 1905 in Lincoln, England, after a broken sand filter and polluted water caused a serious typhoid fever outbreak. Alexander Cruickshank Houston used chlorine to stop the outbreak. He added a strong solution of "chlorinated lime," which is a mix of calcium hypochlorite, calcium hydroxide, and calcium chloride. This solution contained chlorine gas mixed with lime water to create calcium hypochlorite. Chlorination stopped the outbreak and continued until 1911, when a new water supply was built.

In 1908, the United States first used chlorine continuously for disinfection at Boonton Reservoir, which supplied water to Jersey City, New Jersey. Chlorine was added in small amounts (0.2 to 0.35 ppm) using a solution of calcium hypochlorite. The plan was designed by John L. Leal and George Warren Fuller. Over the next few years, chlorine disinfection spread worldwide.

In 1903, a British officer named Vincent B. Nesfield developed a method to use compressed chlorine gas to purify water. He described using a portable container with a fine tube to release chlorine gas into water, making it safe within 10 to 15 minutes. This method was useful for large-scale water treatment.

In 1910, Major Carl Rogers Darnall, a chemistry professor, demonstrated Nesfield’s method, which became the basis for modern water purification systems. Soon after, Major William J. L. Lyster used calcium hypochlorite in a linen bag to treat water. Lyster’s method became the standard for U.S. military water treatment from World War I through the Vietnam War. It was replaced later by reverse osmosis systems, which use pressure to push water through tiny filters to make it safe. Examples include the Reverse Osmosis Water Purification Unit (1980), the Tactical Water Purification System (2007), and the Light Water Purifier, which uses ultrafiltration technology.

In 1913, Charles Frederick Wallace invented a machine called the Chlorinator to use chlorine gas for disinfection at the Belmont filter plant in Philadelphia, Pennsylvania. The Wallace & Tiernan company began manufacturing this machine. By 1941, chlorine gas had largely replaced calcium hypochlorite for disinfecting U.S. drinking water.

Biochemistry

Chlorine is a type of element known as a halogen. It is used as a disinfectant in public water supplies to kill germs that cause diseases, such as bacteria, viruses, and protozoans. These germs can grow in water reservoirs, inside water pipes, and in storage tanks. Before regular disinfection methods were used, germs like those that cause cholera, typhoid fever, and dysentery led to many deaths each year.

Most chlorine is made from table salt (NaCl) through a process called electrolysis. This process is part of the chlor-alkali method. The gas produced is turned into a liquid under high pressure and then transported for use.

As a strong oxidizing agent, chlorine kills germs by removing electrons from organic molecules. Chlorine and its reaction product, hypochlorous acid, are not charged, so they can easily pass through the negatively charged surfaces of germs. Once inside, chlorine breaks down the lipids in the germ’s cell wall and reacts with enzymes and proteins inside the cell, making them unable to function. This causes the germs to die or stop reproducing.

When chlorine dissolves in water, it forms a balance of chlorine, hypochlorous acid (HOCl), and hydrochloric acid (HCl). In acidic water, the main forms are chlorine gas and hypochlorous acid. In alkaline water, the main form is the hypochlorite ion (ClO⁻). Small amounts of other chlorine compounds, such as ClO₂⁻, ClO₃⁻, and ClO₄⁻, may also be present.

Shock chlorination is a method used to clean water in swimming pools, wells, springs, and other water sources. It involves adding a large amount of hypochlorite to the water. Hypochlorite can be in powder form or as a liquid, such as chlorine bleach (a solution of sodium hypochlorite or calcium hypochlorite in water). Water treated with shock chlorination should not be used for swimming or drinking until the sodium hypochlorite level drops to 3 parts per million (ppm) or the calcium hypochlorite level drops to 0.2 to 0.35 ppm.

An alternative to shock chlorination is using a chlorine-generating filter that uses electricity to break down common salt into chlorine. Pools treated this way, called saltwater pools, usually have lower chlorine levels than pools treated directly with chlorine.

Drawbacks

Using chlorine to disinfect water can sometimes cause problems. Chlorine may react with natural organic materials in water to form chemicals called disinfection by-products (DBPs). The most common DBPs are trihalomethanes (THMs) and haloacetic acids (HAAs). Trihalomethanes are the main DBPs created during chlorination. In large amounts, bromoform can slow brain activity, causing symptoms like sleepiness or drowsiness. Long-term exposure to bromoform and dibromochloromethane may increase the risk of liver and kidney cancer, heart disease, unconsciousness, or death in very high doses. Because these chemicals can cause cancer, rules in many countries require regular checks to ensure their levels in public water systems stay safe. The World Health Organization has noted that "the health risks from these by-products are very low compared to the dangers of not properly disinfecting water."

Other concerns about chlorine include its tendency to evaporate quickly from water, which can reduce its effectiveness, and issues like unpleasant taste or smell in treated water.

Dechlorinator

A dechlorinator is a chemical used to remove chlorine or chloramine from water. Tap water is often treated with chlorine, so it must be dechlorinated before being used in an aquarium. This is important because chlorine can harm fish and damage the biological filter in an aquarium, just as it kills tiny microorganisms. Dechlorinators work by using reducing agents that change chlorine into chloride, a substance that is not harmful to living things.

Some chemicals found in commercial dechlorinators include sodium thiosulfate, sodium hydroxymethanesulfonate, and sodium hydroxymethane sulfinic acid.