Chlorine (Cl) in Water Treatment

Quick Answer

Chlorine (Cl) is a halogen with atomic number 17. PubChem lists its standard state as gas and its relative atomic mass as 35.45.

Chlorine compounds are central to disinfection, but chloride is a dissolved salt that can increase corrosion and salinity. Free chlorine, combined chlorine and chloride must be measured separately because their treatment implications differ.

Interesting Facts

Chlorine transformed municipal water safety, but its dose and contact time must be balanced against disinfection by-product formation.
At the same free-chlorine residual, hypochlorous acid and hypochlorite proportions change with pH, changing disinfection effectiveness.
Chloride (Cl⁻) is not free chlorine: dechlorination removes an oxidant residual but does not desalinate chloride.

Water-treatment Design Implications

An element name alone is not enough to select treatment equipment. Confirm chemical form or speciation, total and dissolved concentration, pH, alkalinity, competing ions and the product-water target. Send the water-analysis results to Watermart to identify additional testing and shortlist processes before equipment is specified.

Summary Data Sources

1. Basic Information


Atomic number	17
Symbol	Cl
Atomic mass	35.453 g/mol
Electronegativity	3.0 (Pauling scale)
Density	3.21 x 10-3 g/cm3 at 20°C

Chlorine is a halogen element discovered by Carl Wilhelm Scheele in 1774. In its pure form, chlorine is a yellowish-green diatomic gas with a sharp odor.

2. Physical and Chemical Properties

Chlorine has a melting point of -101°C and a boiling point of -34.6°C. Chlorine gas is 2.5 times heavier than air. Chlorine is highly reactive and reacts easily with almost all other elements. In aqueous solution, chlorine forms hypochlorous acid (HClO) which is a strong oxidizer.

Chlorine dissolves easily in water, forming a solution called “chlorine water”. At high concentrations, this solution is acidic and highly oxidative. Chlorine can also form various inorganic compounds such as chloride salts and chlorinated organic compounds.

3. Presence in Water and Health Effects

Chlorine is rarely found in free form in nature. In water, chlorine is usually present in the form of chloride ions (Cl-) or dissolved chlorine compounds such as sodium chloride (table salt). Seawater contains about 1.9% chloride ions.

Exposure to small amounts of chlorine can cause irritation to the eyes, nose and throat. Higher exposure can cause coughing, shortness of breath and even fluid retention in the lungs. However, these health effects generally do not occur at chlorine levels commonly found in treated drinking water.

4. Water Treatment Applications and Removal Methods

Chlorine is the most commonly used disinfectant in drinking water and wastewater treatment. Some of the main applications include:

Disinfection of drinking water to remove pathogens
Oxidation of iron and manganese in groundwater
Ammonia removal through breakpoint chlorination
Control of algae and biofilm growth in distribution systems

To remove excess chlorine from water, some methods that can be used include:

Aeration or stripping
Adsorption with activated carbon
Chemical reduction using sodium sulfite or hydrogen peroxide
Membrane filtration such as reverse osmosis

To remove chloride ions, ion exchange methods using strong base anion exchange resins can be used. However, due to the low selectivity of chloride, other ions with higher selectivity must be removed first.

5. Industrial Use in Water Treatment

In addition to disinfection, chlorine is also used in various industrial water treatment applications, including:

Bleaching of paper pulp and textiles
Synthesis of chlorinated organic chemicals
Cooling water treatment in power plants
Sanitization in the food and beverage processing industry

8. Environmental Impact and Sustainability Considerations

While effective as a disinfectant, the use of chlorine in water treatment has some environmental impacts that need to be considered:

Formation of disinfection by-products such as trihalomethanes and haloacetic acid, which are potentially carcinogenic
Toxicity to aquatic organisms if chlorinated water is discharged directly into water bodies
Potential formation of persistent organochlorine compounds in the environment

To improve sustainability, some approaches that can be applied include:

Optimization of chlorim doses to reduce the formation of by-products
Use of alternative disinfection technologies such as UV or ozone
Implementation of chloramination systems to minimize the formation of trihalomethanes
Treatment of chlorine-containing wastewater before discharge into the environment

Chlorine was the first widely used water disinfectant, starting in the early 20th century
The use of chlorine in drinking water treatment has reduced the incidence of typhoid disease in the US by 99% between 1900 and 1960
The distinctive “swimming pool” odor is actually not caused by chlorine, but rather by chloramine compounds formed when chlorine reacts with organic contaminants
Some microorganisms, such as Cryptosporidium, are relatively resistant to chlorination and require additional treatment methods
Astronauts on the International Space Station use a water treatment system that incorporates iodine and silver ions as an alternative to chlorine for disinfection