What Is Chlorine in Water and Why Is It Used?

Chlorine is the most widely used water disinfectant in the world. Municipal water treatment plants add chlorine to drinking water to kill pathogenic bacteria, viruses, and protozoa, and to maintain a disinfectant residual throughout the distribution system. This practice has been credited with virtually eliminating waterborne diseases such as cholera, typhoid, and dysentery in developed countries.

Chlorine is applied in several forms: gaseous chlorine (Cl2), sodium hypochlorite (liquid bleach), and calcium hypochlorite (solid). Some utilities use chloramines (a combination of chlorine and ammonia) as an alternative disinfectant that provides a more stable and longer-lasting residual in the distribution system.

While chlorine is essential for water safety, the residual chlorine that reaches consumers' taps can cause taste and odor complaints and, more importantly, can react with naturally occurring organic matter to form potentially harmful disinfection byproducts (DBPs).

Health Effects and Concerns

At the concentrations used in drinking water treatment (typically 0.2-4.0 mg/L), chlorine itself is not considered a significant direct health risk. However, the formation of disinfection byproducts (DBPs) through the reaction of chlorine with natural organic matter is a well-documented concern.

Trihalomethanes (THMs), including chloroform, are the most studied DBPs. The EPA regulates total trihalomethanes (TTHM) at a maximum of 0.080 mg/L. Long-term exposure to elevated THM levels has been associated with increased risk of bladder cancer. Haloacetic acids (HAAs), another major class of DBPs, are regulated at 0.060 mg/L.

Chloramines, while producing fewer THMs and HAAs, form their own class of DBPs called nitrosamines, including N-nitrosodimethylamine (NDMA), which is a probable human carcinogen.

From an aesthetic standpoint, chlorine in water produces a noticeable taste and odor that many consumers find objectionable. The taste threshold is approximately 0.2 mg/L, though sensitivity varies among individuals. Chlorine can also irritate the skin and eyes, and may exacerbate conditions such as eczema in sensitive individuals.

In industrial applications, residual chlorine can damage reverse osmosis membranes, interfere with process water quality, and affect food and beverage production. Dechlorination is a critical pre-treatment step for many industrial water treatment processes.

Regulatory Limits for Chlorine in Drinking Water

MRDL stands for Maximum Residual Disinfectant Level. The EPA notes that some people who use water containing chlorine well in excess of the MRDL could experience irritant effects to the eyes and nose and stomach discomfort.

How to Test for Chlorine in Water

Free and total chlorine can be measured in the field using DPD (N,N-diethyl-p-phenylenediamine) colorimetric test kits or portable chlorine analyzers. These methods provide immediate results and are widely used by water utilities and treatment operators.

For laboratory analysis, EPA Method 330.5 (iodometric titration) and Standard Method 4500-Cl provide accurate chlorine quantification. Disinfection byproducts require specialized analysis: THMs are measured by EPA Method 524.2 (purge and trap GC/MS) and HAAs by EPA Method 552.2.

It is important to test chlorine immediately after sample collection, as chlorine dissipates rapidly from water samples, particularly at elevated temperatures and in the presence of organic matter.

Treatment Methods for Chlorine Removal

Granular Activated Carbon (GAC)

Activated carbon is the most common and effective method for chlorine removal from water. GAC filtration systems remove chlorine through a combination of adsorption and catalytic decomposition. Properly designed GAC systems can reduce chlorine to non-detectable levels. Carbon type, contact time (empty bed contact time of 5-10 minutes is typical), and bed depth are critical design parameters.

Carbon Block Filtration

Carbon block filters offer higher chlorine removal efficiency per unit volume than GAC due to their compressed structure and greater surface area contact. They are commonly used in point-of-use applications and as pre-treatment for RO systems.

Chemical Dechlorination

Chemical dechlorination using sodium bisulfite, sodium sulfite, or sodium thiosulfate is widely used in industrial applications where large volumes of water require chlorine removal. These reducing agents react with chlorine to form harmless chloride ions. Accurate dosing is essential to avoid excess reducing agent in the treated water.

UV Dechlorination

Medium-pressure UV systems operating at wavelengths around 185 and 254 nm can photolytically decompose both free chlorine and chloramines. UV dechlorination is chemical-free and produces no disinfection byproducts, making it suitable for pharmaceutical, electronics, and food processing applications.

Frequently Asked Questions

Why does my tap water smell like a swimming pool?

A swimming pool or bleach-like smell in tap water is caused by chlorine or chloramines added during municipal water treatment for disinfection. The odor threshold for chlorine is approximately 0.2 mg/L, and most municipal systems maintain residual chlorine between 0.2 and 4.0 mg/L. The smell is more noticeable in warm water and in areas closer to the treatment plant where residual levels are higher.

What are disinfection byproducts and why are they a concern?

Disinfection byproducts (DBPs) form when chlorine reacts with naturally occurring organic matter in water. The most common DBPs are trihalomethanes (THMs) and haloacetic acids (HAAs). Long-term exposure to elevated DBP levels has been associated with increased cancer risk and reproductive effects. The EPA regulates total THMs at 0.080 mg/L and total HAAs at 0.060 mg/L.

Does a standard pitcher filter remove chlorine?

Standard activated carbon filters, including those in pitcher-style filters, do reduce chlorine taste and odor. However, their capacity and flow rate are limited. For consistent chlorine removal in commercial and industrial applications, dedicated granular activated carbon (GAC) or carbon block filtration systems with appropriate contact time and media volume are recommended.

Need to Remove Chlorine from Your Water?

ForeverPure provides commercial and industrial dechlorination systems, including GAC filters, carbon block housings, chemical feed systems, and UV dechlorination units. Our engineering team designs systems with proper contact time and media volume for reliable chlorine removal at your required flow rate.

Contact ForeverPure for a customized chlorine removal solution.