What is a Chlorination
Chlorination is the process of disinfecting water by adding chlorine or chlorine-based compounds to destroy harmful microorganisms, including bacteria, viruses and protozoa. It is one of the most widely used and effective methods for ensuring safe drinking water and for disinfecting treated wastewater before discharge.
The technique has been used for more than a century and remains a cornerstone of modern water and wastewater treatment. Chlorination protects public health by preventing the spread of waterborne diseases such as cholera, typhoid, dysentery and hepatitis A. Despite the emergence of alternative disinfection technologies, chlorine remains the most reliable and cost-effective disinfectant for large-scale applications due to its residual activity and proven performance.
The history and importance of chlorination
The use of chlorine for water disinfection dates back to the early 1900s, when it was introduced in the United Kingdom and the United States as a measure to combat outbreaks of waterborne diseases. The success of these early applications quickly led to widespread adoption across Europe and beyond.
By the mid-20th century, chlorination had become the standard practice for municipal water treatment worldwide. It transformed public health, dramatically reducing mortality rates from infectious diseases and contributing to longer life expectancy in urban populations.
Today, chlorination continues to play an essential role not only in drinking water treatment but also in the disinfection of wastewater, swimming pools, industrial water systems and emergency water supplies.
How chlorination works
Chlorination works by introducing chlorine or chlorine compounds into water, where they react to form disinfecting agents known as free chlorine species. The most common forms of chlorine used are chlorine gas (Cl₂), sodium hypochlorite (NaOCl), and calcium hypochlorite (Ca(OCl)₂).
When chlorine is added to water, it undergoes hydrolysis to form hypochlorous acid (HOCl) and hypochlorite ions (OCl⁻). These two species exist in equilibrium, and their relative proportion depends on the pH and temperature of the water.
The chemical reaction can be represented as:
Cl₂ + H₂O → HOCl + H⁺ + Cl⁻
Hypochlorous acid is the primary disinfecting agent because it penetrates microbial cell walls and disrupts vital cellular processes. It oxidises enzymes and cell components, leading to cell death and inactivation of viruses and spores.
The balance between hypochlorous acid and hypochlorite ion is critical. At lower pH levels (around 6 to 7), the more effective HOCl form predominates, while at higher pH values, OCl⁻ becomes dominant, reducing disinfection efficiency. Therefore, pH control is often necessary in chlorination systems to maintain optimal performance.
Types of chlorination systems
Different chlorination systems are used depending on the application, scale, and type of water being treated. The main types include:
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Chlorine gas systems:
These use pressurised cylinders of chlorine gas, which is dosed into water through a gas injector or vacuum feed system. Chlorine gas is highly effective and widely used in large municipal treatment plants but requires strict safety precautions due to its toxic and corrosive nature. -
Sodium hypochlorite systems:
Sodium hypochlorite, commonly known as liquid bleach, is easier and safer to handle than chlorine gas. It can be purchased as a commercial solution or generated on-site by electrolysis of salt water. It is suitable for both drinking water and wastewater treatment. -
Calcium hypochlorite systems:
This form of chlorine is available as solid granules or tablets that dissolve in water to release hypochlorous acid. It is often used for smaller systems, such as rural water supplies, swimming pools and emergency disinfection. -
Chlorine dioxide systems:
Although not a form of free chlorine, chlorine dioxide (ClO₂) is a powerful oxidising agent used as an alternative disinfectant. It is effective against a broad spectrum of microorganisms and produces fewer disinfection by-products, but it must be generated on-site due to instability during storage.
The choice of chlorination method depends on system size, water quality, safety requirements and operational costs.
Applications of chlorination
Chlorination serves multiple purposes in water and wastewater treatment:
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Disinfection of drinking water: Chlorine is used to eliminate pathogens and ensure microbiological safety throughout the distribution system. Its residual effect provides continued protection against recontamination in pipelines and storage tanks.
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Wastewater disinfection: In wastewater treatment plants, chlorination is applied to treated effluent before discharge to natural water bodies. It reduces pathogenic contamination and protects aquatic environments and public health.
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Industrial and commercial systems: Chlorination controls biofouling and microbial growth in cooling towers, process water systems and food processing facilities.
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Swimming pools and recreational water: Chlorine maintains sanitary conditions by killing bacteria and algae.
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Emergency and field use: In disaster relief or military operations, chlorine tablets and liquid solutions provide rapid, reliable water disinfection in low-resource settings.
Because of its versatility, chlorination remains the most universally applicable method of water disinfection.
Control of chlorination and dosage
The effectiveness of chlorination depends on several interrelated factors, including chlorine dose, contact time, water quality and temperature. The key principle governing disinfection efficiency is the CT concept, which represents the product of chlorine concentration (C) and contact time (T).
CT = C × T
A higher CT value generally means better microbial inactivation, but excessive dosing can lead to unwanted chemical reactions and by-product formation. Typical chlorine doses for drinking water disinfection range between 0.2 and 2.0 milligrams per litre (mg/L), while wastewater disinfection may require higher doses due to organic load and ammonia presence.
Monitoring systems continuously measure residual chlorine concentration to ensure proper control. Maintaining a small residual of free chlorine (usually 0.2 to 0.5 mg/L) in the distribution network ensures ongoing protection against contamination.
Advanced systems use automatic feedback loops with sensors and controllers to adjust chlorine dosage dynamically based on flow rate, residual concentration and demand fluctuations.
Chlorine demand and residual
When chlorine is added to water, it first reacts with various substances such as organic matter, iron, manganese, ammonia and hydrogen sulphide. These reactions consume chlorine, reducing the amount available for disinfection. The total amount of chlorine required to satisfy these reactions before a residual is achieved is known as the chlorine demand.
The point at which sufficient chlorine has been added to overcome the demand and maintain a measurable free chlorine residual is called the breakpoint. Beyond this point, any additional chlorine remains available for disinfection and residual maintenance.
Understanding and controlling chlorine demand is essential for effective treatment. In wastewater systems, high organic loads can greatly increase demand, requiring careful adjustment of dosage rates to ensure adequate disinfection without excessive chemical use.
Formation of disinfection by-products
One of the main challenges associated with chlorination is the formation of disinfection by-products (DBPs) when chlorine reacts with natural organic matter or bromide in water. The most common by-products include trihalomethanes (THMs) and haloacetic acids (HAAs). Some DBPs are potentially harmful to human health and are regulated in drinking water standards.
To minimise by-product formation, water utilities employ several strategies:
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Pre-treatment to remove organic matter before chlorination.
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Optimising chlorine dose and contact time.
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Maintaining appropriate pH and temperature conditions.
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Using alternative disinfectants such as ozone or ultraviolet light for primary disinfection, followed by chlorination for residual protection.
Balancing effective disinfection with the control of by-products is a key aspect of modern chlorination practice.
Safety and handling of chlorine
Chlorine is a powerful and potentially hazardous chemical that must be handled with care. Chlorine gas, in particular, poses significant risks due to its corrosive and toxic properties. Inhalation of chlorine vapour can cause severe respiratory damage, and contact with liquid chlorine can result in chemical burns.
Safety measures for chlorine handling include:
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Proper ventilation and gas detection systems in storage and dosing areas.
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Use of corrosion-resistant materials for piping and equipment.
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Installation of emergency shutdown valves and containment systems.
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Training of personnel in safe handling, emergency procedures and first aid.
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Use of personal protective equipment such as gloves, masks and eye protection.
For smaller systems, safer alternatives such as sodium hypochlorite or calcium hypochlorite are often preferred because they are easier to store and handle.
Chlorination in wastewater treatment
In wastewater treatment, chlorination is commonly applied after secondary or tertiary treatment to disinfect effluent before it is discharged or reused. The objective is to destroy pathogens while avoiding excessive chlorine residuals that could harm aquatic life.
Dechlorination is often required after disinfection to remove residual chlorine from treated effluent. This is typically achieved using chemicals such as sodium bisulphite, sodium thiosulphate or activated carbon. Dechlorination ensures compliance with environmental discharge permits and protects sensitive ecosystems.
In some advanced plants, chlorination is combined with other processes such as ultraviolet disinfection or ozonation to achieve higher disinfection efficiency with lower chemical use.
Environmental and operational considerations
While chlorination is effective and economical, it must be managed carefully to avoid environmental and operational issues. Excessive chlorination can lead to equipment corrosion, taste and odour problems in drinking water, and toxicity in aquatic habitats if residual chlorine is not properly controlled.
Environmental regulations in the UK and elsewhere set strict limits on chlorine residuals and by-products in both drinking water and wastewater discharges. Operators must monitor and report chlorine levels to demonstrate compliance with these standards.
Technological advancements, including automated dosing systems and continuous monitoring, help maintain precise control, ensuring safe and environmentally responsible operation.
The role of chlorination in modern water management
Despite the emergence of alternative disinfection technologies such as ultraviolet (UV) irradiation and ozonation, chlorination remains indispensable in most water systems. Its main advantage is the ability to provide a residual disinfectant that continues to protect water during storage and distribution.
In developing regions, chlorination is often the most practical and affordable option for ensuring safe drinking water. Simple systems using chlorine tablets or locally produced hypochlorite solutions can prevent outbreaks of disease where infrastructure is limited.
In advanced systems, chlorination is part of an integrated disinfection strategy that combines physical and chemical processes to maximise safety and minimise environmental impact.
Conclusion
Chlorination is one of the most important and enduring methods of water disinfection. By effectively destroying pathogens and maintaining residual protection, it has safeguarded public health for over a century and continues to do so today.
Modern chlorination systems combine proven chemistry with precise control and monitoring to ensure safety, efficiency and compliance with environmental standards. Although challenges such as by-product formation and chemical handling remain, ongoing innovation in dosing, automation and alternative disinfectants is helping to refine the process.
As a cornerstone of water and wastewater treatment, chlorination remains vital to achieving safe, reliable and sustainable water management, protecting both communities and the environment from the risks of waterborne disease.