Which Is the Oxidant Used for Purification of Drinking Water?Clean and safe drinking water is essential for good health and overall well-being. One of the most important steps in water treatment is disinfection, where harmful microorganisms are eliminated. A common method of achieving this is through the use of oxidants, which destroy or deactivate bacteria, viruses, and other pathogens. Among the various chemicals available, certain oxidants are widely used because of their effectiveness and safety.
This topic explores which oxidant is used for purifying drinking water, how it works, and why it is preferred. It also discusses alternative oxidants and their roles in water treatment.
What Is an Oxidant?
An oxidant, also known as an oxidizing agent, is a substance that gains electrons in a chemical reaction and, in doing so, causes another substance to lose electrons. In water treatment, oxidants help by breaking down organic and inorganic pollutants and killing pathogens through oxidation.
Oxidants are particularly useful because they not only disinfect but can also help in removing unwanted color, taste, and odor from water.
The Most Common Oxidant Chlorine
Chlorine is the most widely used oxidant for the purification of drinking water. It has been used for over a century and remains the standard disinfectant in many countries due to its powerful germ-killing abilities and residual protection.
Why Chlorine Is Preferred
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Highly effective against a broad spectrum of microorganisms
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Provides residual protection, meaning it continues to disinfect as water travels through pipes
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Cost-effective and readily available
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Can also help in oxidizing iron, manganese, and hydrogen sulfide, improving water quality
How Chlorine Works in Water Treatment
Chlorine is usually added to water in the form of gas (Cl₂), sodium hypochlorite (NaOCl), or calcium hypochlorite (Ca(OCl)₂). When chlorine dissolves in water, it forms hypochlorous acid (HOCl), which is the active disinfecting agent.
This acid penetrates the cell walls of bacteria and viruses, disrupting essential cell functions and ultimately leading to the death of the microorganisms.
The chemical reactions are as follows
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Cl₂ + H₂O → HOCl + HCl
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HOCl ⇌ H⁺ + OCl⁻
Both hypochlorous acid (HOCl) and hypochlorite ion (OCl⁻) act as oxidants in water, with HOCl being the more effective form at lower pH levels.
Other Oxidants Used in Water Purification
While chlorine is the most common, other oxidants are also used depending on the specific treatment needs and quality of the water source.
1. Ozone (O₃)
Ozone is a very powerful oxidant and disinfectant. It’s often used in advanced water treatment systems.
Advantages
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Kills bacteria and viruses more rapidly than chlorine
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Removes taste and odor-causing compounds
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No harmful chemical residue
Disadvantages
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More expensive than chlorine
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Does not provide residual protection
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Requires on-site generation
2. Chlorine Dioxide (ClO₂)
Chlorine dioxide is effective over a wide pH range and is known for not forming harmful byproducts like trihalomethanes.
Benefits
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Excellent disinfectant for both bacteria and viruses
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Less affected by organic matter in water
Limitations
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More complex to produce and handle
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Often used in combination with other disinfectants
3. Hydrogen Peroxide (H₂O₂)
Hydrogen peroxide is used mainly in combination with other oxidants or UV light in a process called advanced oxidation.
Advantages
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Breaks down into water and oxygen, making it environmentally friendly
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Effective in removing organic pollutants
Drawbacks
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Limited disinfection capability when used alone
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Not commonly used for primary drinking water disinfection
4. Potassium Permanganate (KMnO₄)
This oxidant is primarily used to remove iron, manganese, and hydrogen sulfide from water, rather than for disinfection.
Usage
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Pre-treatment in water plants
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Not typically used for microbial disinfection
Residual Disinfection and Its Importance
One reason chlorine is so widely used is because of its residual effect. After water leaves the treatment plant, chlorine remains in the water at low levels, continuing to protect it from microbial contamination as it travels through distribution systems.
Other oxidants like ozone and hydrogen peroxide do not provide this benefit, making them less practical for large-scale water supply systems where maintaining water quality over distance is important.
Health and Safety Considerations
The use of chlorine in water purification has raised concerns about disinfection byproducts (DBPs) such as trihalomethanes (THMs) and haloacetic acids (HAAs). These are formed when chlorine reacts with natural organic matter in the water.
To address this, water treatment facilities often
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Control the amount of organic matter before chlorination
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Use combined disinfection methods, such as ozone followed by chlorine
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Monitor and limit the concentration of DBPs to meet safety standards
Regulatory Standards and Global Use
Many countries have established regulations for acceptable levels of disinfectants and byproducts in drinking water. The World Health Organization (WHO) and Environmental Protection Agencies in various countries provide guidelines to ensure that water treatment practices are safe and effective.
Chlorine remains the most commonly approved and widely used oxidant globally due to its balance of effectiveness, affordability, and ability to maintain water safety during distribution.
Choosing the Right Oxidant
The choice of oxidant for drinking water purification depends on several factors
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Source water quality
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Scale of treatment
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Budget constraints
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Desired level of residual protection
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Potential formation of byproducts
In small-scale systems or where water quality is particularly challenging, ozone or chlorine dioxide may be more appropriate. In most municipal systems, however, chlorine remains the oxidant of choice.
Among the various oxidants available for water purification, chlorine is the most widely used and trusted. It effectively eliminates harmful microorganisms, improves water quality, and offers residual protection to ensure the water stays safe as it moves through the supply system.
Other oxidants like ozone, chlorine dioxide, hydrogen peroxide, and potassium permanganate each have their own advantages and applications but are generally used in combination with or as alternatives to chlorine in specific circumstances.
Understanding how oxidants work in water treatment helps in appreciating the complexity behind the simple act of turning on a tap and getting clean, safe water.