Chlorination: Overview | Civil Engineering Optional Notes for UPSC PDF Download

What is Chlorination of water?

Chlorination refers to a method of disinfecting water by using chlorine and its derivatives to eradicate microorganisms. Besides disinfection, it serves as an oxidizing agent for substances such as iron, manganese, and cyanide. Additionally, it helps in addressing taste and odor issues in water and wastewater treatment processes. The primary objective of chlorinating water is to eliminate pathogenic bacteria present in it. While alternatives like chlorine dioxide, ozone, bromine, and iodine are available, the latter two are commonly utilized for personal purposes rather than in public water supplies.

Principle of Chlorination of Water

When chlorine dissolves in water, it forms a balance of chlorine, hypochlorous acid (HOCl), and hydrochloric acid (HCl):

• In an acidic environment, the primary forms are chlorine (\(Cl_2\)) and hypochlorous acid (HOCl). Conversely, in an alkaline solution, predominantly only hypochlorite ion (\(ClO^-\)) is noticeable. Small concentrations of \(ClO^{2-}, ClO^{3-}, ClO^{4-}\) may also be present.

Methods of Chlorination

• Plain Chlorination: Plain chlorination refers to the treatment of water using only chlorine without any prior treatment steps. It is utilized when the water source is already clear, aiming to disinfect the water by neutralizing harmful microorganisms. For instance, in a scenario where a well-operated municipal water system treats water that is sourced from a clean reservoir, plain chlorination might be the sole disinfection process applied.
• Pre-Chlorination: Pre-chlorination involves adding chlorine to untreated raw water before other treatment processes. This method helps in enhancing coagulation by oxidizing specific compounds, which aids in the formation of larger, settleable flocs during subsequent coagulation and flocculation stages. For example, in a water treatment plant dealing with algae-contaminated water, pre-chlorination can assist in breaking down algae cells and improving the effectiveness of subsequent treatment steps.
• Post-Chlorination: Post-chlorination is the addition of chlorine to treated water after all other treatment stages are completed. Its primary purpose is to ensure a residual disinfectant in the water distribution system to destroy any remaining microorganisms and prevent contamination. In practical terms, in a city's water treatment plant, post-chlorination might be the final step before water enters the distribution network, ensuring that water remains safe during its journey to consumers' taps.
• Double or Multiple Chlorinations: Double or multiple chlorinations encompass the utilization of both pre-chlorination and post-chlorination methods. This strategy is implemented in cases where the primary water source is heavily contaminated and contains a high concentration of bacterial organisms. By combining pre-chlorination to address initial contamination and post-chlorination for residual disinfection, this method ensures a thorough treatment of significantly polluted water sources.
• Break Point Chlorination: Break Point chlorination serves as a technique to determine the precise amount of chlorine needed to achieve the desired residual chlorine level in water. This method aids water treatment facilities in identifying the breakpoint, enabling them to ascertain the exact quantity of chlorine necessary for effective disinfection without excessive use that may lead to the formation of undesirable disinfection by-products.
• Super Chlorination: Super chlorination involves intentionally adding chlorine to water beyond the breakpoint stage, often carried out after the filtration process. This practice is commonly adopted in response to waterborne disease outbreaks or when a higher level of disinfection is required. However, it is crucial to highlight that following super chlorination, it is essential to eliminate excess chlorine using suitable dechlorination methods before supplying the treated water to consumers. This precautionary step helps mitigate any potential adverse effects associated with elevated chlorine levels in drinking water.

Advantages of Chlorinating Water

The benefits of chlorination include:

• Elimination of harmful and pathogenic microorganisms: Chlorine effectively kills these dangerous microbes, making water safer to consume.
• Prevention of waterborne diseases: By destroying disease-causing organisms, chlorination helps in reducing the spread of illnesses through water.
• Maintenance of water quality during distribution: Chlorine aids in keeping water clean and safe as it travels through distribution systems.
• Provision of residual protection: Chlorine can continue to safeguard water quality even after treatment, offering ongoing protection.
• Cost-effective water treatment method: Chlorination is a relatively affordable way to treat water compared to other methods, making it accessible for many communities.

Drawbacks of Chlorinating Water

Despite its advantages, chlorination also poses some limitations, such as:

• Unpleasant taste and odor: Chlorinated water can sometimes have a distinct taste and smell, which may be off-putting to consumers.
• Formation of potentially harmful disinfection byproducts: The process of chlorination can lead to the creation of byproducts that could be harmful to health if consumed in high quantities.
• Allergic reactions in certain individuals: Some people may experience allergic responses to chlorine in water, which can range from mild to severe.
• Corrosive effects on specific materials and equipment: Chlorine can be corrosive to certain substances, potentially causing damage to pipes or appliances over time.
• Limited effectiveness against certain parasites and viruses: While chlorine is effective against many pathogens, it may not be as efficient in combating certain types of parasites and viruses, posing a challenge in water treatment.