Chlorination of Water: The Guardian of Public Health

Chlorination is arguably one of the most significant public health achievements of the 20th century. By introducing chlorine to water supplies, civilizations effectively conquered deadly waterborne diseases like cholera, typhoid, and dysentery. Today, it remains the most widely used method for water disinfection globally.

But how exactly does it work, and why is it still the standard despite newer technologies? This article dives into the world of chlorination.

What is Chlorination?

Chlorination is the process of adding chlorine or chlorine compounds (like sodium hypochlorite or calcium hypochlorite) to water. This chemical addition serves two main purposes:

1. Primary Disinfection: Killing or inactivating bacteria and viruses present in the water at the treatment plant.
2. Secondary Disinfection (Residual): Maintaining a "residual" level of chlorine in the distribution pipes to prevent re-contamination as water travels to homes and businesses.

The Chemistry: Why pH Matters

When you add Chlorine to water (Cl₂), it reacts to form Hypochlorous Acid (HOCl). This is the "active" killer. However, HOCl is shy—it dissociates based on the pH of the water:

Cl₂ + H₂O ⇌ HOCl + HCl
HOCl ⇌ H⁺ + OCl⁻

• At pH 5-6: Almost 100% is HOCl (The strongest killer).
• At pH 7.5: It's a 50/50 split between HOCl and OCl⁻ (Hypochlorite).
• At pH 9.0: Almost 100% is OCl⁻ (A much weaker disinfectant).

Conclusion: To disinfect water effectively with chlorine, you must keep the pH below 8.0.

Chlorine Forms: Which one to choose?

Break-Point Chlorination: The Goal

Have you ever noticed that "pool smell"? That’s not chlorine—it's Chloramines (Chlorine + Urine/Sweat).

1. Combined Chlorine: Chlorine reacts with ammonia to form chloramines. They smell bad and are weak killers.
2. The Break-point: You must add enough chlorine to burn off all the ammonia. Once you pass this "Break-point," the smell disappears, and you have Free Residual Chlorine, which is the gold standard for safety.

The Dark Side: Disinfection By-products (DBPs)

If the raw water is very dirty (leafy/organic), chlorine reacts with the organic matter to form Trihalomethanes (THMs).

• The Risk: THMs are suspected carcinogens over long-term exposure.
• The Solution: Remove the organic matter first using Activated Carbon or Coagulation before adding chlorine.

The Importance of "Residual" Chlorine

Unlike UV or Ozone treatment, which provide instant disinfection but leave nothing behind, chlorine stays in the water. This residual chlorine is crucial for municipal systems. If a pipe leaks or breaks, the remaining chlorine helps protect the water and the consumers from soil pathogens entering the system.

Pros and Cons of Chlorination

Advantages

• Proven Efficacy: Decades of success in eliminating waterborne epidemics.
• Cost-Effective: One of the cheapest disinfection methods available.
• Residual Protection: Protects water throughout the distribution network.
• Scalability: Can be used for small wells or massive city supplies.

Disadvantages

• Taste and Odor: High levels can cause a distinct "pool" smell or taste.
• By-products: Chlorine can react with organic matter to form Disinfection By-Products (DBPs) like Trihalomethanes (THMs), which are regulated due to potential health risks.
• Handling Risks: Chlorine gas and concentrated solutions are hazardous chemicals that require strict safety protocols during storage and handling.