Brief description

Water chlorination is the process of adding chlorine or chlorine compounds (such as sodium hypochlorite or calcium hypochlorite) to water to disinfect it. It is one of the most significant public health achievements in human history, designed to neutralize pathogenic microorganisms and prevent the spread of devastating waterborne diseases like cholera, typhoid fever, and dysentery.

Use / Function

• Disinfection: Eliminates bacteria, viruses, and many protozoa from drinking water supplies.
• Disease Eradication: Stops cholera, typhoid, and other waterborne epidemics.
• Residual Protection: Keeps the water supply safe from recontamination during storage and distribution.
• Scale: Domestic (droplets in a bottle), municipal (piped water networks), and emergency/military camps.

Operating principle

Chlorine is a highly reactive and powerful oxidizing agent that destroys pathogens through chemical disruption:

1. Chemical Dissociation: When added to water, chlorine compounds react to form hypochlorous acid ($HOCl$) and hypochlorite ions ($OCl^{-}$). Hypochlorous acid is electrically neutral and highly effective at penetrating cell walls.
2. Pathogen Destruction: $HOCl$ easily penetrates the protective lipid membranes of bacteria and viruses. Once inside, it oxidizes vital enzymes and proteins, disrupting cellular metabolism and structural integrity, killing the pathogen.
3. Chlorine Demand: Some chlorine is immediately consumed by reacting with organic matter, iron, or manganese present in the water. The remaining active chlorine is called free chlorine.
4. Residual Effect: Maintaining a tiny amount of free chlorine (0.2 to 0.5 mg/L) ensures that any pathogens introduced after initial treatment are also destroyed.

How to create it

For emergency or small-scale survival purification:

1. Water Preparation

• If the water is muddy, cloudy, or filled with organic debris, first let it settle and filter it through a water filter, sand, or cloth. Suspended particles shelter bacteria from chlorine contact and consume the disinfectant.

2. Dosing with Household Bleach

• Use fresh, unscented liquid bleach (typically 5% to 6% sodium hypochlorite). Ensure it does not contain added soaps, perfumes, or color-safe additives.
• Dosage for Clear Water: Add 2 drops of bleach per liter (or about 8 drops per gallon) of water.
• Dosage for Cold or Turbid Water: If the water is very cold (under 10°C) or slightly cloudy, double the dosage to 4 drops of bleach per liter.

3. Mixing and Contact Time

• Shake or stir the mixture thoroughly to distribute the chlorine evenly.
• Crucial Wait: Let the water sit covered for at least 30 minutes (60 minutes if the water is very cold). This contact time is absolutely necessary for the chemical reaction to completely kill all pathogens.

4. Verification

• Smell the water after the waiting period. It should have a very faint chlorine scent.
• If there is no smell of chlorine, the initial dosage was consumed by organic impurities. Add one more drop, mix, and wait another 15 minutes.
• If the chlorine smell is unpleasantly strong, aerate the water by pouring it back and forth between two clean containers several times, or let it sit uncovered for a few hours.

Materials needed

• Disinfectant: Unscented liquid bleach (sodium hypochlorite) or calcium hypochlorite powder.
• Dispensing Tool: Clean eyedropper or graduated syringe.
• Containers: Clean, non-reactive vessels (glass, ceramic, or food-grade plastic) to mix and store the water.
• Pre-filtration: Cloth, sand, or a simple gravity filter.

Variants and improvements

• Calcium Hypochlorite (Bleaching Powder): A dry, granular compound ($Ca(OCl)_2$, usually 65-70% chlorine). It is much more stable than liquid bleach, which degrades by up to 50% in a year. Stored in a cool, dry place, calcium hypochlorite powder keeps for years. To use it, a stock solution must be mixed first by dissolving the powder in clean water.
• Electrolytic Generation: Using an electrolysis cell to pass direct current through salt water (salt + water) to generate sodium hypochlorite on-site, bypassing the need for chemical supply chains.

Limits and risks

• Chemical Hazards: Concentrated chlorine compounds are highly corrosive and toxic. Inhaling chlorine gas or swallowing concentrated bleach is lethal.
• Resistant Pathogens: Certain protozoan cysts, notably Cryptosporidium and Giardia lamblia, have thick protective shells and are highly resistant to standard chlorine dosages. Boiling or ultrafiltration is required to eliminate them.
• Disinfection Byproducts (DBPs): When chlorine reacts with high concentrations of organic matter, it can form trihalomethanes ($THMs$), which are carcinogenic over long-term exposure. Pre-filtering the water minimizes this risk.