Chemical Water Treatment: Chlorine and Iodine

Chemical Disinfection Overview

Chemical treatment kills bacteria and viruses effectively. It's lightweight, inexpensive, and requires no fire. The limitations: it doesn't remove turbidity, doesn't work well in cold water, and Cryptosporidium resists standard halogen treatment. Know what you're dealing with before choosing your method.

Bleach (Sodium Hypochlorite)

The most accessible disinfection method. Standard household bleach at 6-8.25% sodium hypochlorite is effective against bacteria and viruses.

Requirements:

• Unscented bleach
• No added cleaners or thickeners
• Sodium hypochlorite as the only active ingredient

Dosing:

| Water Clarity | 6-8.25% Bleach per Gallon | 6-8.25% Bleach per Liter | |---|---|---| | Clear water | 8 drops (~1/8 teaspoon) | 2 drops | | Cloudy water | 16 drops (~1/4 teaspoon) | 4 drops |

For 1% bleach (unusual): use 40 drops per gallon.

The process:

1. Filter turbid water through cloth
2. Add the appropriate amount of bleach
3. Stir well
4. Let stand 30 minutes (clear water) or 60 minutes (cloudy water) before drinking
5. After contact time: treated water should have a faint chlorine odor. If no odor, repeat the dose and wait another 15 minutes.

Temperature matters: Cold water (below 10°C) significantly slows chlorination. Double the contact time in cold conditions.

Bleach shelf life: Sodium hypochlorite degrades over time, particularly after opening. Bleach stored at room temperature loses approximately 20% of its strength per year. Bleach older than 6 months stored at room temperature: use the higher (cloudy water) dose and rely on the smell test.

Calcium hypochlorite (pool shock): More stable than liquid bleach for long-term storage. Pool shock at 68-78% concentration is sold at hardware and pool supply stores. For water treatment: dissolve 1/4 teaspoon in 2 gallons of water to make a stock solution. Then add 1 part stock solution to 100 parts water. This provides the same 1-2 ppm residual chlorine. Pool shock can last 10+ years properly stored.

Iodine Tablets

Iodine tablets (tetraglycine hydroperiodide) are a standard emergency kit item. Compact, lightweight, no measuring required.

Tetraglycine hydroperiodide (Potable Aqua, Globaline):

• Clear water: 1 tablet per quart, wait 30 minutes
• Cold or cloudy water: 2 tablets per quart, wait 60 minutes
• Shake the bottle before opening to verify tablets are dry (crunching sound = good; solid resistance = may be degraded)

The taste problem: Iodine-treated water has a strong medicinal taste. Reduce it by:

• Adding vitamin C (ascorbic acid) after the full contact time — neutralizes residual iodine and the taste. Never add before contact time is complete.
• Adding powdered drink mix or electrolytes

Who should avoid iodine:

• Pregnant women (thyroid effects on fetus)
• People with thyroid conditions
• People with shellfish allergies (iodine cross-reactivity, though this is debated)
• Extended use (more than a few weeks)

Chlorine Dioxide Tablets

Chlorine dioxide (Aquatabs, Potable Aqua with chlorine dioxide, MSR Aquatabs) is the most broadly effective chemical treatment. It works against bacteria, viruses, Giardia, and — given sufficient contact time — Cryptosporidium.

Dosing: Follow the specific product label. Most tablets treat 1 quart or 1 liter.

Contact time: 4 hours for Cryptosporidium effectiveness (not 30 minutes). For bacteria and viruses only: 30 minutes.

The tradeoff: Longer contact time, but the broadest pathogen coverage of any chemical treatment. If Cryptosporidium is a potential concern (wilderness water from areas with cattle, beaver, or other infected animal populations), chlorine dioxide is the chemical disinfectant of choice.

Color: Treated water may have a slight yellow tint. This is normal.

Comparison of Methods

| Method | Bacteria | Viruses | Giardia | Cryptosporidium | Contact Time | Taste | |---|---|---|---|---|---|---| | Bleach (chlorine) | Yes | Yes | Yes | No | 30-60 min | Mild chlorine | | Iodine tablets | Yes | Yes | Yes | No | 30-60 min | Strong medicinal | | Chlorine dioxide | Yes | Yes | Yes | Yes (4 hours) | 30 min - 4 hrs | Minimal | | Boiling (1 min) | Yes | Yes | Yes | Yes | Immediate | Flat | | UV (SteriPen) | Yes | Yes | Yes | Yes | 60-90 seconds | None |

When Chemical Treatment Alone Isn't Enough

Cryptosporidium risk scenarios:

• Wilderness water downstream from cattle grazing
• Beaver-inhabited water (beaver fever = Giardia and Crypto)
• Flood water (fecal contamination from sewage overflow)
• Any water source with evidence of heavy animal use

In these scenarios: use boiling, UV, or chlorine dioxide with the full 4-hour contact time.

Chemical contamination: No chemical disinfectant removes dissolved chemicals, heavy metals, or nitrates. If chemical contamination is suspected, disinfection is not enough.

Turbid water: Chemical treatment is less effective in water with high turbidity. Always pre-filter visibly dirty water before chemical treatment.

Stockpiling Chemical Treatment

Liquid bleach: Short shelf life (6-12 months active), heavy, and bulky. Fine for home storage, poor for go-bags.

Calcium hypochlorite (pool shock): Excellent for long-term storage. A 1-pound container can treat thousands of gallons. Keep dry, sealed, and away from organic material.

Iodine tablets: 4+ year shelf life in the original sealed bottle. Excellent for emergency kits and go-bags.

Chlorine dioxide tablets: 4-5 year shelf life, compact, broadest coverage. The best all-purpose chemical treatment for preparedness stockpiles.

Recommended minimum: One bottle of chlorine dioxide tablets per person in your emergency kit. Calcium hypochlorite for extended home storage.