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Water Sterilization

Practical Approach to Water Purification

  • Camping with high quality source water: heat, mechanical or iodine resin filtration, or halogens in low doses can be used
  • Water with agricultural runoff and/or sewage plant discharge from upstream cities: treat with heat or with a two-step process of filtration to remove Cryptosporidium, then halogens to destroy viruses. A filter containing a charcoal element has the advantage of removing chemicals such as pesticides
  • Surface water in undeveloped countries, even if clear, should be considered highly contaminated with enteric pathogens. Heat alone is effective. Mechanical filtration alone is inadequate, while halogens alone will not kill Cryptosporidium and parasite eggs. A two-stage process of filtration, then halogens to destroy the viruses is needed
  • Cloudy water that does not clear with sedimentation should be pre-filtered or pretreated with alum, and then disinfected with heat or halogens. Filters may clog rapidly with cloudy water

Iodination techniques
added to 1 liter water

Amount for 4ppm
(starting water clear) 

Amount for 8 ppm
(starting water cloudy)

Iodine tabs

0.5 tab

1 tab

2% Iodine solution (tincture)

0.2 mL
5 drops

0.4 mL10 drops

Saturated iodine crystals

In water: 13 mL
In alcohol: 0.1 mL

In water: 26 mL
In alcohol: 0.2 mL

Chlorination techniques
added to 1 liter of water

Amount for 5 ppm
(clear water)

Amount for 10 ppm
(cloudy water)

Household bleach, 5%
Sodium hypochlorite 

0.1 mL
2 drops

0.2 mL
4 drops

sodium dichloroisocyanurate


1 tablet (17mg)

AzuaCure, Aqua Pure, Chlor-floc
Chlorine plus flocculating agent 


8 ppm / tablet

Notes: ppm = parts per million; measure with dropper (1 drop = 0.05mL) or tuberculin syringe

Halogen Contact Time Required to Disinfect Water

Concentration of Halogen Contact time in minutes at various water temperatures

  5ºC 15ºC 30ºC
4 ppm  180 60 45
8 ppm 60 30 15

*Adapted from Becker, H. Field Water Disinfection. In: Wilderness Medicine: Management of Wilderness and Environmental Emergencies. Ed: Aurebach PS.; CV Mosby, third edition, 1995.

Problems with Halogens

  • Taste can be unpleasant when concentrations exceed 4-5mg/l.
  • The potency of some products decreases with time and is affected by prolonged exposure to moisture or heat (tablets) and air (iodine crystals).
  • Liquids are corrosive and stain.

Taste May Be Improved By

  • Adding drink (Cool-Aid) flavoring after adequate contact time.
  • Using activated charcoal to remove halogen after contact time.
  • Reducing the concentration and increase the contact time in clean water.
  • Adding a few granules/liter of ascorbic acid (vitamin C, available in powder or crystal form) or sodium thiosulfate (non-toxic, available at chemical supply) after required contact time. This changes the iodine to iodide (or chlorine to chloride) which has no color or taste.