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Water purification and filtration

Boondockers can use harvested water to stretch the driking water supply and extend trip legnth. With approprate care harvested water can be used for consumption.

Making water drinking water has two components; filtration and purification. Filtration means removing particulate matter such as mud, sand, large plant cells such as algae and phytoplankton. At fine enough filtration levels enteric pathogens may be removed. Purification means killing he pathogens that can be common in natural water sources that can make you sick. Many methods will do both, but not all.

Not all water needs to be both filtered and purified. Some natural water from fast-flowing streams or springs may have very little particulate matter in it and be very clear already, and will only need purification if you're going to drink or cook with it. It can be fine to shower or wash dishes with as long as it's reasonably clear.

See this guide from the National Park Service about how to treat water. Also see REI's guide that wants to sell you stuff (But is still useful).

Mechanical filtration has become more common and less expensive. Straw-like personal filters are now sold, for example.

The main drawback of filtration are clogging (addressed by adquate pre-filtration) and relatively slow filtration rate. Look for filters that can be gravity fed so the filtration time isn't tiresome.

  • 1-micron filters will filter protozoa like giardia spores and Cryptosporidium out of clear water.1) These may be labeled NSF/ANSI Standard 53.2)
  • 0.1-micron will filter all waterborne enteric bacteria from clear water
  • Consumer-grade filtration will not remove viruses because of their tiny size.

The CDC points out the difference between nominal and absolute3) micron ratings: “The absolute 1 micron filter will more consistently remove Cryptosporidium than a nominal filter.”4)

Fabric filtration

Much sediment can clear itself if allowed to rest; Let the water sit still for 30-60 minutes, then pour or siphon off the top and leave the sediment behind.

Alternately you can strain from coarse to fine: through a strainer then a coffee filter, for example.

  1. sieve
  2. bandana, cloth, or paper towel
  3. coffee filter
  4. actual water filter

Note that filtration through layered fabric is much better than nothing. In India, for example, researchers found:

a filter made of four layers of worn cotton material held back more than 99 percent of all cholera bacteria - using more layers or newer cloth slowed water collection too much.5)

Both saris and coffee filters have about a 20-micron pore size:

Typically coffee filters are made up of filaments approximately 20 micrometres wide, which allow particles through that are less than approximately 10 to 15 micrometres6)

The best we should assume for with such ad hoc filtration is a pathogen reduction. Combination with SODIS (below) might be useful.

Further reading: this page is about reducing airborne pathogens with homemade mask materials, but may be useful.


It is possible to disinfect suspect water by several means; it is not practical to purify water that is chemically or mineralogically tainted.

When possible, multiple approaches should be used on the water. The first step in any disinfection routine is to clarify or filter the water to clear it of particulate/foreign matter.


Heat is an effective way to kill waterborne pathogens. Note that not all pathogens that survive heat treatment are found in water:

Sterilisation of water (killing all living containments) is not necessary to make water safe to drink. For example, boiling may not be effective against bacterial spores such as Clostridium which can survive at 100°C (212°F), however, as Clostridium is not a waterborne enteric (intestinal) pathogen, ingestion will not cause infection.
All waterborne enteric pathogens are quickly killed above 60°C (140°F), therefore, although boiling is not necessary to make the water safe to drink, the time taken to heat the water to boiling is usually sufficient to reduce pathogens to safe levels. Allowing the boiled water to cool slowly will also extend the exposure of waterborne enteric pathogens to lethal temperatures. Boiling also gives a simple visual indicator that a high enough temperature has been reached when a thermometer is not available.7),8)

Typical times at temperature:

  • 212F (100C, boiling at sea level) - 0 minutes
  • 158F (70C) - 1 minute.
  • 145F (63C) - 30 minutes to achieve pasteurization.9)
  • 130F (55C) - 10 minutes to kill giardia10)

Given that information, solar ovens or similar devices that can hold 140F-160F for long periods may provide unpowered heat disinfection. Also see SODIS below.

Chemical treatment

Chemical treatments require no power making them especially useful offgrid. Treated water can be run through a charcoal filter (Brita) after treatment to improve flavor. Pouring treated water energetically from container to container may allow chemicals to outgas.

The most famous treatment is chlorine bleach. For these purposes, unscented cheap bleach is better than $$$ bleach with additives. It is used at 8 drops/gallon of 6% bleach (6 drops of 8.25% bleach11) and then wait 30 minutes.12) A 5-gallon bucket would take 1/2 teaspoon of normal bleach. Note that bleach purfication does not reliably kill Cryptosporidium. Increasing wait time to 45mins will kill norovirus and giardia.

Bleach is cheapest by the laundry bottle but is also available as tablets. An eye dropper bottle will store and dispense l1quid bleach accurately.

Iodine tablets were popular with hikers in the past but impart a noticeable flavor to the water. Some come with an additional tablet to be used after the waiting period, intended to remove iodine flavor and color from the water. Iodine treatment should be avoided by people with shellfish allergies.13)


Hybrid chlorinating-flocculating chemicals such as Flo-Chlor show promise for turbid water, but they are not widespread yet. Flocculation means suspended particles are drawn together and either drop out of suspension or (less commonly) float to the top.


SOlar water DISinfection (SODIS) uses sunlight to improve water quality.14) It relies on minimal solar heating of the water, then ultraviolet irradiation in the container. Since UV does not travel far in water, relatively small bottles of 2L or less are used.15) Bottles should be clean and free of surface scratches.

Efficacy: at 86F water temp, ~6 hours of sun (less than 50% cloudy) will significantly reduce diarrhea reduction-causing pathogens in the water. The actual spect is 500w/m2, which is half of the lab spec for solar panel output (1000w/m2) In fully clouded conditions it takes two full days for disinfection to occur. SODIS at normal temperatures might best be thought of as an adjunct method to be used alongside other methods rather than a disinfection method unto itself.

Optimal function may be found by placing small-diameter bottles in a solar oven. This will enhance UV collection and heating.

water/purification.txt · Last modified: 2022/08/14 19:37 by frater_secessus