Water purification

This article was last modified Nov 13th 2011 by Jel Coward

Water is essential for life, but it can and does transmit disease in countries in all continents. The most predominant waterborne disease, diarrhoea, has an estimated annual incidence of 4.6 billion episodes and causes 2.2 million deaths every year (2010 WHO).

Terminology

A number of different terms are used in the field, but fundamentally in a wilderness setting we are interested in two types of water based on usage:

• Potable Water is water considered pure enough to be consumed or used with low risk of immediate or long term harm. This is subjective and quality varies around the global.

• Sterile Water: water for injection  water for parenteral use, prepared by distillation or reverse osmosis and meeting certain standards for sterility and clarity; it may be specified as sterile if it has been sterilised and as bacterio-static if suitable antimicrobial agents have been added.

Use

In the Wilderness setting there are a number of water uses:

• Potable water:

  • Cooking

  • Drinking

  • Wound Cleaning

• Sterile water:

  • For injection

• Untreated water:

  • Washing

  • External Cooling of a person with heat illness
    

For planning purposes a daily individual average is 6 lt for drinking and 4 lt for cooking and washing up. Activity levels, thermal stress, illness and food/cooking will all impact on this estimate.

Types of contamination

There are three types of water-borne pathogens (disease-causing micro-organisms) which can cause significant issues:

Protozoa include cryptosporidium and giardia (the cause of giardiasis or "beaver fever"). Protozoa are increasingly widespread. They are larger than 1.0 micron (one-millionth of a metre.)

Bacteria exist in water all over the world. Most are harmless, but some cause sicknesses such as diarrhoea and dysentery. Most bacteria are about 1.0 micron in size, although some, such as the disease-causing campylobacter, can be as small as 0.2 microns.

Viruses cause hepatitis, polio, and other diseases. They can be present in any water contaminated by human waste. Most viruses are in the 0.1 micron size range, but they can be as small as 0.002 microns.

In addition other pollutants can impact on the body.

Sediment

In-organic and organic matter can enter water from many sources. Often the impact is benign beyond discolouration and aesthetic issues (e.g. Tannins). Glacial streams contain milled rock material that can cause gastrointestinal problems.

Chemical

A number of sources of chemical pollution can impact on wilderness water sources. These include pesticides and heavy metal leaching from underlying rocks.

A simple approach to selecting a water source is to target fast flowing clear water if it is available.

Assessing the risk from a water source

Look smell and taste are unreliable when assessing a water source. For example, Vibrio Cholerae and E.Coli can survive indefinitely in water that may look as though it is good,  and Salmonella, Shigella, Hep A, Crypto can survive frozen for weeks to months.

Contracting illness from the water depends on:

• Number of organisms consumed
• Virulence of organism
• Strength of host defence

Giardia, Cryptosporidium, Shigella, Hep A, E.coli have a small “infectious dose” (Minimum infective concentration or MIC) and are thus easily caught.

Bottled water can be unreliable because it is easily to counterfeited.

Carbonated water is usually harder to ‘fake’ and the process of carbonation actually inactivates many infectious agents.

Given the above, our best source of water is one that we know to be safe.  This is usually water that we have treated ourselves or water from a good municipal supply.

Water testing

There are a number of small portable testing units on the market (e.g. Delagua Oxfam water testing kit). However,  these are still quite bulky and expensive (~ukp2000).  Definitive tests require time and equipment that are not viable in a wilderness setting. Therefore water-borne pathogens must be considered present and the water treated accordingly.

If chemical pollutants are suspected, alternative water sources should be used.

Storage

Storage of water should be minimised in the wilderness setting due to:

Incomplete purification methods and inability to control temperature can result in bacterial levels increasing with time;

A potential solution is the use of halogens which remain active in the water and prevent a build up of bacteria. For example water treated with Chlorine should retain a free residual of 0.2 – 0.5 mg/l after 30 min contact time.

Water purification methods

It is not possible to produce sterile water in the field setting so in this section we have broken down the options available to create potable water in the field. There is no perfect solution and the pros and cons of the options have been listed. It is pragmatic to always have a back up water purification method.

Some people talk about the difference between purification and disinfection

Purification - removal of chemicals and particulate matter which affect colour, taste and odour. Purification may not remove enough micro-organisms to be potable

Disinfection – removal or destruction of harmful organisms to produce “potable” water with minimal microbial hazard

*The boiling point of water is always above 60°C even at 8000m. World Health Organisation (WHO) advice supports the need to bring water to a boil, however concerns around Hep A means that the US Centre of Disease Control (CDC) and US Environment Protection Agency advise boiling for 1 min.

Giardia – need to achieve 70°c for 10 minutes for 100% and 50°c for 10 minutes for 95% eradicatin

Hep A – 98°c for 1 minute for eradication

If fuel is an issue – boil for 1 minute, cover and leave to cool should be adequate for most enteric pathogens

At altitude – CDC recommends 3 minutes if >2000m

[Special mention is needed for the Guinea worm (Dracunculus medinensis).  The larvae of this worm are carried in 'water fleas'. It is present only in some areas of Africa.  90% of the world's guinea worm is now in South Sudan but it is also present in Ethiopia, Mali, Ghana and Chad has recently had its first cases again since 1998.  Globally the battle is being won against this painful and potentially crippling disease.  There are now only 1800 cases, which is down from 100,000 in the late 1980's.  A simple nylon filter removes the water flea and thus prevents disease transmission.]

Summarising water treatment

Bringing water to the boil works well for all pathogens (with the possible need to boil for 3 minutes if at >2000m altitude).  But this is time consuming and requires fuel.

Chlorine dioxide is tasteless and is effective against all pathogens (some longer wait time is needed for cryptosporidium).  The product is cheap and available in different forms.

These two methods are likely to be at the top of the list for effectiveness, palatability and convenience.

Halogens (chlorine or iodine) add taste to the water but this can be improved by adding vitamin C tablets.  They leave residual disinfectant activity in the water, which can be good in some situations.  Iodine should be avoided for more than a very short time in people with thyroid problems and pregnancy. Avoid iodine in iodine allergy.

Filtering is good for particulate matter and although it removes protozoa and bacteria it does not remove viruses.  If weight/effort is not an issue then it is a nice addition to the other methods, or using a filter with built in halogenation or silver as a disinfectant will remove all pathogens.  In Guinea worm endemic areas water should be filtered to remove the water flea which is a vector for that disease, a simple nylon filter suffices for this.

Further reading

SODIS - solar disinfection using clear plastic bottles and the sun

References

Anderson SR et al 2008 Expedition and Wilderness medicine, Oxford medical Publication
Auerbach PS et al 2007 Wilderness Medicine, 5^th Edition, Mosby
CDC: website (http://wwwnc.cdc.gov/travel/page/water-treatment.htm)
Davis J, Lambert R, 1997 Engineering in Emergencies, SRP, Exeter
WHO: website (http://www.who.int/water_sanitation_health/resources/en)
WHO: Water for Health - WHO Guidelines for Drinking-water Quality
WHO: Guidelines for Drinking-water quality

Current affairs:

The debate about how long it is necessary to boil water for at altitude to render it safe to drink.

Myth busters:

The temperature of boiling water at high altitude is too low to kill bacteria.