Water Purification

Gastrointestinal disease (including diarrhoea) is very common in wilderness travelers.  The vast majority of these diseases are transmitted through contaminated food and water.  Several different organisms are responsible for water-borne infection including bacteria (E.coli, Shigella, Campylobacter), Viruses (Hepatitis A, Norovirus), Protozoa (Giardia, Crypotosporidia) and occasionally Helminths (worms).  Whilst in many cases a trail ultramarathoner will be well at home in bed before illness sets in; someone on a longer trek may have a much more difficult time! 

There are many commonly used water purification methods available.  It is over simplistic to say that one is better than the other as there are advantages and disadvantages to each method.  For example whilst boiling is a very reliable method it requires a source of heat (eg. Gas stove) and to wait for the water to cool.  On the other hand iodine drops or tablets are very small in pack size and weight; however are ineffective against Cryptosporidium.  The aim is not to provide ‘sterile’ water but to remove enough pathogens to prevent illness in healthy individuals.  The US Environmental Protection Agency (USEPA) has set a standard to remove 99.9999% of bacterial contaminants (a 6-log reduction), 99.99% of viral contaminants (4-log) and 99.9% of protozoal contaminants (3-log).  When reading about purification methods you will see this standard referred to commonly.

There are some pre-disinfection methods which can also assist including ‘standing’ (allowing sediment to sit on the bottom then separating) and flocculating (using alum to precipitate particles).  General hygiene is critically important – washing hands after toileting, washing cooking pots/pans and keeping disinfection equipment (filters, steripen etc…) clean.  On prolonged or group trips it may be advantageous to bring antibiotics for ‘travellers diarrhoea’ eg. Ciprofloxacin (or azithromycin or bactrim for those pregnant, for children and for those allergic) as well as anti-diarrhoeal medication (eg. Imodium).  These antibiotics will only be effective against bacteria.  It is possible to be vaccinated against Hepatitis A.  Other medication are effective against Protozoa.

There is minimal data comparing different methods for removing different pathogens however there is one trial by MB and EH Lee in 2005 comparing Iodine, Chlorine Dioxide tablets, MIOX and Steripen against Cryptosporidium (an often difficult to remove protozoa).  The Steripen was most effective in this role.  Below is presented a brief description of available methods with some common examples found in Australian Outdoor Stores and a summary table.  Cost price of units is very variable however for comparative fairness in the table below was approximate online cost in US dollars from REI for a ‘midrange’ unit.

1. Heat – Boiling

Boiling water (100 deg C) is a very effective and reliable method of disinfection.  Bacteria and Viruses will die within seconds of reaching boiling point whilst Protozoa take 2 mins at 60 deg C to die.  The CDC in the US recommends boiling for 3mins at altitudes above 2000m.  Assuming you are in a national park and can’t light fires the minimum ‘gear weight’ for boiling water will be an MSR pocket rocket plus a 4 oz gas canister which is 200g (not including mug of course).

2. Filters eg. Katadyn, MSR

These include ‘pump’ filters and filters that attach to bottles.  Viruses are very small (less than 0.1 micron) and cannot be eradicated by filters alone.  Filters can also get clogged with the potential to force pathogens through.

3. Halogenation – Many products eg. Katadyn Micropur

Includes Iodine and Chlorine tablets and drops.  Whilst effective against bacteria and viruses they are not effective against protozoa (especially Cryptosporidium) or helminths.  They also require contact time in order to disinfect properly eg Katadyn Micropur forte quotes 30mins for bacteria and viruses and 2hrs for Giardia (a protozoa).  This will also vary with the turbidity and temperature of water (times quoted are at room temp for clear water).  There is a potential problem with people using large amounts of iodine and thyroid disease.  Iodine should be avoided in those with thyroid disease or who are pregnant.  Whilst these agents can provide an unpleasant taste to water there are several ways of improving this – flavouring, Vitamin C, and ‘antichlorine drops’.

4. Chlorine dioxide eg. MIOX by MSR

Used initially by the military it uses electric current through a salty solution to create mixed oxidants of free chlorine, chlorine dioxide and ozone. The oxidants are then added to the water.  Reported times for disinfection include 15mins for viruses and bacteria; 30mins for Giardia and 4 hours for Cryptosporidia.  There are also liquid and tablet versions containing only chlorine dioxide (eg. Aquamira) with/without an ‘activator’ eg. Phosphoric acid which are mixed together before adding to water. These have also been shown to be effective against Cryptosporidium though times have similarly been reported up to 4 hours especially in cold, turbid water.

5. UV radiation eg. Steripen

Uses UV light to disinfect.  It has been shown to be effective against bacteria, viruses and protozoa including cryptosporidium.  Clearly though similar to drops/tablets it will not remove particulate matter and its effectiveness will be reduced in such circumstances.  Steripen also make a special pre-filter for particulate matter.  It takes approx 90 secs to disinfect 1 litre, so 10 litres would take more time!  Also it should not be used in hydration bladders – it is designed to be used in bottles of 0.5-1 litre.

6. Combination

Clearly with different advantages for different methods this raises the possibility of using 2 combined methods depending on circumstances eg. Filtration (dosn’t remove viruses) plus iodine or chlorine (doesn’t remove cryptosporidium) or filtration (removes particles) plus UV radiation.