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/pathogens 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 2 week long trek will 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 chlorine 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 assist including ‘standing’ (allowing sediment to sit on the bottom then separating) and flocculating (using alum to precipitate particles).  These will be most useful for turbid/cloudy water.

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 is 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.  Times to disinfection for various pathogens is as per the EPA standard as per the manufacturers’ testing/claims.


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.

Most (including the WHO) recommend simply bringing water to the boil.

The CDC and EPA recommend boiling for 1 minute.  Some recommend boiling for 3mins if at altitude (eg. above 2000m).  Assuming you are in a national park and can’t light fires an example of a ‘lightweight kit’ for boiling water will be an MSR pocket rocket plus a 4 oz gas canister which is 200g (not including mug/pot).

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.  They provide a rapid way of removing particles, bacteria, protozoa and some viruses in cloudy/turbid water – but if too turbid can clog the filter rapidly.


3. Halogenation

– Many products eg. Katadyn Micropur forte MF
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).  It is important to use the correct dose ie. Number of tablets/drops per litre of water.  Katadyn Micropur forte MF also contains Silver which helps ‘preserve’ water.  There are many problems with Iodine including allergy and use in thyroid disease and pregnancy.  Some state Iodine (compared with Chlorine) is relatively less effective against Giardia however there is limited evidence of this. Both Chlorine and Iodine (more so) can provide an unpleasant taste this can be alleviated by additional drops.  Iodine is becoming much less frequently used.


4. Chlorine dioxide

eg. MIOX by MSR; Aquamira Water Treatment Drops, Micropur MP1.
The MIOX 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.  The MIOX is scheduled to be discontinued. There are also liquid and tablet formulations (Aquamira, Micropur MP1) containing only chlorine dioxide with/without an ‘activator’ eg. Phosphoric acid (Aquamira) which are mixed together before being added to water. These have also been shown to be effective against Cryptosporidium though times are very variable with turbidity and temperature (30-240mins).

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 which will reduce its effectiveness.  Also in cloudy/turbid water its effectiveness is greatly diminished as the UV light won’t ‘penetrate’ as far.  Steripen make a special pre-filter for particulate matter.  It takes approx 90 secs to disinfect 1 litre, so 10 litres would take more time!  It is designed to be used in bottles of 0.5-1 litre (with wide ‘neck’), so not recommended for hydration bladders.

They provide a rapid, light, effective method of disinfecting clear looking water.

Some models come with rechargeable batteries (including solar charging).


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 chlorine (doesn’t remove cryptosporidium) or for slightly turbid water filtration (removes particles) plus UV radiation or chlorine.


Iodine &Chlorine Chlorine Dioxide MSR MIOX UVR Steripen FiltersPump/bottle Boiling
Removal  Bacteria Y Y Y Y Y Y
Removal Viruses Y Y Y Y N Y
Removal Giardia Y Y Y Y Y Y
Removal Cryptosporidium N Y Y Y Y Y
Taste effected? Y N N N N N
Weight per unit few g few g 99g 103g+ vary Gas+stove
Pack size tiny tiny small small Larger (pump)Smaller (bottle) Medium (gas+stove)
Batteries? N N Y Y N N
Time to workBacteria 30 15 15 Immed Immed Immed
Time to work Protozoa 120 (not crypto) 30- 240 < 240 immed immed Immed
Max Volume any any 4 litres 1 litre any Recepticle size
Short term Cost $10/box $10/box $160 $100 $90/50 Gas/stove




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