A water filter or purifier takes the hassle out of boiling water to make it safe to drink – and you need to do this more often than you might think. Hut water tanks can become contaminated if an animal falls inside and alpine tarns and streams are regularly spoiled by animal droppings.

Water-borne hazards
Water filters and purifiers are designed to protect users from water-borne illnesses like giardia, campylobacter, salmonella and E. coli.

When to use
If you’re carrying a filter or purifier, you should use it every time you collect water for drinking, even in the remote backcountry where the risk of contracting a water-borne illness might be small. The risk increases in more popular areas, where people might go to the toilet near waterways
or where animals are grazed.

In urban environments, a disaster like an earthquake can make a water filter or purifier a life-saver. Hut water should always be boiled or filtered/purified before drinking.

How to filter
Always check – if possible – what’s in the water upstream to avoid contamination. Giardia slowly sinks, so draw your water where the flow is slow-moving, away from the bank. Avoid muddy water as it can contain bacteria stirred up from the streambed and will also clog filters.

Filters
Filters are the first line of defence against larger-than-a-virus protozoa and cryptosporidia such as Giardia Lamblia and parvum. Filters remove sediment and organisms down to 0.4-0.1 microns.

Purifiers
Purifiers kill the much smaller viral and bacterial organisms. UV light is the most common and effective form of purification. It is important to follow instructions precisely to ensure the purification process is adequate.

The full system
Some systems use both a filter and purifier. The filter removes sediment and organisms down to 0.2 or 0.1 micron and the purifier deals with anything smaller. Unless the removal of organisms is at least 99.9%, there is a risk of infection.

Cleaning
To ensure the long life of your filter, clean it regularly. This is usually done by rinsing and can be accomplished in the field as well as at home.

Flow rates
The usual flow (or purification) rate is one to two litres per minute. It’s important to keep filters clean and free of sediment to ensure the flow rate remains high.

MSR Guardian Purifier $699.99
Pump-action purifier with hollow fibre filter to treat filthy water and remove viruses, bacteria, protozoa and particulate. Self-cleaning, impact-resistant, withstands freezing. For use in all environments from popular wilderness areas to developing nations. Flow rate 2.5-litre/min Cartridge life 10,000+ litres Weight 490g. www.ampro.co.nz

SteriPEN Adventurer Opti $239.95
Compact handheld water purifier using ultraviolet light to kill up to 99.9999% of all waterborne bacteria and 99.99% of all viruses. Just insert batteries, push the on button once and insert it into your water container. Uses two CR123A batteries. Purifier life 8000-litres Output 1-litre/90sec Weight 110g. www.outfitters.net.nz

SteriPEN Classic 3 $189.95
Patented handheld water purifier uses ultraviolet light to kill up to 99.9999% ofall waterborne bacteria and 99.99% of all viruses. Pre filter included. Takes four AA batteries. Purifier life 8000-litres Output 1-litre/90sec Weight 225g. www.outfitters.net.nz

MSR TrailShot MicroFilter $129.99
Drink directly from the source or refill drink bottles, quick-deploy design, squeezing action pumps water through filter. Removes 99.9999% of bacteria and 99.9% of protozoa. Flow rate 1-litre+/min Cartridge life 2000 litres Weight 153g. www.ampro.co.nz

MSR MiniWorks EX $229.99
Field maintainable microfilter, removes 99.9999% of bacteria and 99.9% of protozoa and unpleasant tastes and odours caused by organic compounds. Flow rate 1-litre/min Cartridge life 2000+ litres Weight 456g. www.ampro.co.nz