Bluedome

They say travel broadens the mind. It's also fair to say it loosens the bowel as well, something you can do by tucking into dodgy food, but just as easily by drinking unsafe water. My attitude to upland water always used to reasonably cavalier. I heeded the common sense advice about not taking water below farms, but higher up, I'd be quite happy to dip my hand in a stream for an on-the-move slurp without a second thought. The first time I did take a second thought was as I set off on a two week coast-to-coast backpack across the Scottish Highlands, three days after a certain nuclear reactor in the Ukraine showered Western Europe with radio-active fallout.

Official opinions on slurping water directly from upland streams and rivers in the UK are fairly non-committal, but certainly the view from the National Rivers Authority is that you shouldn't be lulled into a false sense of security by drinking water in a wilderness location. "Pesticides get into the water by all sorts of diffuse means," I was told, "and it doesn't necessarily mean that just because there isn't a pipe dumping them straight into the water that they aren't there."

Many prime walking areas overseas have become victims of their own popularity. Mountain streams in many parts of North America, for example, now harbour giardia cysts, directly attributable to human activity. So just what are the dangers of drinking with gay abandon from water sources which may not be as wild and as pure as you think? Some can be seen, others not. They're measured in microns (1/1000th of 1mm), and just to give you an idea as to how far you should be straining your eyes, if not your water, the full stop at the end of this sentence is about 500 microns.

PARTICULATE MATTER
Everything from dirt and sediment (the fine "flour" in glacial rivers is one of the fastest working laxatives around), rust, scale and radioactive particles - 100 microns +
Removed by: filtration, or allowing suspended particles to settle out.

BACTERIA
Cholera, Salmonella, Escherichia Coli, Typhoid - 0.2-10 microns
The list of nasty things you can pick up from bacteria is as long as a roll of toilet paper. Pneumonia, food poisoning, intestinal fever and dysentery are just some of the delights in store for those who drink untreated water.
Removed by: microfiltration, boiling or chemical treatment.

PROTOZOA
Giardia, Cryptosporidia, Entamoeba - 5-15 microns
Undoubtedly the most infamous is the Giardia cyst - responsible for the majority of water-borne illnesses in North America. It survives as a tough-walled cyst for months in water. Drinking as few as six can cause infection, and when they reach the gut, they rapidly break down to produce up to a million baby parasites within 10 days. The classic symptoms are abdominal cramps, diarrhoea, vomiting, and what the Americans politely call "gas" - both ends!
Removed by: microfiltration, chemical treatment or boiling.

VIRUSES
Polio, Hepatitis A - 0.004-0.1 microns
The smallest nasties to be found in water, and undoubtedly the most dangerous. If you're travelling in third world countries where these diseases are prevalent, then you'd be unwise not to have been immunised. But there are other water-borne viruses which can cause diarrhoea, flu and fever.
Removed by: though they often attach themselves to larger hosts, the smallest can't be guaranteed to be removed by filtration. They have to be killed off either by boiling, or by chemical treatment - chlorine or iodine.

CHEMICAL POLLUTANTS
Pesticides, fertilisers, diesel fuel, heavy metals from mines
More likely to be found in lowland areas, but not exclusively so.
Removed by: activated carbon will remove organic chemicals by adsorption, where the molecules stick to the carbon - but not dissolved minerals.

WHY FILTER?
Boiling kills off all water-borne pathogenic organisms. Trouble is, you have to maintain the water at a rolling boil for 5 minutes - an extra burden on your fuel supplies. It's further complicated by the fact that boiling temperatures decrease with altitude, dropping 5 degrees C every 5,000 feet. At 15,000 feet - not untypical for some Himalayan treks - a boiling temperature of 85 degrees C won't guarantee to kill off hardier bugs.
Apart from the resultant taste, chemical tablets work fine, but they don't clean the water, and they do need to be left long enough to work properly. Filter/purifiers are the best option for speed and convenience.

THE KIT
Some companies make gravity-fed filters, but their flow rate can be painfully slow compared to the pump-action models featured here. The flow rate figures are variable dependent on the quality of raw water available. Suck up water from a pool of sludge, and you'll do your filter in very quickly. If, on the other hand, you're selective, taking water from a clear source, using the float where provided to keep the intake tube out of any sediment, you'll prolong the life of the filter considerably.
     Some manufacturers quote a finite life for their products in terms of a maximum volume of water treated. I haven't repeated them here simply because there's no way of knowing what water quality they're based on, and in some instances the figures aren't even available.
     It's all too easy to become pre-occupied with a filter's performance where the contamination is something you can see. What counts is how it handles the nasties that only show up under an electron microscope. Short of taking the whole lot to a public health laboratory, I've worked on the basis of supplied technical data, and judged the different models on their ease of use, and the palatability of the filtered or purified water. In fact, all filter out particulate matter, cysts and bacteria - those with added iodine provide the only sure treatment for viruses.

FILTRATION
The minimum pore size is what counts when it comes to trapping bacteria measuring .3 or .4 microns, or larger protozoa and dirt. Filters are either membrane or surface types, which quickly get coated in gunge but are easy to clean, or depth filters, made of thick porous materials like activated carbon or unglazed ceramic. The latter rely on progressive filtration, and so contaminants move further into the material. Some depth filters are partially cleanable, either by backwashing, or by brushing the outer surface.

PURIFICATION
An additional element of treatment which doses the water with iodine to kill off bacteria and viruses. In areas where viruses are likely to be prevalent - notably third world countries with poor sanitation, this kind of unit provides the best peace of mind. Not everyone likes the taste of iodine, however, and prolonged use beyond three months is not recommended. Activated carbon filters will remove the taste of iodine. The Pur filters can be fitted with an optional carbon cartridge attachment which does this. If you're using a carbon or ceramic filter in such risk areas, use chemical tablets as well to be absolutely certain.