Water Drinking Strategies By Tony Ledenko

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. 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.

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.
 

Types of Bacteria:

                         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.