Water is essential for life and clean water is essential for your health on the trail.  Contaminated water presents such a hazard, you just can't afford to be without a good filtration or purification system.  Many options exist and precisely the one you need depends in part on your water source.  Let's discuss the possibilities.

Chemical Purification:

The least expensive and most portable water treatment method is chemical purification.  Newer chemicals are more effective than those of yesteryear but still have their drawbacks.  The old standby of Iodine is still available, but Iodine has lesser efficacy than some of the other chemicals such as Chlorine Dioxide.  It is considered effective against viruses, bacteria and giardia when used correctly.   Other drawbacks with Iodine include bad taste and discoloration of the water. 

Chlorine Dioxide products such as the Aquamira Drops are stated as effective against viruses, bacteria, giardia and cryptosporidium when correctly applied.  Besides being effective against a greater range of pathogens than Iodine, it has greater pathogen killing power than Iodine as well.  In drop form, you can be drinking treated water in around 20-30 minutes, similar to Iodine.  While tablet form products such as MicroPur are incredibly compact and handy to carry, they can take 30 minutes - 2 hours for effective treatment, quite a long time when you've been hiking in the sun and just reached your watering hole. 

Chlorine Dioxide is intended for use in clear water only.  Cloudy water must be filtered before treatment. Chlorine Dioxide normally leaves no objectionable taste other than a possible slight taste of chlorine.  It is actually stated to improve the taste of water and can be added to water that is to be stored.  Advertised shelf life for chemical agents varies by manufacturer but is generally several years.  It should be noted too that chemicals only address pathogens in the water, not sediments, suspended matter or pollution. 

Filters:

Filters remove sediment, suspended matter and pathogens as well by means of micro-filtration, we're talking filtration down to microns.   Filters use a variety of media and often multiple filtration levels to do the job.  The least expensive use a paper / glass fiber type filter.  The included models from Katadyn filter down to 0.3 microns and include the Hiker, Hiker Pro, Vario and Base Camp. 

Along with the primary glass / fiber filter, these models all include activated carbon.  Carbon is an absorbent which improves water taste and aids in removing certain dissolved pollutants such as pesticides.  In the Hiker, Hiker Pro and Base Camp, the carbon is built into the filter.  Once the carbon is saturated, it becomes ineffective and that layer of protection is lost even if the filter still has life left in it.   The Vario improves on this aspect by allowing replacement of the carbon separately and also includes a ceramic pre-filter disk to extend the fiber filter life. (Carbon life varies depending on the quality of water that passes through it. One rule of thumb is to replace carbon once treated water begins to no longer taste fresh.)

The higher end Katadyn models utilize a ceramic main filter.  Included models are the Pocket, Mini Ceramic, Combi and Gravidyn.  These models filter water nasties down to 0.2 microns.   In addition to finer filtration compared to the glass fiber filters, the ceramic models have insane capacity, as much as 13,000 gallons per element compared to around 200 gal for the glass fiber. 

The Combi and Gravidyn include carbon as well for it's absorbent qualities.  Though the Pocket and Mini Ceramic don't include carbon, it can be added easily with a Carbon Cartridge that attaches to the output hose and provides a replaceable carbon source. Carbon Cartridges can also be used with other models above when their carbon becomes saturated. There is one extra feature to mention with the the Pocket Microfilter and Mini Ceramic.  These models have silver granules impregnated in the ceramic element.  Silver is a natural anti-microbial with pathogen killing action working along with the filtration.

UV Purification:

The new kid on the block is the UV purifier which uses UV light to kill any and all meanies in your drink.  UV light is considered effective against viruses, bacteria, giardia and cryptosporidium.  These units are highly effective and extremely fast.  The MUV treats a liter of water in as little as 75 seconds compared to 20-30 minutes for the fastest chemical agents.  MUV is recommended for clear water, though cloudy water can be treated with longer treatment times and stirring to insure effective exposure to the UV light.  Like chemical purifiers however, the UV light only kills organisms.  Pollutants and suspended matter remain.  For this reason, both chemical and UV purifiers are often used in conjunction with pump filters for maximum safety, especially in 3rd world areas and very dirty water sources. 

Bottom Line:

So now we know the line-up, which one do you need for your application?  As mentioned at the outset, your expected water source dictates your best choice.  We're chiefly working against 3 types of pathogens.  Protozoa are the largest at around 1 -15 microns.  Included in this classification are amoeba, giardia and cryptosporidium.  Next come bacteria at 0.2 - 5 micron.  Bacteria include E-coli, salmonella and cholera.  Last are viruses such as polio, hepatitis and such.  These little boogers measure up at only 0.02 - 0.2 micron.

Viruses tend to come from human contribution, so they appear more prevalently near populated areas.  Protozoa and bacteria often originate from human or animal waste.  As you can see from the numbers above, mechanical filtration is effective against bacteria and protozoa.  They actually filter a large amount of viruses as well since viruses tend to be bound to other solids in the water.  Because of their danger though,

• purification is recommended for water sources near populated areas where viruses may exist 
• Filters are considered ideal for more remote areas and obviously a combination of the two gives optimum security

The Pocket Microfilter is easily our best selling model, accounting for more sales than all our other units combined.  It does present a higher initial cost however.  For folks needing a filter less frequently, and those who normally filter from high, clear streams, some of the less expensive filters work wonderfully too, especially when combined with purifiers.  For those filtering for more than one person or filtering more often, the high capacity of the Pocket makes it the most economical choice. 

There is a great deal of information that can be obtained from the product manufacturers themselves, especially when you wish to be on the cutting edge of the science.  Links to manufacturer web sites can be found on the pages for the individual products mentioned, beneath the manufacturer logo.  Hopefully this brief summary has been helpful in your search for a water treatment solution.   With your first use, raise a canteen and drink to your own health!