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Quality Water Filters 4 You Posting Page
Wednesday, June 26, 2013
Many types of water filter, including carbon blocks and ceramic cartridges, have a rating known as “pore size”, which indicates how large the pores are that penetrate the solid mass of the filter. The water passes along these channels on its way through the filter, either pulled gently by gravity in the case of passive feed water filters or shoved through by water pressure. Since the water must pass through this part of the filter, the pores remove anything of their size or larger through the simple but effective process of physical sieving.
   
The pore size of a filter therefore determines what kinds of solid contaminants it can strain out of the water stream. Large pores provide quick filtration but only remove hefty particulate matter such as rust particles or grains of sand. Small pores can strain parasites from the water, and very fine pores are able to remove bacteria and in some cases viruses from the liquid also.
   
Filter pore size can be rated as absolute or nominal. Absolute pore size is the more important measurement because this is the actual size of the pores. Nominal pore size is based on the idea of the pore “acting like” a pore of that size, even though it is actually larger. The best measure of what filtration a carbon block or ceramic cartridge offers is absolute pore size.
   
The most crucial threshold in water filter pore size is 1 micron. This is the size at which cryptosporidium and giardia cysts are removed from the water. Larger pores allow the cysts to pass through and may allow these tiny intestinal parasites to infest those who drink the poorly-filtered water. Though neither infestation is deadly, both are unpleasant and best avoided – hence only filters with an absolute pore size of 1 micron or less should be used where there is any risk of either of these parasites being found in the water supply.
   
Absolute pores of 1 micron or less remove all or practically all cysts, while nominal 1 micron pores may allow up to 30% of parasitic cysts through. Bacteria are mostly removed by pores of this size, too, though some of the smaller types may occasionally slip through.
   
“Ultrafiltration” occurs when pore size falls in the 0.1 micron to 0.01 micron range. This is enough to remove viruses, especially at the finer end of the scale. Colloidal silica, gelatin, milk proteins, and endotoxin pyrogen can also be removed by pores of this size. Nanofiltration involves pore sizes of 0.001 microns, and takes out sugars, synthetic dyes, and aqueous salts. Reverse osmosis is actually filtration with an even smaller pore size than nanofiltration filters – it removes even the extremely fine metal ions that can be found in solution and leaves literally nothing but water, the only substance able to pass through the filtration membrane.
   
Ultimately, filter pore size is what determines whether or not a water filter can protect you from bacteria, viruses, parasitic cysts, and other particulate contaminants in your drinking water. Knowing generally what pore sizes defeat which threats helps you judge better whether a filter is insufficient, just right, or “overkill” for your particular filtration needs.


by: Chris Tracey

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