07 Water Purification: Physical Treatment: Microfilters
Description
This article is from the Water Treatment FAQ, by Patton Turner with numerous contributions by others.
07 Water Purification: Physical Treatment: Microfilters
Microfilters are small scale filters designed to remove cysts,
suspended solids, protozoa, and in some cases bacteria from water.
Most filters use a ceramic or fiber element that can be cleaned to
restore performance as the units are used. Most units, and almost all
made for camping use a hand pump to force the water through the
filter. Others use gravity, either by placing the water to be
filtered above the filter (e.g. the Katadyn drip filter), or by
placing the filter in the water, and running a siphon hose to a
collection vessel located below the filter (e.g. Katadyn siphon
filter). Microfilters are the only method, other than boiling, to
remove Cryptosporidia. Microfilters do not remove viruses, which many
experts do not consider to be a problem in north America. Despite this
the Katadyn microfilter has seen considerable use around the world by
NATO-member militaries, WHO, UNHCR, and other aid
organizations. Microfilters share a problem with charcoal filter in
having bacteria grow on the filter medium. Some handle this by
impregnating the filter element with silver such as the Katadyn,
others advise against storage of a filter element after it has been
used. The Sweetwater Guardian suggests using a freezer for short term
storage.
Many microfilters may include silt pre filters, activated charcoal
stages, iodine resin. Most filters come with a stainless steel
prefilter, but other purchased or improvised filters can be added to
reduce the loading on the main filter element. Allowing time for
solids to settle, and/or prefiltering with a coffee filter will also
extend filter life. Iodine matrix filters will kill viruses that will
pass through the filter, and if a charcoal stage is used it will
remove much of the iodine from the water. Charcoal filters will also
remove other dissolved natural or manmade contaminates. Both the
iodine and the charcoal stages do not indicate when they reach their
useful life, which is much shorter than the filter element. If you
are depending on the stage for filtering the water you will have to
keep up with how much water passes through it.
New designs seem to be coming out every month. The best selling
brands seem to be the PUR, and Sweetwater Guardian. The Katadyn
doesn't sell as well to outdoor enthusiasts due to its high cost, but
for years it was state of the art for water purification and still has
a loyal following, especially among professionals in relief work.
Below is the data on a few of the more common units, for a excellent
field test of some common units, see the December 96 issue of
Backpacker magazine.
Note that the first price is for the filter, the second for the
replacement filter. The weight is from manufacturers literature if it
was not listed in the Backpacker article. Filter life is from
manufacturers literature and should be taken with a grain of salt.
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