posted on Sep, 4 2009 @ 05:55 PM
Water filtration techniques
As part of my ongoing attempt to detail various skills that can be used in SHTF situations and which I am competent to speak about I present the next
instalment. This is a general overview of filtration with respect to water and it assumes your trying to produce safe water in a SHTF situation. I
would suggest that everyone had a good quality water filter and use this info for emergencies rather than as your preferred method. A pre-built system
is always better than a scavenged one.
The removal of particles and pathogens from water can be done using three very broad methods. The first is chemical means, the second is physical
means and the third is membrane.
This involves adding something to the water to kill the pathogens or to cause the solids to precipitate out of the water. It also covers the technique
of adsorption, in which a material is added to pull something out of the water.
The most common method of chemically treating water is to add chlorine to the water. I will not go into details on this method because so many other
good threads have covered it in depth.
The most common type of Adsorption filters, are called Activated Carbon (AC) filters. The Brita ™ brand of pitchers use an AC filter in them. AC can
be used to remove low molecular weight organics, reduce halogens such as chlorine and improve taste. It will not remove dissolved salts. AC filters
have very low flow rates to give the water sufficient contact time to give up the contaminants to the AC. While AC removes chlorine and fluoride from
water it can not be used as the method of removing bacteria. Some municipal systems use a holding tank where water is mixed with chlorine to kill the
bacteria and then the water is sent through an AC filter to remove the chlorine taste. The other shortfall of AC filters is that they can quickly
become a breading ground for bacteria, because of this they have a short life span once placed in service.
Physical means involve some sort of barrier that has very small holes through which the water can flow yet which are small enough to block the
pathogen from travelling through.
As a general rule water filters need to block items as small as 0.1 microns to stop items like bacteria, though a filter rated for 0.003 is needed for
viruses. This method will not take dissolved solids out of your water but is a good way of making water biologically safe (ie biologics are removed).
Many of the common survival and travel filter use a barrier. The filter industry has several different was of rating filters. The most common is to
give the micron rating of the filter such as 0.1 microns, this info alone is pretty much worthless to you. You would also need to find the efficiency
rating to know how safe the filter is to use. A filter rated at 80% is considered industrial grade, but a 80% efficient 0.1 micron filter would still
allow 20% of the bacteria 0.1 microns and larger to get through. Filters for food are usually rated at 90-95% and true high grade filters are above
99%. Some manufacturers for drinking water filters rate them at 99.99% (basicly 100% but they list it lower in case of law suites). These filters
typically require pressure to force the water through the filter. These filters can be made of numerous materials depending on service, so materials
are not suitable for drinking water and are intended for other services.
These types of filters are very commonly used in drink factories, produce washing facilities (where they clean lettuce etc), car washes (newer ones
where they recycle the water), hospitals and industrial sites. Even fast food restaurants use these to filter the water used in the pop machines. The
materials you should be looking for are ceramic and polypropylene. Some industrial ones are made of cotton or fibreglass. Some fibreglass filters are
food safe, the common orange/rusty colored ones are not safe to use if found. Cotton filters tend to be rated at 50+ micron in size and are way to
coarse of a filter to use in producing safe water. Usually these filters have a sort of plastic or steel enclosure (called a housing) that directs
the water through the filter such that the water doesn’t go around (in industry this is referred to as bypass) the filter.
Many people know of Reverse Osmosis filtration (RO) from buying bottled water in the store. RO uses special cross flow membranes to remove solids and
dissolved solids from water. RO filters are nearly useless without the full system they fit into, so acquiring membranes without the system will not
be helpful. RO systems also require power and pressure to push the water through the membrane and to keep the membrane clean. These systems are some
of the best for water quality in that they can remove 99.99% of viruses and bacteria in some brands and styles. Usually the small home RO units are no
where near this efficient but most hospitals have units like these to provide ultra pure water for immune compromised patients. Also electronic
manufacturers use water this pure in the fabrication of circuit boards. These units provide the highest quality of water.