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10 Water Purification: Physical Treatment: Activated Charcoal Filter:




Description

This article is from the Water Treatment FAQ, by Patton Turner with numerous contributions by others.

10 Water Purification: Physical Treatment: Activated Charcoal Filter:

Activated charcoal filters water through
adsorption, Chemicals and some heavy metals are attracted to the
surface of the charcoal, and are attached to it. Charcoal filters
will filter some pathogens though they will quickly use up the filter
adsorptive ability, and can even contribute to contamination as the
charcoal provides a excellent breeding ground for bacteria and algae.
Some charcoal filters are available impregnated with silver to prevent
this, though current research concludes that the bacteria growing on
the filter are harmless, even if the water wasn't disinfected before
contacting the filter. The only filter I know of that uses only
activated charcoal, and doesn't required pressurized water is the
Water Washer ($59) Available from the
Survival Center.

Activated charcoal can be used in conjunction with chemical treatment.
The chemical (iodine or chlorine) will kill the pathogens, while the
carbon filter will remove the treatment chemicals. In this case, as
the filter reaches its capacity, a distinctive chlorine or iodine
taste will be noted.

Activated charcoal can be made at home, though the product will be of
varying quality compared to commercial products. Either purchased or
homemade charcoal can be recycled by burning off the molecules
adsorbed by the carbon (The won't work with heavy metals of course.)

The more activated charcoal in a filter, the longer it will last. The
bed of carbon must be deep enough for adequate contact with the water.
Production designs use granulated activated charcoal (effective size
or 0.6 to 0.9 mm for maximum flow rate. Home or field models can also
use a compressed carbon block or powered activated charcoal (effective
size <0.01) to increase contact area. Powered charcoal can also be
mixed with water and filtered out later. As far as life of the filter
is concerned, carbon block filters
will last the longest for a given size, simply due to their greater
mass of carbon. A source of pressure is usually needed with carbon
block filters to achieve a reasonable flow rate.

Sol-Air Water Treatment: If sufficient desolved oxygen is available,
sunlight will cause the temporary formation of reactive forms of
oxygen such as hydrogen peroxide and oxygen free radicals.. This form
of water treatment is called solar photooxidative disinfection or
sol-air water treatment. Sol-Air water treatment has been shown to
dramatically reduce the level of fecal coliform bacteria. There is
some evidence that other bacteria and viruses may be affected also.
While not as reliable as other methods, it does offer a low tech
solution in emergencies. Sol-Air treatment requires bright sunlight,
and has been shown to be effective when ever the sun causes a distinct
shadow to be cast. Exposure to 4.5 hours of bright sunlight has been
shown to cause a thousand fold reduction in fecal coliforms in lab
tests

In order for Sol-Air to be effective, oxygen must be present.
Experiments have shown that shaking a bottle filled 3/4 with air will
restore oxygen levels to near saturation. As the treatment continues,
some of the oxygen will come out of solution, while other oxygen will
be consumed by the killed pathogens, so the shaking should be repeated
every few hours. Data shows that maximum activity occurs when the
water temperature is above 50 deg. C (122 deg. F) , so this method may
be unsuitable in colder climates unless special solar collectors are
used.

Either glass or plastic bottles may be used. Plastic bottles will
allow short wave ultraviolet radiation to pass, increasing the rate of
microbial inactivation, but may yellow with age, reducing light
transmission, and may leach plasticizers into the water at the
elevated temperatures that will occur. The leaching out of
plasticizers can be reduced by using bottles of PET (polyethlyene
terephtalate) rather than PVC. Glass bottles on the other hand are
more durable. Research has used bottles with 2 liters of capacity,
but if the water is free of turbidity, larger containers can be used.
Plastic bag, or some sort of flat glass container represent the ideal
container as this maximizes the solar energy received per ounce of
water.

Bottles should be filed 3/4 full in the early morning with water as
free of turbidity as possible. After capping the bottles the should
be shaken vigorously for a few minutes then placed upright in the sun,
where they will be not be shaded later in the day. The shaking should
be repeated at least three times during the day. At the end of the
day the water should be reasonably freed of bacteria, though it is
most practical to let the water cool for consumption the following
day. Each day a new batch should be treated due to the lack of a
residual disinfected.

After consumption of the water the bottle should be air dried to
prevent algae growth with continual use.

 

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