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Did
you know?
Reverse osmosis is the finest water filtration method known. This process will
allow the removal of particles as small as ions from a solution. It is used
to purify water and remove salts and other impurities in order to improve the
color, taste or properties of the fluid. R.O. uses a membrane that is semi-permeable,
allowing the fluid that is being purified to pass through it, while rejecting
other ions and contaminants from passing.
This technology uses a process
known as crossflow to allow the r.o. membrane to continually clean itself. This
is the reason of why an r.o. element can last many years before clogging or
need replacement. This
water purification process requires a driving force to push the fluid through
the membrane, and the most common force is household water pressure or pressure
from a booster pump. The higher the pressure, the larger the driving force and
efficiency.
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OXYGEN IN DRINKING WATER SUPPLY
As rainwater
falls through the atomosphere, it collects oxygen gas. This dissolved
oxygen is not the same as the oxygen in the water molecule. Dissolved
oxygen is present in all rainwaters and surface supplies due to contact
with the atmosphere. Just how much dissolved oxygen a water supply will
contain depends on
Several factors:
1. Under high
pressure relatively large quantities of oxygen dissolve in water. When
the pressure is reduced, a proportionate weight of the gas escapes (Henry's
Law).
Henry's Law. The English chemist, William Henry, formulated a law regarding the effect of pressure on a gas. The law states: The weight of a gas that dissolves in any given liquid is directly proportional to the pressure, providing the temperature remains constant. If one gram of oxygen, for example, dissolves in 100 cubic centimeters of water at atmospheric pressure, two grams of oxygen will dissolve under twice the normal atmospheric pressure, providing there has been no change in temperature.
2. The
amount of minerals in a water affects its ability to dissolve oxygen.
Distilled water can absorb more oxygen than well waters with higher mineral
content. Obviously sea water, for this same reason, holds less dissolved
oxygen than fresh water.
Well
waters usually contain smaller amounts of dissolved oxygen than surface
supplies. In deep wells there may be a total absence of the gas. However,
an article in Science
Magazine, June
11, 1982, pages 1227-30, states:
Contrary
to the prevailing notion that oxygen-depleting reactions in the soil zone
and in the aquifer rapidly reduce the dissolved oxygen content of recharge
water to detection limits, 2 to 8 milligrams per liter of dissolved oxygen
is present in water from a variety of deep (100 to 1000 meters) aquifers
in Nevada, Arizona, and the hot springs of the folded Appalachians and
Arkansas. Most of the waters sampled are several thousand to more than
10,000 years old, and some are 80 kilometers from their point of recharge.
Oxygen
adds to the taste of water. For this reason a small amount of it is desirable
in drinking water. We are all familiar with the "flat" taste which water often possesses
after it has been standing in an open container for some time. The taste
can be improved simply by shaking the water in a partially filled bottle.
This reintroduced oxygen into the water will give it a more appealing
taste. Despite this desirable feature, dissolved oxygen can be a source
of serious trouble in a household water supply. The fact is that oxygen
causes corrosion. In cold water, oxygen normally has little corrosive
effect. In contrast, when the water is heated, the oxygen can cause serious
corrosion problems.
A number of chemicals
are used in industry to remove oxygen from a water supply. Sodium sulfite
(Na2S03) is probably most widely used for this purpose.
It reacts with oxygen at high temperatures to form sodium sulfate (Na2S04),
in this way reducing the oxygen. There are a number of chemicals that
react similarly with oxygen to effect its removal. The degree of success
varies. For household purposes treatment is normally limited to the use
of polyphosphates to coat the insides of water lines to protect the metal
from contact with the oxygen.
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