Sunday, December 5, 2010

Making Water Drinkable

Drinking water can seldom be called “pure” or “clean” in a chemical sense, because it almost always has some gases and minerals dissolved in it. Making water “potable” or safe to drink and pleasing to the taste does not require removal of all impurities. In fact, some of the elements necessary to good health and taste are often found naturally in good drinking water.

Fresh underground water obtained from springs and wells is often—but not always—safe to drink because of the filtering and purifying that takes place as it sinks through layers of soil and porous rock. Even fresh surface waters have self-purifying abilities. As it moves along, running water tends to break up wastes that enter, dissolving and diluting them to harmlessness, allowing the heavier particles to settle out. Wind and turbulence help aerate running water, causing it to release unwanted waste gases and absorb oxygen.

Dissolved oxygen is vital to an amazing step-by-step “digestion” process that occurs in both moving and still waters. The oxygen may directly oxidize or “burn” wastes, neutralizing them, or, more often, it supports bacteria that break down wastes to a harmless residue.

As the process continues, tiny life forms consume the bacteria, clearing the water further. Sunlight penetrates more easily, encouraging the growth of green algae, which, in turn, consume certain contaminating compounds and give off much oxygen in the process. Small water creatures feed on the algae, completing the “digestion” cycle. In this way fresh waters tend to purify themselves in time.

But even this marvelous system can suffer indigestion, as you do when you eat too much of the wrong things. Rainwater runoff from farmlands often contains chemical fertilizer and pesticide residues. New arrays of industrial wastes join them in our water sources, choking these with a variety and volume of chemicals often well beyond the capacity of nature’s purification system. As a result, self-purification, says Preventive Medicine and Public Health, has become “at best a half-truth and has in the past too often been used to justify acceptance of unsafe waters.” Now almost all communities in developed countries treat water in some way before using it.

In so doing, their methods often follow the lead of nature. Thus aeration is usually the first step of a typical purification system. Water is sprayed, cascaded or has air bubbled through it so as to absorb as much purifying oxygen as possible. Then certain chemicals are added that encourage impurities and bacteria to clump together in “flocs.” This coagulation process speeds up natural settling action, which is completed during sedimentation.

Then comes filtration, usually through sand filters, to remove the remaining flocs and most other impurities. Finally, disinfection kills most remaining live organisms, usually by means of chlorine.