WATER TREATMENT IN GENERAL TERMS - WATER AND WATER QUALITY

Water ... what a wonder! People use it in so many ways, it is perhaps unrealistic to expect it to meet all the demands they make of it. Still, with the right treatment, water can and does meet all its obligations.

Public health authorities, industrial firms, commercial firms, hospitals and institutions, farmers and homemakers ... each has special requirements in terms of water quality. And when water quality fails to meet these requirements, trouble begins.

Even the space age scientist gets into the act when he calls for deionized water to clean the metal skins of his satellites. His request for deionized water stems from the fact that it prevents local "hot spot" corrosion and thus unwanted residue weight as his ships soar out into space. More mundane uses are for final rinsing of automobiles, trucks and aircraft.

Deionization is also called demineralization, but not with complete accuracy, since the term deionization specifies the removal of dissolved substances in ionic form. Water treatment processes, such as distillation and reverse osmosis, also remove dissolved substances (dissolved solids) from water. These processes have the advantage of being able to remove not only dissolved substances in ionic form from water, but also substances such as sugar and other organic matter which dissolve in water but do not form ions. Thus, water may contain dissolved impurities or contaminants which are ionized and those which are not. Dissolved substances which form ions make water a better electrolyte (conductor of electricity) and the amount of ionized substances present in water can be measured by its conductivity, and conversely, their absence by increased resistance.

Deionization, distillation, and reverse osmosis are processes which cause direct removal of impurities from water as contrasted with indirect processes which involve conversion of water impurities to their insoluble form as precipitates, with subsequent direct removal by filtration, such as the removal of dissolved iron by oxidation and subsequent filtration.

To provide the right water for any demand, whether it be that of the public health authority, the homemaker or the space age scientist, two all-important factors must be considered:

1. Precisely what does analysis of the raw water supply indicate?
2. To what end use will the water be put?

Analysis of a water may show that it contains

• dissolved minerals,
• dissolved gases,
• turbidity and sediment,
• olor and organic matter,
• taste and odor, and/or
• micro-organisms.

Whether or not any of these impurities are harmful in a given situation in turn depends on:

• The nature and the amount of the impurities;
• The tolerance permissible for each of these impurities;
• The end use of the water.

Water of a quality that may prove unacceptable or unsatisfactory for certain requirements may be quite satisfactory in other instances. To cite an example, water with 15 grains per gallon of hardness (257 milligrams/liter) is objectionable for laundering and bathing. This same water, however, is satisfactory for sprinkling the lawn.

Both the quality of a raw water and its end use must always be determined before it can be treated economically.