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Alkalinity of drinking water

Alkalinity of water may be due to the presence of one or more of a number of ions. These include hydroxides, carbonates and bicarbonates. As discussed in previous articles, hydroxide ions are always present in water, even if the concentration is extremely small. However, significant concentrations of hydroxides are unusual in natural water supplies,

but may be present after certain types of treatment. Small amounts of carbonates are found in natural water supplies in certain sections of the country, rarely exceeding 3 or 4 gpg. They may also be found in water after treatment, such as lime soda ash softening. Bicarbonates are the most common sources of alkalinity. Almost all natural supplies have a measurable amount of this ion, ranging from 0 to about 50 gpg.

Phosphates and silicates are rarely found in natural supplies in concentrations significant in the home. Compounds containing these ions may be used in a variety of water treatment processes. Moderate concentrations of alkalinity are desirable in most water supplies to balance the corrosive effects of acidity. However, excessive quantities cause a number of problems. These ions are, of course, free in the water, but have their counterpart in cations such as calcium, magnesium and sodium or potassium.

You probably will not notice an alkaline condition due to bicarbonate ions except when present in large amounts. In contrast, you should readily detect alkalinity due even to fairly small amounts of carbonate and hydroxide ions.

Strongly alkaline Waters have an objectionable "soda" taste. The EPA Secondary Drinking Water Regulations limit alkalinity only in terms of total dissolved solids (500 ppm) and to some extent by the limitation on pH.

Highly mineralized alkaline waters also cause excessive drying of the skin due to the fact that they tend to remove normal skin oils.

Troublesome amounts of alkalinity can be removed by reverse osmosis, along with other total dissolved solids. Other methods of water treatment remove total dissolved solids and alkalinity, but they are somewhat less suitable for household use than reverse osmosis. These methods are distillation and deionization (demineralization).

Several other methods of water treatment will remove alkalinity, but these methods are not satisfactory for household use. They include:

1.      Lime softening removes hardness. At the same time, this process will precipitate an equivalent amount of alkalinity. Lime softening is usually restricted to industrial and municipal installations.

Lime softening. While reducing total alkalinity, lime softening does convert HC03– to CO3--, a stronger alkalinity ion.

2.      An anion resin regenerated with sodium chloride removes substantially all the anions (carbonates, bicarbonates, and sulfates, as well as nitrates). It replaces these anions with a chemically equivalent amount of chloride ions. The disadvantage of this process is that in almost all cases a high chloride ion concentration results. At the point of exhaustion, the resin has the tendency to unload high concentrations of the anions it carries including the nitrates. For household purposes, such results are almost as undesirable as the original alkalinity.

3. The feed of a mineral acid will neutralize the alkalinity of a water. Hydrochloric acid, sulfuric acid or a combination of these can be used. This process converts the bicarbonates and carbonates present into carbonic acid. At this point, it is advisable to provide some method to permit the resulting carbon dioxide gas to escape into the atmosphere. The disadvantages of this acid feed technique are obvious. There are needs for precise control of the process and caution in handling the strong acid.

 

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