containing calcium and manganese compounds passes over the zeolite, the calcium and manganese are exchanged for the sodium in the zeolite. In this way the water is softened and its sodium content increased. When the sodium of the zeolite is exhausted, it is regenerated by applying a sodium chloride solution. Another exchange is made, and the resulting concentrated solution of calcium and magnesium chloride is discharged to waste. The operating rate varies directly with the thickness of the zeolite bed. The time between regenerations depends on the characteristics of the water and the total amount of water applied. The need for regeneration will be evident when hardness is no longer removed. The zeolite process can only be used on water that has been treated for removal of turbidity.

Ion Exchange. The ion exchange unit, when run on the sodium cycle, will significantly soften water. The ion exchange unit will also remove such undesirable ions as (hose of manganese and lead.

Dissolved Gases

The concentration of a gas in water is directly proportional to the concentration, or partial pressure, of the gas in the atmosphere in contact with the water surface. In general, this involves the water temperature, its salinity, and the altitude. The gases of primary interest to water supply are as follows: .

Oxygen. Large amounts of dissolved oxygen are found in rainwater. The amounts in surface water vary greatly, depending on the amount and type of pollution, the degree of self-purification, the action of algae, and the temperature of the water. Polluted water will exhaust the oxygen supply, while clean water will contain much dissolved oxygen. Cold water contains larger amounts of dissolved oxygen than warm; as water temperature rises, the dissolved oxygen is released to the atmosphere. Decreased pressure on water has the same effect, releasing oxygen to the atmosphere. Dissolved oxygen causes the solution of metals and, especially in the presence of carbon dioxide, causes many metals to corrode. .

Carbon Dioxide. The presence of carbon dioxide in water contributes to the degree of hardness and acidity of the water. Water acquires this gas in four ways: from the air by natural movements of water in contact with the air, such as currents and wave action; by contact with decomposing vegetation, which gives off carbon dioxide freely; by the reaction of ferric chloride and limestone in the coagulation process; and by contact with the gas in underground deposits. A high carbon dioxide content usually makes water more corrosive to metals. .

Hydrogen Sulfide. Hydrogen sulfide in solution lends a disagreeable taste and rotten-egg odor to water. Ground water absorbs sulfides by passing over sulfur-bearing rocks. Hydrogen sulfide is also responsible for the destruction of cement and concrete as well as the corrosion of metals. In small amounts, it is unpleasant but not dangerous. In large amounts it is harmful. Water that smells of hydrogen sulfide should be treated.

Dissolved gases are removed by means of aeration or the use of activated carbon. Aeration exposes as much water as possible to the air. This will release dissolved gases such as hydrogen sulfide and carbon dioxide to the atmosphere. Liberating the dissolved gas from the water by aeration permits the oxygen in the air to come in contact with the finely divided water particles, thereby increasing the dissolved oxygen content of the water. This increase of oxygen content removes the offensive taste and odor imparted by the dissolved gases. Aeration raises the pH by eliminating the carbon dioxide, but increases corrosiveness by increasing the amount of dissolved oxygen. One type of aerator consists of trays containing slats or coke over which the water is sprayed. Other methods of aeration include spraying water up over a shallow receiving basin and forcing air into a basin with diffusers or mechanical pump-type aerators similar to those used in sewage treatment. Operation of most aerators is practically automatic; operators' duties consist essentially of making sure pipes, slots, and surfaces are not clogged and that air has free access to the water. If the water is not to be filtered after aeration, aerators must be screened to keep out insects and other foreign matter.

Activated carbon is a specially treated granular powdered carbon. It absorbs or attracts large quantities of dissolved gases. It is extremely