treated water. The gas making plant in its simplest form consists of four principal parts: a gas furnace, an air compressor, a combination scrubber and dryer, and a diffuser.

In larger installations a more economical plan is to have a plant consisting of a gas producer, a gas burner, a return tubular boiler, a steam driven air compressor, a combination scrubber and dryer, and a diffuser.

In operation, coke is burned in the gas producer, which is a closed furnace with controlled air supply furnished by a blower. The products of combustion pass from the gas producer to a gas burner where they are mixed with air and burned under a boiler. The product of combustion contains from 15 to 17 per cent CO2. The steam produced in the boiler is used for driving the air compressor and blower so as to make a closed cycle for the process.

Cost of softening

At present day prices a combined water softening and purification plant, such as municipalities usually build for softening and filtering their water supplies, costs approximately $40,000 per million gallons capacity. In large installations this figure would be reduced somewhat. If filtration through sand is eliminated the cost per million gallons capacity would be reduced by about one-half the above amount.

The cost of lime for removing each one part per million of noncarbonate hardness in a million gallons of water is 3.1 cents, figuring 90 per cent lime at $12 per ton delivered.

The cost of lime for removing each 1 part per million of magnesium is 13 cents, figuring 90 per cent lime at $12 per ton delivered. The cost of soda ash for removing each 1 part per million gallons of noncarbonate hardness in a million gallons of water is 13.6 cents, figuring soda ash at $30 per ton delivered.

At municipal water softening plants the hardness is usually reduced to 85 to 100 parts per million. The alkalinity is usually reduced to 40 to 60 parts per million and the magnesium to about 10 parts per million. If the alkalinity, non-carbonate hardness and magnesium content of the raw water are known, the cost of producing water of desired hardness may readily be calculated.

ZEOLITE TREATMENT

Base exchange materials, commonly known as zeolites, are silicates containing sodium which may be replaced with other bases, such as calcium and magnesium. Compounds of calcium and magnesium cause the hardness in water. When a hard water is passed through a bed of zeolite, the calcium and magnesium are removed from their compounds in the water and are replaced with sodium. The sodium compounds do not cause hardness, so that as a result the water is softened.

The capacity of a zeolite for softening water is, therefore, limited by the amount of replaceable sodium it contains. When all of the replaceable sodium has been exchanged for calcium and magnesium from the hard water, the zeolite must be regenerated. This is accomplished in practice by treating the material with a solution of common salt, sodium chloride. The sodium of the salt drives out the calcium and magnesium from the zeolite, and replaces them with a fresh supply of replaceable sodium. After regenerating, the salt solution is washed out and the zeolite is again in condition to soften water.

The exchange property of these silicates has been known for many years, but its application to softening water is comparatively recent. Zeolites may be classified as either natural or synthetic. Both materials will soften water. The efficiency depends, however, on the manufacture or the processing of the zeolites, the proper design of the apparatus in which a zeolite is used, and the adaptability of a particular zeolite to meet the requirements of local conditions.

The general design and construction of a base exchange softener resembles that of the well known pressure water filter, in which the bed of zeolite takes the place of the sand. Provision is made for the introduction and removal of the salt solution used in the regeneration process. In the operation of the softener, the water may be passed in at the top and out at the bottom, or in the reverse direction. Each system has its advantages and disadvantages.

The cost of treatment by the zeolite process depends primarily upon the nature of the water treated. Other influencing factors, however, are the cost of salt, the volume of softened water required between regenerations, and the specific requirements placed upon the apparatus.

The salt required for regeneration of the zeolite varies from about one-half to one pound of salt for a thousand grains of hardening salts removed, expressed as calcium carbonate.

The zeolite process of water softening is particularly applicable to the conditions requiring waters practically free from calcium and magnesium compounds, and in which the nature and amount of dissolved solids in the softened water are relatively unimportant. For this reason the process has found particular favor in laundries, textile and dye industries, and in institutions and homes. Under some conditions it is used to soften water for boiler feed purposes, depending principally upon the nature of the water to be treated.

The base exchange softening process cannot be used successfully to soften waters with excessive content of sodium salts, nor with waters containing free acid or large amounts of iron, without previous purification.

CHAPTER XI

ULTRA-VIOLET RAY TREATMENT

The ultra-violet ray process of water disinfection has found favor where the volume of water to be treated is relatively small or where automatic sterilization is of prime importance. Of several hundred ultra-violet installations in this country the majority are used for treatment of swimming pools. Other uses are in connection with bottled water and soft drink establishments, drinking water supplies for hotels and industrial plants. There are about 60 installations on steamers plying the Great Lakes. Of installations treating municipal water supply that of Henderson, Ky., made in 1916-17, is the largest. Advices from Henderson (January 19, 1925) indicate that from 2.5 to 3 million gallons of water were treated per day at a monthly cost of about $125 for electric current and $60 for lamp repairs and renewals. The process has not been in use for over a year and no plans have been made for its future employment, liquid chlorine having been substituted because of the difficulty experienced in keeping the lamps in working order. Other municipal water supply installations are those at Berea, Ohio, 1923, and Horton, Kans., 1924, both treating filtered water at a rate of 0.5 m.g.d.

The process consists in the direct application of ultra-violet rays to water as it flows through a channel or pipe containing a succession of restricted orifices wherein the depth of penetration of the rays can be governed. In each of these orifices or ports, quartz lamps containing mercury are set from which the rays issue and penetrate the water flowing by them. The lamps are arranged in tandem. on the principle that, if the desired result is not obtained from the first lamp, it will be by the succeeding lamps.

In ordinary practice the lamps use a direct current of 220 volts and 3.5 amperes. It usually requires about ten minutes to develop the full voltage across the lamp. The intensity of illumination is a factor dependent upon the power and number of the lamps, their spacing, and the general design of the sterilizing chamber. The effective penetration of the rays in a clear and relatively colorless

water is about 5 inches, the passages in which the lamps are set usually being about 80 square inches in area.

The velocity of flow of water past the lamps (period of contact with the rays) is a function of the head lost by forcing the water through the restricted orifices. The period of contact is of less importance than the distance factor (ray penetration) so far as the sterilizing effect is concerned. The number of contacts required (number of lamps in series) depends upon the degree of sterilization desired. In addition to their normal deterioration, the life of the lamps depends somewhat upon the character of the water.

The efficiency of the ultra-violet ray process is restricted to disinfection. Bacterial reduction is affected by the degree of clearness and color of the water. If the unsterilized water contains over 15 parts per million of turbidity or color the process is relatively ineffective. Where such turbidity is caused by finely divided or clumped suspended matter the direct action of the rays upon bacteria. or other microscopic life is measurably diminished, and bacteria embedded within particles of suspended matter pass by the lamps unharmed. Air bubbles in the flowing water do not affect the disinfecting action. Reflected rays have little, if any, disinfecting action.

If the unsterilized water is not highly polluted, is clear and practically colorless, and if, say, 6 lamps are used to treat 250,000 to 500,000 gallons of water daily, the removal of bacteria, subject to the usual variations, may roughly be assumed at 30, 45, 60, 70, 90, and 99+ per cent after passing each lamp in series.

The process is attractive because nothing is added to the water which may impart offensive tastes and odors. The system requires constant watching, for, if the lamps are not working properly, they will go out automatically. It is more expensive than other forms of water disinfection now in use. The electric current supply must, of course, be uninterrupted, and, as stated before, it is futile to attempt efficient disinfection with the ultra-violet ray process, if the turbidity and color of the untreated water exceed 15 parts per million.

OZONIZATION

Ozone is an active form of oxygen and may be produced by passing a current of air over a brush discharge that takes place between