of turbidity (that is foreign suspended matter) by this natural settling process is known as SEDIMENTA- TION. Sedimentation is accomplished in settling tanks where the water is held for a time to allow the floc to form and settle out turbidity. In conventional treatment, settling immediately follows flocculation. The ideal detention period for settling after slow mixing is about 1 1/2 hours.

Filtration

Not all suspended matter is removed by sedimentation. Therefore, another process known as FILTRATION is used. An effective type of filter used in the filtration process is the diatomite. Because it is lightweight, this filter is widely used at overseas bases. It removes suspended matter from water by passing it through a porous mat of diatomaceous silica. Diatomaceous silica is the skeletal remains of tiny algae, called diatoms, found in marine deposits that have been lifted above sea level. The diatomite (also called diatomaceous earth or filter aid) is supported by a filter element. This supporting base is porous endugh to permit maximum flow. It is also fine enough to support the filter cake that coats the element. Diatomite filters are backwashed by reversing the flow of water and drawing filtered water through the filter to keep the filter output from falling off. The turbidity of the water is largely determined by the frequency of backwash.

Disinfection

Except in rare instances, all water supplies require disinfection. Disinfection is the chemical destruction of bacteria. Because of its economy, dependability, efficiency, and ease of handling, chlorine is almost always used for this purpose. For this reason, the term chlorination generally means the same as DISINFECTION.

Disinfection is a necessary step in ensuring a safe water supply. Ali new, altered, or repaired water- supply facilities must be disinfected before they are placed in service. Water from surface supplies may be disinfected before filtration or before coagulation and sedimentation to prevent the growth of organisms. This procedure is known as precholorination. The water must also be disinfected after filtration to destroy organisms that still remain and to provide a safeguard against recontamination. This procedure is known as postchlorination.

Chlorine is the disinfectant specified for Navy use. In'the form of chlorine gas or of hypochlorites that yield chlorine in water, chlorine is presently the only widely accepted agent that destroys organisms in the water and leaves an easily detectable residual that serves as an indicator of the completeness of treatment. The sudden disappearance of residual chlorine may signal contamination in the system. Under ordinary temperatures and pressures, chlorine gas is greenish yellow and is heavier than air. Its effectiveness as a disinfectant depends on the temperature and the hydrogen-ion concentration (pH) of the water to which it is added. Disinfecting action is faster at higher temperatures, but is retarded by pH. When the pH is above 8.4, the rate of disinfection decreases sharply.

Ozone, potassium permanganate, bromine, and. iodine are also used to a limited extent as disinfectants. If excess lime is used for softening water, it makes the water alkaline and disinfects after about 10 hours of contact. However, the general applicability and economic advantage of chlorine have established it as the preferred disinfectant.

CHLORINE DISINFECTION. - Chlorine disinfectants are available in a number of different forms as described in the following paragraphs

LIQUID CHLORINE is liquefied gas under pressure and is shipped in seamless steel cylinders under the regulations established by the Interstate Commerce Commission. The standard sizes of shipping containers are 150-pound cylinders, 1-ton containers, and 30-ton tank trailers.

Each pound of liquid chlorine produces about 5 cubic feet of chlorine gas at atmospheric pressure and at a temperature of 68F. A standard Chlorine Institute valve and a protective valve hood are screwed into the neck of each cylinder. The valve has a safety plug containing fusible metal that softens between 157F and 162F to protect the cylinder from bursting in case of fire. All cylinders must be factory tested every 5 years; 150-pound cylinders are tested at 500 pounds of pressure; and I-ton containers are tested at 800 pounds of pressure.

HIGH-TEST CALCIUM HYPOCHLORITE is a relatively stable, dry granular solid or powder that is readily soluble to form a chlorine solution. Prepared under a number of trade names, including HTH, Perchloron, and Pittchlor, it is furnished in 3- to 100- pound containers and has 65 to 70 percent available chlorine by weight. Because of its concentrated form and ease of handling, calcium hypochlorite is preferred over other hypochlorites.