WATER PURIFICATION 1. The undesirable qualities which water may possess and the means available for their removal are presented in the following table: Owing to limitations of time and space we will not give fur ther consideration to the methods for the removal of hardness or undesirable metals, but only consider the others.
2. Storage.—The storage of water in large impounding reser voirs or lakes for several months effects a removal.of any in fective agents present, by their exposure to an unfavorable vironment. We have already called attention to the fact that the survival of pathogenic micro-organisms in water is but for a short period. They are exposed to the direct action of the sun's rays and they are devoured by predatory organisms, of which the infusoria are probably the most important. One months storage under these circumstances will result in their destruction. The greatest drawback to the employment of this method is the fact that sufficient space in the shape of a reservoir is frequently not available so as to permit a city to store a month's supply, or else a sufficient reserve of water is not available to permit its aging for a month before consumption (Fig. 30.
3. lowering of the velocity of a stream, such as that effected when it enters a lake or reservoir, permits the fine suspended particles to settle to the bottom. The finer the particles, the lower must be the velocity and the longer the quiescent period before their removal is effected. This method in itself is used chiefly as a means of removing turbidity, and is of but slight value in the removal of bacteria unless the period of quiescence approximates the conditions of storage. Large open tanks or reservoirs are employed for sedimentation. Sedimentation may be employed in conjunction with the process of coagulation.
4. Coagulation is used in conjunction with sedimentation or as a process of treatment preliminary to rapid sand filtration. It is a necessary measure to remove the fine turbidity, parti cularly that produced by colloidal clay, which would not completely settle out no matter how long the period of sedi mentation. Coagulation consists in the employment of cer
tain chemicals, such as sulphate of aluminum, which react with the alkaline carbonates of the water to form aluminum hydrate. This has a large colloidal molecule and being in soluble, is thrown out of solution as a coarse, flocculent pre cipitate in which most of the suspended matter and bacteria become enmeshed (Fig. 32).
In practice only sufficient alum is added to combine with the carbonates, so that no alum remains in solution and all of the precipitate is removed (Fig. 4o). Iron sulphate is similarly used. If sufficient carbonates to break up the alum are not present normally, the required alkalinity is secured by the addition of lime or soda to the water.
5. Slow Sand is accomplished by large filters, either open or covered, each about an acre in extent (Fig. 33) consisting of collecting tiles overlain with gravel and coarse sand (Fig. 34). These are operated with ahead of from two to four feet of water above the sand (Fig. 35). Bacterial removal is not accomplished by any straining action of the sand grains, but by the development of alga; and protozoa in the surface layers of the sand, which reduce the interstices still further and prey upon bacteria. When this biological film is sufficiently developed the water passing a filter is used. Under these conditions it is so operated as to permit the passage of from 2.5 to 5 million gallons of water per acre per day. The biological film continues to develop during the course of several weeks to such an extent that the passage of water gradually becomes seriously hindered, whereupon the filter is drained, and the top layer of sand is removed (Fig. 36). After cleaning, the filter is again placed in commission and the process of cleaning is repeated as often as necessary. When the sand layer is re duced to 12 to 18 inches in depth, the filter is drained and the sand, which in the meantime has been washed, is restored to the filter (Fig. 37).