Preliminary methods of treatment are not required, although the process is not adapted to turbid water. The first cost of installing such a filter plant is large, but the operating cost is low. The filter accomplishes a bacterial removal of about 99 per cent. and is extremely uniform in its action.
6. Rapid Sand Filtration.—This is accomplished by two types of filters, either pressure or gravity, according to the manner in which the head of water is secured upon the filter. The operation of each is based upon the same principle. This method requires only a small installation, with a low initial cost, but a high cost of maintenance. They are very efficient in the removal of color and turbidity, and slightly less efficient (95 per cent. to 99 per cent.) and less uniform in the degree of bacterial removal secured as compared with slow sand filters. Filtration is effected by the formation of a thin film of the precipitated coagulant upon the surface of the sand through which the water passes, either by gravity or pressure. Since the film of precipitate held back by the sand is constantly increasing, the amount of water which passes through is soon finally reduced below efficient limits. When this occurs, usually every 6 to 12 hours, the filter is disconnected and the precipi tate removed by reversing the flow of water, and agitating the sand, either by means of compressed air or mechanical rakes (Figs. 39, 42). The dirty wash water is wasted. When the sand is cleaned, the reverse flow is stopped, the sand settles into place over the gravel, and the coagulant containing water is again sent through the filter. These filters are operated at the rate of zoo to r5o million gallons per acre per day, hence the small installations.
Sedimentation to remove some of the precipitated coagulant is usually used as a preliminary process, in order to prevent the too rapid clogging of the filters.
The gravity filters are open vats or reservoirs or wood or con crete (Fig. 41), while the pressure filters are cylindrical steel tanks (Figs. 38, 39).
7. Disinfection of Water.—In this country chlorine is the only agent extensively employed for this purpose, and is ap plied either as liquid chlorine or a solution of bleaching powder.
Disinfection is applied as a finishing process to water that has been previously filtered or otherwise clarified. The . value of the process from the standpoint of the destruction of the bac teria which pass through the filters, including pathogenic and non-pathogenic germs, has been well demonstrated. From .1 to .5 parts per million of available chlorine are commonly em ployed, requiring the addition of about 5 to 12 pounds of bleach per million gallons of water. Quantities of bleach in excess of 25 pounds can be detected by taste. Of recent years the employ ment of liquid chlorine has been superseding bleach, owing to the lesser cost of the former, as well as its convenience (Fig. 43). The amount required is greater when the content of the water in organic matter is high. Disinfection is probably accomplished by nascent oxygen liberated by the hypochlorous acid formed by the mixture of the bleach with the water.
8. Results of Water Purification and Disinfection.—Since the first serious . attempt at water purification in the United States in 1872, the practice has abundantly justified itself as a sanitary measure of the highest order. Endemic water borne typhoid has either been eliminated or greatly reduced in the municipalities making the change. Iri Table V are presented a few examples to show the typical results which have been secured.
Though a much more recent development, the reduction in typhoid rates effected by chlorination of water are every bit as striking as those secured by purification. The experience of a Evidence is accumulating to indicate that purification in itself did not succeed in completely eliminating water borne typhoid. Cities which experienced a reduction in their typhoid rates following the installation of a purification plant experi enced a still further reduction after the inauguration of dis infection. An example of this experience is shown in Table VII.
In addition, several instances have been observed of typhoid and dysentery outbreaks following emergencies when purifica tion plants were temporarily shut down because of floods or accidents.