WATER AND SEWAGE 619 



into water courses. Then the bacterial content of the water is greatly 

 increased. 



The bacterial flora of surface waters is normally considerably 

 reduced by the action of sunlight which is germicidal at a depth of 

 several feet in quiet, clear water by dilution, sedimentation, oxida- 

 tion, and by the activities of predatory aquatic animals. The average 

 soil pollution of water by surface contamination in sparsely populated 

 drainage areas is not harmful to man, and such waters would ordinarily 

 be suitable for domestic use. 



Unfortunately water courses are convenient channels for the 

 removal of human waste, including excreta, and such waste is poten- 

 tially dangerous because it may contain pathogenic bacteria. Exten- 

 sive epidemics of water-borne excrementitious disease as typhoid 

 and cholera have focused attention upon the potential dangers 

 attending the use of unpurified surface water for domestic purposes, 

 and the statistical evidence of a reduction in the incidence of intes- 

 tinal diseases when water supplies have been purified by filtration 

 or by other methods is conclusive proof of the occasional transmission 

 of excrementitious diseases through polluted water. 



Ground water from deep wells and from springs is usually rela- 

 tively free from bacteria unless surface pollution occurs. The water 

 which feeds these sources is filtered free from bacteria during its 

 passage through the deeper layers of the soil. Ground water is not 

 extensively used for municipal supplies at the present time. Surface 

 waters furnish the principal available sources of this commodity for 

 domestic use, and in thickly settled areas it has been found necessary 

 to purify the water before it is safe for human consumption. 



The objects of water purification are: To eliminate pathogenic 

 bacteria, and to reduce the dissolved and suspended organic matter 

 to a state of complete oxidization and mineralization. It will be 

 remembered that bacteria of the soil effect a mineralization of organic 

 substances, and the purification of water and of sewage, which is 

 grossly polluted water, is ordinarily accomplished by a direct applica- 

 tion of the same natural process. 



For convenience in operation, filters are constructed which are 

 essentially water-tight basins (to prevent the entrance of extraneous, 

 unpurified water) containing underdrains covered with a layer of 

 sand of uniform size, from two to four feet in thickness. 1 The under- 



1 The details of structure and operation of filters designed for the purification of 

 water and sewage are beyond the scope of this volume. 



