Applied Biology 783 



three million gallons per acre; (2) it is not efficient if water contains large 

 amounts of suspended materials because the surface soon becomes coated, 

 thus interfering with the delivery of sufficient, desirable water; (3) it is 

 expensive, especially if large quantities of water are required, because 

 large areas of expensive land are required to build sufficient filters. 



Rapid sand filters, introduced into the United States about 1890, are 

 extensively used in modern water purification plants. The process, in 

 brief, is as follows: (1) screen the raw water to keep out sticks, leaves, 

 animals, etc.; (2) mix the water with flocculating chemicals, such as 

 aluminum sulfate or iron sulfate, and allow the suspended floe to settle 

 out in settling tanks; (3) pass the clarified supernatant water through 

 rapid sand filter beds; (4) disinfect the water by means of chemicals, 

 usually liquefied chlorine gas. 



In recent times the contamination of water supplies by sewage has 

 become a major problem because of increased centers of population and 

 industrialization in cities, because of the increased demand for more 

 water for homes and industries, and because of the greater difficulty in 

 efficiently disposing of large quantities of sewage. Sewage may be con- 

 sidered as the used water supply to which have been added (1) human 

 excrements (urine, feces), water used for bathing, washing, etc., (2) 

 industrial wastes from laundries, creameries, breweries, chemical plants, 

 slaughterhouses, tanneries, and many other similar industries, and (3) 

 water from streets, sidewalks, etc. 



Sewage may be disposed of (1) by dilution, (2) by irrigation, or (3) 

 by stabilization of the sewage through bacterial actions. In the process 

 of disposition by dilution the sewage is placed in a body of water suffi- 

 ciently large to render the sewage more or less harmless. This old 

 method is inexpensive, and if the body of water is large enough to dilute 

 the sewage properly, and if the body of water is not to be used for other 

 purposes, it is reasonably satisfactory. Sewage disposal by irrigation or 

 by running raw sewage over land is not commonly used in the United 

 States. The stabilization of sewage by the actions of various bacteria 

 is based upon the fact that organic and inorganic substances in sewage 

 are excellent foods for bacteria. When bacteria use these substances, 

 they oxidize them more or less completely, forming new substances with 

 less energy and lower molecular weights. When most of the energy of 

 sewage is consumed, bacteria no longer grow rapidly and the sewage is 

 stabilized. The specific method for the bacterial treatment of sewage 

 depends upon many factors, such as the quality of the sewage, the quan- 

 tity to be disposed, the nature of the body of water or soil into which 

 the treated sewage is allowed to run, etc. 



