TECHNICAL AND SANITARY PROBLEMS 1071 



polluted waters and experience has shown that the waters are 

 thereby rendered entirely safe for drinking. 



By the mechanical system of filtration the water is first coagu- 

 lated by the use of chemicals and then filtered rapidly through a 

 small bed of relatively coarse sand at rates which are 20 and 40 

 times as great as in the case of slow sand filtration. This process is 

 likewise effective in the removal of bacteria. The choice of the 

 two systems depends upon the amount of turbidity and color of the 

 water and upon its hardness and upon various local conditions of 

 an engineering character. 



After water has been filtered it must be stored in the dark; other- 

 wise algae and other microscopic organisms are likely to develop and 

 become troublesome. 



Bacteria in water may be killed by the use of liquid chlorine, 

 bleaching powder, ozone, and similar substances. These are poison- 

 ing processes. The quantities of chemicals used are so small that 

 they may be used with entire safety but it is necessary that the 

 chemicals be thoroughly and quickly mixed with water in order to 

 assure efficient sterilization. These processes are especially valuable 

 in cases of emergency and are not to be regarded as substitutes 

 for filtration. Swimming pools should be disinfected regularly in 

 order to prevent the transfer of pathogenic bacteria from person to 

 person. 



Sewage may be treated in various ways to remove bacteria and 

 other objectionable substances. The processes used are screening, 

 sedimentation, chemical precipitation, intermittent sand filtration, 

 contact beds, trickling filters, and disinfection. There are many 

 biological problems involved in these processes and especially in 

 intermittent sand filters and trickling niters. Bacteria play an 

 important part in the disintegration and ultimate absorption of 

 putrescible organic matter while in trickling niters worms and 

 various larvae assist in the process. 



Tastes and Odors in Water Supplies. Water supplies derived 

 from surface sources and stored in reservoirs frequently develop 

 tastes and odors that are very unpleasant. These are largely due 

 to the growth of algae and other microscopic organisms. The matter 

 is one of very considerable importance to waterworks superintend- 



