782 General and Applied Biology 



of health and is desirable from an odor, taste, or appearance standpoint, 

 (2) polluted water, which contains substances not necessarily harmful 

 but of such a character as to offend the senses of sight, taste, or smell 

 (pollution usually refers to such physical characteristics as unpleasant 

 tastes and odors, undesirable color, excessive turbidity, etc.), (3) con- 

 taminated waters, which contain substances harmful to health (patho- 

 genic microorganisms, inorganic or organic poisons, etc.), and (4) pure 

 waters, which are chemically and physically pure; such waters do not 

 exist naturally but can be secured by distillation. 



Waters may become polluted and contaminated by picking up all 

 manner of materials in suspension and solution. They may acquire 

 silt by passing through fertile lands; they may be hard by incorporating 

 chemicals as they flow through limestone; they may have undesirable 

 tastes and odors by contacting decaying plant and animal matters; they 

 may be rendered undesirable by industrial wastes, wastes from oil wells, 

 seepage from mines, domestic sewage, etc. From a sanitary standpoint, 

 human excrements play the most important role in the contamination of 

 water. 



Waters may be purified by (1) filtration through sand filters, with or 

 without previous coagulation induced by the use of chemicals which 

 precipitate undesirable materials, (2) disinfection by the use of certain 

 chemicals, usually chlorine, (3) some kind of water softening process, 

 or (4) a combination of the above processes. The specific method used 

 to purify is determined by the quality of the raw water and the quality 

 of the water expected after treatment. If water contains little dissolved 

 or suspended matter, chemical disinfection may be sufficient; if it is soft 

 but contains suspended matter and microorganisms, filtration and chlo- 

 rination may be necessary; if it contains such dissolved salts as those of 

 calcium or magnesium, it may be necessary to soften it by one means or 

 another. 



Two types of sand filtrations are used for removing pollution from 

 waters: (1) slow sand filtrations and (2) rapid sand filtrations. The 

 former have been used extensively in Europe since 1830 but have not been 

 satisfactory in the United States. Slow sand filters were made of con- 

 crete, covered about one acre each, and were filled with sand to a depth 

 of one to four feet. Bacteria and other microorganisms are removed by 

 mechanical filtration and also by their destruction in the gelatinous, 

 zoogleal mass which covers the filter surface after it has operated for a 

 few days. Criticisms of the slow sand filter include the following: (1) 

 even if operated at full capacity the rate of purification is only two to 



