BACTERIOLOGY OF WATER AND SEWAGE 437 



many cases it is found necessary for cities not wholly to 

 purify their sewage so that no disease-producing bacteria 

 are present, but simply to purify it so that it will be no 

 longer putrescible. Gross pollution of water may also be 

 studied by chemical analysis for the determination of the 

 amount of dissolved organic material (so-called albuminoid 

 ammonia) the presence of nitrates and nitrites, and free 

 ammonia. 



SANITARY QUALITIES OF VARIOUS WATERS 



As noted above, the sanitary quality of any water 

 depends directly upon whether or not it is receiving sewage 

 or other wastes which would make it harmful either to 

 health of the user or to animal life contained. 



Water in flowing streams is potable providing it does not 

 receive sewage or factory waste. Most streams in the more 

 thickly settled districts in America are polluted. There are 

 some exceptions to this. Some of our larger cities use water 

 from streams whose watersheds have been carefully pro- 

 tected and do not permit any sewage to gain access. 



Lake water is in most cases somewhat less apt to be 

 polluted than water in streams. Wherever it receives 

 sewage, however, it is polluted. 



Water from springs usually is of a high quality. Most 

 springs receive water which has filtered through a consider- 

 able depth of soil and in some cases through rock. Some 

 springs, however, particularly intermittent springs, are 

 simply the opening of the channel of an underground 

 stream which has direct communication with the surface of 

 the earth. In limestone regions, for example, so-called sinks 

 are found frequently. These are openings in the ground 

 through which surface water passes. These communicate 

 quite directly with these underground streams which, when 

 they appear again upon the surface on the hillside, may 

 contain all the contaminating materials present in the 



