1070 FRESH-WATER BIOLOGY 



scientific bacteriological literature and especially to the papers 

 which appear in the American Journal of Public Health. 



Numbers of Bacteria in Water. There are two general methods 

 used for determining the numbers of bacteria in water. By the 

 first method nutrient gelatine is used as the culture medium and 

 the period of incubation is 48 hours at 20 C. According to the 

 second the media is nutrient agar and the period of incubation is 

 24 hours at 37 C. Both of these methods are useful but the gela- 

 tine method has been used more than the other. Neither method 

 gives absolute results; the figures are relative in both cases. 



The numbers of bacteria as determined by the gelatine count 

 vary all the way from less than 100 in relatively clean waters to 

 many thousands in waters which are dirty and polluted. For 

 drinking purposes it is generally considered that the number of 

 bacteria determined by this method should be less than 100 per cc. 

 The numbers of bacteria in streams vary greatly according to the 

 rainfall. Very heavy rains wash the surface of the ground and 

 increase the numbers of bacteria in the drainage water. The sew- 

 age of cities contains anywhere from a few hundred thousand to 

 several million bacteria per cc. These bacteria are of many sorts, 

 but most of them are saprophytic in character and in water which 

 contains organic matter, even in small amounts, they are likely to 

 multiply enormously in the course of a day or two. Hence bac- 

 terial counts mean nothing unless the samples are examined im- 

 mediately after collection. They also mean little unless the 

 bacteriologist has a knowledge of the character of the waters. 



Removal of Bacteria from Water. The best method of removing 

 bacteria from water is the process of filtration. There are two 

 general methods in use at the present time, slow sand filtration and 

 mechanical filtration. In the former process the water is filtered 

 slowly downward through a bed of sand at such a rate that the water 

 above the sand descends ten to twenty feet in the course of a day. 

 By sedimentation within the pores of the sand bed and by the 

 adhesion of the bacteria to the sand grains at or just below the 

 surface of the filter the bacteria are removed from the water. The 

 process is partly physical, partly biological. The method is capable 

 of removing upwards of 99 per cent of bacteria from moderately 



