62 BACTERIA IN WATER 



effliieuts from water filters the effect upon filter elfioieney of dilution 

 of the water in winter is less marked than the effect of high tem- 

 perature in summer, the work of a filter in warm weather being, of 

 course, more satisfactory than in cold. 



(/) Sedimeniation.— Whilst Pettenkofer attributes self-purifica- 

 tion to oxygenation and vegetation, most authorities are now agreed 

 that it is largely brought about by the subsidence of impure matters, 

 and by their subsequent disintegration at the bottom of the river. 

 Sedimentation and side-adhesion to the banks in rivers and streams 

 of solids in suspension removes a large number of bacteria in the 

 Severn (Boyce). Sedimentation obviously is greatest in still waters. 

 Hence lake water contains as a rule very few bacteria. "The 

 improvement in water during subsidence is the more rapid and pro- 

 nounced the greater the amount of suspended matter initially 

 present" (Frankland). Tils has pointed out that the number of 

 micro-organisms was invariably smaller in the water collected from 

 the reservoir than in that taken from the source supplying the latter. 

 Percy Frankland has demonstrated the same effect of sedimentation 

 by storage as follows : — 



No. of Colonies in 

 1 c.c. of Water. 



1. Intake from Thames, 25th June 1892 . 1991 



2. First small storage reservoir . . 1703 



3. Second small storage reservoir . . 1156 



4. Large storage reservoir . . . 464 



The large reservoir would of course necessitate a prolonged sub- 

 sidence, and hence a greater diminution than in the small reservoirs. 

 Karlinski gives the following distribution of bacteria in the Borka 

 Lake (Herzegovina) : — 



Bacteria per c.c. 

 Surface water ..... 4000 



Five inches below surface 



Ten inches below surface 



Twelve to sixteen inches below surface 



Bottom when mud was stirred up 



1000 

 600 

 200 



6000 



Del^pine considers that bacteria die rapidly in the deposit, 

 although their large numbers are evidence of the effect of sedimenta- 

 tion. He examined some water mains after the sediment had been 

 disturbed and also with the sediment undisturbed. The results 

 were as follows : — 



Sediment undisturbed. Sediment disturbed. 



1. 51 living bacteria per C.C. 334 living bacteria per c. c. 



2. 356 ,, „ 3164 



3. 10 „ „ 852 



He concludes (1) that sedimentation is a very important factor 

 of bacterial purification in flowing water, and (2) that the effects of 



