138 BACTERIA IN AIR, SOIL, AND WATER. 



latter can always be recognised in .001 grm. of human faeces. 

 The deductions to be drawn from the presence of these in water 

 are the same as those to be drawn from their presence in soil. 

 A further point here is that it is well, wherever practicable, that 

 the indirect evidence as to the potability of a water which is 

 usually derived from chemical analysis should be supplemented 

 by a bacteriological search for the three groups of organisms 

 mentioned. It has been found that in water artificially polluted 

 with sewage containing them, they can be detected by bacterio- 

 logical methods in mixtures from ten to a hundred times more 

 dilute than those in which the pollution can be detected by 

 purely chemical methods. 



Bacterial Treatment of Sewage. — Of late years the opinion 

 has been growing that the most appropriate method of dealing 

 with the disposal of sewage is to imitate as far as possible the 

 processes which occur in nature for the breaking up of organic 

 material. These practically depend entirely on bacterial action. 

 Hence the rationale of the most approved methods of sewage 

 disposal is to encourage the growth of bacteria which naturally 

 exist in sewage, and which are capable of breaking up organic 

 compounds and of converting the nitrogen into nitrates and 

 nitrites. The technique by which this is accomplished is very 

 varied and sometimes rather empirical, but probably the general 

 principles underlying the different methods are comparatively 

 simple. It is probable that for the complete destruction of the 

 organic matter of sewage both aerobic and anaerobic bacteria 

 are required, though on this point there may be some difference 

 of opinion. Certainly very fair results are obtained when ap- 

 parently the conditions chiefly favour aerobic organisms alone. 

 This is usually effected by running the sewage on to beds of 

 coke, allowing it to stand for some hours, slowly running the 

 effluent out through the bottom of the bed, and leaving the bed 

 to rest for some hours before recharging. The final result is 

 better if the effluent be afterwards run over another similar 

 coke-bed. According to some authorities the sewage, as it runs 

 into the first bed, takes up from the air considerable free oxygen, 

 which, however, soon disappears during the stationary period, so 

 that on leaving the first bed the sewage contains little oxygen. 

 In the latter part of its stay it has thus been submitted to ana- 

 erobic conditions. Further, while by the passage of the effluent 



