BACTERIAL TREATMENT OF SEWAGE 139 



never fewer than 100,000 per c.c., and therefore if in a water 

 this organism forms a considerable proportion of the total 

 number of organisms present, then there is grave reason for 

 suspecting sewage pollution. Houston holds that, the nearer the 

 majority of the coli organisms in a water approach to the typical 

 reactions of coli the more likely is sewage contamination to be 

 present. The reactions regarded by him as typical are, gas 

 production in gelatin shake culture, production of indol, 

 clotting of milk, production of fluorescence in neutral-red broth, 

 acid and gas production in lactose peptone solution (v. b. coli). 

 The presence of b. coli in 100 c.c. of deep well water or in 

 10 c.c. of river or shallow well water is sufficient to condemn 

 that water. As the b. coli is fairly widespread in nature, Klein 

 and Houston hold that valuable supporting evidence is 

 found in the presence of the b. enteritidis sporogenes and of 

 the streptococci, both of which are probably constant inhabit- 

 ants of the human intestine. The spores of the former usually 

 number 100 per c.c. in sewage, and the presence of the 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 



