246 OUTLINES OF BACTERIOLOGY. 



pure water and in virgin soils. As a class they are delicate germs that 

 soon lose their vitality under unfavourable conditions, and several of 

 the species are pathogenic to man. Hence if found in drinking water 

 in large quantities, they must have got there by pollution of the water 

 with human faeces or with sewage, in both of which they abound; being 

 delicate organisms their presence in large numbers indicates that the 

 pollution is recent. 



g 3. THE TREATMENT OF SEWAGE. 



It will be seen from the above statements that it would be extremely 

 imprudent to allow sewage to escape in large quantities without sub- 

 jecting it to some treatment which would prevent the possibility of 

 the multiplication of the pathogenic germs contained in it. If not so 

 subjected the sewage of a town with a fairly large population might 

 become a serious menace to the community. There are three obvious 

 methods which present themselves for consideration. We may conduct 

 the sewage, without treating it in any way, to a place where it can 

 do no harm, or we may kill off the contained bacteria by heat or 

 by an antiseptic, or finally we may so change the constitution of 

 sewage that it becomes unfit for the pathogenic bacteria to live in. 

 Of these the first is possible where the quantity of sewage is small; 

 the second is not practicable ; whilst the third method can be used for 

 large and small quantities alike. It will be seen that in all the 

 methods of artificial treatment, the underlying principle is the removal 

 of the bacteria by the removal of their food supply. In connection 

 with this point, it is interesting to note the manner in which the 

 sewage-bacteria are kept under in nature, when they enter into 

 competition with other organisms. In Chap. VI. we have shown 

 how the sewage-bacteria, which enter the Severn in large numbers 

 as the waters of that river pass through Shrewsbury, diminish in 

 numbers to such an extent that the water a few miles lower down 

 is as pure as it was before it reached Shrewsbury. It was shown 

 that one of the chief agencies concerned in this self-purification of the 

 river is the rapid diminution of the food supply due to the fact 

 that the food contained in sewage is keenly competed for, not only 

 by the sewage-bacteria but by a host of other organisms. Amongst 

 these competitors the most formidable are the other non-pathogenic 

 bacteria, for it is a rule in nature that the more nearly allied the 

 organisms, the keener becomes the struggle for existence; and the 



