BACTERIA IN WATER 89 



fying, that is to say, breakers down, by means of putrefac- 

 tion, of organic compounds. The knowledge of this fact 

 has recently been applied, in conjunction with oxidation, to 

 the biological treatment of sewage. As this illustrates in a 

 marked degree some of the facts we have dwelt upon in con- 

 sidering the bacteriology of soil, and as it is likely that the 

 future will witness a still wider application of these same 

 facts, it will be necessary to refer in some detail to the 

 matter. 



Hitherto there has been adopted one of four methods of 

 treatment of sewage. In the first place, in towns situated 

 on the coast the sewage has, by means of a conduit, been 

 carried out to sea. It is clear that such a course, which is in 

 itself open to criticism, is applicable to but few towns. In 

 the second place, methods of chemical treatment have been 

 practised. This has generally been of the nature of a 

 " precipitation " process. Six to twelve grains of quicklime 

 have been added to each gallon of sewage. The process is 

 simple and cheap, but it does not remove the organic matter 

 in solution. On the one hand, it does not produce a valu- 

 able manure ; on the other, it fails to purify the effluent. A 

 dozen other methods have been tried, but all based on the 

 addition of chemical substances to precipitate or change the 

 organic matter of the sewage. Electrolysis, too, has been 

 proposed. The third mode adopted in the past has been 

 that known as intermittent downward filtration. This may 

 be defined as " the concentration of sewage at short inter- 

 vals on an area of specially chosen porous ground, as small 

 as will absorb and cleanse it, not excluding vegetation, but 

 making the product of secondary importance ' ' (Metropolitan 

 Sewage Commission). The action is mechanical and bio- 

 logical, that is to say, due in part to nitrification by bacteria 

 in the upper layers of soil. The fourth plan is that of irri- 

 gation, or " the distribution of sewage over a large surface 

 of ordinary agricultural ground, having in view a maximum 



