250 OUTLINES OF BACTERIOLOGY. 



access is obtained to the oxygen of the air. The oxidation is effected 

 partly in a purely chemical manner and partly through the agency 

 of aerobic bacteria. 



§6. ARTIFICIAL METHODS OF SEWAGE DISPOSAL. 



Until recently, the above-mentioned methods were the only ones that 

 were known for the disposal of sewage. But in thickly populated 

 districts the large amount of land required for the purpose made land 

 treatment very expensive, with the result that a number of artificial 

 methods gradually sprang up, some of a chemical, others of a biological 

 nature. 



With the various chemical methods of disposal we have very little 

 to do in this book, as bacteria play no part in the changes that are 

 effected in the sewage. A score or more of chemical methods are in use, 

 and with one exception all depend upon the precipitation of sewage by 

 means of various chemical substances. A common method is to add 

 6-12 grains of quicklime to each gallon of sewage, the result being the 

 formation of carbonate of lime. As the precipitate falls it carries with 

 it the solid organic matter contained in sewage. Many other sub- 

 stances, and combinations of substances, are in use as precipitants, e.g. 

 lime and ferrous sulphate, a mixture of clay, carbon, blood and salts of 

 alumina, a mixture of the higher oxides and crude sulphate of man 

 ganese and other combinations. The defect from which all chemical 

 methods suffer is that precipitation does not affect the organic matter 

 which is in solution. Of the various methods, many authorities 

 consider that the simplest and cheapest, as well as the most efficient 

 method is lime precipitation. 



The various biological methods aim at the destruction of sewage 

 on the same lines as is effected by land treatment, that is, by getting 

 rid of the organic matter by causing it to be changed into harmless 

 products through the agency of vai'ious non-pathogenic bacteria. 

 The biological methods have this advantage over land treatment 

 that the sewage is under control, and can be changed from one 

 situation to another, so that different kinds of bacteria can have 

 full scope for their acti-s-ity. This ensures a more complete breaking 

 down of the organic matter. It also differs slightly from land treat- 

 ment in that a more important part is played by the sewage bacteria. 

 It may seem strange that the sewage bacteria should be allowed to 

 multiply in this way, but it must be borne in mind that the sewage is 



