160 BACTERIAL TREATMENT OP SEWAGE 



2. Mechanical and Chemical Separations. 



Methods in which this principle is applied are numerous ; they 

 have generally been of the nature of a " precipitation " process. Six 

 to twelve grains of quicklime have been added to each gallon of 

 sewage, forming a precipitate of carbonate of lime, which carries 

 down with it the light, flocculent suspended matter of the sewage. 

 The process is simple and cheap ; it does not, however, remove the 

 organic matter in solution, but merely the solid matters in suspension. 

 On the one hand it does not produce a valuable manure; on the 

 other it fails to purify the effluent. A score of other methods have 

 been tried (e.g. the lime and ferrous sulphate treatment, Hanson's 

 process, " f erozone," amines, electrolysis, etc.), but with the exception 

 of electrolysis, all based on the addition of chemical substances able 

 to precipitate or otherwise change the organic matter of the sewage. 

 All these methods produce large quantities of sludge, the removal 

 of which, by pressing, digging into the land, or sending out to sea, 

 presents many difficulties. But the chemical processes have this 

 advantage, that they remove disease-producing organisms more per- 

 fectly than the bacterial process, though the latter carries further 

 the purification of dissolved organic putrescible matter. 



3. Biological Methods. 



The biological methods, though very various, all have two common 

 features. In the first place, the injurious and putrescible substances 

 in the sewage are not merely " disposed of " nor yet only " separated." 

 They are destroyed. There is a destruction of sewage as sewage, 

 and a building-up of new substance in its place. Secondly, this 

 desired effect is achieved, not by adding anything to the sewage, but 

 ~by the organisms normally present in the sewage or in the medium 

 the land or the " filtering " agent upon which the sewage is treated. 

 In short, all biological processes depend upon the employment of 

 bacteria in some shape or form. Hence each is a bacterial treat- 

 ment of sewage. It may appear at first sight that such a process, 

 involving, as it does, encouragement to the growth of putrefactive 

 bacteria, is not without danger. But we shall be satisfied that this 

 is not really so, when it is remembered that the bacterial treatment 

 of sewage is under control, and may be regulated at will. Moreover, 

 the processes of decomposition and nitrification ultimately destroy 

 the pabulum upon which the organisms in question depend for their 

 existence, and hence lead to their death when they have fulfilled 

 their function. 



Two applications of this principle have long been in vogue, 

 namely, the intermittent downward jiltration and broad irrigation. 



