Purification of Sewage 107 



original sewage. They were incapable, however, of 

 freeing the sewage from most of the putrescible matter, 

 for the reason that' about one-third of the organic nitro- 

 gen and about one-half of the carbonaceous matter 

 in the sewage are contained in the suspended solids. 

 The rest is held in solution. The dissolved substances 

 in the clear effluents were still liable to create serious 

 pollution of surface water. The disease bacteria, also, 

 that were not carried down by the chemicals, still re- 

 tained their virulence. Moreover, the machinery, chemi- 

 cals, and labor required for the chemical treatment of 

 sewage, involved considerable expense, and the sludge 

 removed from the sewage did not prove as valuable for 

 manurial purposes as was anticipated. In some instances, 

 companies that had been organized for the recovery 

 of fertilizer substances from sewage were compelled 

 to dispose of their plants after sustaining large monetary 

 losses. 



More recently, bacteriological methods for the puri- 

 fication of sewage have come into more general use, and 

 have already demonstrated their efficiency when prop- 

 erly applied. Reference has been made to the self- 

 purification of streams, a process dependent upon the 

 activities of microscopic organisms. Similarly, in 

 sewage, the vast number of bacteria attack and destroy 

 both the suspended and dissolved organic matter, 

 and render the liquid non-putrescible and inoffensive. 

 However, this process is gradual and, under natural 

 conditions, gives rise to foul odors in its early stages. 



The artificial methods in the bacteriological purifi- 

 cation of sewage aim to intensify the activities of cer- 



