The Purification of Sewage by Bacteria. 35 



years of study and research Mr. Cameron came to the con- 

 clusion that the polluting matter of sewage might be so changed 

 as to be rendered harmless solely by the operation of natural 

 agencies, provided that these were properly directed and con- 

 trolled. The outcome of his work in this direction is the septic 

 tank, which is probably known by name, at least, to most of 

 those present. 



The septic tank and fillers at Belleisle have now been dealing 

 for more than two years with the sewage of St. Leonards, a 

 suburb of the city of Exeter, having a population of about 1,500. 

 The sewage is turned without any screening or preliminary 

 treatment into the septic tank, in which the solid matter is 

 retained, the clear water then being drawn off between the 

 scum on the surface and the heavy deposit which lies at the 

 bottom of the tank. 



If we did nothing more than arrest the solid matter of the 

 sewage, we should still be confronted with the difficulty of 

 getting rid of the sludge. But, fortunately, sewage contains 

 within itself the seeds of its own destruction. Dr. Poore, 

 among others, has drawn attention to the fact that excrement 

 simply teems with bacteria. Under the conditions provided in 

 the septic tank, these tiny scavengers attack the impurities of 

 the sewage, and gradually resolve them into simpler and 

 simpler forms. The scum which covers the surface is full of 

 bacteria, and serves as the " barm " so to speak, which sets up 

 the fermentation or decomposition by which the solid matter 

 is eventually broken down. 



As the outcome of these operations we get an effluent 

 practically free from solid matter, and showing a marked 

 purification even as regards the impurities in solution. 



Among the final products of the decomposition which takes 

 place in the septic tank, are marsh gas and free hydrogen, both 

 of which are highly inflammable gases. The flame from these 

 gases, though a very hot one, is not luminous ; but it becomes 

 so by the aid of an incandescent mantle. For some months 

 past the works at Exeter have been lighted at night with the 

 gases generated in the tank, 



