ANNIVERSARY ADDRESS. 17 



for some time that any means of treating sewage without the 

 production of sludge, would be hailed by sanitary engineers as a 

 great advance on present methods. 



Experiments have recently been made at Exeter, London and 

 other places. At Exeter the Borough Engineer has constructed 

 closed tanks, large enough for receiving a portion of the sewage 

 from the town. The sewage flows into the tanks in an unscreened 

 condition and remains there from twenty-four to forty-eight hours, 

 according to the extent to which it is diluted by rain-water. 

 When leaving these "Septic Tanks" (as they are called) it is 

 found that an extraordinary change has taken place in the sewage. 

 All the solids are broken up into very minute particles, the bulk 

 becoming soluble. Distinct chemical changes have taken place 

 by the aid of anaerobic germs, which under favourable conditions 

 are present in the tank. 



Running large quantities of sewage into tanks from which light 

 and air are carefully excluded, would, one would think, have the 

 effect of creating a great nuisance, but such is not the case. The 

 gases generated in the tanks undergo chemical decomposition, so 

 that when they are liberated they are not obnoxious. There is 

 no sulphuretted hydrogen present, and the greater part of the 

 gases which are given off are nitrogen, hydrogen and methane or 

 marsh gas. The sewage when discharged from the tanks is a grey 

 liquid with only a slight musty smell, and in such a condition as 

 to be readily purified by filtration, either by passing it direct over 

 land, or through filters of breeze or sand. The experiments 

 show that typhoid germs have little chance of passing through 

 the tank without being destroyed, and should they do so, they are 

 in such a weak condition that they afterwards die. 



Although the tanks are large, having a capacity for at least 

 twenty-four hours average dry weather flow, no settlement takes 

 place in them, and after twelve months use, only a thin layer of 

 mineral ash was found. Should the method prove to be as suc- 

 cessful as it bids to do, the whole difficulty of the sludge is over- 

 come in a very simple way. The tanks at Exeter have been 



B— May 4, 1898. 



