36 The Purification of Sewage by Bacteria. 



During the preliminary decomposition in the septic tank, it 

 was important to exclude oxygen as far as possible ; but having 

 brought the soHds into solution, it becomes necessary to oxidise 

 them ; and this process requires the free access of air. 

 Accordingly the effluent, after flowing through an aerator, 

 passes into the filters, in which the work of oxidation is chiefly 

 accomplished. This duty, like the preliminary liquefaction of 

 the solids in the tank, is the work of bacteria ; but the workers 

 in the filters, unlike those in the tank, which only thrive in the 

 absence of air, require a plentiful supply of oxygen to enable 

 them to perform their functions. Each filter therefore is 

 first filled, then allowed to rest full for a certain time, then 

 emptied, and finally left to drain and aerate. In this way the 

 , nitrifying bacteria obtain the necessary supply of oxygen. The 

 need for constant attention is done away with by means of an 

 alternating gear, which automatically opens and closes the 

 valves in their proper order. The works are thus rendered 

 completely automatic, and one man, visiting ihem for a few 

 minutes on two or three days per week, is able to give all the 

 attention which they ordinarily require. The filtered effluent 

 from these works has been examined at various times by many 

 of ihe foremost chemists in the kingdom, and found to be of a 

 high quality and perfectly inoffensive ; and it remains so when 

 kept for any length of time. 



A marked characteristic of works on this system is their entire 

 freedom from nuisance. At Exeter there is a good house with- 

 in seventy yards of the works, and three other high-class 

 residences within two or three hundred yards. At the Local 

 Government Board Inquiry held at Exeter with reference to 

 the scheme for laying down tanks and filters for the whole 

 city, there was not one word of opposition thereto, although 

 the new works will be some thirty times as large as the in- 

 stallation already laid down, and right under the windows of 

 these houses. 



At Yeovil there is a factory within three paces of the works, 

 and the proprietors state that they have never experienced any 



