92 BACTERIA 



the bottom of the tank perform greater service than those 

 at the top. When the liquid sewage passes out of the tank 

 it differs from the crude sewage which enters the tank in the 

 following particulars : (a) The gravel and particulate debris 

 have been removed ; (b) the organic solids in suspension are 

 so greatly diminished that they are almost absent ; (c) there 

 is an increase of organic matter in solution ; ( d) the sewage 

 is darker in colour and more opalescent ; (e) compounds like 

 albuminoid ammonia, urea, etc., have been more or less 

 completely broken down, and reappear in elementary con- 

 ditions, like ammonia, methane, carbon dioxide, and sul- 

 phuretted hydrogen. These latter bodies may be in solution 

 or may have escaped as gas. 



The cultivation beds are four or five filters, to which the 

 sewage from the tank flows in such a manner as to produce 

 a weir. By an automatic arrangement the fluid is distributed 

 to each filter in turn. When the second filter is full the 

 first is discharged, and remains empty during the time that 

 the third and fourth are being filled. Each filter is thus 

 full, say, about six hours, and has from ten to twelve hours' 

 rest. These filter-beds (at Exeter) have an area of eighty 

 square yards and a depth of five feet ; collecting drains are 

 laid on the bottom of the filters, joining main collectors, 

 the latter terminating in discharging wells. The filtrant is 

 broken furnace clinker or broken coke. 



The changes occurring in these filters are of the nature of 

 oxidation, with the result that the proportion of the oxidised 

 nitrogen increases (as nitrites and nitrates), the ammonia 

 becomes less, and the total solids and organic nitrogen 

 almost disappear. It will thus be seen that the work of 

 these filters is not merely a straining action. It is true that 

 particulate matter in the effluent from the tank is caught on 

 the surface by the film (resulting from previous effluents), 

 but the real work of the bed is nitrification, an oxidation of 

 ammonia into nitrites and nitrates. This change obviously 



