BACTERIA IN WATER 93 



begins when the tank effluent flows over the " weir " on to 

 the filter-beds, and the oxygen thus obtained by the effluent 

 is carried down in solution into the coke-breeze. Upon the 

 surface of the filtrant are oxidising bacteria. When the 

 effluent is on the bed they oxidise its contained products; 

 when the bed is empty and " resting " they oxidise carbon. 

 An advantage arising from the periodical emptying and 

 filling of the filter is that the products of decomposition 

 which would eventually inhibit the action of the aerobic 

 bacteria are washed away, and pass into the nearest stream, 

 where they become absolutely innocuous. 



The " filter " is more correctly termed a cultivation bed, 

 for its purpose is to furnish a very large surface upon which 

 the nitrifying organisms present, as we have seen, in all soils, 

 may flourish, and thus feeding upon the organic matter of 

 the sewage, may perform their function of oxidation. 



It is not possible to lay down exact limits as to where 

 denitrification ends and oxidation begins. To a certain 

 extent, and in varying degree, they overlap each other. 

 But roughly we may say that in the tank there is a breaking 

 down (denitrification and decomposition) and in the filter- 

 beds a building up (nitrification). The case is precisely 

 parallel to similar changes occurring in soil, and which we 

 have dealt with elsewhere. The advantage indeed of this 

 biological treatment of sewage is that it exactly follows the 

 processes of ^nature, in contradistinction to the mechanical 

 and chemical methods hitherto adopted. 



At Sutton and some other places the same principles are 

 applied, that is to say, bacterial filtration, but there is no 

 tank. A metal screen in some measure takes its place, and 

 holds back solid matter from being carried on to the beds. 

 The filtrant is burnt clay, and it is forked over occasionally 

 to let in oxygen. The crude sewage is run over the top 

 of the burnt ballast, where it is left for two or three hours. 

 It is then slowly run off on to a finer filter, where it also 



