168 BACTERIAL TREATMENT OF SEWAGE 



(b) Two feet or so of screened clinkers to pass, a f-inch mesh, 

 but not a ^-inch mesh. 



(c) Three inches of residue from above, which will pass a ^-inch 

 mesh. 



Collecting drains are laid on the bottom of the beds, joining main 

 collectors, which terminate in discharging wells.* 



The change occurring in these bacteria beds is of the nature of 

 oxidation, with the result that the proportion of 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 begins 

 when the tank effluent flows over on to the beds and the oxygen 

 then obtained by the effluent is carried down in solution into the 

 coke breeze. Upon the surfaces 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, or on to the land direct, where they become, 

 of course, absolutely innocuous. The " filter " is perhaps more 

 correctly termed a cultivation bed, for its purpose is to furnish a 

 very large surface upon which nitrifying organisms, present as 

 we have seen in all soils, may flourish, and these feeding upon 

 the organic matter of the sewage, may perform their function of 

 oxidation. 



The solid matter has plenty of time to settle in the tank and be 

 fully operated on by the bacteria, which are not only contained in 

 the sewage, but also grow and multiply in the tanks. This growth 

 is, of course, a question of time; and just as the growth of the 

 nitrifying layer is necessary to a water filter-bed, so the growth of 

 the necessary organisms is required in the septic tank and on the 

 filter-beds. In the tank, however, no rest is necessary, for the supply 

 of organisms is continuous and unlimited, both in supply and in 

 reproduction. Not so the beds. Here there is only a limited 

 amount of oxygen to start with, and consequently a definite limita- 

 tion to the amount of work the filters can perform. Hence the need 

 of rest, in order that the oxygen may be replenished. 



The amount of sludge in the chemical processes has always been 



* An account of the Septic Tank method will be found in the Brit, Med. .Tom:, 

 1900, i., pp. 83-86. 



