Bacterial Treatment of Sewage. 381 



With domestic sewage these tanks will often go for many years 

 without any sludge requiring removal, but with town sewage, in 

 which foreign elements enter, sludge has to be dealt with, and it is 

 then better to build the tanks deeper, and with a conical bottom, so 

 that all the deposit drops towards the centre. This is then discharged 

 through a pipe at the bottom, or pumped up through a chain pump ; 

 by this means the working of the tank is not impeded or delayed. 

 The sludge itself can be run off into shallow trenches, the water 

 soaking into the ground. The sun dries the remainder into a friable 

 cake, which makes excellent top-dressing, and there is scarcely any 

 smell with it. 



After the solid matter has been liquefied in the tanks through 

 the agency of bacteria, the final treatment of the sewage is the 

 oxidation of the ammonia and organic compounds, the oxidation of 

 the ammonia into nitric and organic acids, which combine with lime, 

 soda, and other bases in the sewage to form nitrites and nitrates of the 

 latter. This is called nitrification, and the changes are effected by 

 aerobic bacteria. This change can be effected by applying the 

 effluent to land (known generally as broad irrigation or intermittent, 

 downward filtration), contact beds or percolating filters. 



In contact beds the effluent from the Septic Tank is collected in 

 tanks in which the filtering material is contained. The bed is allowed 

 to fill, and then a period of rest is allowed when filled. It is then 

 discharged and the effluent is allowed to flow away, or a second 

 filtration given if further purification is required. A set of four 

 tanks is constructed, so that when one tank is filling, another resting 

 full, and a further one is being discharged and aerating, and a spare 

 one is also provided, so that each filter in turn can be cut out for two 

 or three days' complete rest, so a complete cycle is formed. The 

 action of the filter is twofold : (i) it collects all gross particles of 

 suspended matter on the filtering material, (2) it effects the oxidation 

 of organic matters by bacteria. It is essential that the discharge be 

 quick and the bed effectually drained and allowed time for aeration. 

 The depths of these filters are constructed from two to six feet, 

 and equally good results have been obtained from either, so that 

 the depth of the material does not seem to make much material 

 difference. 



Various materials have been tried for filtering — broken glass, 

 cinder, coke, coal, gravel, and broken stone, but clinker is generally 

 used when obtainable, that from a destructor being preferable. The 

 most important part about the material selected is that it should not 

 break up or disintegrate, as it then settles down and becomes clogged. 

 In contact beds, clinker broken down to a ?in. mesh is as good as 

 anvthing. but it must be washed and freed from all dust. 



The diverting of the effluent from one filter bed to another can 

 be done by hand, but as this entails careful watching, and has to be 

 done at night time as well as day, it is usually effected by automatic 

 gear, of which there are several kinds. These in their turn require 

 a good deal of attention, but they regulate the period of rest when 

 full and empty far better than can be done by hand. 



