THE SEWAGE DISPOSAL PROBLEM 191 



ground vault (septic tank) of concrete, cemented on exposed 

 surfaces, and holding as many thousands of gallons as are 

 required according to the population. The sewage travels 

 slowly through the tank, so that every particle is supposed to 

 take about twenty-four hours to get through. This period of 

 time is sufficient for such a complete sedimentation and lique- 

 faction of solids that the tank effluent should contain only a 

 very few grains of suspended matter per gallon. The second 

 stage of purification is effected by passing this effluent through 

 an aerator— z>. a long trough, over the edges of which the liquid 

 falls in thin films, thereby coming in contact with a certain 

 amount of air, and from there on to a series of coke breeze 

 filter beds, 4! feet deep. These beds are filled, emptied, and 

 rested like Mr. Dibdin's. 



The advantages claimed for this method were(i) that it solved 

 the sludge difficulty, inasmuch as practically all the organic 

 solid matter was digested in the tank ; (2) that it destroyed any 

 pathogenic organisms which might be present in the sewage ; 

 and (3) that the sewage which had passed through a septic tank 

 was more easily oxidised than sewage from which the solids 

 had been allowed to settle, either with or without the aid of 

 chemicals, in tanks which were frequently cleaned out. 



The Royal Commission, in its Report of 1908, concluded in 

 regard to the above: (1) that it is now clearly established that 

 all the organic solids are not digested by septic tanks, the actual 

 amount of digestion varying with the character of the sewage, 

 size of tank relatively to volume treated, frequency of cleansing, 

 etc. With a domestic sewage, and tanks worked at a twenty- 

 four hours' rate, the digestion is only about twenty-five per cent. 

 (2) The liquor issuing from septic tanks is bacteriologically 

 almost as impure as the sewage entering the tanks ; and (3) 

 domestic sewage which has passed through a septic tank is not 

 more easily oxidised in its passage through filters than domestic 

 sewage which has been subjected to chemical precipitation or 

 simple sedimentation. 



Aerobic filter-beds should be constructed of fine grain 

 material. It does not appear to matter very much what the 

 nature of the material is so long as it is hard and durable — 

 e.g. coal, coke, cinders, clinker, refuse pottery, flint, gravel, 

 burnt ballast, etc., have all been used and proved more or less 

 satisfactory. Local circumstances should determine to a large 



