BACTERIAL PURIFICATION 265 



24 hrs.' stay, but the free ammonia increased from 3 to 34, 

 averaging 17, per cent. [The loss from this cause must have 

 been replaced by the liquefaction of suspended matter.] About 

 50 per cent, of the fats was removed.^ At no time were there 

 offensive odours round the tanks, and the sludge had little 

 odour. In plain sedimentation the deposit averaged 3*3 cub. yd., 

 in the septic tanks 1*33 cub. yd. per million gallons, therefore 

 the sludge liquefied in the latter averaged 60 per cent, of the 

 total entering. The closed tank, receiving crude sewage, 

 showed 50 per cent, reduction in the solid matter deposited as 

 sludge, and this is considered a sound general average. There 

 was no appreciable reduction of sulphates in the tank. It was 

 demonstrated that uncovered tanks and sprinkling filters could 

 be used in the open air in the cold winter climate of Northern 

 Ohio. The scheme selected as best includes open septic tanks 

 holding 8 hrs.' flow, sprinkling filters passing 2 million gals. 

 per acre daily, and final settling basins holding 2 hrs.' flow ; the 

 result being " a non-putrescible effluent of satisfactory appear- 

 ance from which about 90 per cent, of the bacteria in the raw 

 sewage are removed."^ Works are now being constructed for 

 the city under contract for a 20 million gallon plant at £87,000, 

 excluding pumps and mains. Septic tanks are now in regular 

 use in more than thirty cities and towns in the United States.^ 



Details of the working of EngHsh plants are given in the 

 evidence before the R. Comm. on Sewage, Interim Report, 

 vol. ii., igo2* ; also in a Report to the London County Council 

 by Clowes and Houston, Division III., p. 151, 1904. 



At York, two open septic tanks, each 164 ft. long by 40 ft. 

 wide, are used, the flow from the first passing through the 

 second ; the velocity of the flow for some days before and after 

 was 0*28 ft. per minute, or i6| ft. per hour, and the quantity 

 passing, 628,000 gallons per 24 hours. Samples of the flow 

 were taken at the inlet (raw sewage) and at various distances 

 between the inlet and the outlet, where it passed to the con- 

 tinuous filter. The results are given in the following table : — 



^ In 1902 I found the removal of the fats by a septic tank was 78 per cent., the 

 raw sewage containing 308 per cent, of fat, the effluent 068 per cent. 



2 Report on Sewage Purification at Columbus, Ohio ; Nov. 10, 1905. Eiig. 

 Record, N.Y., Dec. 30, 1905. 



'^ Hering, Gray and Stearns, Sewerage Commission, Baltimore, 1906. 



** A full review of this evidence is given in "The Sewage Problem," by A. J. 

 Martin, 1905 ; San. Publishing Co. 



