240 



SEWAGE AND ITS PURIFICATION 



phia, at East Cleveland, Ohio, and at other places in the States. 

 The roughing filters are masonry tanks filled to a depth of 

 2 J feet with Bessemer slag " about egg-coal size." The total 

 area of the strainers (four down-flow and two up-flow beds) is 

 3,630 square feet, designed to receive 150,000 gallons of sewage 

 per day. The aerators are 6,248 square feet, and the rate of 

 application to them is 1,000,000 gallons per day of strained 

 sewage, or 661,000 gallons per day on the total area. The 

 bottom of the aerators is a false floor of half-round drain-pipes 

 through which the air is forced. Nineteen other installations 

 were also designed, but little is now heard of the method. 



At East Cleveland Dr. A. Smith reported "a reduction in 

 the ammonias of 98*8 per cent., in the bacteria of 99 per cent., 

 by double filtration through slag and coke, with aeration under 

 light pressure by a blower." 



Ducat Filter. — Colonel Ducat constructed an aerating filter 

 with walls of 3-inch drain-pipes set nearly horizontally in 

 Portland cement, the inner ends being 3 inches lower than the 

 outer, to prevent the sewage running out. The free exposure 

 to air causes considerable cooling, rendering necessary a special 

 provision by larger pipes for hot-water heating in winter to 

 prevent freezing. It was first tried on crude sewage in 1898 at 

 Hendon, but a large quantity of ammonia was carried off by 

 the air without being nitrified, and sludge collected in the 

 sewer before treatment. Satisfactory continuous working was 

 based on an ample provision of oxygen. The bed was coarse- 

 grained above and fine below, and the action was intended to 

 be exclusively aerobic, as atmospheric oxygen in excess was 

 brought in contact with the contents at once without giving 

 any period of anaerobic incubation, as in a part of the Waring 

 process. I have already observed that in towns with long and 

 old sewers, or where storage is practised, the liquids may have 

 already received sufficient hydrolytic resolution to be quite pre- 

 pared for such strong aeration. This is illustrated by an 

 analysis furnished by Dr. Houston : — 



