304 SEWAGE AND ITS PURIFICATION 



and is less than 54 ft. diameter. Above 54 ft. diameter a third 

 rolhng dram is introduced together with an intermediate track, 

 which is usually carried upon cast-iron columns. This system 

 is suitable for beds up to 120 ft. diameter. The interval 

 between the application of each dose to the filter bed surface is 

 from 3 to 6 minutes, thus giving the bed time for aeration. 

 The standard sizes actually erected so far are on filters from 

 9 ft. (Wolverley, Loughborough) to 100 ft. diameter. Various 

 sizes are in use at Ford House, Shrivenham, Alvechurch, 

 Atherstone, Walsall, Sileby, and a distributor for 100 ft. bed 

 (of clinker, 0"i8 acre, to purify 250,000 gals, per day) has been 



Fig. 52. — FiDDiAN Revolving Distributor at Enfield. 



erected at Enfield (Fig. 52). Prices quoted are : 25 ft. diam., 

 ^^50 ; 50 ft., ;f 106 ; 100 ft., ;f 225 ; not including the iron tracks. 

 It is stated that only 18" of head is required between the sill of 

 the Septic Tank and the top of the filtering medium, not only 

 for driving the distributor, but also to overcome pipe friction 

 and clearance underneath the rolling drum. The flow of 

 sewage may vary within wide limits : the makers state that 

 the test at Tividale shows that the Distributor can work with 

 50 gals, per sq. yd. per diem, and according to Dr. Reid the 

 mean fluctuations over or under the mean delivery per yard of 

 filter only amounted to 10%. No flushing tank or automatic 

 valves are required. 



The Septic Tank Company, Ltd., have a " turbine ring " 

 revolving distributor with graduated trough arms. The sewage 



