28o SEWAGE AND ITS PURIFICATION 



The septic tank is too small, and the "roughing tank" too 

 large.^ 



At Sheffield (population about 400,000, average dry-weather 

 sewage I7'3 million gals, per day, water supply per head 21*89 

 gals. ; hence the sewage contains m.uch surface water), the local 

 Act of igoo allowed a scheme without land treatment, to 

 include open septic tanks, 11 million gals, capacity, 32 acres 

 of coarse-bed filters, 33 acres of fine-bed filters, and a small 

 area of storm filters. Following the satisfactory results from 

 continuous filters at Leeds, the Sheffield Council began experi- 

 ments on their use instead of double contact, as the Leeds 

 experiments showed that it would be possible to replace the 

 65 acres of double contact beds by about 17 acres for continuous 

 filters. 



At Leeds^ the first open septic tank had an area of 6,000 

 sq. ft. and a capacity of 250,000 gallons, or 24 hours' flow, 

 which proved to be the best rate of transit. The advantages 

 found were : " (i) The production of a practically uniform effluent 

 from sewage of such varying composition as that of Leeds : 

 (2) the digestion of part of the solids in suspension, amountmg 

 at Leeds to 40 per cent. : (3) the anaerobic putrefaction which 

 takes place facilitates subsequent filtration, rendering the filtrate 

 less liable to secondary putrefaction." As at Manchester, the 

 results with open and closed tanks were practically identical, 

 the scum itself soon giving a cheap roof and preserving the heat, 

 which averaged only o*8° F. less in the open than in the closed.^ 

 It was recommended that the tanks should be in series, the 

 first being in duplicate so as to be emptied from time to time. 

 The gas produced did not occasion nuisance. 



Continuous filtration of septic effluent over very coarse material,'^ 

 spreading the liquid like rain over coke of not less than i^ in. 

 diameter, well aerated at the bottom and sides, gave, in so short 

 a time as the 15 minutes required to pass through the bed, better 

 results than double contact, if the solids in suspension which 

 pass through are afterwards settled. These solids are largely 

 mineral, non-putrescible, subside easily, and the drying does not 

 give rise to evil odours, while it would seem that their coming 

 through ensures the permanence of the beds. It was found 

 practicable, for long periods, to work continuous beds 10 ft. 



1 Vide the author's remarks in the discussion of the above report, ibid., p. 669. 

 - City Report, 1900. 



'■'' See evidence R. Comra. on Sewage, vol. ii., 1902, questions 7258 and S232. 

 ■^ Chapter X , p. 246 



