278 SEWAGE AND ITS PURIFICATION 



to storm -water " (Chapter VI., p. 148). These requirements 

 were unnecessarily onerous, and in a later report of Dec, 

 1899, the experts confirm their estimate of 60 acres, and state 

 that by " preliminary septic sedimentation " they have obtained 

 an improved effluent, also that they work most successfully with 

 6 hour cycles 6 days per week, but that the exact length of the 

 cycle must vary according to the nature of the sewage. Con- 

 tinuous incubation experiments showed that the effluent im- 

 proved the water of the Ship Canal, that by subsequent land 

 filtration the effluent was actually deteriorated, and that there- 

 fore the land clauses were superfluous. The engineer reported 

 (Sept., 1900) that the experts' scheme would cost ;f337,ooo, 

 the culvert plan ;f350,ooo. The L.G.B. consented to a modi- 

 fication as follows : {a) 46 acres of primary filters at Davy- 

 hulme ; {b) 46 acres of secondary beds at Carrington (which 

 involved the purchase of 213 more acres of land), the primary 

 effluent being conveyed to the secondary beds by means of a 

 culvert ; (c) special storm-filters at Davyhulme for the treat- 

 ment of excess storm-water (say 63,000,000 gallons per diem) at 

 a rate not exceeding 500 gallons per square yard per diem. Cost 

 ;f 487, 000, finally accepted by the Manchester Council. 



Chemical treatment was entirely discontinued in Aug., 1904, 

 and the works now deal by bacterial methods with the whole 

 sewage, averaging 30 to 35 million gals, per day, cr 52-62 gals, 

 per head : daily water supply 29 gals, per head, rainfall 26-34 in. 

 The sewage passes through screens and catch-pits with sub- 

 merged walls, scum boards and outlet sills, into open septic 

 tanks, and then on to the half-acre bacteria beds ; or in exces- 

 sive rains it flows, after simple sedimentation, on to the 26*8 

 acres of storm beds. The latter beds sometimes require forking 

 over to a depth of 4 or 5 inches. By opening each of the valves 

 in turn for a short time, as much as 200 tons of sludge contain- 

 ing 87% of water can be removed from a septic tank without 

 running off the top water. In 1904 and 1905 the sludge removed 

 averaged per million gals, of sewage 8f tons containing 85% 

 water, as distinguished from over 18 tons per million, contain- 

 ing 88% water, during the former chemical treatment. The 

 suspended matter passing away from the bacteria beds has 

 somewhat increased, averaging 5 parts per 100,000, and 

 secondary beds are being constructed.^ 



1 For further details see the annual reports of the Rivers Department, City of 

 Manchester. 



