174 BACTERIAL TREATMENT OF SEWAGE 



and 46 acres of secondary contact beds on land at Flixton.* Dr 

 Fowler, the superintendent and chemist, concludes that the bacterial 

 process is best conducted in three stages: — (a) Settlement and 

 screening out of the grosser solids ; (6) Anaerobic decomposition in 

 the septic tank ; and (c) Oxidation on bacteria beds. He concludes 

 in respect of the septic tanks : — 



That the effluents from closed and open septic tanks are practically indentical in 

 composition, and that with a tank space equal to half the daily flow of Manchester 

 sewage, it is possible to digest about 25 per cent, of the total suspended matter in 

 the sewage. The suspended matter in the septic-tank efiluent is of a granular char- 

 acter, and readily separates out on standing, and when arrested on the surface of a 

 bacteria bed does not seriously impede the free flow of the water into the bed. The 

 organic matter in solution is much more easily nitrified than that present in fresh 

 sewage, so that it is possible with one contact to constantly obtain non-putrefactive 

 filtrates. The blending which takes place in the septic tank is of value in minimis- 

 ing the effect of excessive amounts of manufacturing refuse, and in producing an 

 effluent of fairly constant composition. 



In respect to contact beds he points out that the capacity of contact beds suflers 

 a rapid initial decrease, but afterwards, with careful working, the rate of decrease is 

 very much less. 



The causes of loss of capacity appear to be five, namely — (a) Settling together of 

 the material ; (6) Growth of organisms ; (c) Impaired drainage ; (a) Insoluble 

 matter entering bed ; and (e) Breaking down of material. These matters will be 

 found fuUy discussed in the annual reports of the Rivers Department, and we have 

 not space to enter into them here. We may, however, briefly refer to the conditions 

 of the successful working of contact beds as arrived at as a result of the Manchester 

 experience : — 



(1) The bed should be worked very slowly at first, in order- to allow it to .settle 

 down and the bacterial growths to form. In this way there will be less danger of 

 suspended matter finding its way into the body of the bed, while the material is 

 still loose and open. (2) The burden should not be increased till analysis reveals 

 the presence of surplus oxygen, either dissolved or in the form of nitrates in the 

 effluent. (3) Analyses of the air in the bed may usefully be made from time to time 

 during resting periods. (4) The variations in capacity should be carefully recorded. 

 If the capacity is found to be rapidly decreasing, a period of rest should be allowed. 

 (5) Long periods of rest should be avoided during winter, as when deprived of the 

 heat of the sewage the activity of the organisms decreases. If necessary, the 

 burden on the bed should then be decreased by reducing the number of fillings per 

 day, rather than by giving a long rest at one time. (6J The insoluble suspended 

 matter should be retained on the surface by covering the latter with a layer of finer 

 material not more than three inches in depth. The suspended matter thus arrested 

 should not be raked into the bed, but when its amount becomes excessive it should 

 be scraped off. This should be done if possible in dry, warm weather, after the bed 

 has rested some days. By placing the inlet and outlet penstocks as close together 

 as possible, the suspended matter will tend to concentrate in their vicinity, and its 

 removal will be facilitated, f 



How far the various applications of the bacterial agency in puri- 

 fication will pass the scrutiny of the Eoyal Commission on Sewage 

 Treatment, now sitting, it is impossible to say. But there can be no 



* The particulars as to these new works, their construction, materials, capacities, 

 etc., wiU be found in the Manchester Rivers Department Report, 1902, with plates 

 and charts, pp. 18-24, and more recent extensions in subsequent Reports (1903-4). 



f City of Manchester Rivers Department, Annual Report, 1902, p. 15. 



