170 BACTERIAL TREATMENT OF SEWAGE 



more advanced form at Davyhulme, Manchester. At Sutton there is 

 no tank. A metal screen holds back part of the solids from being 

 carried on to the beds. The filtrant is burnt clay, and it is forked 

 over occasionally to let in oxygen. The beds are 3J feet deep. The 

 bottom of the bed is provided with a 6 -inch main drain with tributary 

 drains. The crude sewage, after passing through a roughing screen 

 to intercept floating paper, etc., is run directly upon the filter 

 without the addition of any chemicals. The filter is charged to 

 within 6 inches from the surface, and the sewage remains in contact 

 for a period of two hours, after which the outlet valve is opened and the 

 filtrate is drawn off to be further purified on fine-grain bacteria beds, 

 after which the effluent is in a fit condition to be discharged into the 

 stream, and is uniformly superior to the effluent obtainable by 

 chemical treatment. The sludge is absorbed by bacterial agency in 

 the beds, and does not accumulate or manifest itself. The beds are 

 free from any offensive odour. At first the beds were seeded with 

 Micrococcus candicans, but it is now known that the necessary bacteria 

 are in the sewage, and seeding is not required. For more effective 

 screening of the sewage an automatic rotary screen may be fixed. 

 This screen may be driven by a Poucelet water-wheel, actuated by 

 the sewage. 



Experiments seem to prove that coarse-grain beds worked on the 

 contact principle may be constructed of a numerous class of materials, 

 and that different districts may adopt materials which are obtainable 

 locally, and often at a small cost, although it may be observed that 

 porous coarse-grained materials such as coke and burnt ballast effect 

 a greater degree of purification than do fine-grained impervious 

 material such as granite, slate, etc. The cost of such a system would 

 be in many cases one-quarter of a chemical precipitation and 

 irrigation system, and yet more effective. It will be understood that 

 the absence of a septic tank does not mean an entire absence of the 

 anaerobic action of the process. It simply takes for granted that 

 this portion of the process has been in part performed in the sewers. 



The Manchester bacterial system is practically the same in 

 principle as at Sutton. But there are two important and interesting 

 differences. First, the quantity of sewage to be dealt with is very 

 much greater. Secondly, there is the added complication that the 

 Manchester sewage contains large quantities of tfnz^e-effluent from 

 breweries, dyeing and bleaching works, galvanising works, tanneries, 

 and derivatives from coal-tar, colour, and naphthalene works. It has 

 been frequently suggested that such chemicals in the sewage would 

 prevent the contained bacteria from fulfilling their rdle in the purifi- 

 cation. Consequently, the trial of the two chief methods of bacterial 

 treatment of Manchester sewage has been followed with much 

 interest. In 1898 three experts were appointed, and requested to 



