CONTACT BEDS 173 



A moment's reflection will make it evident that the problem may 

 be seriously complicated at short notice by the great increase in 

 volume of the sewage following rain-storms. To this matter the 

 experimenters at Manchester have also directed their attention. 

 They draw the necessary distinction between the first flush of a 

 storm and the highly-diluted sewage which follows, designating the 

 latter only as " storm water." They decide that provision must be 

 made for the storage or separate treatment of " first flush " of sewage 

 at the beginning of a storm, and that about two hours after the 

 augmented flow is the time to commence accelerated treatment, the 

 exact procedure varying according to the character and duration of 

 the storm. Short double contacts, or even a single contact, is 

 sufficient to purify storm water, and there is no decrease in the 

 purifying capacity of the bed. 



Summarily, the final conclusions arrived at by Latham, Frank- 

 land, and Perkin were as follows : 



" 1. That the bacterial system is the system best adapted for the 

 purification of the sewage of Manchester. 



" 2. That any doubts which may have arisen in the first instance 

 as to its suitability, owing to the presence in Manchester sewage of 

 much manufacturing refuse, have, through the convincing results of 

 our experimental inquiry, been entirely banished. 



" 3. That inasmuch as a bacterial contact bed can only effect a 

 definite amount of purification in a single contact, it becomes 

 necessary, in order to carry the purification beyond this limit, to 

 apply the effluent to a second bed, in which again a further definite 

 amount of purification can be effected. Hence, for obtaining a high 

 degree of efficiency in bacterial purification of sewage, a system of 

 multiple contact is generally necessary. Thus it may be taken 

 broadly that in the first contact 50 per cent, of the dissolved impurity 

 is removed, and that in the second contact 50 per cent, of the 

 impurity still remaining in the effluent is disposed of, and so on." * 



In subsequent experiments these conclusions were amply con- 

 firmed, and the Manchester Corporation eventually extended their 

 sewage works, laying down five additional tanks, and a large number 

 of contact beds (primary and secondary). These included 92 half -acre 

 primary beds, with 26 acres storm-water filter-beds at Davyhulme, 



* Elvers Department of Manchester, Experts' Report, 1899, p. 53. The 

 Borough Surveyor of Leicester (Special Report, 1900) examined various bacterial 

 systems for the disposal of the sewage of the Belgrave district, and finally 

 recommends as the best method the following : (1) Crude sewage passed into an 

 open or closed detritus tank to remove suspended mineral matters ; (2) then on to 

 clarifying bacteria beds of 4 feet 6 inches depth, and containing crushed and 

 screened clinkers (coarse and fine) from the refuse destructors ; three fillings a day ; 

 (3) finally, land purification of the effluent on old pasture. (Total purification of 

 suspended matter of sewage 99 '12 per cent. ; of albuminoid ammonia, 8676 per 

 cent. ; and of oxygen absorbed 91 '08 per cent.). 



