210 SEWAGE AND ITS PURIFICATION 



An extension of chemical treatment and precipitation having 

 proved to be inadequate as well as costly, the Main Drainage 

 Committee of the County Council in 1891 authorized a series 

 of experiments at Barking outfall, on the lines of the Massa- 

 chusetts researches. From preliminary trials with small filters, 

 coke breeze appeared the most suitable material, although burnt 

 ballast nearly equalled it in purifying efficiency. Sand and 

 gravel effected a greater clarification, but the removal of dis- 

 solved organic matter, as measured by the reduction in the 

 oxygen consumed, given by Mr. Dibdin's report was consider- 

 ably less than with the coarser materials, while there seemed a 

 tendency for this effluent to become putrid, owing to deficient 

 aeration from the closeness of texture, and the filter required 

 frequent scraping and renewals. The average rate of working, 

 including periods of rest, was 411,000 gallons per acre, or 

 250 gallons per square yard in twenty-four hours. For eight 

 hours a day the effluent ran continuously, the filters being kept 

 full ; the filter was then emptied, and allowed to rest for 

 sixteen hours. Mr. Dibdin's figures were : 



Clarification, as measured by the units of depth required to 

 obscure a standard mark : Burnt ballast, i ; coke breeze, i ; 

 pea ballast, if ; sand, 2J. 



Reduction of Organic Matter (oxygen - consumed) : Burnt 

 ballast, 43*3 per cent. ; sand 46*6 ; pea ballast, 52*3 ; coke 

 breeze, 62*2. 



The report adds significantly, " The number of organisms in 

 the tank efiluent before filtration, and in the filtrates, was 

 found to vary very considerably, those in the filtrate being 

 generally present in larger numbers; but it soon became appa- 

 rent . . . that the presence of a large number of organisms 

 was evidence of the activity of the process of splitting up the 

 organic compounds in the sewage matters passing through the 

 filters. Here it is clear that the main purification was bacterial, 

 and only the beginning of a further resolving change to be 

 carried on in the river. It would undoubtedly have been an 

 advantage if the biological process so initiated could have been 

 allowed to develop a further stage in the filter, but the pre- 

 scribed object of the experiments was ' the attainment of the 

 highest rate of speed consistent with such purification as 

 would remove the obvious objectionable characters such as 

 odour, colour, and liability to putrefaction.' " 



At this time, the importance of the surface contact action as 



