1900.] oil Bacteria and Sewage. 321 



London Sewage. This bed lias been receiving screened and sedi- 

 mented sewage up to the present time, the process of sedimentation 

 having been assisted by the addition of a small proportion of 

 solutions of lime and of ferrous sulphate. Two years ago the bed 

 was deepened to about six feet. Its purifying action, as measured 

 by the amount of oxidisable matter present in the raw sewage and 

 in the clear effluent, amounts to 92 per cent, and if the purification 

 is calculated from the clear sewage and effluent, it amounts to 84 

 per cent. More recent experiments have proved that the treatment 

 of raw roughly-screened sewage in such coke-beds is satisfactory, 

 but that the capacity of the bed becomes continuously reduced by 



Fig. 9. — Proteus vulgaris, x 1000. 



the deposition upon the coke of mineral matter from road detritus, 

 of particles of straw, chaff and woody matter from the horse-traffic 

 and from the wood pavements. It was, therefore, evident that these 

 matters must be deposited by sedimentation before the sewage was 

 brought into the coke-beds. A comparatively rapid process of sedi- 

 mentation suffices to remove these matters, since even the cellulose 

 matters arrive in the sewage in a heavy and waterlogged condition. 



It was found advantageous to use coke in comparatively large 

 fragments, about the size of walnuts, since this facilitated the rapid 

 draining of the liquid from the coke, and at the same time increased 

 the sewage capacity of the bed and promoted its efficient aeration. 

 The depth of the beds has been augmented from 4 to 13 feet, and 

 the increase of depth seems to be attended with increase of efficiency. 



