I 



BACTERIAL PURIFICATION 213 



The use of ordinary gas coke, in pieces about the size of walnuts, 



[seems to be attended with the following advantages, as compared with 



the use of smaller coke. The larger coke enables the bed to hold a 



larger volume of sewage. The beds now in use had an original 



capacity for sewage which was nearly equal to the volume of the 



[coke which they contained, in place of only 20 or 30 per cent, of that 



^volume, as is shown by beds containing smaller coke. The use of 



the larger coke also allows the bed to be more rapidly filled and 



jmptied, and to be more completely emptied and aerated." 



The increase of depth of the beds beyond 5 feet, as I had 

 redicted in my Cantor Lectures, has not been attended by 

 igher efficiency. The report states that " coke-beds similar 

 n character, but differing in depth, have been found to give 

 Tactically identical purifying effects . . . with a 4 - feet and 

 a 6-feet bed. A bed 13 feet in depth . . . has given a purifi- 

 cation approximately equal to that effected by the 4-foot bed." 

 The depth is always of great importance, both as to fall, volume, 

 and cost. In the intermittent system the bed is really used at 

 intervals as a storage tank, so that in this sense greater depth 

 means higher capacity. 



In the report of July 28, 1900 (p. 59), Dr. Houston says : 



" It must be admitted that the 13-feet coke-bed at Crossness yielded 

 very unsatisfactory results from the bacteriological point of view. 

 Thus, although the effluents usually contained fewer bacteria and 

 less of B. coli and spores of B. enteritidis than the crude sewage, the 

 reduction was not well marked, and, indeed, was immaterial from 

 the epidemiological point of view, considering the actual number still 

 remaining. For, as has been already pointed out, the effluents 

 usually contained more than 1,000,000 microbes, more than 100,000 

 B. coli, and at least 100, but less than 1,000 spores, of B. enteritidis 

 sporogenes per c.c." 



An important point is that the capacity of the 4-foot bed had 

 during ten months been reduced from 50 to 33 per cent, of the 

 whole volume of the bed, *' mainly due to fragments of straw 

 and chaff, apparently derived from horse-dung, and to woody 

 fibre, derived from the wear of the wood pavements. . . . The 

 original capacity is not restored in any degree by prolonged 

 aeration, which proves that the deposit on the coke surface was 

 not organic matter of animal origin ; but it has been found that 

 the vegetable tissue, which seems to be the main cause of the 

 difference in capacity, can be in great measure separated from 

 the raw sewage by a brief period of sedimentation before the 

 sewage is allowed to flow into the coke-bed."^ It should be 



^ Compare Waring's and Lowcock's experiments. 



