PURIFICATION OF WATER FOR DRINKING PURPOSES 157 



unfiltered water was much greater in this second series 

 of experiments than in the first. The greater efficiency 

 manifested by this second series of filters is attributable 

 to the more thorough coating of the sand with a stratum 

 of slime. 



The careful and constant testing of these experi- 

 mental filters showed, what has been demonstrated by 

 one of us on the large scale in the case of the London 

 filter beds (see p. ISQ, New Eiver table), that the water 

 which first passes through the filter is but slightly 

 affected, only little reduction taking place in the number 

 of micro-organisms, and that it is only after the filter has 

 been at least one day in use that the full effect is obtained. 



These experiments also conclusively establish that the 

 rate of filtration is the factor of commanding import- 

 ance ; for by increasing the rate not only is the filtrate 

 deteriorated, but the filter becomes quickly clogged, 

 necessitating constant renewal, which, seeing that it does 

 not attain its normal efficiency at once, must necessarily 

 reduce the reliability placed in the purifying power of 

 sand-filtration. 



It will, however, be seen from the table (p. 156) that 

 the quick filter (a) had in eight days, when cleaning was 

 first necessary, passed much more water than the slow 

 filter in the course of the whole month, so that with an 

 insufficient area for filtration it is absolutely necessary 

 to employ high pressures, rapid filtration, and frequent 

 cleaning, leading of course to imperfect bacterial purifi- 

 cation. 



One of the most important results arrived at in these 

 experiments of Fraenkel and Piefke is the proof which 

 they furnish that the sand filters, even under the most 

 favourable conditions of working, do not form a com- 

 plete obstacle to the passage of micro-organisms. Thus, 

 even when the rate of filtration was reduced to a 



