136 



Elements of Water Bacteriology. 



It must be remembered that in the Washington plant 

 filtration is supplemented by thorough sedimentation, 

 preliminary and subsequent. The entire credit for the 

 good effluent obtained is not therefore due to the filters. 

 At Lawrence it has been shown that removal of colon 

 bacilli in storage reservoirs and pipe systems may be 

 considerable. The figures obtained in 1900 at various 

 points in the distribution system may be cited as an 

 example. 



PERCENTAGE OF SAMPLES OF WATER CONTAINING 



B. COLI. 



Lawrence Experiment Station (Massachusetts State Board of Health, igoi.) 



Effluent 

 of Filter. 



Outlet of 

 Reservoir. 



Tap, 

 City Hall. 



Tap,Ezperi- 



ment 



Station. 



In I c.c. 

 In 100 c.c. 



18.14 

 38-12 



8-57 

 23-30 



4.07 

 IS -54 



1.87 

 IS -54 



In regard to the proportion of positive colon tests per- 

 missible ia a filter effluent, Clark and Gage (Clark and 

 Gage, 1900) reported some specially instructive obser- 

 vations made when certain of the underdrains of the 

 Lawrence filter were relaid in the autumn of 1898. In 

 doing this work the sand on some of the beds was seriously 

 disturbed; and ia December, after the work was com- 

 pleted, B. coli was found in i c.c. of the ffltered effluent 

 in 72 per cent of the samples examined. In January and 

 February the organisms were foimd in 54 per cent and 62 

 per cent of the samples, respectively, while in March the 



