Quantitative Bacteriological Examination. 53 



Washington, for example, prolonged sedimentation gener- 

 ally reduces the bacterial numbers to less than a thou- 

 sand, and it is almost impossible to secure a 99 per cent 

 removal. The actual numbers of bacteria in the effluent 

 are, however, much lower than at Lawrence. The 

 monthly average results obtained for a year at these two 

 plants are tabulated on page 54. 



Mechanical filtration gives similar results. Fuller at 

 Cincinnati (Fuller, 1899) records 27,200 organisms per c.c. 

 in the w;ater of the Ohio River between September 21, 

 1898, and January 25, 1899, while the average content of 

 the effluent from the Jewell filter was 400. Data with 

 regard to the operation of mechanical filters are now 

 abundant, since all over the world the operation of these 

 plants is controlled by bacteriological methods. Recently 

 Johnson (1907) has reported some interesting sesults from 

 the far East. At Osaka, Japan, an average of 200 bac- 

 teria per c.c. in the raw water of the Yodo River was 

 reduced, in 1905, to an average of 25 by slow sand 

 filters; at Bethmangala, India, in 1906, mechanical filters 

 treated the water of the Palar River, containing 4350 bac- 

 teria per c.c, and yielded aln effluent with only 13 per c.c. 

 (Johnson, 1907). 



The average monthly results obtained with the new 

 mechanical filter plant at Harrisburg, Pa., are included in 

 the table on page 54 for comparison with the figures 

 recorded at Washington and Lawrence; and these may be 

 taken as typical since the Harrisburg plant is the latest 



