QUANTITATIVE BACTERIOLOGICAL ANALYSIS 57 



Bacteria in Filtered Waters. The process of slow 

 sand filtration for the purification of unprotected 

 surface-water is essentially similar to the action which 

 takes place in nature when rain soaks through the 

 ground to appear in wells and springs; and it is in the 

 examination of the effluent from such municipal plants 

 that the quantitative bacteriological analysis finds, 

 perhaps, its most important application. The chemical 

 changes which occur in the passage of water through 

 sand at a rate of 1,000,000 or 2,000,000 gallons per acre 

 per day are so slight as to be negligible. The bacteria 

 present should, however, suffer a reduction of 98 or 

 99 per cent, and their numbers furnish the best standard 

 for measuring the efficiency of such filtration plants. 

 At Lawrence, in 1905, Clark found an average of 12,700 

 bacteria per c.c. in the raw water of the Merrimac 

 River, while the number present in the filtered water 

 was only 70 (Massachusetts State Board of Health, 

 1906). Where the number of bacteria in the applied 

 water is smaller it is difficult to obtain so high a per- 

 centage efficiency. At Washington, for example, pro- 

 longed sedimentation generally reduces the bacterial 

 numbers to less than a thousand and it is almost impos- 

 sible 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 58. 



Mechanical filtration gives similar results. Fuller 

 at Cincinnati (Fuller, 1899) records 27,200 organisms 

 per c.c. in the water of the Ohio River between 



