TO COUNT BACTERIA IN WATER 281 



demic of typhoid, the number of bacteria is of great value in 

 locating the place of infection. 



The efficiency of niters in large municipal water supplies is 

 known only by the bacterial content of the effluent. In good 

 sand and mechanical (alum) niters, the reduction in the number 

 of bacteria is often over 95 percent (sometimes 99 percent). 

 Plate cultures should be made daily from every filter in order to 

 determine how each filter is performing. Sand filters should not 

 filter more than 1,000,000 gallons per acre a day. They should be 

 at least i metre thick; the upper Y^ inch of the sand performs over 

 90 percent of the filtration, due to a certain zooglea, or growth 

 of bacteria. Cracks, or imperfections in the filter beds are 

 quickly detected by the rapid increase of the number of the bac- 

 teria in the effluent. It is supposed that not only are bacteria 

 filtered by the sand but that destructive changes occur in the 

 filter which greatly diminish the number of bacteria. A filter 

 must be used for a few days before it becomes efficient or "ripe. " 

 After a time it becomes inefficient and it must then be scraped, 

 finally the sand must be removed and washed. 



A sand filter is a highly efficient means of water purification. 

 It often converts a foul dirty water into a bright, clean, whole- 

 some water of low bacterial content. 



Mechanical filters depend for their efficiency upon the addition 

 of aluminum sulphate to the water. This is decomposed by the 

 carbonates and aluminum hydroxide is produced, which is a 

 white jelly-like flocculent precipitate, which mechanically en- 

 tangles bacteria and removes them from the water. Mechanical 

 filters, as a rule, are highly efficient. Domestic filters, even the 

 Pasteur, are often unreliable. 



In time of epidemics of cholera and typhoid even filtered water 

 should be boiled before use, as it was found by experiments in the 

 Medico-Chirurgical Laboratories that typhoid bacilli live longer 

 in filtered water than in bouillon; they may even live three months. 



