144 APPLIED BACTERIOLOGY FOR NURSES 



When, in 1892, the river water became polluted with the 

 discharges of a cholera patient, Altona, which used filtered 

 water, had but a few cases, most of which could be traced 

 to Hamburg; while the latter city, whose sand-filters were 

 not yet completed, paid a toll of 8000 lives. Bacterial 

 examination of the water at Albany, N. Y., has shown 

 that the sand-filters in use there remove from 98 to 99 

 per cent, of all the bacteria in the water. In Albany, 

 as in other cities, the introduction of filter plants has 

 enormously reduced the number of deaths from typhoid 

 fever. 



Filtration of Water Supplies. Briefly, these filters are 

 huge reservoirs which hold a layer of coarse, broken stone, 

 upon this a layer of smaller stone and gravel, then a layer 

 of coarse sand, and at the top one of fine sand. As the 

 dirty water percolates through these different layers it 

 gradually deposits its gross impurities at the top, and coats 

 the individual sand grains with a slimy covering. This 

 "dirt cover" forms the really efficient filter, but it finally 

 becomes too tenacious to allow any water to pass through, 

 and must be scraped off about once a month, hence it is 

 necessary always to have at least two filtering beds. 

 The water must not flow through the filters faster than 

 4 to 4J inches per hour, as otherwise infectious material 

 may be carried through. Waters which are very dirty 

 require sedimentation to rid them of the grossest im- 

 purities before filtering. When, at the same time, the 

 color is muddy, it is often advisable to add a chemical, 

 usually alum, which acts much as egg-white in clearing 

 fluids, and then to remove the jelly-like aluminum hydrate 

 plus impurities through a relatively thin sand-filter. 



Domestic Filters. Household filtration ought to be only 



