Polluted Water Can Kill You 



229 



of debris becomes thicker, filtration slows down to the point where 

 the filter must be cleaned. A slow sand filter can be expected to 

 treat between three and six million gallons of water per acre of 

 filter surface per day. The combination of mechanical trapping 

 of the organisms, electrical charges, and the natural eating habits 

 of protozoa and other organisms accounts for the removal of up to 

 98^ of the bacteria in the water. All pathogens are generally re- 

 moved or die during this operation. 



RAPID SAND FILTER 

 Until the colloidal layer becomes well established on a slow 

 sand filter, the operation is not too efficient. And when the filter 





Raw water inlef 

 \ 



Filtered 



outlet 



Smgie 

 muifiport va've 



Bockwrssr- ^./os"c 

 Rinse waste Istie 



Inlet baffle 



Fine sand 



Coarse sand 



Graded grave! 



■Deflector pjate 

 Adfustable jack legs 



Permutif Vertical Pressure Filter 



(Mulfiporr Volve Conrro?) 



Tvoe T 



Fig. 37. A vertical pressure filter. Water is forced under pressure 

 through successive layers of fine sand, coarse sand, and graded gravel. 

 (Courtesy of the Permutit Company, New York, New York.) 



