82 



WATER 



attached to faucets, because of the large volume of water 

 that passes through them daily. 



85. Filtering City Water. The best natural filter is 

 clean sand. It is loose, and contains much air; hence 

 oxidizing bacteria can penetrate far into it. If the sand 

 is not kept soaked too long at a time, impure water that 



passes through it will be 

 made fit for drinking. 

 A soil containing much 

 clay does not make a 

 good filter, because clay 

 is too compact. 



FIG. 70. 

 Filter Beds of a City System; Evanston, 111. 



In filtering water for cities 

 (Fig. 70) men run the water 

 of rivers or lakes through 

 beds of sand. After soaking 

 through the sand the water 

 enters reservoirs, from which 

 it is distributed through the 

 water " mains " of the city. 

 But the large filters, like the 

 small ones, need to be emptied 



often, and allowed to lie idle, so that the sand may be purified by 



the direct action of sunlight and air. 



Coagulation Filters. In some cases a coagulating, or clotting, 



substance, such as alum, is added to the water before it enters the sand 

 filters. The alum causes the clay particles, which of themselves settle 

 only very slowly, to flock together and to come down rapidly, carrying 

 decaying matter and germs with them. Coke dust is often mixed 

 with the sand to improve the quality of the filter. 



