WATER FILTRATION. XXXVII. 



the filtration works through a 66 inch steel main, discharg- 

 ing into a concrete standpipe, 10 feet in diameter, located 

 at the north, end of the coagulating basin. In this stand 

 pipe the water is treated with a solution of coagulant and 

 after tliorough mixing by the natural agitation in the stand 

 pipe, it passes from the bottom of same into the coagulating 

 basin, and thence to the south end, where near the surface, 

 it is collected in a perforated pipe through which it passes 

 to the filters. After passing through the filters the water 

 enters the clear-water basin, from which it flows through 

 a 66 inch steel suction main, encased in concrete, leading 

 to the main pumps." 



Washing of filters. — ■" Wash water is applied to the filters 

 by centrifugal pumps, having a capacity of 3,000 gallons 

 per minute. When the wash water is applied to the filters, 

 it enters the filtered water outlet pipe, passes through the 

 the manifold pipe of the strainer system, and thence through 

 the strainers themselves into and through the sand layer. 

 The dirty wash water is removed in part through lateral 

 gutters about 60 inches in cross-section, placed along the 

 sides of each filter tank. The dirty wash water leaves the 

 filter through the inlet pipe, to which there is a branch 

 leading to the sewer. The wash water is applied with a 

 vertical velocity of about 1 foot per minute, equal to about 

 1\ gallons per square foot per minute. 



" Compressed air under a low pressure is used in agitat- 

 ing the sand layer and facilitating the removal of the 

 accumulated materials during the washing, in place of 

 mechanical stirring devices with rake arms. Air is sup- 

 plied by two No. 3 Root's rotary blowers, driven direct by 

 electric motors. Each has a guaranteed capacity of 1,500 

 cubic feet of free air per minute under pressure of 5 lbs. 

 The ordinary working rate is 1,000 cubic feet per minute 

 under a pressure of 3 lbs." 



