MODERN METHODS OF WATER PURIFICATION. 335 



also give entrance upwards into the layers of the filters to the 

 wash water, and in some installations, e.g.. Little Falls, they 

 liave a third function : they admit the air under pressure from 

 the blowers, which in these modern plants has displaced the 

 mechanical rotating rakes as sand agitators, which used to be 

 the sand-stirring mechanism. In another installation the air 

 for sand agitation had a separate pipe system. 



Thus the filter from below upwards has the following layers : 

 On the floor of the filter are the pipes, screwed into the pipes 

 are the little inverted cones of brass perforated at top and 

 sides, above them the broken quartz or gravel for some inches 

 • — from 6 to lo inches, above the quartz or gravel layer, which 

 varies in size from i inch pieces below to i-i6th inch pieces 

 above, lies the sand of the filters. This is carefully screened 

 quartz sand, and a great deal of attention is paid to the sand, 

 and trouble and expense incurred to get suitable sand, both 

 as regards character of sand, cleanness, and freedom from 

 soft particles and soluble grains, and the size of the grains. 

 In the Columbus works it was specified to have a uniformity 

 ■co-efiicient of i'5, and an effective size between o'36 and 

 o"44 millimetre, and not more than i per cent, of it may be 

 smaller than o'25 m.m. The depth of the sand layers varies in 

 different places, but 2 feet 6 inches is the usual. At the Pitts- 

 burg experimental filters the Jewell had ^/ inches of sand, the 

 Warren had 27 inches. The filtered water which has passed 

 the filter now reaches the effluent pipe. 



To regulate the discharge from the filters, which work under 

 a free head of about 10 feet in the particular installation I am 

 referring to, each filter is provided with an automatic con- 

 troller, called the Weston Controller. The standard form of 

 this controller consists essentially of a nicely-constructed cham- 

 ber, in which there is a moving orifice supported by a float, 

 wdiich is also connected to a set of butterfly inlet valves worked 

 by the float. The action of these controllers is to maintain a 

 uniform rate of discharge of water independent of the " head " 

 of water on the outlet pipe. Accordingly the filters are always 

 worked during filtration with the valve on the outlet pipe wide 

 open, and the flow is controlled automatically by this device. 

 The orifice discs are various in size to provide different yields, 

 e.g., at Little Falls, 1,000,000, 1.250,000 or 1,500,000 gallons 

 per filter per 24 hours, and exceptionally to give only a daily 

 yield of 650.000 gallons for each 24 ft. by 15 ft. filter. If too 

 much water tends to pass the automatic controller lessens it; 

 if too little it increases it. 



The control of the working of these filters was very elegantly 

 arranged. On the working floor of the filter-houses in front 

 of each filter is a little marble-topped table with a small plated 

 lever that looks almost ornamental, which works the filter 

 valves. Hydraulic and electric power are used for starting and 

 stopping. There were no 30 turns of a rusty spindle by Kafir 

 power there, but touch a button, push a handle and the work is 

 ■done. Taps and tubes for examining the different effluents 

 are also there. On each table is a " loss of head gauge," the 

 dial worked by a float in the water above the sand layer, and a 



