284 PURIFICATION OF PUBLIC WATER SUPPLIES. 



the sand. Sometimes instead of broken stone, bricks 

 laid dry, witli open joints, are used, and sometimes drain 

 pipes of virtrified clay, laid also with open joints, are 

 used. The purpose of the stones, bricks or drain pipes 

 is to conduct the filtered water to the main drain usually 

 running lengthwise of the basin through its center. The 

 thickness of sand employed varies from two feet to four 

 feet. The area of filtering surface should be about one 

 acre for every 2,000,000 gallons filtered in 24 hours. 

 The filters recently built at Hamburg, Germany, were 

 designed for one acre for each 1,600,000 gallons in 24 

 hours. Other German filters have one acre for each 

 2,500,000 gallons in 24 hours, and a few have a much- 

 higher rate. The London filters, and that recently 

 built at Lawrence, Mass., have an area of one acre for 

 each 2,000,000 gallons in 24 hours. The impurities re- 

 moved from the water are collected on the surface of the 

 sand, and when this has accumulated to such an extent 

 as to prevent the passage of the desired quantity of 

 water, the water, which is usually four or five feec in 

 depth over the sand, must be drawn off and a thin layer 

 of the sand, about one-half inch, together with the ac- 

 cumulated silt, removed by means of laborers with 

 shovels, after which filtration proceeds as before. The 

 period of time elapsing between these cleanings varies 

 ordinarily with the condition of the water, from two 

 weeks to two months. An essential requirement of this 

 meihod of filtration is an ample area always in good, 

 order. 



At the present time the efficiency of this method of fil- 

 tration is judged, principally, from the results of 

 bacteriological examinations of the water before and. 

 after passing the filters. la European filtration systems 

 from 98 per cent, to 99f per cent, of the bacteria in the 

 applied water are removed by filtration. The new filter 

 bed at Lawrence, Mass., is said to remove something over 



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