WATER FILTRATION. LIII. 



vide better control over filtering operations. Its use will 

 be described later on. 



The filtering medium is clean river sand 2 feet inches 

 in depth, resting on 6 inches of fine gravel, under which 

 are two layers of loose bricks arranged to form subdrainage 

 ducts, the whole being contained in a water tight tank 7 

 feet in depth, the floor of which slopes to a central channel 

 leading to the effluent outlet pipe. 



Each bed has two water level gauges, side by side — one 

 is in communication with the water above the sand, and 

 the other with the water in the channel under the sand. 

 When a bed is working, the difference of level of the water 

 in the two gauges is the filtering head, and indicates the 

 condition of the sand. The least difference is seen when 

 the bed is clean, and the greatest after it has been in pro- 

 longed use and the interstices in the surface layer of sand 

 have been choked with deposits of vegetable matter and 

 fine clay. When this stage is reached the bed is put out 

 of action, dried and the surface pared off, after which it is 

 ready to start again. At each paring a portion of sand is 

 unavoidably removed and the thickness of the sand bed is 

 gradually diminished. When the reduction amounts to 9 

 inches, the bed is made up again to its original thickness 

 by the addition of new sand. 



When it is required to recharge a bed which has been 

 dried off, the main effluent valve at the clear water tank 

 is shut and the effluent outlet valve of the dry bed is opened. 

 This allows the filtered water from the active beds to flow 

 into the bottom of the dry bed, and gradually rise up to the 

 surface ; as soon as this stage is reached, the valve which 

 controls the supply of unfiltered or raw water is opened, and 

 the filling of the bed above the sand is completed from this 

 source ; the main effluent valve is then reopened and ad- 

 justed, and filtration is resumed. 



