64 The Filtration Works for the Improvement of the 



spreading trough on the top of the sand, by means of which it 

 is delivered gently, and disturbance of the sand prevented. 

 There are pipes laid in the concrete slope from the bottom 

 drains to allow the upward passage of the air as the water 

 gradually works down from the top of the sand. Provision is 

 made at the outlet wells, by means of a partition of stop planks, 

 for regulating the quantity of water to be passed through the 

 filter, or in other words keeping the proper head of water 

 according to the condition of the filter. The upper edge of the 

 partition is formed with a notch or check and a scale attached, 

 so that the foreman can tell at a glance the rate per day at which 

 the filter is working, and regulate the valve accordingly. 



As will be recognised by those present, great care is necessary 

 in washing all the coarse materials and sand put into the filters. 

 This washing is done in tanks having perforated bottoms, and 

 a supply of water laid on under pressure. Each tank holds 

 about three tons of sand, and when the latter is laid in, the 

 six-inch valve is opened. The water rushes up through the 

 perforated bottom, and flows away at one end, which is made a 

 little lower than the top of the tank. A few minutes of vigorous 

 stirring up with shovels soon frees the sand from all earthy or 

 vegetable matter, and it is then wheeled away to the filters. A 

 permanent washing tank of the same kind is placed in the 

 centre of each filter, and it is in these that the periodic sand- 

 washing will be carried on from time to time. After being in 

 work for a period varying from three to six weeks, a sand filter 

 requires to be redressed. This operation consists in turning off 

 the water, and then scraping the upper half-inch of sand in 

 which the sediment from the water is generally found collected. 

 The dirty sand is then passed through the washing tank, and 

 afterwards respread on the top. With this the operation of 

 cleansing or redressing is complete, and the filter is again ready 

 for use. 



But some one may, perhaps, now ask, does not a sand filter 

 become foul all through its layers, after long-continued use ? 

 I answer no, provided the filter is regularly cleansed as I have 



