14 VETERINARY HYGIENE 



the winter. The tanks or troughs should be of iron or glazed 

 stoneware. Wood troughs should not be used as they become foul. 



FILTRATION OF WATER. 



While the mere storage of water in reservoirs and service tanks 

 considerably improves its quality and appearance, it is not in itself 

 a sufficient safeguard when the water is to be used for domestic 

 purposes. The completion of the purifying process is done by 

 filtration, and in addition it is sometimes sterilised. 



All surface waters, those collected from a watershed and im- 

 pounded, all river waters and sub-soil waters must be considered 

 as exposed to the risk of contamination, and therefore requiring 

 filtration. 



Filtration may be natural, as when the water finds its way 

 to the deeper strata through porous soil. 



During the percolation the organic matter and bacteria are 

 arrested and removed, and, if the percolation has lasted a sufficient 

 time and the water gone through a sufficient depth of soil of suitable 

 character free from any possible contamination, it becomes purified. 

 But natural filtration is not always to be depended upon, and some 

 spring waters that theoretically should be pure are not uncommonly 

 found to be polluted. Foul water has been known to travel long 

 distances underground along the top of an impervious ledge to 

 find an outlet in a previously pure supply. 



Artificial Filtration as at present practised is of two kinds, 

 Open Sand Filtration and Mechanical Filtration. 



Sand Filtration. Sand filtration may be described as a copy 

 of natural filtration. While there is considerable variation in the 

 details of filter bed construction, in principle they are the same. 

 The bed itself is made of cement concrete throughout, and is pro- 

 vided with an inlet and a run-off. At the bottom of the bed is 

 placed a layer of bricks between which run the outlet channels ; on 

 the bricks is laid a layer of broken stones or large gravel of about 

 1 inch mesh to a depth of 6 or 7 inches ; above this a layer of finer 

 gravel, usually of -| inch mesh, to a depth of 4 or 5 inches; then 

 comes a layer of coarse sand 2 feet 6 inches deep, and on the top 

 of this is a layer of finer sand 8 or 9 inches deep. Such is the con- 

 struction of a filter bed when newly made. As it is, however, 

 it is not an efficient filter, as the sand in the clean state will let 

 organic matter and bacteria pass through. As soon as the filter bed 

 is completed water is let in gently, so as not to disturb the sand, 

 until it is 4 or 5 feet deep. Filtered water is let in from below, 

 and unfiltered from above. 



