J& BACTEI^IA 



and the double mechanical action were believed to be the com- 

 plete process, and its efficiency was tested by chemical oxida- 

 tion and alteration, and absence of the suspended matter. 



Now, however, it is recognised that the second portion 

 of the chemical action is vastly the more important, indeed, 

 the only vital, part of the process. This is the chemical 

 effect of the layer of scum and mud on the surface of the 

 sand at the top of the filter-bed. The mechanical part of 

 this layer is, of course, the holding back of the particulate 

 matter which has not subsided in the reservoir; the vital 

 action consists in what is termed nitrification of unoxidised 

 substance, which is accomplished in this layer of organic 

 matter. We shall deal at some length with the principles 

 of nitrification when we come to speak of soil. But we may 

 say here that by nitrification is understood a process of 

 oxidation of elementary compounds of nitrogen, by which 

 these latter are built up into stable bodies which can do 

 little harm in drinking water. From what has been said it 

 will be seen that the action of a filter-bed is of a complicated 

 nature. There is (i) subsidence of the grosser particles of 

 impurity in the water; (2) mechanical obstruction to impuri- 

 ties in the interstices of the scum, sand, and gravel in the 

 filter; (3) oxidation of organic matter by the oxygen held in 

 the pores of the sand and gravel; (4) nitrification in the 

 vital scum layer, which is accomplished by micro-organisms 

 themselves. This latter is now considered to be incompar- 

 ably the most important part of the filter. That being so, 

 its removal, except when absolutely necessary, is to be 

 avoided as detrimental to the efficiency of the filter. New 

 filters have obviously but little of this action. Hence it is 

 wise to allow a new filter-bed to act for a short period (say 

 twenty-four to forty-eight hours) before the filtered water is 

 used for domestic purposes, in order to allow the organic 

 layer to be formed. This must also be borne in mind after 

 a filter-bed has been cleaned. 



