FILTRATION OF WATER 67 



is led by suitable channels to the surface of the filter-bed. Having 

 passed through the 3 or 4 feet of the bed, it is collected in a storage 

 reservoir and awaits distribution. Such being the principles of 

 construction, it wiU. be apparent that the action of the whole process 

 is both mechanical and chemical. Mechanically by subsidence, 

 much suspended matter is left behind in the reservoir. Again, 

 mechanically, much of that which remained suspended in the water 

 when it reached the filter-bed is waylaid in the substance of the 

 sand and gravel of the filter-bed. The next change is a chemical 

 one. Oxidation of the organic matter occurs to some extent as the 

 water passes through the sand. Until recently this chemical action 

 and the double mechanical action (sedimentation and straining) was 

 believed to be the complete process, and its efficiency was tested by 

 chemical oxidation 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 httle or no harm in drinking-water. The action 

 of a filter-bed may, therefore, be summarised as follows : — There is 

 (1) subsidence of the grosser particles of impurity in the settling 

 tank ; (2) mechanical obstruction to impurities 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 

 incomparably 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. Ktimmel found that when a filter had new 

 sand placed upon it the number of bacteria in the filtered water was 

 as follows : — p^j, ^ ^ 



Before cleaning .... 42 



One day after cleaning . . . 1880 



Two days after cleaning . . . 752 



Three days after cleaning . . 208 



Four days after cleaning . . 156 



Five days after cleaning . . 102 



Six days after cleaning . . - . 84 



