Water Supply of Belfast 65 



described, and that it is not overworked. By passing too much 

 water through a filter the dirt may be driven down deep into 

 the sand, and if this is continued long enough, the whole sand- 

 layer may have to be taken out and rewashed. With proper 

 working, however, experience has shown that all the visible 

 dirt is caught in the upper half-inch of sand. The lower part 

 of the sand-layer and the gravel and stones remain clean for 

 years. In fact, I have known filters made over thirty years ago 

 where the lower layers have never been disturbed. 



Another question, however, arises at this stage, and it is this, 

 " Why employ at great expense a depth of four to six feet of 

 material, if the work is all done by the upper half- inch of sand ? " 

 This is a natural question, but it is one that has never been 

 satisfactorily answered. Many attempts have been made to 

 filter with a thin body of material, but they have been failures. 

 With the thin layers the water is not rendered so clear, and 

 even when this has been done, the resultant water when 

 analysed does not compare favourably with water filtered 

 through the deeper stratum. What, then, is the nature of the 

 eifect produced by the lower layers of material upon the water 

 passing down ? Observation by means of the ammonia process 

 shows a greater reduction of the organic matter in the water 

 than if a thin layer be employed, and yet there is no visible 

 deposit on the lower sand and stones. Some authorities attri- 

 bute this beneficial result to oxidation, but the answer to this 

 appears to me to be that the air in the interstices of the sand 

 and gravel has all been driven out by the water passing down. 

 Again, the good work done by the lower strata of the filtering 

 medium has been explained by other authorities in the follow- 

 ing ingenious way : — In the water passed on to the filter there 

 is a certain amount of free oxygen. This is not immediately 

 operative in attacking the organic matter in the water ; but the 

 presence of and contact with a third body — viz., the particles of 

 sand or gravel — immediately renders operative the oxygen in 

 the water, which then attacks the organic matter. This is 

 known in chemical science as the catalytic process, and the 



