WATER FILTRATION. LV. 



show that a very great improvement in the water is effected 

 by the filters. The storage water, which before filtration 

 is unsuitable for domestic purposes, is after aeration and 

 filtration converted into a water of satisfactory purity. 

 The beneficial effect of filtration is shown by the following 

 results of analysis of storage water : — 



Parts per million— 

 Before filtration. After filtration 



Oxygen absorbed in 4 hours ... 2*10 ... '92 



„ „ 15 minutes ... 1*05 ... *46 



Combined ammonia ... ... '54 ... *14 



Free ammonia *09 ... *00 



It is only under exceptional circumstances that storage 

 water is alone supplied to the filters. Usually equal volumes 

 of storage and river water are taken. Better results are 

 obtained in this way, as the following analyses of mixed 

 water will show : — 



Befoi ' 



Oxygen absorbed in 4 hours 



„ ,, 15 minutes ... 

 Combined ammonia 

 Free ammonia 



The average results throughout the year are as follows 

 for filtered water : — 



Oxygen absorbed in 4 hours '64 



,, ,, 15 minutes ... ... *31 



Combined ammonia '10 



Free ammonia ... '00 



The rate of filtration is not allowed to exceed 4| inches 

 per hour; this corresponds to the passage of a volume of 

 21*05 gallons per superficial yard of surface per hour. 



In the practical working of the filter beds they are as 

 far as circumstances will admit, supplied with equal volumes 

 of storage and river water mixed together in the* supply 

 pipe to the aerating fountain. This ensures a more uniform 

 quality of water and mitigates the severity of the change 

 which would otherwise occur in passing direct from river 

 water to storage or vice versa. 



Parts 



filtratic 



per million — 



m. After filtration 



1*00 



... -47 



•50 



... *24 



*20 



... -07 



*01 



... -oo 



