METHODS OF PURIFYING WASH-WATER 209 



fine-sand layer. In a general way, the slower the rate of fil- 

 tration is, the more thorough it is; and, vice versa, the more rapid 

 the rate of filtration, the more incomplete is the removal of 

 the bacteria. If the filter-bed is constructed as described 

 above, the rate of filtration will be about 18 gallons per hour, 

 and about 96 per cent of all the germs present will be removed, 

 together with the impurities present in suspension. 



(4) The filter used at the Iowa Experiment Station was in 

 constant use for about three months, without having been 

 changed. At the end of this time it did as efficient work as 

 at any previous time. The length of time a filter-bed can be 

 used without being changed depends upon the purity of the 

 water to be filtered, and also upon which kind of filtration is 

 used, the continuous or the intermittent. The more impure 

 the water which has to be filtered, the oftener the filter-bed 

 should be changed. Whenever the rate of filtration is decreased 

 to such an extent as to make the process impracticable, the 

 filter-bed should be taken out and cleaned. If the water to be 

 filtered is of average purity, a change of the filtering-material 

 once every four months is ordinarily sufficient, no matter whether 

 continuous or intermittent filtration is used. A filter-bed may 

 do efficient work even a longer time than this. The same filter- 

 ing-material can be used again providing it is thoroughly washed 

 previous to replacing it in the filtering-can. 



Kinds of Filtration. The two kinds of filtration in use are 

 (i) Continuous, and (2) Intermittent. 



By the continuous method of filtration the inflow of water 

 into the can is constant during night and day. The stream of 

 water admitted into the filter-can is sufficient to cause the 

 surface of the filter-bed to be covered with water all the time. 

 This method excludes all oxygen from the filter-bed, except 

 that which is in solution in the water. 



During the process of filtration a slimy coat is deposited on 

 the fine sand. This seems to be the real agent absolutely 

 necessary in order to eliminate bacteria by a process of filtration. 

 A filter-bed without this slimy deposit on it simply takes out the 

 coarse organic and inorganic matter held in suspension, without 



