METHODS OF PURIFYING WASH-WATER 267 



almost entirely free from germs, and in some cases entirely 

 sterile. It used to be thought by older German investigators 

 that these different filtering substances had almost miraculous 

 power of removing organisms from water. 



The factors which are to be considered in successful filtration 

 are: 



(i) Storage capacity for unfiltered water. 



(2) Construction of filter-beds. 



(3) Rate of filtration. 



(4) Renewal of filter-beds. 



(i) Concerning the storage capacity, nearly all creameries 

 have storage-tanks overhead in the creamery; so far as that is 

 concerned, however, filtration can be successfully carried on 

 continuously as well as intermittently. 



(a) The construction of the filter-bed used in the experi- 

 ment carried on at the Iowa Experiment Station, Ames, Iowa, 

 is as shown in Fig. loi. The approximate proportionate depth 

 of each layer in the bed is as follows, beginning at the bottom: 



Two inches small flint stones; 22 inches fine sand; 12 inches 

 fine coke; 9 inches charcoal; 2 inches fine stone or coarse gravel. 

 The layer of fine sand should not be less than 15 inches. It has 

 been asserted that a few pieces of old iron mixed in the filter-bed 

 are beneficial. Alum, lime, and copperas have been recom- 

 mended for clarifying and deodorizing very impure water. As 

 these substances are soluble they should not be used in filter- 

 beds, which are intended for the filtration of water for potable 

 purposes. The filtering-can was made from 22 galvanized iron. 

 The height of can is 48 inches; diameter, 18 inches. The bottom 

 of the can is slanting towards the faucet, or opening. Thus 

 no water is permitted to stand on the bottom and afford oppor- 

 tunities for germs to accumulate. On the inside are three plates. 

 One lies horizontally, near the bottom, and upon it the filtering- 

 material rests. Another hes on the top of the fine sand. Both 

 of these plates were perforated with many small holes. Near the 

 top is placed a concave plate with a hole near the center. This 

 plate directs all the water to the center of the filter-bed, and thus 

 the water gets the full benefit of the filtering process. The 



