270 



CHURNING AND WASHING BUTTER 



removing the bacteria. If some bacteria are removed with 

 the matter held in suspension, others are carried along from the 

 filter-bed. Owing to this, a new filter-bed must be kept in 

 operation a few days before the filtered water can be considered 

 pure and ready for use. The following table illustrates how the 

 germ-content of water is decreased as the process of filtration is 

 carried on during the first few days : 



It will be seen from the table that during the first three days 

 the filter-bed was in use the filtered water contained more 

 germs than the unfiltered. Good results were not obtained 

 until the seventh day. In order to be on the safe side it is 

 best to expose the filter-bed to continuous filtration for about 

 nine days before the water is used. 



The slimy coat referred to above is formed by certain germs. 

 These germs then constitute the real agent of filtration. In 

 order that these micro-organisms may do efficient work oxygen 

 is essential. Well-water of average purity contains enough 

 oxygen in solution without employing an intermittent process of 

 filtration, and consequently for creamery purposes the con- 

 tinuous method of filtration is to be recommended. 



Intermittent. The intermittent process of filtration is used 

 where comparatively impure water is being purified, such as in 

 purifying water for large cities. If the continuous process 

 of filtration were employed in such instances, the filtered water 

 would not be free from germs, due to the fact that impure river- 



