82 Bacteria in Relation to Country Life 



only means for the bacterial purification of the Potomac 

 water used in the city, showed a bacterial efficiency of 

 70 to 90 per cent. The city of Baltimore still depends 

 upon the settling basins and the prolonged storage of 

 the drinking-water for the elimination of the typhoid 

 germs from its water. 



Notwithstanding the marked decrease of the bacteria 

 in stored water, it is generally conceded that settling 

 basins alone should not be depended upon for the pro- 

 tection of the community against typhoid fever. A 

 small proportion of the disease germs might survive, 

 and, probably, do survive, the sedimentation process. 

 It has become necessary, therefore, to supplement the 

 latter with more efficient means for the elimination of 

 typhoid and other intestinal germs. As a result of this 

 conviction, many modern water-purification plants 

 include in their equipment both settling reservoirs and 

 filter beds, and, likewise, additional reservoirs for the 

 storing of filtered water. 



Sand- filtration. — After remaining in the settling reser- 

 voirs for a greater or shorter length of time, the water 

 is passed through the filters in order to destroy most of 

 the remaining organic matter, including most of the 

 sewage bacteria. It should not be supposed, however, 

 that the action of the filters is purely mechanical. .On 

 the contrary, they owe their efficiency to the bacteria 

 that establish themselves in the upper layers of the bed 

 and act there as very vigorous scavengers. For this 

 reason, the sand filters do not attain their highest effi- 

 ciency until some days after they are put in operation. 

 It is found then that the grains of sand which compose 



