574 A MANUAL OF BACTERIOLOGY 



(2) Thickness of fine sand in the filter-beds. Efficient 

 sand nitration removes quite 99 per cent, of the organisms 

 originally present. The fine sand only has to be taken 

 into account in estimating the removal of organisms and 

 efficiency of a filter bacteriologically. It probably should 

 form a layer not less than 3 ft. to 3 ft. 6 in. in thickness. 

 Moreover, a filter-bed is not efficient at first, but becomes 

 so when the surface film forms, composed of sedimented 

 particulate matter, and of a zoogloeal mass of bacteria and 

 algae. 



(3) The rate of filtration. The removal of organisms is 

 less perfect when the rate of filtration is increased ; this 

 should not exceed about 1*5 gallons per square foot per 

 hour. 



(4) The renewal of the filter-beds. New, or recently 

 cleaned, filter-beds allow a greater number of organisms 

 to pass through. The beds must be cleaned from time 

 to time by raking up and clearing away the surface layer 

 of sand, for as time goes on the rate of filtration becomes 

 slower and slower, though the bacterial efficiency of the 

 filter-beds does not appear to be reduced by prolonged 

 use. The normal bacterial efficiency seems to be 

 rapidly regained after cleaning within two or three 

 days. 



Besides storage and filtration, sedimentation in the 

 presence of fine particles, either naturally present or 

 artificially added, may also effect a marked removal of 

 micro-organisms from water. Thus, by the addition of 

 alum, an old method of clarifying turbid water, a large 

 number of the organisms present are carried down in the 

 precipitate. 



The Clark process of softening water may also reduce 

 the number of organisms present, but is very uncertain 

 (Moor and Hewlett). By the Porter-Clark rapid process, 

 however, in which the precipitate of calcium carbonate is 



