QUANTITATIVE BACTERIOLOGICAL ANALYSIS. 37 



again filled with water. The best plan is to fill the bed from 

 below with filtered water to the top of the sand and then run on 

 raw unfiltered water, which is allowed to stand for ten to twelve 

 hours before filtration is started. Kiimmel found that when a 

 filter had new sand placed on it the numbers of bacteria in the 

 effluent were as follows : 



Before cleaning . . . > . 42 bacteria per c.c. 



One day after cleaning . . . . 1,880 



Two days after cleaning .... 752 ., 



Three days after cleaning .... 208 ., 



Four days after cleaning . . . .156 ., 



Five days after cleaning .... 102 ,, 



Six days after cleaning . . . . 84 



He therefore insists on wasting the water for several days after 

 filling. The Massachusetts Board, however, considers that it is 

 best to have the fine sand in a filter of such a depth that it can 

 be cleaned and used for a year without fresh sand being put on 

 it, otherwise there is a tendency for clogging to occur at the 

 junction of the new and old sand. The Board found in their 

 experiments that it was not necessary to waste the water, pro- 

 vided there was no disturbance in the main body of the filter. 

 Piefke, however, considers that it is best to waste the water for 

 twenty-four hours after scraping. 



As regards the influence of the depth of the filtering material 

 upon bacterial efficiency, it has been found that filters contain- 

 ing two feet, and even one foot, of sand yield satisfactory results 

 so long as there are no disturbing influences. But if these 

 occur, their effects on the efficiency of the filtration are more 

 marked arid of longer duration in shallow than in deep filters. 

 The depth of fine sand in a filter should never be reduced below 

 twelve inches. The size of the sand grains has a powerful 

 influence on bacterial efficiency ; this is shown by differences in 

 the length of time required by new filters to yield effluents of 

 normal bacterial contents. As the size of the sand grains 

 increases the effect of deep scraping and unequal rates of filtra- 

 tion become more marked even in filters which have been some 

 years in operation. Good results may be obtained with grains 

 ranging from 0'14 to 0'38 mm. ; but the size must be uniform 



