406 APPLIED BACTERIOLOGY 



the number of organisms became after a short time much 

 larger in the filtrate than in the unfiltered water. Filters 

 once polluted with the cholera or typhoid bacillus were also 

 found to convey the bacillus to sterile water passed through 

 them at considerable periods — up to six weeks or more — 

 after pollution. This fact has been responsible for several 

 epidemics, such as that of Lucknow in 1894, in which, out 

 of 646 officers and men in the East Lancashire Regiment, 

 143 were attacked by cholera, and 92 died. This epidemic 

 was conclusively traced to the infection of the barrack-room 

 filter by the cholera microbe. 



Sand-filtration. — The working of sand-filters on a large 

 scale depends on the facts described above. A sand filter- 

 bed consists of a layer of sand from 2 to 4 feet deep, 

 supported on gravel. The fineness of the grains of sand, 

 the depth of the filter, and the rate of filtration, all aflfect 

 the working of the filter in the removal of organisms. The 

 coefficients given for safe working are filtration through a 

 sand-layer not less than 30 centimetres thick, at a rate not 

 exceeding 100 millimetres per hour, and giving a filtrate 

 containing not more than 100 bacteria per c.c. These 

 coefficients, however, take no account of the class of sand 

 used or character of water filtered, and they are no longer 

 regarded as trustworthy. When a filter-bed is freshly 

 constructed, organisms are washed through it with great 

 rapidity, but after a certain quantity of water has passed 

 through, or the water has been allowed to stand upon it for 

 a certain time, a slimy coating of detritus and bacteria is 

 formed on the surface. If water is slowly passed through 

 the filter when this coating has been formed to a sufficient 

 extent, which will occur after a period varying mainly with 

 the composition of the water, the majority of the bacteria 

 will be retained by this surface, either by sticking to it or 

 by being strained off. The increasing thickness of this 



