68 BACTERIA IN WATER 



Hence it is necessary to allow a new filter-bed to act for a short 

 period (say four days) before the filtered water is used for domestic 

 purposes, in order to allow a fresh film, the organic layer, to be formed. 

 This must also be borne in mind after a filter-bed has been cleaned.* 



To maintain this nitrifying action of a filter in efficiency, Koch 

 suggested, in the second place, that the rate of filtration must not 

 exceed four inches per hour. At the Altona water-works this rate of 

 filtration was maintained, and the number of organisms always 

 remained below 100 per c.c, which, as we have seen, is the standard. 

 Thirdly, it is important that periodic hacteriological examination^ 

 should he made. Koch's emphasis upon this point is well known, and 

 the cumulative experience of bacteriologists only further supports such 

 a course being taken. Clark and Gage of the Lawrence Experimental 

 Station, claim that the test for the presence of £. coli is a more delicate 

 indication of filter efficiency when filtering polluted water than tests for 

 the total number of bacteria present. Fourthly, Koch maintaine'd that 

 the thickness of the sand of the filter-bed should never be less than one 

 foot. Fifthly, if it be true that efficient sand filtration is a safeguard 

 against putrefactive and disease-producing germs, then there can be 

 but one criterion of efficiency, viz., their absence in the filtered water, 

 which can only be ascertained by regular examination. But it is 

 not alone for pathogenic germs that filtration is proposed. Hence 

 Koch laid down that filtered water containing more than 100 micro- 

 organisms of any kind per c.c. is below the standard of purity, and 

 should not, if possible, be distributed for drinking purposes. In this 

 country chemical analysis, with a more or less cursory microscopic 

 examination, has been almost invariably accepted as reliable indication 

 of the condition of the water. But such an examination is not really 

 any more a fair test of the working of the filter than it is of the 

 actual condition of the water. It is true, the quantity of organic 

 matter can be estimated and the condition in which it exists in 

 combination obtained; but it cannot tell us what a bacteriological 

 examination can tell us, viz., the quantity and quality of living 

 micro-organisms present in the water. Upon this fact, after all, an 

 accurate conclusion depends. There is abundant evidence to show 

 that no valuable opinion can be passed upon a water except by both 

 a chemical and a bacteriological examination, and further by a 

 personal investigation, outside the laboratory, of the origin of the 

 water and its liabilities to pollution. 



So convinced was Koch of the efficiency of sand filtration as 

 protection against disease-producing germs, that he advocated an 

 adaptation of this plan in cases where it was found that a well 

 yielded infected water. Such pollution in a well may be due to 



*See also Thirty-fourth Ann. Bep. State Bd. of Health, Massachusetts, 1903, 

 p. 228. 



