368 SOME PROBLEMS IN WATER BACTERIOLOGY 



ice are stated in a somewhat different manner.' The agar plate count is not recog- 

 nized. The following statement includes only the bacteriological requirements. 



1. Of all the standard (lo cc.) portions examined in accordance with the procedure speci- 

 fied below, not more than lo per cent shall show the presence of organisms of the B. 

 coli group. 



2. Occasionally three or more of the five equal ( lo cc.) portions constituting a single stand- 

 ard sample may show the presence of B. coli. This shall not be allowable if it occurs in 

 more than — 



o) Five per cent of the standard samples when twenty (20) or more samples have been 



examined; 

 b) One standard sample when less than twenty (20) samples have been examined. 



The series of samples must conform to both (i) and (2). The "5. coli group" is defined 

 on the basis of the 1923 edition of Standard Methods of Water Analysis. A standard 

 portion is 10 cc, and a standard sample is five standard portions. 



The foregoing standard seems a reasonable one at the present time. It is not so 

 rigid as to be impossible of attainment. On the other hand, it is sufficiently exacting to 

 protect the public against water-borne bacterial infections while at the same time 

 making allowance for slight variations in laboratory results which are occasionally en- 

 countered. It would be unwise to predict how long it will be before even more rigid 

 standards are proposed. Certainly, it is hoped there will be no retrogression. 



Attempts have been made to express the results of the colon tests in a mathemat- 

 ical manner on the basis of the theory of probabilities. A discussion of these attempts 

 seems to the writer to be out of place in this summary. It is doubtful if the laboratory 

 methods employed for the detection of Bad. coli or other members of that group are 

 sufficiently accurate from a quantitative standpoint to warrant the application of 

 mathematical theory. The reader is referred to the original articles.^ 



CHLORINATION 



The rapid extension, since 1911, of chlorination in the treatment of public water 

 supplies has been nothing less than remarkable. In 1924^ it was estimated that more 

 than 3,750 million gallons were being treated daily. Relatively pure waters, both bac- 

 terially and chemically, lend themselves to such treatment. The dosage varies from 

 0.15 to i.o part of chlorine per million gallons of water. 



The chlorine dosage is a function not only of the number of bacteria present in the 

 water but also of the total amount of organic matter present. Such studies as that re- 

 cently made by Meadow and Hale^ on the chlorine absorption of water are likely to be 

 valuable in this connection. 



The bacteriological results obtained are, on the whole, satisfactory. One of the 



' Advisory Committee on Official Water Standards: loc. cit. 



'McCTa.dy,M.I{.: J. Infed. Dis., 17, 183. 1915; Pub. Health J., 9, 201. 1918; Wolman, A.: 

 J.A.W.W.A., 4, 200. 1917; ibid., 6, 444. 1919; ibid., 7, 927. 1920; Wolman, A., and Weaver, H. L.: 

 J. Infect. Dis.,21, 287. i9i7;Wells,W.F.:5'«e«ce(N.S.), 47.46. \9i^;J.A.W.W.A.,Z, 187. 1921; Stein,] 

 M. F.: /. Bad., 4, 243. 1919; J.A.W.W.A., 8, 182. 1921. 



3 Water Works Practice, p. 174. 1925. 



1 Meadow, J. R., and Hale, H.: J.A.W.W.A., 18, 75. 1927. 



