54 ILLINOIS ACADEMY OF SCIENCE 



ed to omit purification of such a water. The disastrous 

 results that may come from this ommission is well illustrated 

 by the experience in Plymouth, Pa., in 1885. In this case the 

 source of water supply was a mountain stream such as above 

 described, with but two houses upon the entire drainage basin. 

 Nevertheless a single case of typhoid in one of these houses 

 was responsible for 1104 cases and 114 deaths among the con- 

 sumers of the water. 



Notwithstanding the great difificulty in defining that de- 

 gree of pollution which is permissible in streams which are 

 to be used as public water supplies after purification there 

 would seem to be an advantage in attempting to approximate 

 a general rule for the control of such streams. A rule has 

 been formulated in the light of the present available evi- 

 dence, but it must be admitted that this rule is not based upon 

 any very scientific data and it can, therefore, only be put for- 

 ward tentatively with the expectation that it will be modi- 

 fied from time to time as more and more experience is ac- 

 quired. This rule may be stated as follows: The time in 

 hours required for the passage of a particle of water 

 from the sewer outlet to the point of water works intake dur- 

 ing high water, multiplied by the dilution available during 

 low water in cubic feet per second per 1000 persons tributary 

 to the sewers, should equal *a constant and this constanr 

 should not equal less than -K). This may be expressed mathe- 

 matically as follows : 



T X D equals C. 



In which T equals time in hours required for the passage 

 of a particle of water from the sewer outlet to the water 

 works intake at high water; 



D equals dilution available during low water in cubic 

 feet per second per 1,000 persons tributary to the sewers; 

 and 



C equals constant, which it is recommended be not less 

 than 40. 



The above formula applies to streams in which there is 

 no appreciable increase in volume of flow between sewer out- 

 let and the point of water works intake. In the case of 

 streams which receive the discharge of large tributaries be- 

 tween the point of sewer outlet and the point of water works 

 intake, the formula must, of course, be modified. Generally 

 it will be merely necessary to assign a value D which repre- 

 sents the mean of the quantity of water flowing past the sew- 

 er outlet and that flowing past the water works intake. If 

 the factor of safety proves to be more than 40, purification of 

 the sewage will not be necessary for the protection of the 

 water supply. If the factor of safety is less than 40, some 



