(i) Bacterial.—From a hygienic standpoint the information secured by a bacterial examination of water is probably of greatest assistance and hence of greater value than any other determination. Unfortunately, since the sources vary so widely, it is impossible to establish numerical standards of the number of bacteria permissible. The greatest value of the numerical determination of bacteria in water is in the study of surface waters or in the control of purification plants. The detection of fecal bacteria is of greatest hygienic value since they give evidence of recent contamination. The group of colon like bacilli are the chief ones sought. Their detection in ground water is of great significance, while in surface waters in quanti ties less than i per c.c., their significance is slight. Owing to their extreme dilution and for other reasons the detection of specific pathogenic organisms in water is not practicable. The detection of bacteria characteristic of feces indicates that fecal contamination is occurring, which also indicates that the ex creta from an individual discharging specific infective agents might as easily gain introduction.
It is impossible to form an accurate idea of the hygienic character of a water supply from single isolated examinations. The character of raw natural waters, particularly surface waters, is subject to considerable fluctuation, while the operations of a water purification plant can only be accurately controlled by frequent routine examinations of the water. Fur thermore, contamination of a supply deemed to be above suspicion may actually occur, and unless its quality is regularly followed by laboratory examination, an epidemic may be the first indication of an unnoted contamination. Such periodic routine examinations should be at least daily at water purifi cation plants, and with untreated water from satisfactory sources, at not less than weekly intervals.
A rigid bacterial standard for potable waters is the Treasury Department standard for water furnished common carriers in interstate traffic. It is as follows: (a) The total bacteria per cubic centimeter shall not exceed zoo, when inoculated into standard agar and incubated at 37 degrees C. for 24 hours.
(b) Not more than one out of five, io c.c. portions of any sample shall show the colon bacillus.
A water that meets these requirements is probably from a satisfactory source, but on the other hand, the standard is so rigid that water from many safe sources of supply would be excluded.
The following are suggested by Bartow as the permissible limits for impurities in Illinois waters, expressed in parts per million (P.P.A1).
6. Water Consumption.—The water consumption in cities where a municipal supply is available under pressure in the majority of dwellings, when considered on a per capita basis is quite variable, ranging from 25 to 37o gallons and perhaps averaging somewhere near 120 gallons per capita per day. It is ordinarily considered, that per capita, common domestic uses require from ten to seventeen gallons daily. The difference in the above figures is due to the use of water in manufacturing, irrigation, and losses by wastage. In European cities the per capita consumption is much lower, varying from 24 to 69 in a number of larger cities, and perhaps averaging about 4o gallons. These figures probably represent an unduly low con sumption of water, but on the other hand, the American figures undoubtedly represent a serious waste of this indespensible necessity. Our aim should be to encourage a generous use of water, but to discourage waste.