Disposal of sewage by dilution is the most common method at the present time, and no doubt will continue to be for some time to come where the stream or body of water is not used for domestic purposes. Under certain conditions this affords a satisfactory solution of the problem. However, if the water into which the sewage is discharged is a source of water supply, two very important questions arise. They are: 1—How large must the dilution be? 2—How far must the polluted stream flow before the water way may be used with safety to public health?

In 1867 Dr. Letheby and others in England made the statement that if sewage is diluted at least twenty times its volume, it will not only be made inoffensive but be thoroughly destroyed after flowing a dozen miles or more.

The Rivers Pollution Commission of Great Britain in 1878 maintained that no river in England was long enough to allow of a complete disappearance of sewage matter discharged into it.

Mr. F. P. Stearns in an article on The Pollution and Self-Purification of Streams in the Massachusetts State Board of Health Report 1890 gives a table showing, “The calculated composition of sewage for different degrees of dilution in a running stream”. A comparison was made of the Blackstone and Merrimack rivers, and extended over three years. It was from this investigation that the Commissioner of the City of Chicago reported that the flow of the Chicago Drainage Canal should be four cubic feet per second per 1000 inhabitants.

The purification by dilution may be due to three things:

1—

Oxidation of organic matter.

2—

Subsidence.

3—

Destruction of the organic matter by small animals and plants.

The oxidation is a very slow process and depends mainly upon bacterial life and conditions favorable to it. Subsidence depends upon the difference in the specific gravity of the suspended matter, and the clarification is increased as the velocity is diminished.

In regard to the destruction of the organic matter by small animals and plants Dr. Sorby says, (Journal Royal Microscopic Society, 1884 p. 988.) “It appears to me that the removal of impurities from rivers is more of a biolytic than a chemical question, and it is most important to consider the action of minute animals and plants, which may be looked upon as being indirectly most powerful agents.”