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.
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.”
Irrigation
Although the method of broad irrigation has been carried on successfully in several of the largest cities of the old world, it has not been used to any great extent in the United States, except at a few state institutions in the east, and in the arid districts of the west.
Several years ago this method of disposal was given much attention as it was thought the sewage if used would yield large profits, but more recent information shows conclusively that such is not the case.
Perhaps the most noticeable examples of this system that are now in operation in the United States are the plants at the State Hospital of the Insane, Worcester, Mass., at the Rhode Island State Institute, and at the Reformatory at Concord, Mass. In all cases the labor was done by the inmates so that it is not possible to get a fair statement of the cost. General information regarding the use of sewage for irrigation in the west may be obtained from Baker and Rafters’ “Sewage Disposal in the United States”.
Undoubtedly the best example of sewage farming is at Berlin. The following summary is given by James H. Fuentes, M. Am. Soc. C. E., in No. 2. Vol. 40 of the Engineering Record. The average quantity of sewage pumped to the farms and distributed amounts to from 6,000,000 to 7,000,000 gallons per acre per year, or in other words each acre serves for 750 people. The area farmed is divided into small beds about 150 by 200 feet separated from each other by slight embankments and ditches for distributing the sewage over the surface. The sewage is admitted into the carriers from the force mains through checking chambers made of woven willow and posts driven into the sand.