The belief that the disposal of sewage by turning it into running streams is only a temporary expedient which will in the near future be abandoned, is growing rapidly. Our cities are constantly increasing in population. The quantity of sewage is growing larger, and the degree of contamination of running streams is becoming so great as to be practically beyond control. In addition to the septic tank, which is a rather expensive method of disposing of sewage, it has lately been discovered that the deposits of sewage known as sludge may be utilized largely for promoting purification. By innoculating the sludge with the organisms which destroy protein and cellulose, under certain conditions which are controllable, the purification of the sewage resting upon the sludge is greatly promoted. The principles upon which this destruction depends are the activating of the sludge by the introduction of the proper bacteria and the securing of proper aeration and movement to bring the sewage into contact with this activating material. This method has been exhaustively studied by the Water Commission of Illinois.

One of the chief objections to the purification of sewage in Northern cities is due to the fact that the low temperatures of winter interfere seriously with bacterial activity. If the pools in which the sewage is stored can be protected from the freezing weather either by cover or by sinking them deeper into the earth, the temperature suitable to the destruction of organic matter is more easily maintained. In addition to this the bacterial activity itself is always attended with the evolution of heat, which may be so conserved as to continue to promote the activity of the sludge even in cold weather.

Professor Bartow of the University of Illinois has experimentally determined the high value of an activated sludge in hastening the purification of sewage. I have had the opportunity of seeing this process near Urbana. His reports are found in the publications of the State Water Commission. His work is now temporarily suspended while he is engaged in field duty with our Army in France. Pearse has studied the results of utilizing activated sludge on the sewage of the Chicago packing houses and concludes that:1

The activated sludge process offers the best promise of a solution of the problem of treating packing-house waste of any suggested up to the present time. For the study of conditions in Chicago the construction of a unit plant has been recommended to handle 1.5 million gallons per 24 hours, to obtain working results on a large enough scale to determine the design of the final plant."

1 American Journal of Public Health, Vol. VIII, No. I, Jan., 1918, page 55.