Sewage disposal . trogen value at Grafton is only four per cent of the value atLockport. Here, as in the case of chlorine, it is probable thatdilution is responsible for most of the improvement; but someof the ammonia nitrogen not affected by dilution has been re-moved by oxidative processes. The nitrite and nitrate figuresshow the progress of the changes which are at work. Where the SELF-PURIFICATION IN DES PLAINES, ETC., RIVERS 49 process is most active, from Ottawa to Henry, nitrites rise toa maximum. From Ottawa to Wesley nitrates are Pekin, the discharge of sewage from Peori
Sewage disposal . trogen value at Grafton is only four per cent of the value atLockport. Here, as in the case of chlorine, it is probable thatdilution is responsible for most of the improvement; but someof the ammonia nitrogen not affected by dilution has been re-moved by oxidative processes. The nitrite and nitrate figuresshow the progress of the changes which are at work. Where the SELF-PURIFICATION IN DES PLAINES, ETC., RIVERS 49 process is most active, from Ottawa to Henry, nitrites rise toa maximum. From Ottawa to Wesley nitrates are Pekin, the discharge of sewage from Peoria causes some tem-porary reducing changes, followed again by an increase of nitratesat Kampsville and Grafton. The bacterial self-purification which accompanies these chem-ical changes is also indicated in Fig. 8. A steady decrease isapparent from Lockport to Wesley. At Pekin there is a notableincrease, due to the sewers of Peoria, followed by a secondself-purification. Comparison of the total improvement effected. Fig. 9. View of the Chicago Drainage Canal at Willow Springs(Courtesy of Robert R. McCormick). shows a bacterial reduction of nearly ninety-nine per cent fromLockport to Grafton. The reduction here is considerably greaterthan in the case of the organic constituents. Aside from dilu-tion, true self-purification has played an important part. It .is interesting to notice, as epidemiological evidence of thevalue of self-purification, that no striking excess of typhoidfever at St. Louis followed the opening of the Chicago drainagecanal. The experts for the plaintiff were able to show a slightincrease in typhoid fever; but this might well have been due tothe pollution of the Mississippi River from sources other thanChicago, or to causes other than water. The Supreme Court 50 DISPOSAL OF SEWAGE BY DILUTION finally dismissed the case without prejudice, on the ground thatdamage to St. Louis was not proved. Subsequent History of the Chicago Drainage Canal. Thesuccess of Chicag
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