In cities with combined sewers, the influent flow rate to wastewater treatment p
ID: 1921014 • Letter: I
Question
In cities with combined sewers, the influent flow rate to wastewater treatment plants can increase dramatically after a heavy rainfall. As long as the rain water runoff is relatively clean, and the plant can handle the volume of water coming in, the F/M (and therefore the treatment efficiency) of the activated sludge process should not change.
a. Assuming that the runoff from a particular storm carries a high organic load and that the plant operators cannot divert flow away from the plant, would you expect the treatment efficiency to increase or decrease? Justify your answer by describing how the F/M and the mean cell residence time (? c) will change during such an event.
b. Other than waiting for the flow to subside, what action could you immediately take as the plant operator to help bring the F/M back to its original value?
Explanation / Answer
(a) The treat efficiency tends to decrease and not increase in case of high organic load coming through a particular storm. The reason behind this is heavy flow of organic matter where the mass tends to increase. This will decrease the F/M ratio. It will also decrease the mean cell residence time (c).
(b) The method which should be imposed in order to bring back the original value of F/M is to decrease the content of mass of the organic load.
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