PROBLEM TO BE SOLVED: To rapidly decrease the concentration of dissolved oxygen in a denitrification tank to make its atmosphere a reducing one in a biological nitration denitrification treatment so that the nitrogen removing capability can be stabilized, the amount of a hydrogen doner to be used can be decreased, and the load of operation management can be reduced.;SOLUTION: There is provided a nitration denitrification system comprising a nitration tank, a denitrification tank, and a reducing agent adding part for adding a reducing agent to the denitrification tank, and characterized in that the reducing agent is a ferrous compound. The nitration denitrification system can make the atmosphere in the denitrification tank a reducing one by letting dissolved oxygen in the denitrification tank be rapidly consumed making use thereof that a divalent iron ion of the ferrous compound is oxidized to become a trivalent iron ion. As a result, the efficiency of the denitrification reaction in the denitrification tank can be improved without increasing the amount of the hydrogen doner added thereinto. Further, the influence of the inflow of the dissolved oxygen into the denitrification tank can be eliminated regardless of the fluctuation of the quality of water to be treated, and thereby the operation management can be made easier.;SELECTED DRAWING: Figure 1;COPYRIGHT: (C)2019,JPO&INPIT
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