An aeration amount control system and a water treatment method capable of reducing the operation cost of the aeration amount control system are obtained. In the air diffusion amount control system 100 that performs air diffusion on the separation membrane 3 in the membrane separation tank 2 storing the treated water based on the target air diffusion amount, the first target air diffusion amount is determined as the target air diffusion amount. The control device 7 that determines the second target air diffusion amount after determining the first target air diffusion amount, and the air diffusion device that supplies the gas based on the target air diffusion amount and performs air diffusion 5 and a measuring device 6 that measures the amount of change in the transmembrane pressure difference of the separation membrane 3 with respect to the gas supplied by the air diffusing device 5, and the air diffusing device 5 calculated by the measuring device 6 is the first target air diffusing. The first change amount of the transmembrane pressure difference of the separation membrane 3 during the aeration based on the amount was calculated by the measuring device 6, and the aeration device 5 performed the aeration based on the second target aeration amount. When it is larger than the second change amount of the transmembrane pressure difference of the separation membrane 3 between, the control device 7 is smaller than the second target aeration amount as the third target aeration amount. And determining the a values.
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