Operating method for membrane separation device and membrane separation device

A membrane separation device comprising:

• a separation membrane immersed in water to be treated;
• an air diffusion device positioned below the separation membrane, the membrane separation device providing treated water that has permeated through the separation membrane while diffusing air from the air diffusion device towards the separation membrane;
• a target value setting section that sets a target value of an amount of diffusion air diffused from the air diffusion device based on the transmembrane pressure difference;
• and
• a diffusion air amount controlling section that controls the air diffusion device so that the amount of the diffusion air becomes the target value,
• wherein the target value setting section sets an absolute value of a change or a rate of change in the target value for increasing the amount of the diffusion air greater than an absolute value of a change or a rate of change in the target value for decreasing the amount of the diffusion air.

A membrane separation device, comprising:

• a separation membrane immersed in water to be treated;
• an air diffusion device positioned below the separation membrane, the membrane separation device providing treated water that has permeated through the separation membrane while diffusing air from the air diffusion device towards the separation membrane;
• a reference transmembrane pressure difference setting section that updates, in every predetermined period of time, a reference transmembrane pressure difference for evaluating a measured transmembrane pressure difference;
• and
• a target value setting section that sets a target value of an amount of the diffusion air diffused from the air diffusion device based on a differential value between the reference transmembrane pressure difference and the measured transmembrane pressure difference.