Method for operating membrane filtration unit and membrane filtration unit

A method of operating a membrane filtration unit, the membrane filtration unit comprising plural hollow fiber membrane modules connected to each other in parallel, each hollow fiber membrane module comprising a container which has a raw liquid inlet, a filtrate outlet, and a raw liquid outlet and is filled with hollow fiber membranes,

• the method comprising:
• a filtration step of introducing a raw liquid into the container from the raw liquid inlet, filtering the raw liquid through the hollow fiber membranes from a primary side to a secondary side, and leading out resulting filtrate to outside the container from the filtrate outlet;
• a collection step of causing the filtrate existing in the container to flow in a reverse direction through the hollow fiber membranes from the secondary side to the primary side and leading out resulting backflow liquid to outside the container from at least one of the raw liquid inlet and the raw liquid outlet;
• and
• a recovery step of introducing the backflow liquid into the container again from at least one of the raw liquid inlet and the raw liquid outlet, filtering the backflow liquid through the hollow fiber membranes from the primary side to the secondary side, and leading out resulting recovery liquid to outside the container from the filtrate outlet,
• wherein a relation of n1≥n2>n3 is satisfied, where n1 is the number of the hollow fiber membrane modules simultaneously used in executing the filtration step, n2 is the number of the hollow fiber membrane modules simultaneously used in executing the collection step, and n3 is the number of the hollow fiber membrane modules simultaneously used in executing the recovery step.