WATER TREATMENT APPARATUS AND WATER TREATMENT METHOD

Disclosed are a water treatment device and a water treatment method, both of which can prevent the clogging of a membrane using a chemical substance containing a chlorine component and can also prevent the increase in the amount of the chlorine-component-containing chemical component to be used. Specifically disclosed is a water treatment device which is equipped with a membrane unit comprising a membrane and is so adapted that water to be treated can be separated through the membrane in the membrane unit. The device is characterized by being so adapted as to conduct a first filtration step and a washing step or a second filtration step alternatively, wherein the first filtration step comprises separating residual-free-chlorine-containing water (i.e., water containing a residual free chlorine) to be treated that has the same or lower residual free chlorine concentration as or than a predetermined value through the membrane in the membrane unit, the washing step comprises immersing the membrane in the membrane unit in a wash solution having a higher residual free chlorine concentration than the predetermined value, and the second filtration step comprises separating residual-free-chlorine-containing water that has a higher residual free chlorine concentration than the predetermined value through the membrane in the membrane unit.