Water treatment method

A method for treating water, the method comprising:

• adding, as coagulants, a polymer compound having a phenolic hydroxyl group and an iron-based coagulant to water to be treated;
• filtering coagulated water with a clarification filter;
• and
• subjecting filtered water to a reverse osmosis membrane treatment,
• wherein the addition of the coagulants is controlled such that a ratio between an addition concentration of the polymer compound having the phenolic hydroxyl group and an addition concentration of the iron-based coagulant is constant and is 0.01 to 0.25,
• the addition concentration of the polymer compound having the phenolic hydroxyl group is 1 to 15 mg/L,
• the addition concentration of the iron-based coagulant is controlled according to a TOC concentration of the treated water, and the polymer compound having the phenolic hydroxyl group is added following the addition concentration of the iron-based coagulant, and
• the addition concentration of the iron-based coagulant is between 5 and 300 mg/L when the TOC concentration varies in a range between 0.1 and 5.0 mg/L.