Water treatment method and water treatment agent composition

A water treatment method comprising subjecting a water to be treated to a membrane filtration treatment using a separation membrane, and treating a membrane filtration treated water obtained during the membrane filtration treatment using a reverse osmosis membrane to obtain a concentrate and a permeate, the method comprising either:

• adding, to the water to be treated containing at least one of hypochlorous acid, hypobromous acid, and salts of these acids, at least 1 mol of iodide ions per 1 mol of a free chlorine content and free bromine content in the water to be treated with the proviso that a free iodine CT value, represented by a free iodine concentration (mg/L) produced in the water to be treated containing the added iodide ions×an iodide ion addition time (h), is not more than 1.25 (mg/L·h), or
• adding, to the water to be treated containing iodide ions, at least one of hypochlorous acid, hypobromous acid, and salts of these acids in such an amount that a free chlorine content and free bromine content does not reach more than 1 mol per 1 mol of the iodide ions in the water to be treated and with the proviso that a free iodine CT value, represented by a free iodine concentration (mg/L) produced in the water to be treated to which at least one of the chlorine-based oxidizing agent and the bromine-based oxidizing agent has been added×an addition time (h) for at least one of the chlorine-based oxidizing agent and the bromine-based oxidizing agent, is not more than 1.25 (mg/L·h).