Membrane separation method

A membrane separation method in which a combined-chlorine-based oxidizing agent is contained to water of interest, and the water of interest containing the combined-chlorine-based oxidizing agent is allowed to pass through a membrane separation apparatus, wherein

• a ratio of [a Free 300-second value/a total chlorine concentration] in the combined-chlorine-based oxidizing agent is 5% to 15%,
• a molar ratio of a chlorine stabilizer to 1 mol of the total chlorine concentration in the combined-chlorine-based oxidizing agent is 1.0 to 1.5 mol,
• the water of interest containing the combined-chlorine-based oxidizing agent is the water of interest having the Free 300-second value of 0.036 mg/L-Cl2 or more,
• a stabilized combined chlorine concentration in the water of interest containing the combined-chlorine-based oxidizing agent is 0.1 to 5.0 mg/L-Cl2, and
• the water of interest containing the combined-chlorine-based oxidizing agent is the water of interest having [Free 300-second value/Log10 (biofouling potential)] of 0.008 or more,
• wherein the Free 300-second value is a chlorine concentration (mg/L-Cl2) obtained by containing a combined chlorine-based oxidizing agent to the water of interest and performing measurement after 300 seconds using a free chlorine measurement reagent in the water of interest containing the combined chlorine-based oxidizing agent, and
• the biofouling potential is a number of bacteria (cfu/mL) in a sample when stored for 7 days when water of interest before containing a combined-chlorine-based oxidizing agent is stored under dark conditions at 30° C. for 7 days.