A method for cleaning separating membranes uses a uniquely formulated detergent. The detergent includes a percarbonate-containing, material, existing in the condition in which hydrogen peroxide is adducted in carbonate and is desirably an alkali metal salt. The percarbonate decomposes the organisms deposited on the membrane surfaces while being environmentally desirable. The detergent preferably also includes at least one bivalent iron salt, which improves the cleaning capability of the percarbonate via the so-called Fenton's reaction in which hydrogen peroxide adducted in the percarbonate reacts with the iron ions to form OH— radicals which have a very high oxidative reactivity. The detergent may also contain other oxidizers, a surfactant, a chelating agent, an acid and a pH regulator. The detergent is dissolved in water prior to use.