Process for scrubbing porous hollow fiber membranes in hollow fiber membrane module

A plural stage process for scrubbing including sequentially removing an outer layer of easier to remove particles and then an inner layer of harder to remove particles from porous hollow fiber membranes fouled by power plant water in a hollow fiber membrane module including a large number of hollow fibers bundled together therein as said porous hollow fiber membranes:

• comprising the filtration step of passing raw water containing fine particles through the pores of said hollow fibers from the outsides thereof to the insides thereof to trap said fine particles on the outer surfaces of said hollow fibers while draining filtrate obtained on the insides of said hollow fibers out of said hollow fiber membrane module, the scrubbing step of charging bubbles into water around the outer surfaces of said hollow fibers dipped in water to agitate said water around the outer surfaces of said hollow fibers to vibrate said hollow fibers, thereby exfoliating said fine particles trapped on the outer surfaces of said hollow fibers, and the drain step of draining the resulting waste water containing the exfoliated fine particles out of said hollow fiber membrane module;
• wherein the preliminary drain step of draining waste water containing fine particles exfoliated off the outer surfaces of said hollow fibers in the initial stage of said scrubbing step is added in the course of said scrubbing step to simultaneously effect scrubbing of said porous hollow fiber membranes and drain of said waste water in the initial stage of said scrubbing step.