FILTERING METHOD

PURPOSE:To obtain a permeating liquid in a high recovery ratio without lowering the amount of the permeating liquid over a long period of time, by applying ultrafiltration treatment to a liquid containing a minute amount of a fine particle in a range of predetermined liquid recovery ratio. CONSTITUTION:At first, raw water is treated by a central system consisting of a filter apparatus, a RO apparatus, a mixed bed type ion exchange apparatus, an ultraviolet sterilizing apparatus and a polisher to reduce the numbers of fine particles to 10 /ml or less. This raw water 1 is supplied to an ultrafiltration membrane module 3 with a fractionation M.W. of 6,000-300,000 under supply pressure of 0.4-15kg/cm by a pump 2 and the outlet flowline 4 of a concentrate side is throttled to filter the raw water in a recovery ratio of 97-99.9% to obtain a membrane filtrate 5. By this method, because a liquid extremely reduced in the content of fine particles can be stably obtained with the elapse of time, this method is suitable as a membrane filtering method in the course point of ultra-pure water.