Patent ID: 7767077
Filing Date: 2010-08-03
Classification: B01D,C02F

Abstract:
1. A membrane filtration system comprising: a filtering membrane portion having a reverse osmosis membrane for removing impurities in feed water; a water supply line connected to a primary side inlet port of the filtering membrane portion; a feed pump provided in the water supply line; an inverter controlling an operation frequency of the feed pump; a permeate line connected to a secondary side port of the filtering membrane portion; a flow rate sensor provided in the permeate line; a concentrate line connected to a primary side outlet port of the filtering membrane portion; one or more drain lines branched off from the concentrate line for draining a part of concentrate from the filtering membrane portion to an exterior of a system; one or more drain valves provided in each of said drain lines; a concentrate return line branched off from the concentrate line for returning a remainder of the concentrate from the filtering membrane portion to the water supply line on an upstream side of the feed pump; a proportional control valve provided in the concentrate return line for adjusting a return flow rate of the concentrate; a temperature sensor provided either in the water supply line or in the permeate line; and a control portion controlling an opening/closing of the drain valves and an opening of the proportional control valve based on a detection value of the temperature sensor together with outputting a flow rate signal to the inverter based on a detection value of the flow rate sensor, (i) wherein said control portion controls a rotational speed of the feed pump with the use of proportional-integral-differential (PID) control function of the inverter so that said detection value of the flow rate sensor attains a constant target value, (ii) wherein said control portion changes a water recovery rate by controlling the number of the opened drain valves in response to said detection value of the temperature sensor so that no impurities concentration in excess of the solubility of impurities occurs in said concentrate, (iii) wherein said control portion decreases the return flow rate so as to maintain the concentrate flow rate of the concentrate line at the set value by decreasing the opening of the proportional control valve when the water recovery rate decreases, and (iv) wherein said control portion increases the return flow rate so as to maintain the concentrate flow rate of the concentrate line at the set value by increasing the opening of the proportional control valve when the water recovery rate increases.