Apparatus and method for membrane filtration with enhanced net flux

• US 5,958,243 A
• Filed: 03/03/1997
• Issued: 09/28/1999
• Est. Priority Date: 07/11/1996
• Status: Expired due to Term

• Current Assignee: Zenon Environmental Incorporated (Veolia Environnement SA)
• Original Assignee: Zenon Environmental Incorporated (Veolia Environnement SA)
• Inventors: Hellenkamp, Theo A. A., Lawrence, Darren P.
• Primary Examiner(s): Walker, W. L.
• Assistant Examiner(s): Cecil, Terry K.
• Application Number:
  US08/804,279
• Time in Patent Office: 939 Days
• Field of Search: 210/137, 210/637, 210/321.65, 210/739, 210/741, 210/416.1, 210/636, 210/321.69, 210/412
• US Class Current: 210/636
• CPC Class Codes: B01D 2321/04 Backflushing
• CPC Class Codes: B01D 2321/2066 Pulsated flow
• CPC Class Codes: B01D 61/20 Accessories; Auxiliary operations
• CPC Class Codes: B01D 65/02 Membrane cleaning or sterilisation ; Membrane regeneration
• CPC Class Codes: B01D 65/08 Prevention of membrane fouling or of concentration polarisation

Abstract

A membrane filtration system which uses pump speed controllers to control flow rate and pressure during both the filtration cycle and the backwash cycle. The pump speed controllers operate by changing the frequency of AC current delivered to the pump motors, which changes the flow rate by changing the speed of the pump motor. The speed controllers operate by modulating the pump input power frequency in response to a signal from a flow or pressure signal received from a flow or pressure meter located on a liquid input pipe. This pump speed control method avoids the cyclical dampening patterns observed when trying to adjust pressure and/or flow rates by adjusting regulating valves. The system can also reduce or prevent formation of pressure spikes when switching from a filtration cycle to a backwash cycle, and vice versa. This reduces the likelihood of damage to the membrane from hydraulic pressure pulses. Finally, it offers the flexibility to provide pressure and flow pulses throughout the filtration and backwash cycles. These pulses have been found to dislodge foulants from the membrane while improving the net flux across the membrane, resulting in an increase in system productivity.

Claims

1. A method of filtering a liquid at a controlled rate of flow using a membrane filtration system to provide enhanced net flux having:

(a) a source of liquid to be filtered;

(b) a feed pump having a controllable speed adapted to pump liquid to be filtered from the source of liquid through a feed conduit to the system at a controlled rate;

(c) a filtration module comprised of a nano-, micro- or ultra-filtration membrane, the module adapted to receive liquid to be filtered from the feed conduit through an inlet opening and direct the flow of the liquid to be filtered across a porous membrane mounted in the filtration module;

(d) a permeate conduit adapted to carry liquid which has been filtered through the membrane in the filtration module out of the filtration module through a permeate outlet opening wherein said permeate conduit can be closed with a first valve;

(e) a concentrate conduit adapted to carry unfiltered liquid out of the filtration module through a concentrate outlet opening;

(f) a backwash conduit through which liquid enters the permeate chamber during a cleaning operation wherein said backwash conduit can be opened with a second valve;

(g) a backwash pump for pumping backwash liquid through the backwash conduit into the permeate chamber during a cleaning operation;

(h) a means for closing the first valve and opening the second valve when the backwash pump in running; and

(i) backwash liquid pressure controlling means for controlling the pressure of the backwash liquid in the backwash conduit, the method comprising;

(i) pumping liquid to be filtered through the feed conduit into the filtration module;

(ii) withdrawing filtered liquid from the filtration module through the permeate conduit and withdrawing unfiltered liquid through the concentrate conduit;

(iii) measuring the rate at which liquid to be filtered flows through the feed conduit to provide a measured flow rate;

(iv) comparing the measured flow rate to a preset flow rate to determine whether the measured flow rate differs from the preset flow rate;

(v) if the measured flow rate is different from the preset flow rate, adjusting the rate at which liquid to be filtered flows through the feed conduit until the rate at which liquid flows through the feed conduit is equal to the preset flow rate by adjusting the speed of the pump; and

periodically backwashing said membrane by;

(vi) closing said permeate conduit and opening said backwash conduit;

(vii) pumping backwash liquid through said backwash conduit into said permeate chamber; and

(viii) controlling the pressure of the backwash liquid in the backwash conduit using said backwash liquid controlling means, said backwash liquid controlling means comprising a backwash pump speed controller for adjusting the pressure of the backwash liquid by adjusting the speed of backwash pump, said speed controller controlling said backwash pump and generating pulses in the backwash liquid to dislodge foulants from said membrane to provide said enhanced net flux.

2. The method of claim 1 including providing a pump having an electric motor which runs on AC current, and adjusting the speed of the pump by changing the frequency of AC power supplied to the pump motor.

3. The method in accordance with claim 1 including the step of generating periodic pressure pulses or flow pulses in the liquid entering the filtration module through the feed conduit using a feed pump speed controller to vary the speed of the pump in a periodic fashion.

4. The method in accordance with claim 1 including:

(a) starting the feed pump at a predetermined speed;

(b) using the feed pump to pump liquid to be filtered through the feed conduit into the filtration module;

(c) measuring pressure or rate of liquid flowing through the feed conduit immediately after the pump starts to provide a measured flow rate or pressure;

(d) comparing the measured flow rate or pressure to a preset flow rate or pressure to determine whether the measured flow rate or pressure differs from the preset flow rate or pressure; and

(e) if the measured flow rate or pressure is greater than the preset flow rate or pressure, adjusting the rate or pressure of liquid flowing through the feed conduit by decreasing the speed of the pump, or increasing the speed of the pump if the measured flow rate or pressure is less than the preset flow rate or pressure, thereby substantially eliminating large hydraulic pressure pulses upon reversal of flow for backwashing.

5. A method of membrane filtration with enhanced flow rate control, using a membrane filtration system having:

(a) a source of liquid to be filtered;

(b) at least one filtration membrane separating the liquid to be filtered from a permeate chamber; and

(c) a suction pump having a controllable speed adapted to withdraw liquid to be filtered through the membrane to provide a permeate to be removed as filtered fluid from the permeate chamber through a permeate conduit which can be closed by a first valve, a backwash conduit through which liquid enters the permeate chamber during a cleaning operation wherein said backwash conduit can be opened with a second valve, a backwash pump for pumping backwash liquid through the backwash conduit into the permeate chamber during a cleaning operation;

a means for closing the first valve and opening the second valve when the backwash pump in running; and

backwash liquid pressure controlling means for controlling the pressure of the backwash liquid in the backwash conduit, the method comprising;

(i) using the suction pump to withdraw the filtered liquid from the permeate chamber through the permeate conduit;

(ii) measuring the rate or pressure of liquid flowing through the permeate conduit to provide a measured flow rate or pressure;

(iii) comparing the measured flow rate or pressure to a preset flow rate or pressure to determine if the measured flow rate or pressure differs from the preset flow rate or pressure; and

(iv) if the measured flow rate or pressure differs from the preset flow rate or pressure, adjusting the rate or pressure of liquid flowing through the permeate conduit until the flow rate or pressure equals the preset flow rate or pressure by adjusting the speed of the pump, periodically backwashing said membrane by;

a. closing said permeate conduit and opening said backwash conduit;

b. pumping backwash liquid through said backwash conduit into said permeate chamber; and

c. controlling the pressure of the backwash liquid in the backwash conduit using said backwash liquid controlling means, said backwash liquid controlling means comprising a backwash pump speed controller for adjusting the pressure of the backwash liquid by adjusting the speed of backwash pump, said speed controller controlling said backwash pump and generating pulses in the backwash liquid to dislodge foulants from said membrane to provide said enhanced net flux.

6. The method of claim 5 including providing the pump with an electric motor which runs on AC current, and adjusting the speed of the pump by changing frequency of AC power supplied to the motor.

7. In a membrane filtration system comprising:

(a) a filtration module comprising a concentrate chamber and a permeate chamber separated by a filtration membrane;

(b) a pump having a controllable speed for driving liquid to be filtered through the membrane from the concentrate chamber to the permeate chamber;

(c) a permeate conduit through which liquid which has been filtered by passage through the membrane is removed from the permeate chamber during filtration wherein said permeate conduit can be closed with a first valve;

(d) a backwash conduit through which filtered liquid enters the permeate chamber during a cleaning operation wherein said backwash conduit can be opened with a second valve;

(e) a backwash pump for pumping backwash liquid through the backwash conduit into the permeate chamber during a cleaning operation;

(f) a means for closing the first valve and opening the second valve when the backwash pump is running; and

(g) backwash liquid pressure controlling means for controlling the pressure of the backwash liquid in the backwash conduit, said pressure controlling means comprising a backwash pump speed controller for adjusting pressure of the backwash liquid by cyclically varying the speed of the pump in a predetermined manner, the backwash speed controller having the ability to generate pulses in the backwash liquid by cyclicalling varying the speed of the pump.

8. The system of claim 7 wherein the pressure controlling means comprises:

(a) a pressure sensor adapted to measure the pressure of the backwash liquid flowing through the backwash conduit to provide a measured pressure; and

(b) a backwash pump speed controller for;

(i) comparing the measured pressure to a preset pressure to determine whether the measured pressure differs from the preset pressure; and

(ii) if the measured pressure differs from the preset pressure, adjusting the pressure of the backwash liquid in the backwash conduit until the backwash liquid pressure matches the preset pressure by adjusting the speed of the backwash pump.

9. The system of claim 7 wherein the pressure controlling means comprises a backwash pump speed controller adapted to vary the speed of the pump to cyclically vary the pressure of the backwash liquid flowing through the backwash conduit.

10. A method of cleaning a filtration membrane with increased efficiency using a system having:

(a) a filtration module comprising a concentrate chamber and a permeate chamber separated by a filtration membrane;

(b) a pump for pumping liquid to be filtered through the membrane from the concentrate chamber to the permeate chamber;

(c) a permeate conduit through which liquid which has been filtered by passage through the membrane is removed from the permeate chamber during filtration wherein said permeate conduit can be closed with a first valve;

(d) a backwash conduit through which filtered liquid enters the permeate chamber during a cleaning operation wherein said backwash conduit can be opened with a second valve;

(e) a backwash pump having a controllable speed for pumping backwash liquid through the backwash conduit into the permeate chamber during a cleaning operation; and

(f) a means for closing the first valve and opening the second valve when the backwash pump is running, the method comprising;

(i) closing the first valve and opening the second valve;

(ii) starting the backwash pump and pumping backwash liquid through the backwash conduit, into the permeate chamber, and through the membrane for a predetermined period of time to dislodge foulants which have collected on the membrane;

(iii) measuring the pressure of the backwash liquid in the backwash conduit;

(iv) comparing the pressure of the backwash liquid to a preset pressure to determine if they differ;

(v) if the pressure of the backwash liquid differs from the preset pressure, adjusting the pressure of the backwash liquid by adjusting the speed of the backwash pump until the pressure of the backwash liquid is equal to the preset pressure;

(vi) during said pumping of said backwash liquid, varying the pressure of the backwash liquid flowing through the backwash conduit to create pressure pulses in a periodic fashion by cyclically varying the speed of the backwash pump; and

(vii) at the end of the predetermined period of time, stopping the backwash pump, closing the second valve, and opening the first valve.

11. The method of claim 10 including performing steps (iii), (iv), and (v) immediately after the backwash pump is started.

12. A method of cleaning a filtration membrane with increased efficiency using a system having:

(a) a filtration module comprising a concentrate chamber and a permeate chamber separated by a filtration membrane;

(b) a pump which drives liquid to be filtered through the membrane from the concentrate chamber to the permeate chamber;

(c) a permeate conduit through which liquid which has been filtered by passage through the membrane is removed from the permeate chamber during filtration wherein said permeate conduit can be closed with a first valve;

(d) a backwash conduit through which filtered liquid enters the permeate chamber during a cleaning operation wherein said backwash conduit can be opened with a second valve;

(e) a backwash pump having a controllable speed for pumping backwash liquid through the backwash conduit into the permeate chamber during a cleaning operation; and

(f) a means for closing the first valve and opening the second valve when the backwash pump is running, the method comprising;

(i) closing the first valve and opening the second valve;

(ii) using said backwash pump and pumping backwash liquid through the backwash conduit, into the permeate chamber, and through the membrane for a predetermined period of time to dislodge foulants which have collected on the membrane; and

(iii) during said pumping of said backwash liquid, varying the pressure of the backwash liquid flowing through the backwash conduit to create pressure pulses in a periodic fashion by cyclically varying the speed of the backwash pump.

13. In a membrane filtration system comprising:

(a) a source of liquid to be filtered;

(b) a filtration pump having a controllable speed adapted to pump liquid to be filtered from the liquid source through a feed conduit at a controlled rate;

(c) a filtration module having at least one concentrate chamber and at least one permeate chamber which are separated by a filtration membrane wherein said concentrate chamber is adapted for receiving liquid to be filtered from the feed conduit through a liquid inlet opening;

(d) a permeate conduit for carrying liquid which has been passed through the membrane from the concentrate chamber into the permeate chamber out of the permeate chamber through a permeate outlet opening;

(e) a conduit adapted to carry unfiltered liquid out of the concentrate chamber through a concentrate outlet opening;

(f) a source of backwash liquid comprised of filtered liquid;

(g) a backwash pump adapted to pump backwash liquid into the permeate chamber through a backwash opening at a controlled flow rate;

(h) a means for closing the backwash opening and opening the liquid inlet opening and permeate outlet opening when the filtration pump is running, and closing the liquid inlet opening and permeate outlet opening and opening the backwash opening when the backwash pump is running; and

(i) first means for controlling the rate at which liquid to be filtered flows into the concentrate chamber when the filtration pump is running, and second means for controlling pressure of the backwash liquid flowing into the permeate chamber when the backwash pump is running;

the first means of controlling the flow of liquid to be filtered comprises a filtration pump speed controller for adjusting the rate at which liquid to be filtered flows to the concentrate chamber by adjusting the speed of the filtration pump, said first means adapted for generating periodic pressure pulses or flow pulses in the liquid entering the concentrate chamber when the filtration pump is operating, said first means comprising a filtration pump speed controller for varying the flow rate of the liquid to be filtered in a periodic fashion by cyclically varying the speed of the filtration pump; and

the second means of controlling the pressure of the backwash liquid comprises a backwash pump speed controller adapted to adjust the pressure of the backwash liquid by adjusting the speed of the backwash pump, said second means for generating periodic pressure pulses or flow pulses in the backwash liquid entering the permeate chamber when the backwash pump is operating said second means comprising a backwash pump speed controller for varying the flow rate of the backwash liquid in a periodic fashion by cyclically varying the speed of the backwash pump to create pulses.

14. The system of claim 13 wherein the filtration pump is powered by an electric motor which runs on AC current, and the filtration pump speed controller adjusts the speed of the filtration pump by adjusting the frequency of AC current supplied to the filtration pump motor;

and wherein the backwash pump is powered by an electric motor which runs on AC current, and the backwash pump speed controller adjusts the speed of the backwash pump by adjusting the frequency of AC current supplied to the backwash pump motor.