Abstract:
A method and apparatus for chemical cleaning microfiltration and ultrafiltration membranes immersed in a tank involves backwashing a chemical cleaner through the membranes while the tank is empty of tank water. A backwash pump which drives the chemical cleaner is controlled by a speed controller which is in turn connected to a programmable logic control and, preferably pressure and flow indicators. The backwash pump is operated to supply the chemical cleaner to the membranes in pulses. The pressure of the pulses is high enough to reduce the relative size of pressure differentials between membranes or portions of membranes in varying places in the tank. The duration and frequency of the pulses is chosen to provide an appropriate contact time of the chemical, preferably without allowing the membranes to dry between pulses and without using excessive amounts of chemical.

Description:
This application claims the benefit of Provisional Application No. 60/146,154, filed Jul. 30, 1999. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to cleaning immersed ultrafiltration or microfiltration membranes with a chemical cleaner. 
     BACKGROUND OF THE INVENTION 
     Membranes are used for separating a permeate lean in solids from tank water rich in solids. Typically, filtered permeate passes through the walls of the membranes under the influence of a transmembrane pressure differential between a retentate side of the membranes and a permeate side of the membranes. Solids in the feed water are rejected by the membranes and remain on the retentate side of the membranes. The solids may be present in the feed water in solution, in suspension or as precipitates and may further include a variety of substances, some not actually solid, including colloids, microorganisms, exopolymeric substances excreted by microorganisms, suspended solids, and poorly dissolved organic or inorganic compounds such as salts, emulsions, proteins, humic acids, and others. 
     Over time, the solids foul the membranes which decreases their permeability. Any solid can contribute to fouling and reduced membrane permeability, and the fouling may occur in different ways. Fouling can also occur at the membrane surface or inside of the pores of the membrane. To counter the different types of fouling, many different types of cleaning regimens may be used. Such cleaning usually includes both physical cleaning and chemical cleaning. 
     For physical cleaning, permeation through the membranes is typically stopped momentarily. Air or water are flowed through the membranes under pressure to backwash the membranes. The force of the backwash physically pushes solids off of the membranes. Typically, the membranes are simultaneously agitated, for example by aerating the feed water around the membranes with large, scouring bubbles to assist in shearing solids from the surface of the membranes. Such back washing and agitation is partially effective in removing solids from the surface of the membranes, but is not very effective for removing solids deposited inside the membrane pores and is almost ineffective for removing any type of solid chemically or biologically attached to the membranes. 
     Accordingly, fouling continues despite regular physical cleaning. This continued fouling is countered by cleaning with a chemical cleaner. For example, the membranes may be soaked in one or more cleaning solutions either in the process tank (after it has been drained and filled with chemical cleaner) or in a special cleaning tank. These methods, however, require either large volumes of chemical cleaner (to fill the process tank) or the expense of providing special cleaning tanks and means to move the membranes to the cleaning tank. These methods also disrupt permeation for extended periods of time. 
     Other methods involve backwashing the membranes with a chemical cleaner. Examples of such methods are described in U.S. Pat. No. 5,403,479 and Japanese Patent Application No. 2-248,836 in which chemical cleaning is performed without draining the tank or removing the membranes from the tank. Permeation is stopped and the membranes are cleaned by flowing a chemical cleaner in a reverse direction through the membranes while the membranes are simultaneously agitated. Although effective, these methods leave residual chemicals in the tank. In wastewater applications, the chemicals interfere with useful biological process in the tank water. In drinking water applications, the chemicals pass through the membranes when permeation is resumed resulting in unwanted concentrations of chemicals in the permeate. Further, some chemical cleaner disperses in the tank water during the cleaning event thus increasing the amount of chemical cleaner required. 
     French Patent No. 2,741,280 describes another method of backwashing membranes with a chemical cleaner. In this method, the tank water is drained before the chemical backwash begins. When the chemical backwash is over, the cleaner is drained from the tank and the tank is refilled. In this way, the chemical cleaner does not contaminate the tank water or permeate. In a typical municipal installation, however, the tank may range from 1 m to 10 m in depth. The chemical cleaner inside the lower membranes or the lower portions of vertical membranes may be subject to a local pressure up to 100 kPa higher than the local pressure of the chemical cleaner inside the upper membranes or the upper portions of vertical membranes. Since the flow of chemical cleaner through the membranes is dependant on the local pressure of the chemical cleaner inside the membranes, the flow rate of chemical cleaner varies considerably between the upper and lower membranes. As a result, either insufficient cleaner is supplied to the upper portions of the membranes or excess cleaner is supplied to the lower portions of the membranes. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method and apparatus for chemical cleaning of immersed microfiltration and ultrafiltration membranes. 
     According to an embodiment of the invention, the tank is first drained, a chemical cleaner is backwashed through the membranes, the cleaner is preferably removed from the tank, and the tank is refilled so that permeation may continue. A backwash pump which drives the chemical cleaner is controlled by a speed controller which is in turn connected to a programmable logic control and, preferably, pressure and flow indicators. The backwash pump is operated to supply the chemical cleaner to the membranes in pulses. 
     The pressure of the pulses is selected to be high enough to reduce the relative size of the local pressure differentials in the system, including local pressure differentials between upper and lower membranes or portions of membranes. The duration and frequency of the pulses is chosen to provide an appropriate contact time of the chemical cleaner, preferably without allowing the membranes to dry between pulses and without using excessive amounts of chemical cleaner. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the present invention will now be described with reference to the following figures. 
     FIG. 1 is a schematic diagram of an embodiment of the invention. 
     FIGS. 2,  3  and  4  show alternate arrangements of portions of the embodiment in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Referring now to FIG. 1, a reactor  10  is shown for treating a feed water  14  having solids. A feed pump  12  pumps feed water  14  to be treated from a water supply  16  through an inlet  18  to a tank  20  where it becomes tank water  22 . In an industrial or municipal reactor  10 , the tank  20  is typically between 1 m and 10 m deep. During permeation, the tank water  22  is maintained at a level which covers one or more membranes  24 . Each membrane  24  has a permeate side  25  which does not contact tank water  22  and a retentate side  27  which does contact the tank water  22 . 
     Membranes  24  made of hollow fibres are preferred although the membranes  24  may be of various other types such as tubular, ceramic, or flat sheet. For hollow fibre membranes  24 , the retentate side  27  of the membranes  24  is preferably the outside of the membranes and the permeate side  25  of the membranes  24  is preferably their lumens. The membranes  24  are held between two opposed headers  26 . The ends of each membrane  24  are surrounded by potting resin to produce a watertight connection between the outside of the membranes  24  and the headers  26  while keeping the lumens of the hollow fibre membranes  24  in fluid communication with at least one header  26 . The membranes  24  and headers  26  together form a membrane module  28 . Similar modules can be created with tubular membranes in place of the hollow fibre membranes  24 . For flat sheet membranes, pairs, of membranes are typically attached to headers or casings that create an enclosed surface between the membranes. A plurality of modules may also be joined together and referred to as a cassette. 
     To collect permeate, the headers  26  are connected to a permeate collector  30  and a permeate pump  32  through a permeate valve  34 . Referring to FIG. 2, a plurality of membrane modules  28  may be connected to a common permeate collector  30 . Depending on the length of the membranes  24  and the depth of the tank  20 , the membrane modules  28  shown in FIG. 2 may also be stacked one above the other. Referring to FIGS. 3 and 4, membrane modules  28  are shown in alternate orientations. In FIG. 3, the membranes  24  are oriented in a horizontal plane and the permeate collector  30  is attached to a plurality of membrane modules  28  stacked one above the other. In FIG. 4, the membranes  24  are oriented horizontally, in a vertical plane. Depending on the depth of the headers  26  in FIG. 4, the permeate collector  30  may also be attached to a plurality of these membrane modules  28  stacked one above the other. 
     Although a small number of hollow fibre membranes  24  are illustrated in each membrane module  28  of FIGS. 1 through 4, a typical module may have a large number of fibres arranged in skeins. For example, the membrane modules  28  illustrated in FIGS. 2 through 4 comprise rectangular skeins  8  each typically having a mass of hollow fibre membranes  24  between 2 cm and 10 cm wide. The hollow fibre membranes  24  typically have an outside diameter between 0.4 mm and 4.0 mm and are potted at a packing density between 10% and 40%. The hollow fibre membranes  24  are typically between 400 mm and 1,800 mm long and mounted with between 0.1% and 5% slack. The membranes  24  have an average pore size in the microfiltration or ultrafiltration range, preferably between 0.003 microns and 10 microns and more preferably between 0.02 microns and 1 micron. 
     Referring again to FIG. 1, when permeate pump  32  is operated and permeate valve  34  opened, a negative pressure is created on the permeate side  25  of the membranes  24  relative to the tank water  22  surrounding the membranes  24 . The resulting transmembrane pressure, typically between 1 kPa and 150 kPa, draws tank water  22  (then referred to as permeate  36 ) through membranes  24  while the membranes  24  reject solids which remain in the tank water  22 . Thus, filtered permeate  36  is produced for use at a permeate outlet  38  through an outlet valve  39 . Periodically, a storage tank valve  64  is opened to admit permeate  36  to a storage tank  62 . Tank water  22  which does not flow out of the tank  20  through the permeate outlet  38  flows out of the tank  20  through a drain valve  40  in a retentate outlet  42  to a drain  44  as retentate  46  with the assistance of a retentate pump  48  if necessary. The retentate  46  may be withdrawn from the tank  20  either continuously or periodically. 
     During permeation, solids accumulate on the surface of the membranes  24  and in their pores, fouling the membranes  24 . Physical techniques may prevent some of this fouling. Firstly, the membranes  24  may be agitated, possibly by mechanically agitating the tank water  22  near the membranes  24  but preferably by aerating the tank water  22  near the membranes  24 . For this, an aeration system  49  has an air supply pump  50  which blows air from an air intake  52  through air distribution pipes  54  to one or more aerators  56  located generally below the membrane modules  28  which disperses air bubbles  58  into the tank water  22 . The air bubbles  58  agitate the membranes  24  and create an air-lift effect causing tank water  22  to flow upwards past the membranes  24 , all of which inhibits fouling of the membranes  24 . 
     In addition to aeration, the membranes  24  are backwashed periodically. For this, permeate valve  34  and outlet valve  39  are closed while backwash valves  60  are opened. Permeate pump  32  pushes filtered permeate  36  from storage tank  62  through a backwash pipe  63  to the headers  26  and through the walls of the membranes  24  in a reverse direction thus pushing away some of the solids attached to the membranes  24 . At the end of the backwash, backwash valves  60  are closed and permeate valve  34  and outlet valve  39  re-opened. Such backwashing may occur for a period of 15 seconds to one minute approximately every 15 minutes to an hour. Permeate  36  may be stored in a permeate tank  37  to even out minor disruptions in the flow of permeate  36 . As an alternative to using the permeate pump  32  to drive the backwash, a separate pump can also be provided in the backwash line  63  which may then by-pass the permeate pump  32 . 
     As mentioned earlier, backwashing and the use of air bubbles  58  to clean the membranes  24  fails to effectively inhibit all types of fouling, particularly fouling caused by solids deposited inside the membrane pores and solids chemically or biologically attached to the membranes. This type of fouling is countered by chemical cleaning. 
     To clean the membranes  24  with chemical cleaner, permeation is temporarily stopped, permeate valve  34 , outlet valve  39  and backwash valves  60  are all closed and permeate pump  32  is turned off. Feed pump  12  is turned off and tank water  22  is drained out of the tank  20  by opening drain valves  40  and turning retentate pump  48  on if necessary. When the level of the tank water  22  is below the membranes  24 , chemical cleaner is flowed through the walls of the membranes  24 . The chemical cleaner used may be any chemical appropriate for the application and not overly harmful to the membranes  24 . Typical chemicals include sodium hypochlorite, citric acid and sodium hydroxide. The chemical cleaner may be used in a non-liquid form such as by flowing chemical in a gaseous state to the headers  26  or introducing it as a solid into the backwash line  63 . Liquid chemical cleaners are preferred, however, because they are easier to handle and inject in the proper amounts. 
     To flow chemical cleaner through the walls of the membranes  24 , chemical valve  66  is opened and chemical pump  67  turned on to flow chemical cleaner from chemical tank  68  to backwash line  63 , headers  26  and through the walls of the membranes  24 . A lower header cut-off valve  110  is preferably closed so that chemical cleaner flows only into the upper header  26 . In each cleaning event, the chemical pump  67  is turned on and off repeatedly to provide the chemical cleaner in pulses. In each pulse, the chemical pump  67  is turned on for between 10 seconds and 120 seconds, preferably about 60 seconds for drinking water applications, and turned off for between 30 seconds and five minutes, preferably about three minutes for drinking water applications. 
     Preferably, the time that the chemical pump  67  is turned off approximates the time required for a dose of chemical to either flow out of the pores of the membranes  24  or to be substantially consumed through reactions with solids such that the membranes  24  are no longer effectively wetted with chemical cleaner. This time may vary with the packing density and configuration of the membrane module  28 , the diameter of the membranes  24  and other factors. Providing too short a time between pulses wastes chemical cleaner by forcing it into the tank  20  prematurely while providing too long a time between pulses wastes process time because the chemical cleaner is not sufficiently efficacious for the entire time. Conversely, the time that the chemical pump  67  is turned on preferably approximates the time required to effectively re-wet the membranes  24  to an initial wetness. In this way, chemical cleaner contacts the membranes  24  for substantially the duration of the cleaning event. 
     The pressure of the pulses is preferably high enough to substantially reduce the relative size of head losses in the system, differences in head loss across parts of membranes  24  with different permeabilities because of uneven fouling and differences in local pressure inside the lumens of the membranes  24  caused by differences in elevation in the tank  20 . With less variation in the flow of chemical cleaner from one part of the membranes  24  to another, less chemical cleaner is required to achieve a minimum level of cleaning throughout the membranes  24 . The pressure for the backwash typically ranges from between 10 and 55 kPa. 
     The pulsed chemical cleaner delivery is particularly beneficial for modern submerged outside-in hollow fibre membranes  24  which may be between 1 meter to 3 meters in length, resulting in significant pressure drop in the lumens of the membranes  24 , but having unfouled permeability of a few hundred litres per square meter per hour per bar of transmembrane pressure (L/m 2 /h/bar) or more. In particular, with chemical cleaner flowing into the upper header  26  only of a membrane module  28  with vertical hollow fibre membranes  24 , the head loss in the lumens of the membranes  24  assists in reducing the flow of chemical cleaner through the lower portions of the membranes  24  which, as explained above, tend to receive too much chemical cleaner. With such membranes  24  and chemical cleaner flowing into upper headers  26  only, and depending on the expected fouled permeability of the membranes  24 , the pressure of an effective backwash can be near the lower limit of the range specified above and corresponds to an average flux of between 30 and 55 L/m 2 /h. 
     For example, a ZW 500 membrane module manufactured by ZENON Environmental Inc. has vertical hollow fibre membranes approximately 1650 mm in length. In a test with partially fouled fibres having a permeability of 250 L/M 2 /h/bar and backwashing from the top header only, backwashing at 7 kPa resulted in a flux of chemical cleaner through the membranes varying from about 17 L/m 2 /h at the top of the membranes to about 39 L/m 2 /h at the bottom of the membranes. Backwashing at 22 kPa resulted in a flux of about 54 L/m 2 /h at the top, about 50 L/M 2 /h near the middle and about 61 L/M 2 /h near the bottom of the fibres. Thus backwashing at 22 kPa substantially reduced the variation in flux across different parts of the membranes. 
     The pressure of the pulses is controlled by altering the speed of the chemical pump  67  with a speed controller  100 . Based on the expect ed permeability of the membranes  24  when fouled, the flux through the membranes at a given pressure can be calculated. From this flux the speed of the chemical pump  67  can also be calculated. The speed controller  100  can thus be set to run the chemical pump  67  at this speed during the parts of the chemical backwash cycle during which the chemical pump  67  is on. 
     Preferably, the speed controller  100  is controlled by a programmable logic controller  102 . The programmable logic controller (PLC)  102  is programmed to turn the chemical pump  67  on and off in repeated cycles for the duration of the cleaning event. The PLC  102  starts each on portion of a cleaning event with the chemical pump  67  at the speed calculated above. Optionally, a pressure gauge  104  senses the pressure in the backwash line  63  and converts this information to an analog current or potential signal, preferably a 4-20 mili-amp current signal, proportional to the pressure. The PLC  102  converts this signal to a pressure reading and compares the pressure reading to the desired pressure which is entered into the PLC  102  by an operator. Based on the comparison, the PLC  102  in turn sends an analog current or potential signal, preferably a 4-20 mili-amp current signal, to the speed controller  100 . The speed controller  100  changes the frequency of the electric current to the chemical pump  67  in proportion to the signal presented by the PLC  102 , which changes the speed of the chemical pump  67 , and hence, the chemical cleaner flux and pressure. If the pressure is below the desired value, the speed of the chemical pump  67  is increased by the PLC  102  and conversely decreased if the pressure is too high. In this way, increases in the permeability of the membranes  24  as they are cleaned are compensated for by increasing the speed of the chemical pump  67 . 
     Further optionally, a flow sensor  106  in the backwash line  63  measures the increase in chemical flux caused by such increases in speed of the chemical pump  67  and converts this information to an analog current or potential signal, preferably a 4-20 mili-amp current signal proportional to the flux. The, PLC  102  converts this signal to a flux reading. As the chemical flux increases, the time taken to re-wet the membranes  24  decreases. Accordingly, the PLC  102  is programmed to shorten the length of time during which the chemical pump  67  is turned on as the flux of chemical cleaner increases. 
     After the chemical cleaning is completed, chemical pump  67  remains, turned off and chemical valve  66  is closed. Preferably, the backwash valves  60  are opened and permeate pump  32  operated to provide a rinsing backwash to remove chemical cleaner from the backwash line  63  and permeate collectors  30 . Drain valves  40  are then closed and feed pump  12  turned on to refill the tank  20 . 
     The effectiveness of a chemical cleaning event may be approximated by multiplying the concentration “C” of the chemical cleaner and the time, “T”, that the chemical cleaner effectively wets the membranes  24  to create a third parameter “CT”. The preferred CT for each event is selected by an operator according to his or her preferred chemical cleaning regimen, for example a maintenance cleaning regimen as will be described below. Once the CT is selected, a concentration of chemical cleaner is selected. In possible alternative embodiments, the chemical cleaner may be diluted before it reaches the membranes  24 . For example, with appropriate modifications to the procedure and apparatus above, backwash valves  60  can also be opened and permeate pump  32  used to flow permeate  36  through backwash line  63  where it mixes with chemical cleaner from the backwash line  63 . The concentration of the chemical cleaner is therefore measured as the chemical cleaner meets the permeate side  25  of the membranes  24  and is typically between 20 and 200 mg/L when NaOCl is used. Once C is known, T can be calculated and entered into the PLC  102  which is programmed to start a timer with the first pulse of chemical cleaner and continue to provide chemical cleaner pulses until T is reached on the timer. More typically, however, T is made to be an even multiple of a selected time between pulses and the PLC is programmed to provide a selected number of pulses. 
     In a maintenance cleaning regimen, the cleaning events are started before the membranes  24  foul significantly and are repeated between one and seven times a week, preferably between 2 and 4 times a week. For drinking water applications, each cleaning event involves between 5 and 30 pulses, preferably between 6 and 10 pulses times, with a total duration between 10 and 100 minutes, preferably about 30 minutes. Since the cleaning events may be repeated with varying frequency for different applications or concentrations of solids in the feed water  14 , a parameter called the weekly CT is used as a basis for some calculations. The weekly CT is the sum of the CT parameters for the cleaning events performed during a week. 
     The desired weekly CT is preferably chosen to maintain acceptable permeability of the membranes  24  or to reduce the rate of decline in permeability of membranes  24  over extended periods of time, preferably between 15 days and three months, so as to reduce the frequency of intensive recovery cleanings rather than to provide recovery cleaning itself. In some drinking water applications, however, intensive recovery cleanings can be postponed almost indefinitely. There may be a slight instantaneous increase in permeability of the membranes  24  after a cleaning event, but this permeability gain is typically lost before the next cleaning event and is not significant enough to be considered recovery cleaning. 
     For drinking water applications, the weekly CT is preferably in the range of 2,000 min*mg/L to 20,000 min*mg/L when NaOCl is the chemical cleaner and more preferably between 5,000 min*mg/L and 10,000 min*mg/L of NaOCl. When other chemical cleaners are used, the concentration of the chemical cleaner is expressed as an equivalent concentration of NaOCl that has similar cleaning efficacy. For example, for citric acid preferred values are approximately 20 times those given for NaOCl and for hydrochloric acid preferred values are approximately 4 times the values given for NaOCl. The precise weekly CT to use in a given application is preferably chosen to achieve a gradual decline in permeability over an extended period of time. 
     It is to be understood that what has been described are preferred embodiments to the invention. The invention nonetheless is susceptible to certain changes and alternative embodiments fully comprehended by the spirit of the invention as described above, and the scope of the claims below.