Abstract:
Various filtration processes using low amounts of aeration are disclosed. One process comprises a cycle of permeating and then backwashing, aerating, partially draining the tank and refilling the tank. Another process comprises steps of (a) permeating and withdrawing retentate; (b) after (a), backwashing; and (c) during (a), providing gentle aeration. Another process comprises a cycle of (a) permeating; (b) after (a), backpulsing; (c) during (b) and extending into a portion of (a), aerating; and, (d) during a portion of (a), withdrawing retentate

Description:
[0001]     This is an application claiming the benefit under 35 USC 119(e) of U.S. Provisional Patent Application Ser. No. 60/633,432 filed Dec. 7, 2004. U.S. Ser. No. 60/633,432 is incorporated herein, in its entirety, by this reference to it. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates to membrane separation devices and processes as in, for example, water filtration using membranes.  
       BACKGROUND OF THE INVENTION  
       [0003]     Typically a batch filtration process has a repeated cycle of concentration, or permeation, and deconcentration steps. During the concentration step, permeate is withdrawn from a fresh batch of feed water initially having a low concentration of solids. As the permeate is withdrawn, fresh water is introduced to replace the water withdrawn as permeate. During this step, which may last from 10 minutes to 4 hours, solids are rejected by the membranes and do not flow out of the tank with the permeate. As a result, the concentration of solids in the tank increases, for example to between 2 and 100, more typically 5 to 50, times the initial concentration. The process then proceeds to the deconcentration step. In the deconcentration step, which is typically between 1/50 and ⅕ the duration of the concentration step, a large quantity of solids are rapidly removed from the tank to return the solids concentration back to the initial concentration. This may be done by completely draining the tank and refilling it with new feed water. To help move solids away from the membranes themselves, air scouring and backwashing may be used before or during the deconcentration step.  
         [0004]     An alternate process is a feed and bleed process. In a feed and bleed process, feed water flows generally continuously into a tank. Permeate is withdrawn generally continuously, but may be stopped from time to time for example for backwashing. Retentate is generally continuously bled out of the tank. The average flow rate of retentate may be 1-20% of the feed flow rate, the remainder of the feed flow being removed as permeate. Aeration may be provided continuously or intermittently during permeation.  
       SUMMARY OF THE INVENTION  
       [0005]     It is an object of the invention to provide an apparatus and method for treating water. It is another object of the invention to provide a membrane separation device and process. The following summary is intended to introduce the reader to the invention and not to define the invention, which may reside in a sub-combination of the following features or in a combination involving features described in other parts of this document, for example the claims.  
         [0006]     The invention provides various filtration processes. The filtration processes may be used, for example, in new plants or as a retrofit for existing plants such as feed and bleed plants with continuous aeration. After retrofitting an existing feed and bleed plant with continuous aeration, the invention may reduce the amount of aeration required at an acceptable cost to implement the changes.  
         [0007]     In one aspect, the invention provides a cyclical process in which, after a dead end permeation period, the membranes are backpulsed and aerated. After the backpulsing, a portion of the tank, for example about 10-25% of the tank, is drained. Aeration may continue during this partial drain. After the partial drain, the tank is refilled and permeation begins in the next cycle.  
         [0008]     In another aspect, the invention provides a process having a generally continuous reject bleed. Permeation is also generally continuous, but is stopped periodically, for example for backwashing. Aeration is provided during this backwash and intermittently between backwashes.  
         [0009]     In another aspect, the invention provides a cyclical process in which permeation is generally continuous but for periodic backwashing. Aeration is provided during the backwash but continues for a period of time after the backwash. Retentate flow occurs during the backwash and continues beyond the backwash and aeration but for less than the entire cycle duration. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     Embodiments of the invention will now be described with reference to the following figures.  
         [0011]      FIG. 1  is a schematic diagram of a filtration apparatus.  
         [0012]      FIGS. 2, 3 , and  4  are representations of various membrane cassettes.  
         [0013]      FIGS. 5, 6  and  7  are diagrams of processes according the embodiments of the invention. 
     
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
       [0014]     The following description of a filtration apparatus applies generally to the embodiments which are described further below unless inconsistent with the description of any particular embodiment.  
         [0015]     Referring now to FIGS.  1  to  4 , a reactor  10  is shown for treating a liquid feed having solids to produce a filtered permeate with a reduced concentration of solids and a retentate with an increased concentration of solids. Such a reactor  10  has many potential applications, but will be described below as used for creating potable water from a supply of water such as a lake, well, or reservoir. Such a water supply typically contains colloids, suspended solids, bacteria and other particles or substances which must be filtered out and will be collectively referred to as solids whether solid or not.  
         [0016]     The first reactor  10  includes a feed pump  12  which 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 . Alternatively, a gravity feed may be used with feed pump  12  replaced by a feed valve. Each membrane  24  has a permeate side  25  which does not contact the tank water  22  and a retentate side which does contact the tank water  22 . The membranes  24  may be hollow fibre membranes  24  for which the outer surface of the membranes  24  is the retentate side and the lumens of the membranes  24  are the permeate side  25 .  
         [0017]     Each membrane  24  is attached to one or more headers  26  such that the membranes  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 permeate side  25  of the membranes  24  in fluid communication with a permeate channel in at least one header  26 . Membranes  24  and headers  26  together form an element  8 . The permeate channels of the headers  26  are connected to a permeate collector  30  and a permeate pump  32  through a permeate valve  34 . Air entrained in the flow of permeate through the permeate collectors  30  becomes trapped in air collectors  70 , typically located at at least a local high point in a permeate collector  30 . The air collectors  70  are periodically emptied of air through air collector valves  72  which may, for example, be opened to vent air to the atmosphere when the membranes  24  are backwashed. 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 . The filtered permeate  36  may require post treatment before being used as drinking water, but should have acceptable levels of colloids and other suspended solids.  
         [0018]     In a large reactor  10 , a plurality of elements  8  are assembled together into cassettes  28 . Examples of such cassettes  28  are shown in  FIGS. 2,3  and  4  although a cassette  28  would typically have more elements  8  than shown. Each element  8  of the type illustrated may have a bunch between 2 cm and 10 cm wide of hollow fibre membranes  24 . Other sorts of elements  8  and cassettes  28  may also be used. The membranes  24  may have an average pore size in the microfiltration or ultrafiltration range, for example between 0.003 microns and 10 microns or between 0.02 microns and 1 micron.  
         [0019]     Referring to  FIG. 2 , for example, a plurality of elements  8  are connected to a common permeate collector  30 . Depending on the length of the membranes  24  and the depth of the tank  20 , multiple cassettes  28  as shown in  FIG. 2  may also be stacked one above the other. Referring to  FIGS. 3 and 4 , the elements  8  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 elements  8  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 cassettes  28  stacked one above the other. The representations of the cassettes  28  in  FIGS. 2, 3 , and  4  have been simplified for clarity, actual cassettes  28  typically having elements  8  much closer together and many more elements  8 .  
         [0020]     Cassettes  28  can be created with elements  8  of different shapes, for example cylindrical, and with bunches of looped fibres attached to a single header or fibers held in a header at one end and loose at the other. Similar modules or cassettes  28  can also 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 and allow appropriate piping to be connected to the interior of the enclosed surface. Several of these units can be attached together to form a cassette of flat sheet membranes. Commercially available cassettes  28  include those made by ZENON Environmental Inc. and sold under the ZEE WEED trademark, for example, as ZEE WEED 500 or ZEE WEED 1000 products.  
         [0021]     Referring again to  FIG. 1 , tank water  22  which does not flow out of the tank  20  through the permeate outlet  38  flows out of the tank  20  at some time through a drain valve  40  and a retentate outlet  42  to a drain  44  as retentate  46  with the assistance of a retentate pump  48  if necessary.  
         [0022]     To provide air scouring, alternately called aeration, an air supply pump  50  blows ambient air, nitrogen or other suitable gases from an air intake  52  through air distribution pipes  54  to aerator  56  or sparger which disperses scouring bubbles  58 . The bubbles  58  rise through the membrane module  28  and discourage solids from depositing on the membranes  24 . In addition, where the design of the reactor  10  permits it, the bubbles  58  also create an air lift effect which in turn circulates the local tank water  22 .  
         [0023]     To provide backwashing, permeate valve  34  and outlet valve  39  are closed and backwash valves  60  are opened. Permeate pump  32  is operated to push filtered permeate  36  from retentate tank  62  through backwash pipes  61  and then in a reverse direction through permeate collectors  30  and the walls of the membranes  24  thus pushing away solids. At the end of the backwash, backwash valves  60  are closed, permeate valve  34  and outlet valve  39  are re-opened and pressure tank valve  64  opened from time to time to re-fill retentate tank  62 .  
         [0024]     To provide chemical cleaning from time to time, a cleaning chemical such as sodium hypochlorite, sodium hydroxide or citric acid is provided in a chemical tank  68 . Permeate valve  34 , outlet valve  39  and backwash valves  60  are all closed while a chemical backwash valve  66  is opened. A chemical pump  67  is operated to push the cleaning chemical through a chemical backwash pipe  69  and then in a reverse direction through permeate collectors  30  and the walls of the membranes  24 . At the end of the chemical cleaning, chemical pump  67  is turned off and chemical pump  66  is closed. Preferably, the chemical cleaning is followed by a permeate backwash to clear the permeate collectors  30  and membranes  24  of cleaning chemical before permeation resumes.  
         [0025]     To fill the tank  20 , a feed pump  12  pumps feed water  14  from the water supply  16  through the inlet  18  to the tank  20  where it becomes tank water  22 . The tank  20  is filled when the level of the tank water  22  completely covers the membranes  24  in the tank  20  but the tank  20  may also have tank water  22  above this level.  
         [0026]     To permeate, the permeate valve  34  and an outlet valve  39  are opened and the permeate pump  32  is turned on. 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 the membranes  24  while the membranes  24  reject solids which remain in the tank water  22 . Thus, filtered permeate  36  is produced for use at the permeate outlet  38 . Periodically, a storage tank valve  64  is opened to admit permeate  36  to a storage tank  62  for use in backwashing. As filtered permeate  36  is removed from the tank, the feed pump  12  is operated to keep the tank water  22  at a level which covers the membranes  24  accounting for retentate removal during permeation, if any, or removal of foam or other substances, if any.  
         [0027]     To backwash the membranes, alternately called backpulsing or backflushing, with permeation stopped, backwash valves  60  and storage tank valve  64  are opened. Permeate pump  32  is turned on to push 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 . The volume of water pumped through the walls of a set of the membranes  24  in the backwash may be between 10% and 40%, more often between 20% and 30%, of the volume of the tank  20  holding the membranes  24 . At the end of the backwash, backwash valves  60  are closed. 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 . By either means, the backwashing may continue for between 15 seconds and one minute. When the backwash is over, permeate pump  32  is then turned off and backwash valves  60  closed. The flux during backwashing may be 1 to 3 times the permeate flux and may be provided continuously, intermittently or in pulses.  
         [0028]     To provide scouring air, alternately called aeration, the air supply pump  50  is turned on and blows air, nitrogen or other appropriate gas from the air intake  52  through air distribution pipes  54  to the aerators  56  located below, between or integral with the membrane elements  8  or cassettes  28  and disperses air bubbles  58  into the tank water  22  which flow upwards past the membranes  24 .  
         [0029]     The amount of air scouring to provide is dependant on numerous factors but is preferably related to the superficial velocity of air flow through the aerators  56 . The superficial velocity of air flow is defined as the rate of air flow to the aerators  56  at standard conditions (1 atmosphere and 25 degrees Celsius) divided by the cross sectional area effectively scoured by the aerators  56 . Scouring air may be provided by operating the air supply pump  50  to produce air corresponding to a superficial velocity of air flow between 0.005 m/s and 0.15 m/s. At the end of an air scouring step, the air supply pump  50  is turned off. Although air scouring is most effective while the membranes  24  are completely immersed in tank water  22 , it is still useful while a portion of the membranes  24  are exposed to air. Air scouring may be more effective when combined with backwashing.  
         [0030]     Air scouring may also be provided at times to disperse the solids in the tank water  22  near the membranes  24 . This air scouring prevents the tank water  22  adjacent the membranes  24  from becoming overly rich in solids as permeate is withdrawn through the membranes  24 . For this air scouring, air may be provided continuously at a superficial velocity of air flow between 0.0005 m/s and 0.015 m/s or intermittently at a superficial velocity of air flow between 0.005 m/s and 0.15 m/s.  
         [0031]     To drain the tank  20 , also called rejection, reject removal or bleed, the drain valves  40  are opened to allow tank water  22 , then containing an increased concentration of solids and called retentate  46 , to flow from the tank  20  through a retentate outlet  42  to a drain  44 . The retentate pump  48  may be turned on to drain the tank more quickly, but in many installations the tank will empty rapidly enough by gravity alone, particularly where a reject bleed is desired during permeation. It may take between two and ten minutes to drain the tank  20  completely from full and less time to partially drain the tank  20 .  
         [0032]      FIG. 5  shows a first process. Permeation begins at T 0  and continues to T 1 . The time between T 0  and T 1 , which may be 15 to 40 minutes for example, may be dead end permeation, that is permeation without withdrawal of retentate. At T 1 , permeation stops and backpulsing and aeration begin. Backpulsing and aeration continue for 15 seconds to 5 minutes or 30 seconds to 90 seconds until T 2 . At T 2 , backwashing stops and a partial drain or refill of the tank begins. During the drain/refill, a portion, for example 10-25%, of the normal volume of tank water  22 , for example the average volume of water present during permeation, is drained from the tank and then replaced with fresh feed water. Parts of the membranes may be exposed during these steps. These steps may take for example from 30 seconds to 5 minutes and end with T 0  at the start of the next cycle. Aeration may continue until a time T 3  occurring during the drain/refill step. Compared to a continuously aerated feed and bleed process, the process of  FIG. 5  may allow a 90-95% reduction in the amount of aeration required while still handling medium to high solid loadings, for example a TSS of 1000 mg/L. Although the plant must be modified or built to provide for rapid partial drains and refill, the process requires less modification or drain and feed capacity than a batch process having a complete tank drain and refill steps.  
         [0033]      FIG. 6  shows another process. At T 0 , the membranes are backwashed and aerated until T 1 . The time between T 0  and T 1  may be about, for example 10 seconds to 60 seconds or about 15 seconds. The backpulse and aeration need not occur exactly at the same time, or for the same duration of time, as shown. At T 1 , permeation and aeration for resuspension begin. As shown, the aeration may be intermittent, for example 5-20 seconds or about 10 seconds every 1 to 4 minutes or about 2 minutes at the regular aeration rate. Alternately, continuous aeration at a reduced rate may be provided. A generally continuous bleed or reject is provided generally throughout the cycle. The cycles may last, for example for between 10 and 20 minutes or about 15 minutes.  
         [0034]     Compared to a continuously aerated feed and bleed process, aeration may be reduced by about 80-85%. Only modifications to the aeration system are required. However, the process may result in reduced fluxes or occasional sludging of the membranes in medium or high solids concentration plants, although it may be adequate for low to medium solids concentration plants.  
         [0035]      FIG. 7  shows another process. Backpulsing, aeration and rejection begin at T 0 . Backpulsing stops, for example after 10-30 seconds or, about 15 seconds, at T 1  and permeation begins. Aeration continues until T 2 , which may be, for example about 60-120 seconds or about 90 seconds after T 0 . Reject removal continues until T 3 . After T 3 , reject removal stops while permeation continues to T 0  of the next cycle. T 3  is chosen to include a period after T 2  when the TSS concentration in the reject remains elevated due to the backpulsing and aeration, which may be, for example about 5 to 10 minutes or about 7.5 minutes after T 0 . The rate of reject removal may be chosen, or T 3  extended, to achieve a desired volumetric removal of retentate. Alternately, if reject removal until T 3  does not remove enough volume of tank water, rejection may begin again prior to T 0 . The total cycle time may be, for example about 10-20 minutes or about 15 minutes and reject may be withdrawn for, for example about ⅔ or ½ or less of the duration of the cycle.  
         [0036]     Compared to a continuously aerated feed and bleed process, this method may reduce aeration by 80% or more. The plant or design must be modified to accept increased reject flow rates, for example 150% or twice or more of the design flow of a continuous bleed plant, but those modifications are less than for a batch process with full tank drainings. The process can handle medium to high solids loadings.  
         [0037]     In the paragraphs above, comparisons with a continuously aerated feed and bleed process assumed that the continuously aerated feed and bleed process uses aeration in a 10 seconds on 10 seconds off cycle throughout permeation. A low solids level has an after flocculation feed solids level of less than 5 mg/L. A high solids level has an after flocculation feed solids level of over 25 mg/L. A medium solids level is between these two.  
         [0038]     The preceding description was of exemplary embodiments only and does not limit the scope of the invention, which may be practiced with various modifications.