Abstract:
Carbon dioxide, liberated by introducing fouling inhibiting bubbles into a tank containing an immersed membrane module, is captured and returned to the tank by way of the bubbles to minimize increases in pH in the tank water caused by carbon dioxide stripping. Minimizing the pH increase reduces the amount of acid required to produce a desired pH in the tank water or, with scaling feed water, reduces the rate of membrane fouling.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to the use of filtering membranes to treat water, and more particularly to the design and operation of reactors which use membranes immersed in tanks and aerated to inhibit fouling.  
         BACKGROUND OF THE INVENTION  
         [0002]    An immersed membrane apparatus and process is described in U.S. Pat. No. 5,639,373. The immersed membranes are used for separating a permeate lean in solids from tank water rich in solids. Feed water having an initial concentration of solids flows into an open tank containing the immersed membranes to keep the membranes submerged. Filtered permeate passes through the walls of the membranes under the influence of a suction applied to the inside of the membranes. As filtered water is permeated through the membranes and removed from the system, the solids are rejected and accumulate in the tank. These solids are removed from the tank by draining appropriate amounts of tank water containing a high concentration of solids.  
           [0003]    Over time, solids foul the pores of the membranes and reduce their permeability. To inhibit this fouling, the membranes in U.S. Pat. No. 5,639,373 are backwashed from time to time and are aerated from beneath the membranes either continuously or periodically. Bubbles rise past the membranes to scrub and agitate them. Although backwashing and aeration inhibit fouling, fouling is not eliminated completely and still occurs. In feed waters of various types, fouling remains a serious problem that interferes with the use of immersed filtering membranes.  
         SUMMARY OF THE INVENTION  
         [0004]    The inventors have noticed that aerating filtering membranes immersed in a tank liberates carbon dioxide and thereby causes an increase in the pH of the tank water. In some process, such as coagulation, which require a certain and generally low pH, increased acid may need to be applied through to maintain a desired pH. In other process, particularly filtration of well water where the feed water is hard, removing carbon dioxide causes scaling due to CaCO 3  precipitation.  
           [0005]    It is an object of the present invention to provide a process and apparatus which captures and recycles gases, particularly carbon dioxide, liberated by aerating an immersed membrane module. It is a further object of the invention to minimize increases in pH in the tank water surrounding an immersed membrane module caused by aeration and, more particularly, by carbon dioxide stripping resulting from aeration. Increases in pH are undesirable for various reasons. For example, membrane performance often suffers at a pH above about 8.0. Processes such as coagulation provide better organic matter removal (which is desirable itself but also improves membrane performance) within certain pH ranges which may be equal to or lower than the pH of the feed water. Hard or scaling feed water (for example, feed water with a Langelier Scaling Index of greater than 0.5) fouls membranes rapidly if its pH is increased. Minimizing a further pH increase through aeration reduces these undesirable effects or reduces the amount of acid required to produce a desired pH in the tank water.  
           [0006]    In one aspect, the invention provides a reactor having one or more modules of filtering membranes located within a tank. Feed water is introduced to the tank through a feed inlet. A source of transmembrane pressure to the one or more modules produces a permeate on the insides of the immersed membranes. An aeration system supplies bubbles to the tank to inhibit fouling of the membranes. Retentate is removed from the tank through a retentate outlet. A gas recirculation system collects the off-gas from the tank and returns the collected gases to the tank, typically by returning the collected gases to the aeration system.  
           [0007]    Preferably, the gas recirculation system includes a lid closely fitted to the tank so as to collect gases liberated from preferably substantially the entire surface area of the feed water in the tank. Optionally, the lid may be substantially sealed to the tank. The collected gases include carbon dioxide. Preferably, 80% or more of the carbon dioxide liberated from the water in the tank is returned to the tank, preferably through the bubbles. The aeration system may include a gas dryer operable to dry collected gases before they are returned to a blower of the aeration system. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    Preferred embodiments of the invention will now be described below with reference to the following FIGURE:  
         [0009]    [0009]FIG. 1 is a schematic representation of a reactor according to an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0010]    [0010]FIG. 1 shows a reactor  10  having a tank  12  which is filled with feed water  14  through an inlet  16 . The feed water  14  may contain microorganisms, suspended solids or other matter which will be collectively called solids, although some rejected matter may not actually be solid. The feed water  14  is typically supplied to the tank  12  through a variable speed feed pump or by gravity through a valve. Once in the tank  12 , the feed water  14  may still be referred to as feed water  14  but will be also referred to below as tank water  18  because it typically has increased concentrations of the various solids. When treating various feed waters  14 , chemical additives  15  may be added through a chemical inlet  17 . For example, chemicals may be added to flocculate or coagulate solids or otherwise alter the tank water  18  to make it easier to separate filtered permeate from the solids.  
         [0011]    One or more membrane modules  20  are mounted in the tank  12 . The membrane modules  20  are made so as to separate an inner surface of the membranes from an outer surface of the membranes. A suitable membrane module  20  is described in U.S. Pat. No. 5,639,373 which is incorporated into this document by this reference. The membrane module 20 described in the &#39;373 patent uses hollow fibre membranes suspended generally vertically between rectangular headers. Other membrane modules  20  may have one or two headers of various shapes and may orient the hollow fibres generally horizontally. Yet other membrane modules  20  may use flat sheet membranes which are typically oriented vertically in a spaced apart pair with headers on four sides and means to communicate with the resulting interior surface. Further, many membrane modules  20  may be joined together to form larger membrane modules  20 , or cassettes, but all such configurations will be referred to as membrane modules  20 . The membranes in the membrane modules  20  preferably have a pore size in the microfiltration or ultrafiltration range, more preferably between 0.003 and 10 microns.  
         [0012]    Commercially available membrane modules  20  include those based on ZW 500 or ZW 650 units made by ZENON Environmental Inc. and referred to in the examples further below. Each ZW 500 or ZW 650 unit has two rectangular skeins of hollow fibre membranes having a pore size of approximately 0.1 microns oriented generally vertically with a total membrane surface area of approximately 47 and 61 square meters respectively.  
         [0013]    Filtered water called permeate  24  flows through the walls of the membranes in the membrane modules  20  under the influence of a transmembrane pressure and is transported to a permeate outlet  26  through a permeate line  28 . The transmembrane pressure is preferably created by a permeate pump  30  which creates a partial vacuum in a permeate line  28 . Feed water  14  flows into the tank  12  as required to keep the membrane modules  20  immersed in tank water  18  typically at all times while the permeate pump  30  is on. The permeate pump  30  or another pump may also be used to backwash the membranes as is known in the art.  
         [0014]    As filtered permeate  24  is produced, the membranes in the membrane modules  20  reject solids which remain in the tank water  18 . These solids may be removed by draining the tank  12  periodically or continuously to remove a portion of the tank water  18  which is replaced with new feed water  14 . To drain the tank, a drain valve  32  is opened in a drain conduit  34  at the bottom of the tank  12 .  
         [0015]    An aeration system  37  has one or more aerators  38  connected by an air delivery system  40  to one or more air blowers  42  and produces bubbles  36  in the tank water  18 . The aerators  38  may be of various types known in the art, for example holes drilled in conduits. The aerators  38  are located generally below the membrane modules  20 . The bubbles  36  agitate the membranes which inhibits their fouling or cleans them. In addition, the bubbles  36  also decrease the local density of tank water  18  in or near the membrane modules  20 . This creates an air-lift effect and causes tank water  18  to flow upwards past the membrane modules  20  and then downwards along the sides or other parts of the tank  12 . The bubbles  36  typically burst at the surface and do not generally follow the tank water  18  back down to the bottom of the tank  12 .  
         [0016]    The bubbles  36  have an average diameter between 0.1 and 50 mm. Individual large bubbles  36  are believed to be more effective in cleaning or inhibiting fouling of the membranes, but smaller bubbles  36  are more efficient in transferring oxygen to the tank water  18  and require less energy to produce per bubble  36 . Bubbles  36  between 3 mm and 20 mm, and more preferably between 5 mm and 15 mm in diameter, are suitable for use in many wastewater applications. If the reactor  10  is used to create potable water or for other applications where oxygen transfer is not required, then bubbles between 5 mm and 25 mm are preferred.  
         [0017]    The amount of aeration provided is dependant on numerous factors but is preferably related to the superficial velocity of air flow if aeration is continuous. The superficial velocity of air flow is defined as the rate of air flow to the aerators  38  at standard conditions (1 atmosphere and 20 C) divided by the cross sectional area of aeration. The cross sectional area of aeration is determined by measuring the horizontal area effectively aerated by the aerators  38  which is often roughly one half of the horizontal area of the tank. Superficial velocities of air flow of between 0.01 m/s and 0.15 m/s are preferred with the air supplied continuously or intermittently in cycles of less than about 120 seconds in duration. An average superficial velocity is preferably chosen to achieve a desired effect against fouling without regard to the amount of carbon dioxide that may be released through aeration, because that carbon dioxide will be mostly recycled and returned to the tank water  18 .  
         [0018]    While scouring the membranes, the bubbles  36  also strip carbon dioxide from the tank water  18  as long as the partial pressure of carbon dioxide in the bubbles is less than that corresponding to the carbon dioxide concentration in the tank water  18 , the partial pressure and the concentration being related by Henry&#39;s law. The amount of carbon dioxide removed from the tank water  18  if the carbon dioxide is released to the atmosphere is primarily a function of the dissolved carbon dioxide present in the raw water, the hydraulic retention time of the tank  12  and the amount of aeration. For example, carbon dioxide stripping is often significant when filtering groundwater since groundwater is often very high in dissolved carbon dioxide or when filtering surface waters that have had acid added to them.  
         [0019]    Shifts in pH of the tank water  18  resulting from carbon dioxide stripping are minimized, however, by capturing and recycling a substantial portion of the carbon dioxide that would otherwise be liberated by aeration. A lid  50  is placed over top of the tank  12 . The lid  50  may be a single piece or may be made of several plates, preferably made of aluminum or fiber reinforced plastic, placed over the tank  12  to cover its surface. The lid  50  preferably closely covers the tank  12  but does not need to create an air tight seal with the tank  12 . Optionally, however, the lid  50  may be sealed to the tank  12 . A recycle line  52  connects the space in the tank  12  between the tank water  18  and the lid  50  with an inlet  43  of the blower  42 . Optionally, the inlet  43  of the blower  42  can also intake air from the atmosphere generally through an outside air inlet  62  and an outside air inlet valve  60 . Further optionally, gases may be exhausted from the air delivery system  40  through an air exhaust port  61  and an exhaust valve  63 . An air dryer  54  is optionally provided in the recycle line  52  upstream of the intake to the blower  42 . Further optionally, a drain line  56  which may be opened with a drain valve  58  to release liquids collected by the air dryer  54 .  
         [0020]    The amount of carbon dioxide recovered can vary depending on the tightness of the lid  50 , the extent to which atmospheric air is taken into or gases are exhausted from the aeration system  37  or the tank  12 , and the amount of dissolved gases removed from the system by the air dryer, if any. For example, the aeration system  37  can be configured such that the pressure of the gases above the tank water  18  is slightly above atmospheric which causes some carbon dioxide to escape if the lid  50  is not sealed to the tank  12 . In this case, the outside air inlet  60  may be used to control the flow of air from the atmosphere into the tank  12  while the exhaust valve  63  is omitted or kept closed. Alternatively, the aeration system  37  can be configured such that the pressure of the gases above the tank water  18  is slightly below atmospheric which causes some air from the atmosphere to enter if the lid  50  is not sealed to the tank  12 . In this case, the exhaust valve  63  may be used to control the flow of gases to the atmosphere while the outside air inlet  60  is omitted or kept closed. With the lid  50  sealed to the tank  12 , the gases above the tank water  18  may be either slightly above or slightly below atmospheric pressure and the exhaust valve  63 , if any, and/or outside air inlet  60 , if any, adjusted, if desired, to reduce the amount of carbon dioxide recycled or account for matter removed by the air dryer  54 .  
         [0021]    Even without a lid  50  completely sealed to the tank  12 , typically 80% or more, and more typically 90% or more, of the carbon dioxide liberated to the upper part of the tank  12  is recycled to the aerators  38 . While sealing the lid  50  to the tank  12  helps achieve high rates of carbon dioxide recycle, it is also mechanically difficult and costly to achieve in a tank  12  generally designed to hold tank water  18  at ambient pressure. Accordingly, it is often preferable to size and configure the aeration system such that the pressure in the tank  12  above the tank water  18  is very close to ambient pressure and use a lid  50  that is not completely sealed to the tank  12 . Further, it is not always desirable to achieve highest possible rate of carbon dioxide recycle. In some cases, particularly when the reactor  10  is the last stage in a treatment process, some carbon dioxide stripping is desirable to reduce the corrosiveness of the permeate  24 . In these cases, a more moderate rate of carbon dioxide recycle may be preferred.  
         [0022]    Water vapour and will also be entrained in the flow through the recycle line  52 . The water vapour is optionally removed by the air dryer  54 . The air dryer  54  has a cooling coil which condenses water vapour and rejects the water collected although other types of air driers may be used. Removing water vapour from the recycle line  52  reduces corrosion of the blower  42  but also removes some carbon dioxide which might otherwise be recycled to the tank  12 . Thus, alternatively, exposed parts of the blower  42  may be coated with or made of corrosion resistant materials and the air dryer  54  omitted.  
       EXAMPLE  
       [0023]    A reactor containing 60 ZW 650 ultrafiltration membrane modules in a single tank was used to filter feed well water ultimately intended for use as drinking water. The characteristics of the feed water were as follows:  
                                                       pH   7.4 to 7.45           Hardness   350-500 mg/L as CaCO 3             Alkalinity   250-350 mg/L as CaCO 3             Turbidity   0.1-0.4 NTU           Color   &lt;5 Pt Co. units                      
 
         [0024]    The Langelier Saturation Index of the feed water was greater than 0.5 indicating that the feed water had a tendency to scale. Air was provided continuously at a rate of 15 cubic feet per minute (at standard conditions) of 900 cubic feet per minute total.  
         [0025]    During the first two weeks of operation, the pH of the permeate averaged approximately 8.3. The increased pH (over that of the feed water) was believed to be rapidly fouling the membranes by scaling. A lid and recycle loop were installed as described above. Thereafter, the average pH of the permeate dropped to 7.55. The flux was kept substantially constant at between 27 and 29 gfd both with an without carbon dioxide recycle. Before carbon dioxide recycle was added, the average increase in transmembrane pressure to maintain the selected flux was approximately 0.2 psi/day. Following the installation of the carbon dioxide recirculation system, the average rise in transmembrane pressure reduced to approximately 0.11 psi/day.  
         [0026]    The invention is not limited to the embodiment described above. For example, the inventors believe that the invention could be adapted to a fully closed system in which the transmembrane pressure is created by pressurizing the feed water. The scope of the invention is defined by the following claims.