Abstract:
A gas sparger produces an intermittent flow of bubbles even if provided with a continuous gas flow. The sparger has a housing to collect a pocket of gas and a conduit to release some of the gas from the pocket when the pocket reaches a sufficient size. Optionally, a cover over an outlet from the conduit may break up or distribute the released gas. A large sparger for use with a commercial membrane module can comprise a plurality of smaller units.

Description:
FIELD 
     This specification relates to a gas sparger and to gas scouring to inhibit fouling of an immersed membrane. 
     BACKGROUND 
     The following background discussion is not an admission that anything discussed below is citable as prior art or common general knowledge. 
     International PCT publication WO/2000/021890 describes an aeration system for a submerged membrane module that has a set of aerators connected to an air blower, valves and a controller adapted to alternately provide a higher rate of air flow and a lower rate of air flow in repeated cycles to individual aerators. In an embodiment, the air blower, valves and controller, simultaneously provide alternating air flows to two or more sets of aerators such that while the total system air flow is constant, allowing the blower to be operated at a constant speed, each aerator receives a flow of air that varies over time. In some embodiments, the flow of air to an aerator occurs in repeated cycles of short duration. Transient flow conditions result in the tank water which helps avoid dead spaces and assists in agitating the membranes. WO/2000/021890 is incorporated herein in its entirety by this reference to it. 
     INTRODUCTION 
     The following discussion is intended to introduce the reader to the more detailed discussion to follow, and not to limit or define any claim. 
     The air cycling process described in WO/2000/021890 has proven to be very effective at reducing the amount of air or other gas, and therefore energy, required to operate an immersed membrane system. It was noted in WO/2000/021890 that rapid valve movements result in very large bubbles being created for a brief period of time, and that these very large bubbles might help inhibit membrane fouling. However, it was also noted in WO/2000/021890 that creating these large bubbles required producing undesirable pressure spikes in the aeration system. A gas sparger, alternately called an aerator, will be described below that produces an intermittent flow of bubbles even when provided with a continuous gas flow. The flow of bubbles can be in the form of short bursts of very large bubbles, but rapid valve movements are not required. 
     The sparger has a housing to collect a pocket of gas and a conduit to release at least some of the gas from the pocket when the pocket reaches a sufficient size. Optionally, a cover over an outlet from the conduit may distribute the released gas, and may also break up the gas into bubbles, or smaller bubbles, if the gas was initially released in a more nearly bulk form. A large sparger for use with a commercial membrane module or cassette can comprise a plurality of smaller units. Even if fed with a continuous supply of gas, the sparger produces discrete periods of bubble flow, optionally in the form of short bursts of large bubbles. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  shows a partially sectioned side elevation view of a sparger. 
         FIG. 2  shows a top view of the sparger of  FIG. 1 . 
         FIG. 3  shows an end view of the sparger of  FIG. 1 . 
         FIG. 4  shows an isometric view of the sparger of  FIG. 1 . 
         FIG. 5  shows a schematic side view of four spargers immersed in a liquid at various stages in an aeration process. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 to 4  show a sparger  10  in various views. Sparger  10  has a housing  12  defining an interior chamber bounded by an upper surface. The housing  12  shown is elongated, with its length being more than twice its width, and has a generally inverted “U” cross section, although other shapes may also be used. The housing  12  shown has a connection  14  at one end. Connection  14  can be fit into or over a port in a gas supply manifold (not shown) to provide gas to the sparger  10  and to hold one end of the sparger  10  in a selected position immersed in a liquid. The other end of the sparger  10  may be held in a selected position immersed in a liquid by a pin  16  extending from the housing  12 . 
     The connector  14  is connected to one or more distribution pipes  18 . Distribution pipes  18  extend generally along the length of the sparger  10  and have gas outlets  20  along their length. The size of the gas outlets  20  may be made sufficiently small relative to the gas flow rate so as to (a) create a head loss that encourages an even distribution of gas flow from the gas outlets  20  even if the distribution pipe  18  is not exactly level and (b) cause a sufficient velocity of gas flow through the gas outlets  20  to inhibit liquid entry into the distribution pipe  18 . The Distribution pipes  18  may be located near the bottom of sparger  10  as shown or at other elevations. For example, distribution pipes  18  may be located along the top of housing  12 , with the outlets  20  in an area that always contains a pocket of gas. Further optionally, different parts of the housing  12  may receive gas from separate gas tubes connected to a gas supply manifold located further away from the housing  12 . 
     The sparger  10  has a plurality of discharge conduits  22  along its length. Discharge conduits  22  have first outlets  24  in communication with an area inside and near the top of the housing  12 , and second outlets  26  open to the outside of the housing  12 . At least a portion of the conduit  22  extends downwards between the first opening  24  and the second opening  26 . Another portion of conduit  22  extends upwards again before reaching the second opening  26 . Gas leaving the housing  12  through the conduit  22  must pass through a low point in the conduit  22  between the first opening  24  and the second opening  26 , as in the generally J or U shaped conduits  22  shown. Second opening  26  may have an area of 1 to 10 square cm or 3 to 6 square cm. The cross-sectional area of a pocket of gas in communication with a conduit  22  is preferably larger than the area of the second opening  26  by a factor of 10 or more, for example by a factor in the range of 20 to 35. As shown for example in  FIG. 1 , each discharge conduit  22  forms a closed channel between first outlet  24  and second outlet  26 . 
     Adjacent conduits  22  are preferably separated from each other, for example by dividers  28 . The dividers  28  prevent one conduit  22  from depleting a pocket of gas in housing  12  to the extent that gas is rarely or never discharged from another one of the conduits  22 . With solid dividers  28  extending to below the lowest expected extent of a gas pocket in housing  12  as shown, gas pockets associated with different conduits  22  are fluidly separated from each other. The sparger  10  acts as if it is a number of distinct smaller spargers. Over a period of operation, the timing of gas discharges from different conduits  22  in a sparger  10  may vary or scatter such that gas is not discharged from each conduit  22  at the same time. However, the pattern of gas discharge from an individual conduit appears to follow a generally regular cycle having a short burst of gas followed by a period in which gas is not discharged, or is discharged in only small amounts. 
     A cover or distributor  30  may optionally be provided over the housing  12 . Cover  30  receives gas from one or more discharge conduits  22  and discharges gas in the form of bubbles from holes  32  in the cover  30 . Cover  30  may have a plurality of holes  32  per conduit  22  to disperse the gas flow over a larger horizontal area. The cover  30  may also break a burst of gas leaving conduit  22  into bubbles or smaller bubbles if desired. As shown, the cover  30  may have dividers generally aligned with dividers  18  in the housing  12  to keep a flow of bubbles near the conduit  22  that released the gas for those bubbles. Optionally, holes  32  may be distributed either along the length of the housing  12  or across the width of housing  12  or both to spread the flow of bubbles as desired for one or more immersed membrane modules intended to be scoured by the bubbles. A module may be located above the sparger  10  in a tank. Optionally, the tube sheet of a module having air passages through the tube sheet between the membranes can function as the cover  30 . 
     Cover  30  shown snaps over the housing  12  without making a gas tight seal with the top of housing  12 . However, in the embodiment shown, the housing  12  and cover  30  both have a dome shape in cross section such that a gap between the cover  30  and housing  12  is located below the top of housing  12 . With this arrangement, gas does not escape through the gap between the cover  30  and housing  12  at the gas flow rates tested by the inventors. The volume contained within cover  30  is preferably small, for example about 50% or less, or 33% or less, of the volume of an associated air pocket in housing  12 . This tends to maintain the short burst characteristics of the gas leaving a conduit  22 . 
     The operation of a sparger  10  immersed in a liquid  34  is illustrated schematically in  FIG. 5 . Parts A, B, C and D of  FIG. 5  show a sparger  10  at four different points in a sequence of events that occurs in the sparger  10  as a gas is fed into it. The sequence progresses from condition A to B to C to D and then returns to condition A and repeats for as long as a supply of a gas is provided to a sparger  10 . In Part A of  FIG. 5 , a conduit  22  is flooded with liquid  34 , although a pocket of gas  36  may be trapped in the housing  12 . In Part B, the pocket of gas  36  grows in size as gas from distribution pipe  18  is collected in housing  12  and displaces liquid  34 . Liquid  34  leaves the housing  12  through an opening to the bottom of the housing  12  and through conduit  22 . In Part C, after the expanding pocket of gas  36  extends below the upper bound of a low point in conduit  12 , a path is created for gas to flow from the pocket  36  and through the conduit  22 , and gas is discharged outside of the housing  12 , for example in bubbles  38 . In Part D, gas continues to flow through the conduit  22 , liquid  34  re-enters the housing  12  and the pocket  36  becomes smaller. Returning to Part A, the liquid  34  inside of the housing  12  eventually reaches the conduit  22 , the conduit  22  floods, and gas flow through the conduit  22  stops. The process then repeats, producing discrete periods of gas discharge even when gas is supplied continuously. The periods of gas discharge tend to be near an average duration and frequency. However, the precise timing, volume and duration of a gas discharge may vary within a range around the average, for example, with waves or other movement of the liquid or the discharge of gas from other spargers  10 . 
       FIGS. 1 to 4  are drawn to scale. The sparger  10  is 85 mm wide, 139 mm high and 770 mm long. These dimensions are given to provide an example of a workable sparger, but the invention is not limited to these dimensions. The sparger  10  shown was designed to replace an aerator tube normally provided below a cassette of ZeeWeed™ 500 membrane modules by GE Water and Process Technologies, and to use the same fittings. These modules are intended for immersed, suction driven operation. The module has many hollow fiber membranes with a total surface area of about 200 to 525 square feet. The membranes are oriented vertically between a pair of elongated potting heads. The modules are generally rectangular in plan view, having a length about the same as the length of the sparger  10 . The modules are arranged into cassettes in which several modules placed side by side in a frame separated by vertical gaps between adjacent modules. One sparger  10  is placed about 1 to 10 cm below every second module and oriented parallel with the module. Holes  32  are positioned to direct bubbles into the gaps on either side of the module. Each sparger  10  provides bubbles to the both sides of the module above it, and to one side of the adjacent modules on both sides of that module. When fed with air at about 4 cubic feet per minute, the sparger  10  shown releases bursts of bubbles lasting for about 1 or 2 seconds about every 8 seconds. Increasing or decreasing the rate of gas flow to the sparger  10  has very little, if any, effect on the duration of the burst of bubbles, but decreases or increases the time between bursts. Dimensions, ratios of dimensions, gas flows and process parameters within a range of plus to minus 50% of the values provided in this document are expected be suitable for typical commercial immersed suction driven membrane applications but other dimensions, relative proportions and gas flow rates may also be useful. Other variations are also possible. For example, a square or circular sparger  10 , optionally divided into sections appropriate for those shapes, may be used for modules of other shapes. Conduit  22  may be one of a variety of shapes that provide the required passage.