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. The housing is integrated with the potting head of a module. The conduit passes through the potting head.

Description:
FIELD 
     This specification relates to a gas sparger and to gas scouring to inhibit fouling of a filtering 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 burst of large bubbles can be used to break up a fouling film, gel or cake formed on a membrane, or accumulated around the membrane. Once the fouling structure is ruptured, less intense aeration at the end of a burst, or provided by other aerators between bursts, can continue to remove the foulants. The instantaneous gas flow rate during a burst may be 1.25 to 10 times that of conventional gas sparging. The duration of the burst of gas may be between 1 and 10 seconds. The frequency of the bursts may be from once every 2 seconds to once every 24 hours. Bursts may be created by temporarily increasing the gas pressure or flow of an existing aeration system, by a secondary gas sparging system or, as will be described below, by accumulating gas in a device configured to periodically release the accumulated gas. 
     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. One or more gas spargers may be integrated or combined with a membrane module. Bubbles can be released in bursts within or at the sides, or both, of a bundle of hollow fiber membranes. 
     A potting head, or a permeate collector or gas conduit below a potting head, provides the top of a housing to collect a pocket of gas. A conduit passing through the potting head releases at least some of the gas from the pocket when the pocket reaches a sufficient size. Optionally, a cover or diffuser above the potting head and over an outlet from the conduit may direct the released gas or break up the released gas into smaller (though still large) bubbles or both. Even if fed with a continuous supply of gas, the sparger produces discrete periods of bubble flow, typically in the form of short bursts of large bubbles. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  shows a schematic side view of four spargers immersed in a liquid at various stages in an aeration process. 
         FIG. 2  shows an isometric view of an alternate conduit as in a sparger of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows four spargers  10  integrated with the potting heads  40  of four membrane modules A, B, C and D. A potting head  40  may alternately be called a header. Each potting head  40  is typically a block of a potting material such as a hardened resin. The ends of a plurality of hollow fiber membranes  42  are potted into the potting head. In the case of modules A, B and C, the ends of the membranes  42  are plugged in the potting head  40 . A second potting head (not shown) is provided at the other ends of the membranes  42  to withdraw permeate from the lumens of the membranes  42 , for example by way of a suction applied to a permeate cavity in communication with the lumens of the membranes  42 . In module D, the ends of the membranes  42  are open to a permeate cavity  44  which is in turn connected to a permeate withdrawal pipe  46 . The upper ends of the membranes  42  in module D may be individually plugged and loose (not held in a potting head), plugged collectively into one or more upper potting heads, or potted in a second permeating potting head. The bottom of the potting head  40 , or a mold for the potting head  40 , or the bottom of the permeate cavity  44 , or the bottom of a gas distribution conduit (not shown) below the permeate cavity  44 , defines the top of a housing  12  below the potting head  40 . The housing  12  also has walls extending below the potting head  40  to define an open bottomed plenum below the potting head  40 . 
     A sparger  10  receives a flow of a gas, typically air, from a gas distribution pipe  18 . The gas is discharged below or directly into the sparger  10  through one or more gas outlets  20  in communication with the distribution pipe  18 . The distribution pipe  18  may be located near the bottom of sparger  10  as shown or at other elevations. For example, an alternative distribution pipe  18 ′ may be connected to a gas conduit  50 ′ formed by placing a horizontal wall below and parallel to the bottom of the potting head  40  or the bottom of the permeate cavity  44  as shown for module D. In the case of module D, the gas conduit  50 ′ and permeate cavity  44  may each be connected to one or more adjacent modules such that the gas pipe  18 ′ and permeate pipe  46  serve multiple modules without being directly connected to all of them. The gas distribution pipe  18  may also be located above the module, with a gas line dropping down to the sparger  10 . 
     A sparger  10  has a discharge conduit  22  passing through the potting head  40 . The discharge conduit has a first outlet  24  in communication with an area inside and near the top of the housing  12 , and a second outlet  26  open to the outside of the housing  12  above the potting head  40 . 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. If the cross-sectional area of a pocket of gas is small relative to the area of the second opening  26 , then the low point of the conduit  22  and the walls of the housing  12  may be made lower to increase the volume of air in a pocket of gas in communication with the conduit  22 . 
     A cap  48  or diffuser  52  may optionally be provided over the potting head  40  in communication with the second opening  26 . The diffuser  52  may be, for example, a chamber with a plurality of holes  54  to cause a flow of air from the conduit  22  to break up into smaller bubbles. The cap  48  directs the flow of gas from the conduit downwards to the upper face of the potting head  40  or across the potting head  44  and may also cause gas flowing from the conduit  22  to break up into smaller bubbles. A solid cap  48  extending below the second opening  26  as shown may tend to trap a pocket of gas below the cap  48 , which may interfere with the re-flooding of the conduit  22 . If this occurs, holes may be provided in the cap  48  above the second opening, the lower edge of the cap  48  may be scalloped to provide horizontal openings near or above the height of the second opening  26 , or the second opening may be lowered relative to the bottom of the cap  48 , or the cap  48  may be raised relative to the second opening  26 . 
     The operation of a sparger  10  immersed in a liquid  34  is illustrated schematically in  FIG. 1  in that parts A, B. C and D each show a sparger  10  at four different points in a sequence that occurs in a single sparger  10  as a gas is fed into it. The sequence progresses from the conditions shown for 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. 1 , 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. 
     Optionally, the conduit  22  may have a third opening, or an open tube  56  pointing downwards. Such an opening or tube  56  may help the conduit flood between the stages of Parts C and D but is typically not necessary. A third opening may also allow for an air-lift to be created in the part of the conduit from the third opening to the second outlet  26  to create a two phase gas-liquid discharge from the conduit  22 . This may be useful if, for example, a module has a problem with liquid circulation near the top of the potting head  40 . However, the inventors believe that creating a two phase flow also reduces the cleaning effect of the bubbles and so prefer a discharge that consists essentially of gas and any liquid that must be initially forced out of the conduit  22  to allow the gas to flow through the conduit. 
     The features of modules A, B, C and D, and the additional optional features described below, may be selected, mixed or combined together into any possible permutation or combination. The potting heads  40  may be round, square or rectangular in plan view for example. A second opening  26  may be located in the center, in plan view, of a round potting head, either as the only second opening  26  or in combination with a ring of additional second openings  26 . A large module may have a large potting head that supports multiple spargers  10 , for example as if the modules A, B, C and D were merged together to have a common potting head  44  but multiple spargers  10 . A large potting head  44  with a plurality of conduits  22  may have the conduits distributed along the length of the potting head  44 , across the width of the potting head  44 , or both. A single conduit  22  may have two or more second openings  26 , for example an opening on each side of a module or an opening in the middle and at each side of a module.  FIG. 2  shows a further alternate conduit  22  having multiple second openings  26 .