Patent Description:
This specification relates to a gas delivery device for use, for example, in supplying bubbles to inhibit fouling of an immersed filtering membrane.

International Publication Number <CIT>, Gas Sparger for a Filtering Membrane, describes a gas sparger that 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. A large sparger can be divided into a plurality of units each having a conduit. A gas supply pipe has at least one hole aligned with each unit to deliver air to each of the units. <CIT> is directed to a gas sparger for an immersed membrane and claims the same priority as International Publication Number <CIT>. <CIT> is directed to an inverted cavity aerator for a membrane module which instead of a sparger, makes use of an aerator shell in the shape of an inverted box which has a series of holes. Similar to the sparger of <CIT> and International Publication Number <CIT>, a gas supply pipe having a plurality of holes is used to deliver air to the inverted cavity aerator. More simply, International Publication Number<CIT> discloses the simple use of aerators comprising holes in a gas supply pipe.

A method of air scouring an immersed membrane is described in this specification. The method comprises scouring an immersed membrane module comprising adjusting one or more aeration parameters; between successive permeation cycles, successive back pulse cycles or successive relaxation cycles; during a permeation cycle; between a permeation cycle and a backpulse cycle; or, between a permeation cycle and a relaxation cycle; further comprising steps of,.

the inlet <NUM>, port <NUM> and outlets <NUM> being located at generally the same elevation but spaced horizontally and the pressurized gas flows generally horizontally in the channels. method may be used with a gas delivery device described in this specification in which a supply of gas is provided to a manifold with multiple ports connected to multiple conduits.

In a gas sparger as described in International Publication Number <CIT>, a unit of the sparger that receives a larger flow rate of input gas will produce pulses of bubbles at a higher frequency. In order to uniformly clean a membrane cassette, it may be desirable to have each unit operate at near the same frequency. The holes of the gas supply pipe are made small to help equalize the gas flow rate between holes feeding different units of the sparger. However, if the gas supply pipe is installed out of level by as few as <NUM> over a length of about <NUM>, the holes at higher elevation will have a noticeably larger gas flow rate. In addition, solids entering the gas supply pipe during maintenance periods when the gas supply is turned off can dry out or agglomerate when the gas is turned back. Occasionally, a solid particle is formed in the gas supply tube that is large or rigid enough to be lodged into one of the holes and to restrict or block the hole. A partially or completely blocked hole will in turn lead to poor distribution of gas to the membranes and allow solids to accumulate on the membranes. A gas delivery device will be described below that can be used as an alternative to such a gas supply pipe either with or without a further gas sparger.

<FIG> show different views of a gas delivery device <NUM>. Alternatively, the gas delivery device <NUM> may be called an aerator or a sparger. In use, the gas delivery device <NUM> is immersed in a liquid, typically water, for example activated sludge. Pressurized gas is supplied to an inlet <NUM> of the gas delivery device and is emitted as bubbles from a plurality of outlets <NUM>. The gas is typically air, but in some applications another gas, for example biogas, nitrogen, ozone or oxygen may be used. The gas delivery device <NUM> shown has four outlets <NUM>, but there may alternatively be more or less outlets <NUM>.

The inlet <NUM> is separated from the outlets <NUM> by a plurality of ports <NUM>. Each outlet <NUM> communicates with a port <NUM> through a channel <NUM>. Part of the gas delivery device <NUM> from the inlet <NUM> to the ports <NUM> functions as a manifold <NUM>, alternatively called a plenum, to distribute the gas entering through the inlet <NUM> among the channels <NUM>. The inlet <NUM>, ports <NUM> and outlets <NUM> are located at generally the same elevation but spaced horizontally. The gas flows generally horizontally in the channels <NUM>.

The area of the ports <NUM> is less than the area of the channels <NUM>, or less than the area of the smallest of the channels <NUM> if they have different areas. For example, the channels <NUM> may have a cross sectional area that is three times or more than the cross sectional area of the ports <NUM>. The ports <NUM> restrict the flow of gas into the channels <NUM>. The restriction provided by the ports <NUM> helps to distribute the total airflow more nearly equally among the channels <NUM>. Decreasing the area of the ports <NUM> produces a more nearly equal flow in the channels <NUM> but also increases head loss through the ports <NUM>. The ports <NUM> may be made all of the same area. The area of the ports <NUM> may be reduced until the flow is adequately distributed among the channels <NUM>. Optionally, a port <NUM> opening into a long or narrow channel <NUM> may be larger than a port <NUM> opening into a short or wide channel <NUM> to help equalize the flow among the channels <NUM>. Alternatively, one or more ports <NUM> may be made larger than other ports <NUM> to intentionally increase the relative airflow through one or more channels <NUM>. This may be done, for example, to provide more air to the extremities of an immersed membrane cassette to counteract a tendency for water to be lifted preferentially through the center of a cassette.

As shown in <FIG>, the ports <NUM> are located close to each other in the horizontal direction. In this way, if the gas delivery device <NUM> is mounted a few degrees out of level, there is very little difference in elevation between the ports <NUM>. In particular, the largest horizontal distance between two ports <NUM> is less than the average horizontal distance between adjacent outlets <NUM>, or less than half of the average horizontal distance between adjacent outlets <NUM>. The largest horizontal distance between the ports <NUM> is also less than <NUM>%, or less than <NUM>%, of the largest distance from a port <NUM> to an outlet <NUM>. This helps produce a more nearly equal distribution of the gas among the channels <NUM> compared to an ordinary aerator in the form of a tube with holes when the gas delivery device <NUM> is mounted out of level. Because the ports <NUM> are primarily responsible for equalizing flow between channels <NUM>, the outlets <NUM> can be made larger, for example as large as the cross sectional area of the channels <NUM>, so that any solids that accumulate in a channel <NUM> are unlikely to block the outlet <NUM>.

The gas delivery device <NUM> has its outlets <NUM> spaced generally in a line. Alternatively, other configurations may be used. For example, channels <NUM> could extend along a line but in both directions from the inlet <NUM>. In another example, the channels <NUM> could radiate from the inlet <NUM> like spokes from a wheel hub.

Optionally, the top of the channels <NUM> may be pointed slightly upwards. In this way, if the gas delivery device is inadvertently mounted with a slightly downwards slant, then gas will not be trapped in the channels <NUM> when the supply of gas is off. A slight upwards slant may also help compensate for differences between the lengths of the channels <NUM>.

Referring to <FIG>, the gas delivery device <NUM> may be used, for example, to provide bubbles for scouring an immersed membrane module <NUM>. The membrane module <NUM> contains a plurality of ultrafiltration or microfiltration membranes. Permeate is withdrawn from the membrane module by way of a suction pump in communication with the inside of the membranes. A typical filtration cycle comprises periods of filtration interrupted by backwashing, alternatively called backpulsing, procedures or relaxation periods in which there is no filtration. A device with a line of outlets <NUM> is particular suited for providing bubbles to membrane modules with rectangular elements such as flat sheet modules or ZeeWeed™ hollow fiber elements sold by GE Water & Process Technologies.

The gas delivery device <NUM> is immersed in a tank <NUM> containing one or more membrane modules <NUM>. The gas delivery device <NUM> may be mounted separately in the tank <NUM> or attached to the membrane modules <NUM>. Gas may be brought down into the tank from a riser pipe <NUM> and then spread horizontally through as header <NUM>. Saddles <NUM> attached to the header <NUM> receive gas from the header and carry the gas to a line of gas deliver devices <NUM> oriented perpendicularly to the header <NUM> in a generally horizontal plane. Optionally, a gas delivery device <NUM> may be connected directly to a header <NUM> or riser pipe <NUM>. Streams of bubbles <NUM> are discharged from the outlets <NUM> at various lateral positions relative to a membrane module <NUM>. The gas flowing to each lateral position bypasses any intermediate lateral positions. The bubbles <NUM> may be allowed to rise directly to the membranes to clean them or inhibit fouling. Alternatively, a transducer may be placed above the gas delivery device <NUM> to modify its output before the bubbles reach the membranes. For example, a diffuser may be placed over an outlet to disperse the bubbles over a wider area.

<FIG> illustrates another transducer option in which an intermittent gas sparger <NUM>, for example of the type shown in International Publication Number <CIT>, is associated with the gas delivery device. Pressurized gas <NUM> is split in the gas delivery device into four bubble streams <NUM>. Each bubble stream <NUM> rises into a different cavity <NUM> of the intermittent air sparger <NUM>. Gas flowing through a conduit <NUM> to a particular cavity <NUM> bypasses any intervening cavities <NUM>.

Each cavity <NUM> has a discharge conduit <NUM>, in the form a J-shaped tube in the example of <FIG>, which acts like an inverted siphon to discharge intermittent pulses of air from the cavity <NUM>. Bubbles emitted from the gas delivery device <NUM> first collect in the cavity <NUM> forming a pocket of gas in the top of the cavity <NUM>. No gas is emitted from the cavity <NUM> until the pocket of gas expands to reach the low point of the discharge conduit <NUM>. At that time, the pocket of gas empties out of the cavity <NUM> through the conduit <NUM> and the process repeats. In this way, a continuous stream of bubbles <NUM> from the gas delivery device <NUM> is converted into an intermittent flow of bubbles from the intermittent gas sparger <NUM>.

In <FIG>, the gas delivery device <NUM> is shown mounted separately and below the intermittent gas sparger <NUM>. Alternatively, the gas delivery device <NUM> may be mounted to the intermittent gas sparger <NUM>. In the example shown, the inlet <NUM> may be fitted into a receptacle <NUM> of the intermittent gas sparger <NUM>. A fastener (not shown) is then placed through an eyelet <NUM> on the gas delivery device <NUM> and into an abutment <NUM> on the intermittent gas sparger <NUM>. This results in the gas delivery device <NUM> being located partially within the intermittent gas sparger <NUM>. However, the outlets <NUM> are below the conduits <NUM> and still discharge into water below the lower limit of the pockets of gas in the cavities <NUM>.

<FIG> is an isometric cross sectional view of the bottom of an alternative intermittent sparger <NUM>. In this example, multi-port conduits <NUM> provide two or more outlet paths extending upwards from the low point of each multi-port conduits <NUM>. A divider <NUM> between adjacent multi-port conduits <NUM> has a slot <NUM> extending from the bottom of the divider <NUM> to above the low point of the multi-port conduits <NUM>. Each cavity with a multi-port conduit <NUM> replaces two cavities with a single outlet conduit and so avoids a need to balance the supply of gas between the two replaced cavities. The slot <NUM> in the divider <NUM> helps equalize the air supply to the cavities. Gas may flow in either direction through the slot <NUM> but the net flow will be from a cavity that receives a larger gas flow to a cavity that receives a lower air flow.

The gas delivery device <NUM> is preferably an open-bottomed structure. For example, the channels <NUM> are formed by side walls and a top. The channels <NUM> are open at the bottom and, preferably, at their ends. The outlets <NUM> are defined by the open end of the channels <NUM>. The manifold <NUM> between the inlet <NUM> and the ports <NUM> is preferably also open at the bottom. The ports <NUM> are preferably slots also open at the bottom of the gas delivery device <NUM>. In this way, solids caught anywhere in the gas delivery device <NUM> beyond the inlet <NUM> can fall or be expelled downwards out of the gas delivery device <NUM>. Having such a short and simple pathway for solids to leave helps prevent fouling in the gas delivery device <NUM>. In the event that solids somehow still accumulate in the gas delivery device, the open-bottomed structure makes it easy to locate and remove the solids, for example by spraying water into the bottom of the gas delivery device <NUM>.

The open-bottomed construction of the gas delivery device <NUM> also helps accommodate a range of input gas flow rates. At low flow rates, water enters into the gas delivery device <NUM> and reduces the size of the ports <NUM> and channels <NUM>. At higher gas flow rates, less water enters into the gas delivery device <NUM> and the ports <NUM> and channels <NUM> increase in size. The gas delivery device <NUM> can be made to provide a well distributed flow of air at supplied air flow rates that vary, for example, from a low flow rate to a high flow rate that is two or more times as large as the low flow rate. The gas delivery device <NUM> may also be operated at one or more distinct intermediate flow rates or at flow rates that vary smoothly over time. This is done without the gas delivery device <NUM> fouling rapidly or providing excessive back pressure at either flow rate. In comparison, aerators in the form of a horizontal tube with a series of holes can foul and provide a poor distribution of air at low flow rates and excessive back pressure at high flow rates.

<FIG> shows a method of operating a membrane filtration system having a membrane module <NUM> immersed in a tank <NUM> as shown in <FIG>. The filtration system may be part of a membrane bioreactor (MBR). The operation consists of periods of permeation followed by periods of either relaxation or backpulse, alternatively call backwash. The periods of permeation may be from <NUM> to <NUM> times as long as the periods of backwashing or relaxation. The membranes are scoured with bubbles from a gas delivery device <NUM>, optionally in combination with an intermittent gas sparger <NUM>. Scouring is used during both the permeation and the backpulse/relaxation cycles to control the accumulation of solids on the membranes and to reduce membrane fouling. <FIG> illustrates of a method of air scouring in which the air flow rate remains the same throughout the permeation cycle and between permeation and relaxation cycles. During the permeation cycle, transmembrane pressure (TMP) slowly builds until the backpulse/relaxation cycle where at least some solids fouling the membrane are removed from the membrane module <NUM>. When the next permeate cycle is begun, the TMP is reduced but begins to rise throughout the permeate cycle as more solids foul the membranes.

<FIG> show alternative methods of operating a membrane filtration system having a membrane module <NUM> immersed in a tank <NUM> as shown in <FIG>. When the gas delivery device <NUM> is used alone, the aeration flow rates in <FIG> represent both the air flow input to the gas delivery device <NUM> and the output of the gas delivery device <NUM>. When the gas delivery device <NUM> is used in combination with an intermittent gas sparger <NUM>, the aeration flow rates in <FIG> represent the air flow input to, and output from, the gas delivery device <NUM> and the time-averaged output of the intermittent gas sparger <NUM>. However, the instantaneous flow rate of the output of the intermittent gas sparger <NUM> does not change much, or at all, with the input flow rate. Instead, the frequency at which bursts of bubbles are released from the intermittent gas sparger <NUM> increases with the input gas flow rate. Accordingly, in a system having intermittent gas spargers <NUM>, the aeration flow rates shown in <FIG> may alternatively be thought of as representing the frequency of bursts of bubbles being released from the intermittent gas sparger <NUM>.

In at least some situations, air scouring during backpulse/relaxation cycles is more effective in preventing solids accumulation and controlling membrane fouling compared to air scouring during permeation cycles. In one method, illustrated in <FIG>, the aeration flow rate remains constant at a first aeration flow rate during the permeation cycle. The aeration flow rate is increased during the backpulse/relaxation cycle to a second aeration flow rate which is greater than the first aeration flow rate. After the completion of the backpulse/relaxation cycle and at the beginning of a new permeation cycle, the aeration flow rate is decreased to the first aeration flow rate. In some cases, the amount of energy consumed by a membrane filtration system may be reduced by using the method of <FIG> rather than the method of <FIG>.

Solids build up in the module during the permeation cycle. The aeration flow rate can also be increased during the permeation cycle so that the aeration rate is higher during a later part of the cycle. <FIG> illustrates a method wherein the aeration flow rate at the beginning of the permeation cycle is gradually increased from a first aeration flow rate to a second aeration flow rate over the course of the permeation cycle and as solids build up. Alternatively, the aeration flow rate can go through one or more abrupt or step form changes from the first aeration flow rate to the second aeration flow rate within the permeation cycle. At the end of the permeation cycle and at the beginning of the backpulse/relaxation cycle, the aeration flow rate is increased to a third aeration flow rate. After the completion of the backpulse/relaxation cycle and at the beginning of a new permeation cycle, the aeration flow rate is decreased to the first aeration flow rate.

<FIG> illustrates a method wherein the aeration flow rate varies one or more times within a permeation cycle from a first aeration flow rate to a second aeration flow rate. During the backpulse/relaxation cycle, the second aeration flow rate or an even higher third aeration flow rate may be used. In the example illustrated, the aeration flow rate remains constant at a first aeration flow rate from the beginning of the permeation cycle. After a predetermined period of time during the course of the permeation cycle, the flow rate is increased from the first aeration flow rate to a second aeration flow rate for a predetermined period of time. After the completion of the predetermined period of time, the aeration flow rate is reduced to the first aeration flow rate. At the end of the permeation cycle and at the beginning of the backpulse/relaxation cycle, the aeration flow rate is increased to a third aeration flow rate, wherein the third aeration flow rate is greater than the second aeration flow rate. After the completion of the backpulse/relaxation cycle and at the beginning of a new permeation cycle, the aeration flow rate is decreased to the first aeration flow rate.

While <FIG> illustrates two occurrences of an increase of the aeration flow rate from the first flow rate to the second flow rate, any number of occurrences of an increase of the aeration flow rate may be applied during the permeation cycle. The first aeration flow rate illustrated in <FIG> may be any flow rate. For example, the first aeration flow rate of the air scouring method illustrated in <FIG> may be zero.

<FIG> illustrates a method wherein aeration is provided in distinct periods of time during the permeation cycle. The aeration flow rate increases from one distinct period of time to the next and then the aeration flow rate may be further increased during the backpulse/relaxation cycle. Optionally, a lower aeration flow rate, or no air flow, may be provided between the distinct periods of time. In the example shown, the aeration flow rate remains constant at a first aeration flow rate from the beginning of the permeation cycle. After a predetermined period of time during the course of the permeation cycle, the flow rate is increased from the first aeration flow rate to a second aeration flow rate for a predetermined period of time. After the completion of the predetermined period of time, the aeration flow rate is reduced to the first aeration flow rate. After another predetermined period of time, the flow rate is increased from the first aeration flow rate to a third aeration flow rate for a predetermined period of time, wherein the third aeration flow rate is greater than the second aeration flow rate. At the end of the permeation cycle and at the beginning of the backpulse/relaxation cycle, the aeration flow rate is increased to a fourth aeration flow rate, wherein the fourth aeration flow rate is greater than the third aeration flow rate. After the completion of the backpulse/relaxation cycle and at the beginning of a new permeation cycle, the aeration flow rate is decreased to the first aeration flow rate.

While <FIG> illustrates two occurrences of an increase of the aeration flow rate during the permeation cycle, any number of occurrences of an increase of the aeration flow rate may be applied during the permeation cycle. The first aeration flow rate illustrated in <FIG> may be any flow rate. According to one embodiment, the first aeration flow rate of the air scouring method illustrated in <FIG> may be zero.

<FIG> illustrates a method wherein air scouring is only used during the backpulse/relaxation cycle and all aeration is discontinued during the permeation cycle. As illustrated, an aeration flow rate of zero is applied during the permeation cycle. At the end of the permeation cycle and at the beginning of the backpulse/relaxation cycle, the aeration flow rate is increased to a first aeration flow rate. After the completion of the backpulse/relaxation cycle and at the beginning of a new permeation cycle, the aeration flow rate is decreased to zero.

In the description above, one or more aeration flow rates are said to begin with the beginning of a backwash/relaxation cycle and to end with the completion of a backwash/relaxation cycle. This is meant to be approximate. The stated aeration flow rate preferably includes at least the period of time occupied by the backwash or relaxation but may begin before the backwash or relaxation, or continue after the backwash or relaxation, or both.

<FIG> do not have specific time scales. However, permeation cycles typically last for <NUM> minutes or more, sometime <NUM> minutes or more. Backwash/relaxation cycles are typically at least one minute long and sometimes more than two minutes long. Changes in aeration flow rate between discrete aeration flow rates during the permeation period are preferably applied for at least one minute, more preferably for at least two minutes or five minutes. In contrast, an intermittent gas sparger <NUM> typically releases bursts of bubbles once every <NUM> to <NUM> seconds, more often once every <NUM> to <NUM> seconds. Accordingly, the changes in aeration flow rate shown in <FIG> do not represent individual bursts of bubbles from an intermittent gas sparger <NUM>. Instead, if an intermittent gas sparger <NUM> is used, it typically releases a plurality of bursts of bubbles within a period of time shown as having a particular aeration flow rate.

The aeration flow rate (or frequency of bursts of bubbles from an intermittent gas sparger <NUM>) could also be controlled considering one or more properties of feed water, of water in a process tank of a bioreactor or the tank <NUM> containing a membrane module <NUM>, or considering the performance of the membrane module <NUM>. Viscosity, mixed liquor suspended solids (MLSS) concentrations, extracellular polymer (ECP) concentrations, soluble microbial product (SMP) concentrations, a fouling index, temperature, and a rate of membrane fouling or recovery of flux after backwashing are some of the properties that could be used to control aeration flow rate, to determine when to change between one or more of the methods shown in <FIG>, or to control the timing of changes between aeration flow rates in a method shown in <FIG>. For example, a higher aeration flow rate, or more time at a higher aeration flow rate, or a change to a more intensive aeration process may be applied as the viscosity of water being fed to the filtration system of the tank <NUM> increases or its temperature decreases. In another example, a higher aeration flow rate, or more time at a higher aeration flow rate, or a change to a more intensive aeration process may be applied at higher MLSS concentrations. In another example, a higher aeration flow rate, or more time at a higher aeration flow rate, or a change to a more intensive aeration process may be applied when electricity may be obtained at a lower cost.

Optionally, to reduce the frequency of changes in the speed of a blower proving gas to a plurality of gas delivery devices <NUM> (alone or in combination with intermittent gas spargers <NUM>), a valve set may be provided between a blower and the gas delivery devices <NUM>. The valve set distributes the flow provided by the blower between two or more distinct branches of an air delivery system connected to the gas delivery devices such that flow in one distinct branch may vary in a time period during which the blower output does nto vary. For example, at one period of time a higher aeration flow rate is applied to a first distinct branch and a lower aeration flow rate is applied to a second distinct branch, while at a second period of time the higher aeration flow rate is applied to the second distinct branch and the lower aeration flow rate is applied to the first distinct branch. Alternatively or additionally, the blower may provide air at a generally constant rate to a pressure tank or other accumulator while a valve between the pressure tank and the gas delivery device <NUM> is modulated to provide the required variation in aeration flow rate.

Optionally, gas delivery devices <NUM> may also be connected to a cyclic aeration system as used in some ZeeWeed MBRs sold by GE Water and Process Technologies. In this case, even without an intermittent gas sparger <NUM>, the gas delivery device <NUM> produces a flow of bubbles for a period of about <NUM> to <NUM> seconds followed by a period in which the gas delivery device <NUM> produces a lesser flow of bubbles or no bubbles for a period of about <NUM> to <NUM> seconds, these periods alternating in repeated cycles over time. For example, one gas delivery device <NUM> can produce bubbles for <NUM> seconds and then be off for <NUM> seconds while a second gas delivery device is off for <NUM> seconds and then produces bubbles for <NUM> seconds. In this case, in the methods of <FIG>, the aeration flow rate can be interpreted as a time averaged aeration flow rate.

Claim 1:
A method of air scouring an immersed membrane module comprising adjusting one or more aeration parameters; between successive permeation cycles, successive back pulse cycles or successive relaxation cycles; during a permeation cycle; between a permeation cycle and a backpulse cycle; or, between a permeation cycle and a relaxation cycle; further comprising steps of,
a) bringing a flow of pressurized gas into a tank to below the bottom of a membrane module;
b) supplying the pressurized gas to an inlet <NUM> and splitting the flow of pressurized gas with a plurality of ports <NUM> into multiple flows of pressurized gas below the membrane module;
c) directing each of the multiple flows of pressurized gas through channels <NUM> to a different lateral position having an outlet <NUM>, the outlet <NUM> being defined by an open end of the channel <NUM>;
d) releasing bubbles only from the outlet <NUM> of the different lateral positions,
the inlet <NUM>, port <NUM> and outlets <NUM> being located at generally the same elevation but spaced horizontally and the pressurized gas flows generally horizontally in the channels.