Patent Application
20130015130
The present disclosure relates to submerged membrane filtration and particularly to using scouring air bubbles produced by an aeration system to clean or inhibit the fouling of membranes in a submerged membrane filter.
The following discussion is not an admission that anything discussed below is common knowledge or citable as prior art.
Immersed membranes are used, for example, for filtering water in municipal water supply treatment plants and in wastewater treatment plants. The membranes may be microfiltration or ultrafiltration membranes made in the form of modules of hollow fiber membranes. In large plants, many modules are immersed in the water to be filtered in an open tank and filtered water, also called permeate, is withdrawn through the membranes by suction. In a typical filtration cycle, a period of permeation, for example for 15 minutes to an hour, is followed by a relatively short period of backwashing or relaxation, and then the cycle repeats. In wastewater plants, and also in some water supply plants, the membranes are scoured with bubbles during permeation to clean or inhibit fouling in the membranes. The bubbles are provided according to an aeration regime that is applied throughout most, and possibly all, of the permeation part of the filtration cycle, and possibly during the backwash or relaxation part of the cycle as well. The aeration regime uses a significant amount of energy, adding to the cost and environmental impact of the plant. Accordingly, there have been various attempts to reduce the energy required by the aeration regime while still maintaining an acceptable fouling rate in the membranes.
A cyclic aeration system is described in International Publication Number WO 2000/021890, published April 20, 2000. In a version of this system used by GE Water and Process Technologies with its ZeeWeed™ hollow fiber membrane modules, air from one or more blowers is split through a set of controlled valves into two or four sets of aerators. The valves distribute the air to the sets of aerators in turn. For example, with two sets of aerators the air may be sent first to one set of aerators and then to the other set of aerators. Several filtration modules are connected together in a frame to provide a larger unit called a rack or cassette. There are typically several aerators below a cassette, but all of these aerators are part of the same set of the aerators. In this case, the cassette experiences an aeration regime wherein bubbles are provided for a period of time, and then not generally provided for a period of time, in repeated cycles. For example, with two sets of aerators, bubbles may be provided to a cassette for 10 seconds out of every 20 seconds. This is also called a 10 seconds on, 10 seconds off cycle, or simply a 10, 10 cycle. With four sets of aerators, bubbles may be provided to a cassette for 10 seconds out of every 40 seconds, or in a 10, 30 cycle. In some plants, the valve set allows the system to switch between these two regimes depending, for example, on variations in the fouling qualities of the water or the feed flow rate.
In the Mempulse™ system by Siemens Water Technologies Corp., each module is provided with an integrated pulsing air lift pump. Each module in a rack or cassette also has an air tube connecting its air lift pump to a common air supply pipe. The air tube discharges air into an inverted cup in the pulsing air lift pump. A pocket of air grows inside the cup until it extends downwards to a critical elevation, at which point most of the air in the pocket discharges into a vertical passageway. The passageway is open at its bottom end to draw in water in the tank, and its top is open to gaps in a lower potting head of the module. An air lift is formed in the passageway, and bubbles and water are discharged through the gaps into the module. The module experiences an aeration regime that, as in the cyclic aeration system, consists essentially of an air on—air off cycle. The precise timing of the cycle in commercial systems is not known to the inventor, but the related International Publication Number WO 2008/153818, published Dec. 18, 2008 suggests a cycle of roughly 2 seconds of bubbles, with entrained water, followed by 8 seconds without bubbles.
The cyclic aeration system has proven to be generally at least as effective as continuous aeration in inhibiting fouling while requiring only one half the amount of air. The Mempulse™ system avoids using controlled valves, but the pulsing air lift pumps might sometimes be fouled by solids in the wastewater. Their location at the bottoms of the modules would make the air lift pumps difficult to monitor and service if they did foul. The 2-8 cycle appears to use less air than a 10-10 or 10-30 cycle because the air is on for a lesser percentage of the entire cycle time but the air flow rate in the air on time, which is unknown to the inventor, would also need to be considered in determining how much air is used in each regime.
In International Publication Number WO 2008/153818, the inventors of the air cycling system noted that a burst of large bubbles produced at the start of an air on period might be particularly effective at cleaning the membranes. The Mempulse™ system might seem to take advantage of such a burst of bubbles, but if so the effect is diminished by other aspects of the system. In particular, some of the energy of the bubbles is spent in drawing water through the air lift pumps. Further, the air-on periods cannot be made to start at the same time for any particular set of modules. Considering several modules in a rack each with its own air lift pump, slight variations in the size or shape of components in the air lift pumps, or in the depth of water over the air lift pumps, or movement of water in the tank, will cause slight variations in the cycles. One air lift pump might operate according to a 2-8 cycle, while another operates according to a 2.1-8 cycle, another at 1.9-8.1, another at 2-8.1 and so on. There are thousands of cycles in a day, and so differences of even one tenth of a second would accumulate to the point where one air lift pump releases bubbles in the air-off period of another air lift pump. Considering, for example, 10 modules in a rack, on average 2 air lift pumps could be releasing bubbles at all times. The center to center distance between modules is very small, possibly one fifth or less, relative to the height of the modules and so a pulse of water and air rising through one module is likely to also entrain some water in an adjacent module. The release of air from 2 air lifts pumps in a rack on average at all times is likely to create a persistent upwards flow of water through the rack. The effect observed in the context of the air cycling system, however, was of a burst of bubbles being released into essentially still water. Releasing a burst of bubbles into upwards moving water is not likely to be as effective.
In a process described herein, an aeration regime comprising bursts of bubbles is applied simultaneously to one or more aerators located in association with an assembly of filtering membranes, such as a rack or cassette of membrane modules. The bursts are provided by accumulating gas under pressure in a plenum and then releasing a burst of the gas from the plenum to the one or more aerators. The one or more aerators may provide bubbles to an area that has a span of at least one quarter of the height of the assembly of membranes. The aeration regime may comprise, for example, a series of air-on or high flow rate periods, each in the range of 0.5 to 4 seconds long, separated by longer air-off or low flow rate periods, each in the range of 5 to 40 seconds long. Alternatively, longer air—on periods of 4 to 20 seconds may also be provided. The plenum may be located outside of a tank holding the rack or cassette, and it may be fed with a continuous supply of gas.
In an apparatus described herein, one or more aerators associated with a plurality of membrane modules are connected to the output side of a shared plenum. The plurality of modules may be connected together in a cassette or rack. The plenum may be located outside of a tank of water containing the membranes. The plenum may be fed with a continuous supply of gas, for example from one or more air blowers operating at a generally continuous speed. The release of gas may be provided from a pressure sensitive automatic valve or from a controlled valve.
By way of any of the processes or apparatus described above, an aeration regime is provided with a gas flow rate that varies in time. However, the plenum which produces the variation in flow rate may be located outside of the wastewater in the membrane tank where it is not exposed to wastewater and where it may be easily monitored and serviced. Further, a single plenum may supply gas to a set of aerators under an assembly of modules, such as a cassette or rack. The set of aerators associated with a rack or cassette may all receive a burst of air at the same time. If the set of aerators has a sufficient span relative to its depth of submergence, most of the water in the area of the cassette or rack may be allowed to become generally still between bursts. At the start of a burst, bubbles are released into still water which maximizes the velocity of the bubbles relative to the water.
One or more embodiments of the invention will now be described, by way of example, with reference to the attached Figures.
The tank 4 has one or more membrane assemblies 8 shown in dashed lines in
To provide air to the aerator assembly 10, air is drawn in through an air inlet 12 to a set of blowers 14. The blowers 14 may operate at a generally constant speed, thus providing a generally constant flow rate of air. By turning off one or more of the blowers 14, or reducing the speed of one or more of the blowers, a different flow rate of air can be provided. However, turning a blower 14 on an off frequently, or varying its speed often, can reduce the life of the blower 14. Accordingly, it is preferable to leave the blowers 14 either on or off, and operate them at a constant speed, for at least an hour or more at a time. Alternatively, systems can be provided to supply other gases, such as nitrogen, oxygen or oxygen enriched air or biogas.
The gas travels through a blower outlet pipe 16 to one or more manifolds 22. The manifolds 22 deliver the gas to one or more inlets 26 of one or more plenums 30. Gas accumulates in the plenum 30 by increasing in pressure, optionally in combination with expansion of the plenum 30. When gas is released from the plenum 30, the gas flows through a plenum outlet 32 to one or more of the aerator assemblies 10. As will be described further below, gas is released from the plenum 30 in bursts, causing corresponding bursts of bubbles to be emitted from the one or more aerator assemblies 10 connected to the plenum 30.
As shown in
The span 11 is preferably significant in length relative to the depth of submergence of the one or more aerator assemblies, which is typically also related to the height of membrane assemblies 8. For example, the span 11 may be one quarter or more, or one third or more, or one half or more, of either the depth of submergence of the aerator assembly 10 or the height of a membrane assembly 8. The extent to which a column of bubbles rising through one membrane assembly 8 could lift water in an adjacent membrane assembly 8 depends in part on the vertical distance that the bubbles rise through. Providing a significant span 11 relative to that vertical distance inhibits bubbles produced from one aerator assembly 10 from causing water to rise over an adjacent aerator assembly 10. Water in a membrane assembly 8 can therefore come closer to being still between bursts of air from its associated aerator assembly 10. When a burst of bubbles is provided to a membrane assembly 8, the bubbles are released into water that does not already have a significant upwards velocity. In addition to the comment mentioned previously in International Publication Number WO 2008/153818, the potential benefit to releasing bubbles into still water, rather than rising water, was discussed in Reexamination of the Gas Sparging Mechanism for Membrane Fouling Control, by Masao Kondo et al.
In operation, gas is emitted from a plenum 30, and bubbles are provided from an aerator assembly 10, at a rate that varies in repeated cycles. Each cycle may include a period during which gas accumulates in the plenum 30, followed by the release of a burst of the gas. Alternatively, a cycle may be described as having an “air-on” time and an “air-off” time. The flow rate during the burst, or air on time, could be generally constant or might rise to a peak flow rate and then decline again. There may be an abrupt transition to the accumulation stage, or air off time, during which the flow of gas may be completely stopped. Alternatively, the flow rate in the air on time may decrease gradually and the air air-off time may be deemed to occur when the rate of gas flow from the outlet 32 of a plenum 30 is 10% or less of the peak gas flow rate in the air on time. The duration of the burst, or air on time, may be between 0.5 and 20 seconds or between 0.5 and 4 seconds. The duration of the accumulation stage, or air off time, may be between 5 and 40 seconds.
In the system 2 shown in
A potting head 44 may have a spigot 46 which is connected to one or more permeate collection pipes 48. The permeate connection to only one of the modules 40 is shown to simplify
A membrane assembly 8 can also be made according to other configurations. For example, modules 40 of vertically oriented membranes 42 may be round or square and connected together in rows or grids to form a assembly 8. Module 40 may have also have hollow fiber membranes 42 oriented horizontally. Modules 40 with horizontal membranes may arranged into an assembly 8 by placing them side by side or in grids, or by stacking them on top of each other, or both. Modules 40 may also have flat sheet membranes, and may be arranged into an assembly 8 by placing them side by side or in grids, or by stacking them on top of each other, or both.
As shown in
This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.
