US20060091075A1 - Water filtration using immersed membranes - Google Patents
Water filtration using immersed membranes Download PDFInfo
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- US20060091075A1 US20060091075A1 US11/302,271 US30227105A US2006091075A1 US 20060091075 A1 US20060091075 A1 US 20060091075A1 US 30227105 A US30227105 A US 30227105A US 2006091075 A1 US2006091075 A1 US 2006091075A1
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- membrane modules
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- solids
- membranes
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- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 title claims abstract description 211
- 238000001914 filtration Methods 0.000 title abstract description 82
- 238000000034 method Methods 0.000 claims abstract description 56
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- 239000013505 freshwater Substances 0.000 abstract 1
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- 238000005273 aeration Methods 0.000 description 47
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- 238000004140 cleaning Methods 0.000 description 27
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Images
Classifications
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- B01D65/00—Accessories or auxiliary operations, in general, for separation processes or apparatus using semi-permeable membranes
- B01D65/08—Prevention of membrane fouling or of concentration polarisation
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- B01D61/00—Processes of separation using semi-permeable membranes, e.g. dialysis, osmosis or ultrafiltration; Apparatus, accessories or auxiliary operations specially adapted therefor
- B01D61/14—Ultrafiltration; Microfiltration
- B01D61/145—Ultrafiltration
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- B01D61/14—Ultrafiltration; Microfiltration
- B01D61/145—Ultrafiltration
- B01D61/146—Ultrafiltration comprising multiple ultrafiltration steps
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- B01D61/14—Ultrafiltration; Microfiltration
- B01D61/147—Microfiltration
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- B01D61/14—Ultrafiltration; Microfiltration
- B01D61/149—Multistep processes comprising different kinds of membrane processes selected from ultrafiltration or microfiltration
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- B01D61/14—Ultrafiltration; Microfiltration
- B01D61/18—Apparatus therefor
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- B01D61/22—Controlling or regulating
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- B01D63/043—Hollow fibre modules comprising multiple hollow fibre assemblies with separate tube sheets
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- B01D65/02—Membrane cleaning or sterilisation ; Membrane regeneration
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- C—CHEMISTRY; METALLURGY
- C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F1/00—Treatment of water, waste water, or sewage
- C02F1/44—Treatment of water, waste water, or sewage by dialysis, osmosis or reverse osmosis
- C02F1/444—Treatment of water, waste water, or sewage by dialysis, osmosis or reverse osmosis by ultrafiltration or microfiltration
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- C—CHEMISTRY; METALLURGY
- C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F3/00—Biological treatment of water, waste water, or sewage
- C02F3/02—Aerobic processes
- C02F3/12—Activated sludge processes
- C02F3/1236—Particular type of activated sludge installations
- C02F3/1268—Membrane bioreactor systems
- C02F3/1273—Submerged membrane bioreactors
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- B01D2311/04—Specific process operations in the feed stream; Feed pretreatment
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- B01D2321/04—Backflushing
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- B01D2321/16—Use of chemical agents
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- B01D2321/167—Use of scale inhibitors
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- B—PERFORMING OPERATIONS; TRANSPORTING
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- B01D2321/18—Use of gases
- B01D2321/185—Aeration
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D2321/00—Details relating to membrane cleaning, regeneration, sterilization or to the prevention of fouling
- B01D2321/20—By influencing the flow
- B01D2321/2066—Pulsated flow
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
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- B01D2321/20—By influencing the flow
- B01D2321/2083—By reversing the flow
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02W—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO WASTEWATER TREATMENT OR WASTE MANAGEMENT
- Y02W10/00—Technologies for wastewater treatment
- Y02W10/10—Biological treatment of water, waste water, or sewage
Definitions
- This invention relates to the use of ultrafiltration or microfiltration membranes to treat water, and more particularly to the design and operation of reactors which use immersed membranes as part of a substantially continuous process for filtering water containing low concentrations of solids, for example for producing potable water.
- Immersed membranes are used for separating a permeate lean in solids from tank water rich in solids.
- Feed water flowing into a tank containing immersed membranes has an initial concentration of solids.
- Filtered permeate passes through the walls of the membranes under the influence of a transmembrane pressure differential between a retentate side of the membranes and a permeate side of the membranes.
- the solids are rejected and accumulate in the tank.
- retentate In a continuous fully mixed process, there is typically a continuous bleed of tank water rich in solids, which may be called retentate. Unfortunately, while this process preserves a mass balance, the tank water must contain a high concentration of pollutants or the process will generate large volumes of retentate.
- a fully mixed continuous bleed process is operated at a recovery rate of 95% (ie. 95% of the feed water becomes filtered permeate)
- 95% of the feed water becomes filtered permeate
- the retentate must have a concentration of pollutants 20 times that of the feed water.
- the concentration of solids in the retentate is the same as the concentration of solids in the tank since the retentate is drawn from the tank water. Accordingly, the tank water has a high concentration of pollutants at all times.
- Operating at a lower recovery rate, 80% for example results in tank water having a lower concentration of solids but the cost of transporting excess feedwater and then disposing of excess retentate also increases.
- PCT Publication No. WO 98/28066 describes a process in which retentate is not withdrawn continuously. Instead, the tank water is drained to remove the accumulated solids from time to time. The tank is then refilled with fresh feed water and operation continues. While regular operation is interrupted in this method, there is a period directly after the tank is refilled in which the membranes are operated in relatively solids lean tank water. For feed water with low suspended solids, the intervals between drainings may be long enough that the benefit gained by emptying the tank offsets the loss in production time.
- the solids in the tank water foul the membranes.
- the rate of fouling is related to the concentration of solids in the tank water and can be reduced but not eliminated in a fully mixed continuous bleed process by lowering the recovery rate.
- the solids may be present in the feed water in a variety of forms which contribute to fouling in different ways.
- cleaning regimens may be required. Such cleaning usually includes both physical cleaning and chemical cleaning.
- backwashing and aeration The most frequently used methods of physical cleaning are backwashing and aeration. These methods are typically performed frequently and thus may influence the filtering process.
- backwashing permeation through the membranes is stopped momentarily. Air or water flow through the membranes in a reverse direction to physically push solids off of the membranes.
- aeration bubbles are produced in the tank water below the membranes. As the bubbles rise, they agitate or scrub the membranes and thereby remove some solids while creating an air lift effect and circulation of the tank water to carry the solids away from the membranes.
- aeration may be provided continuously and the membranes backwashed periodically while permeation is temporarily stopped.
- PCT Publication No. WO 98/28066 mentioned above describes a process in which permeation continues for 15 minutes and then stops while the membranes are aerated for 2 minutes and 15 seconds. After the first minute of aeration, the membranes are backwashed for 15 seconds.
- Chemical cleaning is typically performed less frequently than backwashing or aeration. According to one class of methods, permeation is stopped and a chemical cleaner is backwashed through the membranes. In some cases, the tank is emptied during or after the cleaning event so that the chemical cleaner can be collected and disposed of. In other cases, the tank remains filled and the amount of chemical cleaner in a cleaning event is limited to an amount that is tolerable for the application.
- the invention provides an improvement to a process for filtering water using membranes immersed in an open tank.
- the improvement includes reducing the concentration of solids in the water in the tank from time to time through deconcentrations.
- the deconcentrations are performed by withdrawing retentate rich in solids and simultaneously replacing it with a similar volume of feed water such that the membranes remain immersed during the deconcentration and permeation is not interrupted.
- the volume of retentate removed in a deconcentration is between 40% and 300% of the volume of water normally in the tank.
- the water in the tank has 40% or less of the average concentration of solids in the tank before the deconcentration.
- one or more of aeration or backwashing are biased towards a later part of a period between deconcentrations.
- the invention provides an immersed membrane filter.
- One or more membrane modules are placed in an open tank spaced consecutively along a general flow path between an inlet and an outlet.
- the distance between membrane modules (measured along the flow path) is less than one half of the length of each membrane module (measured along the flow path).
- the total length of all of the membrane modules (measured along the flow path) excluding the distance between them (along the flow path) is at least twice the width of the membrane modules (measured perpendicular to the flow path).
- Agitators preferably aerators, are provided below the membrane modules and operated substantially throughout permeation to entrain tank water around the membrane modules and flow the water containing solids upwards through the modules.
- Tank water flows through a plurality of membrane modules sequentially in relation to the flow path before leaving the tank at the outlet.
- one or more of aeration, backwashing and packing density are biased towards the outlet end of the tank.
- the tank may be deconcentrated from time to time as described above.
- the invention provides an open tank divided into a plurality of sequential filtration zones. Partitions between the filtration zones substantially prevent mixing between the filtration zones but for permitting water containing solids to flow from the first filtration zone to the last filtration zone through the filtration zones in sequence.
- One or more membrane modules are placed in each filtration zone and a similar permeate flux is withdrawn from each filtration zone.
- a non-porous casing around the one or more membrane modules in each filtration zone provides a vertical flow channel through the one or more membrane modules.
- Tank water flows downwards through the one or more membrane modules in each filtration zone.
- a plurality of passages connect the bottom of the vertical flow channel in one filtration zone to the top of the vertical flow channel of another filtration zone and permit the tank water to flow from the first filtration zone to the last filtration zone through the filtration zones consecutively.
- the passages include a weir at the tops of the partitions.
- packing density, aeration and backwashing are biased towards an outlet end of the tank.
- the tank may be deconcentrated from time to time as described above.
- the last filtration zone may be deconcentrated by draining and refilling it while permeation from the last filtration zone is stopped.
- FIG. 1 is a schematic representation of a general immersed membrane reactor.
- FIGS. 2, 3 and 4 are representations of various membrane modules.
- FIG. 5A is a schematic representation of an embodiment of the invention with a long aerated filtration train.
- FIG. 5B is a schematic cross section of the embodiment of FIG. 5A .
- FIG. 6 is an elevation view of a membrane module adapted for use with a filtering reactor having membrane modules in series.
- FIG. 7 is a plan view of the membrane module of FIG. 2 .
- FIG. 8 is a schematic representation of a filtering reactor having membrane modules in series.
- FIGS. 9 and 10 show tanks with alternate shapes.
- FIGS. 11 through 16 are charts showing the results of modelling experiments performed according to an embodiment similar to that of FIG. 5 .
- FIG. 17 is a chart showing the results of an experiment performed with an embodiment similar to that of FIG. 5 .
- a first reactor 10 for treating a liquid feed having solids to produce a filtered permeate substantially free of solids and a consolidated retentate rich in solids.
- a reactor 10 has many potential applications such as separating clean water from mixed liquor in a wastewater treatment plant or concentrating fruit juices etc., but will be described below as used for creating potable water from a natural supply of water such as a lake, well, or reservoir.
- a water supply typically contains colloids, suspended solids, bacteria and other particles which must be filtered out and will be collectively referred to as solids.
- the first reactor 10 includes a feed pump 12 which pumps feed water 14 to be treated from a water supply 16 through an inlet 18 to a tank 20 where it becomes tank water 22 .
- a gravity feed may be used with feed pump 12 replaced by a feed valve.
- the tank water 22 is maintained at a level which covers a plurality of membranes 24 .
- Each membrane 24 has a permeate side which does not contact the tank water 22 and a retentate side which does contact the tank water 22 .
- the membranes 24 are hollow fibre membranes for which the outer surface of the membranes 24 is preferably the retentate side and the lumens 25 of the membranes 24 are preferably the permeate side.
- Each membrane 24 is attached to at least one but preferably two headers 26 such that the ends of the membranes 24 are surrounded by potting resin to produce a watertight connection between the outside of the membranes 24 and the headers 26 while keeping the lumens 25 of the membranes 24 in fluid communication with a permeate channel in at least one header 26 .
- Membranes 24 and headers 26 together form of a membrane module 28 .
- the permeate channels of the headers 26 are connected to a permeate collector 30 and a permeate pump 32 through a permeate valve 34 . When permeate pump 32 is operated and permeate valve 34 opened, a negative pressure is created in the lumens 25 of the membranes 24 relative to the tank water 22 surrounding the membranes 24 .
- the resulting transmembrane pressure is typically between 1 kPa and 150 kPa and more typically between 10 kPa and 70 kPa and draws tank water 22 (then referred to as permeate 36 ) through membranes 24 while the membranes 24 reject solids which remain in the tank water 22 .
- filtered permeate 36 is produced for use at a permeate outlet 38 through an outlet valve 39 .
- a storage tank valve 64 is opened to admit permeate 36 to a storage tank 62 .
- the filtered permeate 36 may require post treatment before being used as drinking water, but should have acceptable levels of colloids and other suspended solids.
- each having a plurality of membranes 24 are assembled together into larger units called membrane modules 28 which may also be referred to as a cassette.
- Examples of such membrane modules 28 are shown in FIGS. 2, 3 and 4 in which the discrete units are rectangular skeins 8 .
- Each rectangular skein 8 typically has a bunch between 2 cm and 10 cm wide of hollow fibre membranes 24 .
- the hollow fibre membranes 24 typically have an outside diameter between 0.4 mm and 4.0 mm and are potted at a packing density between 10% and 40%.
- the hollow fibre membranes 24 are typically between 400 mm and 1,800 mm long and mounted with between 0.1% and 5% slack.
- the membranes 24 have an average pore size in the microfiltration or ultrafiltration range, preferably between 0.003 microns and 10 microns and more preferably between 0.02 microns and 1 micron.
- the preferred number of membrane modules 28 varies for different applications depending on factors such as the amount of filtered permeate 36 required and the condition of the feed water 14 .
- a plurality of rectangular skeins 8 are connected to a common permeate collector 30 .
- the membrane modules 28 shown in FIG. 2 may also be stacked one above the other.
- the rectangular skeins 8 are shown in alternate orientations.
- the membranes 24 are oriented in a horizontal plane and the permeate collector 30 is attached to a plurality of rectangular skeins 8 stacked one above the other.
- the membranes 24 are oriented horizontally in a vertical plane.
- the permeate collector 30 may also be attached to a plurality of these membrane modules 28 stacked one above the other.
- the representations of the membrane modules 28 in FIGS. 2, 3 , and 4 have been simplified for clarity, actual membrane modules 28 typically having rectangular skeins 8 much closer together and a large cassette often having many more rectangular skeins 8 .
- Membrane modules 28 can be created with skeins of different shapes, particularly cylindrical, and with skeins of looped fibres attached to a single header. Similar modules or cassettes can also be created with tubular membranes in place of the hollow fibre membranes 24 .
- pairs of membranes are typically attached to headers or casings that create an enclosed surface between the membranes and allow appropriate piping to be connected to the interior of the enclosed surface. Several of these units can be attached together to form a cassette of flat sheet membranes.
- Each ZW 500 unit has two rectangular skeins of hollow fibre membranes having a pore size of approximately 0.1 microns oriented as shown in FIG. 2 with a total membrane surface area of approximately 47 square metres. In plan view, each ZW 500 unit is about 700 mm long and about 210 mm wide. Typically, several ZW 500 units are joined together into a cassette to provide a plurality of parallel rectangular skeins 8 . For example, a membrane module 28 of 8 ZW 500 units is about 1830 mm by 710 mm, some additional space being required for frames, connections and other related apparatus.
- tank water 22 which does not flow out of the tank 20 through the permeate outlet 38 flows out of the tank 20 through a drain valve 40 and a retentate outlet 42 to a drain 44 as retentate 46 with the assistance of a retentate pump 48 if necessary.
- the retentate 46 is rich in the solids rejected by the membranes 24 .
- an air supply pump 50 blows ambient air, nitrogen or other suitable gases from an air intake 52 through air distribution pipes 54 to aerator 56 which disperses scouring bubbles 58 .
- the bubbles 58 rise through the membrane module 28 and discourage solids from depositing on the membranes 24 .
- the bubbles 58 also create an air lift effect which in turn circulates the local tank water 22 .
- permeate valve 34 and outlet valve 39 are closed and backwash valves 60 are opened.
- Permeate pump 32 is operated to push filtered permeate 36 from retentate tank 62 through backwash pipes 61 and then in a reverse direction through permeate collectors 30 and the walls of the membranes 24 thus pushing away solids.
- backwash valves 60 are closed, permeate valve 34 and outlet valve 39 are re-opened and pressure tank valve 64 opened from time to time to re-fill retentate tank 62 .
- a cleaning chemical such as sodium hypochlorite, sodium hydroxide or citric acid are provided in a chemical tank 68 .
- Permeate valve 34 , outlet valve 39 and backwash valves 60 are all closed while a chemical backwash valve 66 is opened.
- a chemical pump 67 is operated to push the cleaning chemical through a chemical backwash pipe 69 and then in a reverse direction through permeate collectors 30 and the walls of the membranes 24 .
- chemical pump 67 is turned off and chemical pump 66 is closed.
- the chemical cleaning is followed by a permeate backwash to clear the permeate collectors 30 and membranes 24 of cleaning chemical before permeation resumes.
- aeration and backwashing clean the membranes sufficiently so that permeation can continue over extended periods of time.
- Permeate backwashes typically last for between 5 seconds and two minutes and are typically performed between once every 5 minutes and once every 3 hours. If such permeate backwashes are performed between more intensive restorative cleaning events, the filtering process is still considered continuous since permeation is only stopped momentarily.
- chemical cleaning is performed in short duration chemical backwashes while the tank 20 remains full of tank water 22 , the process is still considered continuous.
- flow rates of permeate 36 , retentate 46 and feed water 14 are calculated as average flow rates over a day or such longer period of time as appropriate.
- flow rates of processes that are periodically interrupted as described above are measured as average flow rates unless they are described otherwise.
- the filtration process proceeds as a number of repeated cycles which end with a procedure to deconcentrate the tank water 22 , the procedure being referred to as a deconcentration.
- the cycles usually begin at the end of the preceding deconcentration. Some cycles, however, begin when a new reactor 10 is first put into operation or after intensive restorative cleaning or other maintenance procedures which require the tank 20 to be emptied. Regardless, the cycle begins with the tank 20 filled with membranes 24 submerged in tank water 22 with an initial concentration of solids similar to that of the feed water 14 .
- permeate pump 32 is turned on and sucks tank water 22 through the walls of the membranes 24 which is discharged as filtered permeate 36 .
- Drain valves 40 initially remain closed and the concentration of solids in the tank water 22 rises. While drain valves 40 are closed, the feed pump 12 continues to pump feed water 14 into the tank 20 at about the same rate that filtered permeate 36 leaves the tank such that the level of the tank water 22 is essentially constant during permeation. Aeration and backwashing are provided as required.
- the tank water 22 is deconcentrated.
- the desired period of time between deconcentrations may be based on the concentration of solids in the tank water 22 but preferably is chosen to achieve a desired recovery rate.
- a recovery rate of 95% ie. 95% of the feed water becomes filtered permeate
- This recovery rate results in a concentration of solids in the tank water 22 at the start of the deconcentrations approximately 20 times that of the feed water.
- the deconcentrations comprise a rapid flush of the tank water 22 while maintaining the level of tank water 22 above the level of the membranes 24 and continuing permeation.
- the drain valves 40 are opened and retentate pump 48 rapidly draws retentate 46 rich in solids out of the tank 20 if gravity flow alone is insufficient.
- feed pump 12 increases the flow rate of feed water 14 into the tank 20 by an amount corresponding to the flow rate of retentate 46 out of the tank 20 .
- the retentate 46 is removed at a sufficient rate, assisted by retentate pump 48 if necessary, such that the tank water 22 is not diluted significantly by mixing with incoming feed water 14 before it is flushed out of the tank 20 .
- aeration and any other source of mixing are turned off to minimize dilution of the retentate 46 and between 100 % to 150 % of the average volume of the tank water 22 is discharged during the rapid flush deconcentration. If aeration must be left on to provide continued cleaning, a higher volume of tank water 22 is discharged.
- the tank water 22 preferably has less than 40% of the concentration of solids that was present in the tank water 22 prior to the deconcentration.
- the tank water 22 after a deconcentration preferably has less than 20% of the concentration of solids that was present in the tank water 22 prior to the deconcentration.
- Retentate 46 is typically disposed of down a drain 44 to a sewer or to the source of water where it initially came from.
- a reduced flow rate of air bubbles 58 is initially supplied to the tank 20 when the concentration of solids is low and the membranes 24 foul more slowly.
- the flow rate of air is also increased.
- aeration is only provided directly before the deconcentration. In this way, excess air is not supplied while the concentration of solids is low in the tank water 22 .
- the frequency or duration of backwashing may be decreased when the concentration of solids in the tank water 22 is low to minimize loss in production due to backwashing. To the extent that aeration can be made to coincide with backwashing, the effectiveness of the aeration is increased since is does not have to work against the transmembrane pressure.
- intensive cleaning is preferably done when the transmembrane pressure exceeds 54 kPa or the permeability drops below 200 litres per square metre per hour per bar (L/m2/h/bar) at normal operating temperatures.
- the tank is typically emptied during the intensive maintenance cleaning, but this is independent of the periodic deconcentrations and occurs only infrequently, between once every two weeks to once every two months.
- a second reactor 70 has a rectangular (in plan view) second tank 120 with an inlet end 72 and an outlet end 74 .
- the inlet end 72 is at one short end (as seen in plan view) of the second tank 120 and has an inlet 18 and the outlet end 74 is at the opposite short end of the second tank 120 and has a retentate outlet 42 .
- the second tank 120 is filled with tank water 22 which moves generally in a general flow path 76 between the inlet 18 and the retentate outlet 42 , the word general meaning that the actual flow path of a volume of tank water 22 may deviate substantially from the flow path 76 as will be described below, but the average flow of tank water 22 has at least a component in the direction of the flow path 76 .
- Membrane modules 28 are arranged in the second tank 120 in series along the flow path 76 .
- the membrane modules 28 are spaced apart horizontally along the flow path 76 to allow room for associated apparatus, installation and maintenance, and to provide a small movable volume of tank water 22 between each membrane module 28 .
- This space is preferably less than one half of the length (measured along the flow path 76 ) of the membrane module 28 and for ZW 500 units is typically about 20 cm.
- greater space is provided above, below and beside the membrane modules 28 .
- the distance between the membrane modules 28 and the long walls of the second tank 120 is typically about one half of the width of the membrane modules 28 (measured perpendicular to the flow path 76 ).
- each membrane module 28 is typically of the size of a cassette of 6 to 12 ZW 500 units.
- the total length of all of the membrane modules 28 (measured along the flow path 76 ) excluding the space between them (also measured along the flow path 76 ) is at least twice, and preferably at least four times, the width of the membrane modules 28 (measured perpendicular to the flow path 76 ).
- Feed water 14 continuously enters the second tank 120 at the inlet 18 .
- Permeate pump 32 continuously withdraws filtered permeate 36 through membranes 24 of each membrane module 28 and consolidated retentate 46 continuously leaves the second tank 120 through retentate outlet 42 .
- the path of a volume of tank water 22 passes in series through some or all of the membrane modules 28 .
- the concentration of solids in the volume of tank water 22 increases downstream of each membrane module 28 it passes through.
- the concentration of solids in the volume of tank water 22 increases from the inlet 18 to the retentate outlet 42 along its flow path.
- the tank water 22 Downstream of the membrane module 28 nearest to the retentate outlet 42 , the tank water 22 has a high concentration of solids of at least five times that of the feed water 14 , preferably at least 14 times that of the feed water 14 and more preferably at least 20 times that of the feed water 14 . Conversely, tank water 22 near the inlet 18 has a concentration of solids similar to that of the feed water 14 . In long trains of membrane modules 28 in which the length of the membrane modules 28 (excluding the spaces between them) is four or more times their width, up to 75% of the membrane modules 28 operate in tank water with minimal solids concentration, the concentration of solids rising sharply only near the outlet 42 .
- membrane modules 28 near the inlet 18 operate in water that has a substantially lower concentration of solids than the consolidated retentate 46 which flows out of the retentate outlet 42 .
- the last membrane modules 28 (in the direction of the flow path 76 ) have a higher concentration of solids in the tank water 22 around them and are therefore likely to have reduced permeabilities.
- the permeate pump 32 may be placed near the outlet 42 so that the last membrane modules 28 will receive higher transmembrane pressures (relative to more distant membrane modules 28 ) to overcome their reduced permeability and provide more nearly even permeate flux from the set of membrane modules.
- the average concentration of solids in the tank water 22 is an intermediate concentration in relation to the concentration of solids in the feed water 14 and consolidated retentate 46 . If the length of all of the membrane modules 28 (excluding spaces between them) is more than twice their width, the area of significantly reduced concentration can include more than half of the second tank 120 . Thus consolidated retentate 46 can be withdrawn having a high concentration of solids but the average concentration of solids in the tank water 22 is significantly less than the concentration of solids in the consolidated retentate 46 . The average permeability of the membrane modules 28 is increased as fouling occurs less rapidly.
- membrane modules 28 operate in tank water 22 having a concentration of solids less than 14 times that of the feed water and more preferably less than 10 times that of the feed water.
- the path of a volume of tank water 22 passes in series through some or all of the membrane modules 28 .
- This effect would not occur if the second reactor 70 operated like a completely stirred tank reactor.
- aeration is provided during the entire permeation cycle. While aeration is normally considered to be a mixing agent, in the second reactor 70 the inventors believe that the aeration (or alternately an agitator such as a rotating propeller) provided substantially throughout permeation encourages tank water 22 to flow through a plurality of membrane modules 28 sequentially in relation to the flow path when as will be explained below.
- the tank flow 76 must have an average substantially horizontal flow from inlet 18 to outlet 42 .
- the membrane modules 28 significantly resist such horizontal flow. Accordingly, the bulk of the horizontal flow has a tendency to by-pass the membrane modules by flowing beneath, over or beside them. The inventors believe that if tank water 22 readily by-passed the membrane modules 28 , it would be difficult to avoid substantial mixing in the tank 20 .
- Typical vertical velocities of tank water 22 upwards through the membrane module 28 are of a comparable magnitude, typically 0.05 to 0.2 m/s.
- a cassettes flow 78 is created in which tank water 22 is drawn up into the bottom of a membrane module 28 released from the top of the membrane module, flows towards the outlet 42 while descending to the bottom of the tank 20 where it is entrained in a second membrane module 28 and so on.
- the cassette flow 78 has a component flowing downwards besides the membrane modules 28 (as shown in FIG.
- Cassette flow 78 created within a first membrane module 28 and flowing downwards between membrane modules 28 likely mixes in part with tank water 22 similarly flowing downwards in the cassette flow 78 of an adjacent membrane module 28 and becomes part of the cassette flow 78 of the adjacent membrane module 28 .
- a mixing flow 80 of tank water 22 circulating around a membrane module 28 may be drawn towards the inlet 18 by an upstream membrane module 28 or towards the retentate outlet 42 by a downstream membrane module 28 .
- the degree of mixing in the second tank 120 may be expressed in relation to a recirculation rate defined as the flow rate of the cassette flow 78 through the centre of the membrane modules 28 divided by the flow rate of feedwater.
- the second tank 120 can be made of a plurality of filtering zones wherein the outlet of a first filtering zone is connected to the inlet of a downstream filtering zone.
- the filtering zones may be created by breaking the second tank 120 into a plurality of containers or with baffles 82 at the upper upstream edge or lower downstream edge of a membrane module 28 to restrict backflows 80 flowing towards the inlet 18 .
- baffles are installed only on membrane modules 28 located near the retentate outlet 42 where the rate of flow in the flow path 76 is reduced.
- FIGS. 6 and 7 another second membrane module 110 having hollow fibre membranes 24 is shown in elevation and plan view respectively.
- the membranes module 110 is similar to that shown in FIG. 4 but the perimeter of the second membrane module 110 is surrounded by a non-porous casing 124 which defines a vertically oriented flow channel 126 through the second membrane module 110 .
- Similar modules can be created with membrane modules 28 as shown in FIGS. 2, 3 and 4 or with tubular or flat sheet membranes as described above.
- a third reactor 128 has a plurality of second membrane modules 110 in a plurality of filtration zones 130 .
- the third reactor 128 has a feed pump 12 which pumps feed water 14 to be treated from a water supply 16 through an inlet 18 to a third tank 140 where it becomes tank water 22 .
- the feed pump 12 is operated to keep tank water 22 at a level which covers the membranes 24 .
- the permeate collector 30 of each second membrane module 110 is connected to a set of pipes and valves as shown including a pair of permeate valves 144 and a pair of backwash valves 60 .
- TMP transmembrane pressure
- the membranes 24 admit a flow of filtered permeate 36 which is produced for use or further treatment at a permeate outlet 38 .
- a permeate storage valve 64 is opened to maintain a supply of permeate 36 in a permeate storage tank 62 .
- the permeate pumps 32 are operated to produce a similar flux of permeate 36 from each filtration zone 130 . Since solids concentration in each filtration zone 130 differs, as will be explained further below, this typically requires each permeate pump 32 to be operated at a different speed.
- the second membrane modules 110 in different filtration zones 130 can be connected to a common permeate pump 32 . This will result in some variation in flux between the filtration zones 130 (because the downstream second membrane modules 110 are likely to foul faster), but the amount of variation can be minimized by locating the permeate pump 32 near the outlet 42 as described above or by variations in aeration, backwashing and packing density to be described below. With any of these techniques, the second membrane modules 110 can be made to have similar permeate fluxes.
- Tank water 22 which does not flow out of the third tank 140 through the permeate outlet 38 flows out of the third tank 140 through a drain valve 40 and retentate outlet 160 to a drain 44 as consolidated retentate 46 .
- Additional drains in each filtration zone 130 are also provided to allow the third tank 140 to be drained completely for testing or maintenance procedures.
- the consolidated retentate 46 is rich in the solids rejected by the membranes 24 . Flow of the consolidated retentate 46 may be assisted by a retentate pump 48 if required.
- the inlet 18 and retentate outlet 160 are separated by the filtration zones 130 .
- Partitions 176 at the edges of the filtration zones 130 force the tank water 22 to flow sequentially through the filtration zones 130 in a tank flow pattern 178 .
- the partitions 176 have decreasing heights in the direction of the tank flow pattern 178 such that a difference in depth from one filtration zone 130 to the next drives the tank flow pattern 178 .
- the difference in depth between partitions 176 varies with different applications, but is unlikely to be more than 1 m between the first and last partition 176 .
- flow from one filtration zone 130 to the next could be through conduits and driven by differences in depth from one filtration zone 130 to the next or driven by pumps.
- feed pump 12 While in normal operation, feed pump 12 substantially continuously adds feed water 14 to the third tank 140 while one or more permeate pumps 32 substantially continuously withdraw permeate 36 .
- the process is typically operated to achieve a selected recovery rate defined as the portion of feed water 14 removed as permeate 36 (not including permeate 36 returned to the third tank 140 during backwashing to be described further below) expressed as a percentage.
- the selected recovery rates is typically 90% or more and preferably 95% or more.
- the third reactor 128 shown in FIG. 8 is operated at an overall recovery rate of 95%.
- 100 flow units of feed water 14 having a concentration of 1 enters the third tank 140 at the inlet 18 .
- 95 flow units leave the third tank 140 as permeate 36 while 5 flow units leave the third tank 140 as consolidated retentate 46 .
- 19 flow units leave the third tank 140 as permeate 36 in each filtration zone.
- the tank water 22 would have a concentration 20 times that of the feed water 14 throughout.
- concentration of solids in the tank water 22 in most of the filtration zones 130 is significantly reduced.
- the reduced concentration of solids results in significantly reduced fouling of the second membrane modules 110 in the applicable filtration zones 130 .
- less chemical cleaning is required for these second membrane modules 110 .
- reduced aeration and backwashing routines are sufficient for individual filtration zone 130 or groups of filtration zones 130 with reduced concentrations of solids.
- aeration is not required to prevent tank water 22 from by passing the membrane modules and so less or even no aeration can be provided during substantial periods. Further, by forcing tank water 22 to flow through the casings 124 , aeration is not required to create local circulation of tank water 22 around second membrane modules 110 . Accordingly, space in the third tank 140 is not required for downcomers and the second membrane modules 110 can occupy 80% or more of the plan area or footprint of the tank 140 .
- an air supply 50 associated with each filtration zone 130 is operable to blow air, nitrogen or other suitable gases through air distribution pipes 54 to a header 170 attached to a plurality of aerators 56 below the second membrane module 110 .
- the aerators 56 emit scouring bubbles 58 below the second membrane module 110 which rise through the membranes 24 .
- aeration can be provided to each filtration zone 130 individually.
- the second membrane module 110 in each filtration zone 130 can also be backwashed individually by closing its associated permeate valves 144 and opening its associated backwash valves 60 .
- the associated permeate pump 32 (or alternatively, a separate pump) is then operated to draw permeate 36 from the permeate storage tank 62 and pump it through the permeate collector 30 and, ultimately, through the membranes 24 in reverse direction relative to permeation.
- the second membrane modules 110 in adjacent filtration zones 130 are not backwashed at the same time.
- the backwash typically lasts for between 15 seconds and one minute and involves a flux one to three times the permeate flux but in a reverse direction.
- the level of the tank water 22 in the backwashed filtration zone 130 rise temporarily causing more tank water 22 to flow to the next filtration zone 130 .
- the downstream partition 176 in each filtration zone is sufficiently lower than the upstream partition 176 such that tank water 22 does not flow over an upstream partition 176 during backwashing.
- the second membrane modules 110 are sized to nearly fill each filtration zone. Further, the second membrane modules 110 are positioned such that tank water 22 or feed water 14 flowing into a filtration zone 130 must flow first through the flow channel 126 of the second membrane module 110 .
- the tank flow 178 thus generally flows downwards through each second membrane module 110 then upwards outside of each second membrane module 110 and over the downstream partition 176 . Accordingly, the tank flow 178 is transverse to the membranes 24 and generally inhibits solids-rich zones of tank water 22 from forming near the membranes 24 .
- the tank flow 178 may temporarily flow upwards through the second membrane module 110 if the top of the casing 24 around the second membrane module 110 is located near the normal level of the tank water 22 . Such reverse flow does not significantly effect the general tank flow 178 but it is preferred if during backwashing the tank water 22 does not overflow the second membrane module 110 . In this way, after backwashing stops, there is a momentarily increased tank flow 178 which assists in moving solids from near the bottom of the second membrane module 110 to the next filtration zone 130 . For second membrane modules 110 with minimal aeration, the tank flow through a second membrane module 110 approaches a plug flow and there is an increase in concentration of solids as the tank water 22 descends through the second membrane module 110 .
- membranes 24 near the top of the second membrane module 110 experience a concentration of solids even lower than that predicted by the chart above, and comparatively more solids attach to the lower membranes 24 .
- the bubbles 56 rise upwards against the tank flow 178 and no space for downcomers is required in the filtration zones 130 .
- the embodiments described with reference to FIGS. 5 and 8 are operated in cycles including rapid flush deconcentrations.
- the resulting temporal reduction in concentration of solids produced by the deconcentrations works to further the effect of the spatial reductions in concentration of solids.
- FIGS. 5A and 5B or 8 at the start of a cycle, the second tank 120 or third tank 140 is filled with tank water 22 .
- Filtered permeate 36 is withdrawn from the second tank 120 or third tank 140 while drain valves 40 remain at least partially and preferably completely closed so that the tank water 22 becomes more concentrated with solids until a deconcentration is indicated as described above.
- Permeation continues while the second tank 120 or third tank 140 is deconcentrated by simultaneously withdrawing consolidated retentate 46 from the second tank 120 or third tank 140 and increasing the rate that feed water 14 enters the second tank 120 or third tank 140 to maintain the level of tank water 22 above the membranes 24 during the flushing operation.
- the volumes of water removed from the second tank 120 or third tank 140 can be the same as those described above.
- lower flush volumes may be used since only the downstream part of the tank water 22 requires deconcentration.
- Deconcentrations can also be performed by stopping permeation and the flow of feed water 14 into the second tank 120 or third tank 140 while retentate 46 is withdrawn.
- the level of the tank water 22 drops and so the second tank 120 or third tank 140 must first be refilled before permeation can resume.
- this process avoids dilution of the retentate 46 with feed water 14 but also interrupts permeation.
- the last filtration zone 130 can be drained separately while permeation is stopped in that filtration zone 130 only. Compared to a process in which a tank is emptied, such deconcentrations are performed more frequently but involve less volume each which reduces the capacity of the drain 44 required.
- this technique advantageously allows tank water 22 rich in solids to be withdrawn while permeating through most membrane modules 28 and without diluting the retentate 46 .
- the flow path over the last partition 176 is preferably fitted with a closure such as a gated weir 180 or a valved conduit. The closure is shut at the start of the deconcentration which prevents tank water 22 from flowing over the partition 176 after the drain valve 40 is opened.
- Retentate pump 48 may be operated to speed the draining if desired.
- Feed water 14 continues to be added to the third tank 140 during the deconcentration until the level of the tank water 22 rises in the downstream filtration zones 130 to the point where appreciable reverse flow may occur across the partitions 176 .
- retentate pump 48 is turned off (if it was on) and drain valve 40 is closed. The closure is opened releasing an initially rapid flow of tank water 22 which fills a portion of the last filtration zone 130 .
- the flow of feed water 14 is increased until the remainder of the last filtration zone 130 is filled.
- baffles are preferably installed above the second membrane modules 110 to direct the flow and dissipate its energy.
- FIGS. 5A and 5B is fitted with a separate aeration system for each membrane module 28 as shown in FIG. 8 , the connection between the air distribution pipes 54 and selected aerators 56 are fitted with restricting orifices or, preferably, each aerator 56 has a flow control valve associated with it.
- Membrane modules 28 or second membrane modules 110 operating in tank water 22 with low concentration of solids are aerated less forcefully, preferably based on the concentration of solids 22 in the tank water surrounding each membrane module 28 or second membrane module 110 .
- the furthest upstream membrane module 28 or second membrane module 110 is exposed to the lowest concentration of solids and thus receives the least amount of air, subject in the embodiment of FIGS. 5A and 5B to the need to entrain tank water 22 that would otherwise by-pass the membrane modules 28 .
- the most downstream membrane module 28 or second membrane module 110 is exposed to the highest concentration of solids and receives the most aeration.
- all aerators 56 are built to the same design and are rated with the same maximum air flow that can be passed through them.
- the minimum amount of air flow is typically about one half of the rated maximum air flow, below which the aerator 56 may fail to aerate evenly.
- the upstream one half or two thirds of the membrane modules 28 or second membrane modules 110 are aerated at 50% to 60% of the rated capacity of the aerators 56 and the remaining membrane modules 28 or second modules 110 are aerated at 80% to 100% of the rated capacity, the increase being made either linearly or in a step form change.
- Such a variation approximately follows the increase in solids concentration in the tank water 22 .
- tapered backwashing may be employed.
- Membrane modules 28 or second membrane modules 110 operating in tank water 22 with low concentration of solids require less backwashing.
- the furthest upstream membrane module 28 or second membrane module 110 is exposed to the lowest concentration of solids and receives the least amount of backwashing whereas the most downstream membrane module 28 or second membrane module 110 is exposed to the highest concentration of solids and receives the most backwashing.
- the amount of backwashing is typically increased between these extremes using a lower amount of backwashing for the upstream one half or two thirds of membrane modules 28 or second membrane modules 110 and then increasing either linearly or in step form to a higher amount for the remaining membrane modules 28 or second membrane modules 110 .
- the apparatus in FIGS. 5A and 5B is fitted with a separate backwashing system for each membrane module 28 as shown in FIG. 8 .
- Backwashing can be varied in both frequency or duration. Precise parameters depend on the feed water 14 and other variables but typically range from a 10 second backwash once an hour to a 30 second backwash once every five minutes, the lower amount being near the former regime and the higher amount being near the latter.
- the direction of tank flow 78 may be reversed periodically by providing an inlet 18 and retentate outlet 46 at opposite ends of the second tank 120 or third tank 140 .
- the reversal is done after periodic chemical cleaning which is required approximately every two weeks to two six months and often requires draining the second tank 120 or third tank 140 .
- Such flow reversal allows the membranes 24 near the ends of the second tank 120 or third tank 140 to be operated at times in solids lean tank water 22 which substantially increases their useful life.
- Such flow reversal can be accomplished in the embodiment of FIG. 8 with some modification but is inconvenient, the method being more suited to the embodiment of FIGS. 5A and 5B .
- membrane modules 28 or second membrane modules 110 with lower packing density are preferred in solids rich tank water 22 .
- the reduced packing density allows bubbles 58 to reach the membranes 24 more easily and increases the cleaning or fouling inhibiting effect of aeration.
- higher packing density is desirable as more membrane surface area is provided for a given volume of second tank 120 or third tank 140 .
- the packing density of downstream membrane modules 28 or second membrane modules 110 is reduced relative to upstream membrane modules 28 or second membrane modules 110 with a corresponding change in the size of the filtration zones 130 .
- Preferred upstream packing densities vary from 20% to 30%.
- Preferred downstream packing densities vary from 10% to 20%.
- a round tank 220 is used.
- Inlet 18 is located at one point on the circumference of the tank 220 and the retentate outlet 42 is located in the middle of the tank 220 , or alternately (as shown in dashed lines) at another point on the circumference of the tank 220 .
- Membrane modules 28 or second membrane modules 110 are placed in a ring around the centre of the tank 220 in a horizontally spaced apart relationship.
- An internal divider 222 in the tank 220 is used to create a circular flow path 276 between the inlet 18 and the retentate outlet 42 .
- a low aspect ratio or square tank 320 is used.
- Inlet 18 is located at one point on the tank 320 and the retentate outlet 42 is located at another point on the tank 320 .
- An internal divider 322 in the tank 320 is used to create a flow path 376 between the inlet 18 and the retentate outlet 42 .
- Membrane modules 28 or second membrane modules 110 are placed in series along the flow path 376 in a horizontally spaced apart relationship.
- the internal divider 322 is a wall between separate tanks joined in series by fluid connector 324 .
- partitions 176 are provided between second membrane modules 110 .
- a submerged membrane reactor according to FIGS. 5A and 5B was modelled using experimental data from tests under a continuous process and assuming that the local flow around the membrane modules is symmetrical in the upstream and downstream directions—ie. the overall tank flow towards the outlet was discounted.
- the system comprises a tank 16.4 metres long, 3.28 metres wide with an average depth of water of about 2.5 metres.
- the tank of the reactor contains 12 membrane modules each being a cassette of 8 ZW500 membrane modules.
- Each cassette is approximately 1.82 metres high, 1.83 metres wide and 0.71 metres long along the flow path and placed in the tank so as to leave approximately 0.75 m between the edge of the cassettes and the long walls of the tank.
- the cassettes are spaced evenly between the inlet end and outlet end of the tank.
- Transmembrane pressure is maintained at a constant 50 kPa throughout the model and the permeability of the membranes at any time is determined by a chart based on experimental data relating sustainable permeabilities to the concentration of solids in the water surrounding the membranes.
- the flow rate of feedwater and consolidated retentate were adjusted as necessary for a recovery rate of 95%.
- the feed water is assumed to have an initial concentration of solids of 10 mg/l.
- the membrane modules were assumed to be continuously aerated at a constant rate that would result in a total cassette flow of about 3800 litres per minute (for a velocity of 0.05 m/s) upwards through the centre of each cassette and a downward flow of 1900 litres per minute down each of the upstream and downstream edges of each membrane module.
- the model assumes that all of this cassette flow flows downwards between adjacent membrane modules.
- the model also assumes that the water between adjacent membrane modules mixes completely such that 50% of the water flowing downwardly along the edge of a cassette, or 950 litres per minute is entrained in the flow moving upward through each adjacent membrane module.
- the model further assumes that any by-pass flow around the membrane modules 28 along the sides of the tank is negligible.
- the test reactor was modelled in continuous bleed operation, that is filtered permeate, consolidated retentate and feed water are all flow continuously.
- the concentration of solids at each cassette is shown in FIG. 11 and increases from approximately 20 mg/l to 200 mg/l. As shown, the average concentration of solids surrounding the cassettes is significantly reduced while the consolidated retentate has a concentration of solids of 200 mg/l.
- the expected permeability of the membrane modules is also shown in FIG. 11 which suggests that such a reactor will operate continuously with an average permeability of over 200 L/m 2 /h/bar with 8 of 12 membrane modules operating with permeabilities above that average.
- the concentration of solids throughout the tank would be 200 mg/l and all membrane modules would operate at a permeability of approximately 155 L/m 2 /h/bar which would exceed the recommended operating conditions of the ZW500 membrane modules.
- the model of the first modelling experiment was repeated without deconcentrations assuming a varying number of cassettes between 1 and 16. As shown in FIG. 15 , the average concentration in the tank is reduced with even 2 or 4 cassettes and that with 6 or more cassettes, the average concentration of solids in the tank is nearly half of the concentration (200 mg/l) that would occur in the model with a conventional fully mixed process.
- a sixth modelling experiment the model of the first modelling experiment was repeated without deconcentrations but with the recovery rate varying from 90% to 99% and compared to a model of a conventional fully mixed continuous bleed process operating at the same recovery rates.
- a conventional process operating at a 95% recovery rate will have an average concentration of solids in the tank of 200 mg/l.
- the process and apparatus modelled for a long aerated filter train could be operated at a recovery rate of approximately 97.5% with the same average concentration of solids which would result in 50% less consolidated retentate to be disposed of.
- FIGS. 5A and 5B an actual experimental apparatus was constructed and operated similar to FIGS. 5A and 5B .
- the dimensions of the tank were as described for the modelling experiments above, but 16 cassettes of 8 ZW500 membrane modules each were installed consecutively 20 cm apart from each other in the direction of the flow path and used with constant aeration.
- the apparatus was run continuously without deconcentrations at a recovery rate of 91%.
- the yield was maintained at a constant 93 litres/second with 9.4 litres/second of consolidated retentate continuously leaving the tank.
- Colour was monitored at each cassette along the tank as an indicator of the concentration of solids at each cassette. As shown in FIG. 17 , the concentration of solids increased significantly only in the downstream 20% of the tank with most cassettes operating in relatively clean water.
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Abstract
A process for operating filtering membranes submerged in a tank involves, in one aspect, periodically deconcentrating the tank by partially emptying and refilling the tank with fresh water. The emptying and refilling may be performed generally simultaneously or sequentially. In another embodiment, the membrane modules are arranged in a series of filtration zones between a feed water inlet and a retentate outlet of a tank and the zone adjacent the outlet is emptied and refilled.
Description
- This is a continuation of U.S. patent application Ser. No. 11/006,626 filed Dec. 8, 2004 which is a continuation of U.S. patent application Ser. No. 10/098,365, filed Mar. 18, 2002; which is a division of U.S. patent application Ser. No. 09/444,414, filed Nov. 22, 1999; which is an application claiming the benefit under 35 USC 119(e) of U.S. provisional patent application No. 60/109,520, filed Nov. 23, 1998. U.S. application Ser. Nos. 11/006,626, 10/098,365, 09/444,414 and 60/109,520 are incorporated herein, in their entirety, by this reference to them.
- This invention relates to the use of ultrafiltration or microfiltration membranes to treat water, and more particularly to the design and operation of reactors which use immersed membranes as part of a substantially continuous process for filtering water containing low concentrations of solids, for example for producing potable water.
- Immersed membranes are used for separating a permeate lean in solids from tank water rich in solids. Feed water flowing into a tank containing immersed membranes has an initial concentration of solids. Filtered permeate passes through the walls of the membranes under the influence of a transmembrane pressure differential between a retentate side of the membranes and a permeate side 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 must be removed from the tank in order to prevent rapid fouling of the membranes which occurs when the membranes are operated in water containing a high concentration of solids.
- In a continuous fully mixed process, there is typically a continuous bleed of tank water rich in solids, which may be called retentate. Unfortunately, while this process preserves a mass balance, the tank water must contain a high concentration of pollutants or the process will generate large volumes of retentate.
- For example, if a fully mixed continuous bleed process is operated at a recovery rate of 95% (ie. 95% of the feed water becomes filtered permeate), only 5% of the feed water leaves the tank as retentate. To preserve a mass balance of solids, the retentate must have a concentration of
pollutants 20 times that of the feed water. The concentration of solids in the retentate, however, is the same as the concentration of solids in the tank since the retentate is drawn from the tank water. Accordingly, the tank water has a high concentration of pollutants at all times. Operating at a lower recovery rate, 80% for example, results in tank water having a lower concentration of solids but the cost of transporting excess feedwater and then disposing of excess retentate also increases. - Another process involves filtering in a batch mode. For example, PCT Publication No. WO 98/28066 describes a process in which retentate is not withdrawn continuously. Instead, the tank water is drained to remove the accumulated solids from time to time. The tank is then refilled with fresh feed water and operation continues. While regular operation is interrupted in this method, there is a period directly after the tank is refilled in which the membranes are operated in relatively solids lean tank water. For feed water with low suspended solids, the intervals between drainings may be long enough that the benefit gained by emptying the tank offsets the loss in production time.
- With either process, as filtered water is permeated through the membranes the solids in the tank water foul the membranes. The rate of fouling is related to the concentration of solids in the tank water and can be reduced but not eliminated in a fully mixed continuous bleed process by lowering the recovery rate. Further, the solids may be present in the feed water in a variety of forms which contribute to fouling in different ways. To counter the different types of fouling, many different types of cleaning regimens may be required. Such cleaning usually includes both physical cleaning and chemical cleaning.
- The most frequently used methods of physical cleaning are backwashing and aeration. These methods are typically performed frequently and thus may influence the filtering process. In backwashing, permeation through the membranes is stopped momentarily. Air or water flow through the membranes in a reverse direction to physically push solids off of the membranes. In aeration, bubbles are produced in the tank water below the membranes. As the bubbles rise, they agitate or scrub the membranes and thereby remove some solids while creating an air lift effect and circulation of the tank water to carry the solids away from the membranes. These two methods may also be combined. For example, in a fully mixed continuous bleed process as described above, aeration may be provided continuously and the membranes backwashed periodically while permeation is temporarily stopped. Alternately, PCT Publication No.
WO 98/28066 mentioned above describes a process in which permeation continues for 15 minutes and then stops while the membranes are aerated for 2 minutes and 15 seconds. After the first minute of aeration, the membranes are backwashed for 15 seconds. - Chemical cleaning is typically performed less frequently than backwashing or aeration. According to one class of methods, permeation is stopped and a chemical cleaner is backwashed through the membranes. In some cases, the tank is emptied during or after the cleaning event so that the chemical cleaner can be collected and disposed of. In other cases, the tank remains filled and the amount of chemical cleaner in a cleaning event is limited to an amount that is tolerable for the application.
- Known fully mixed continuous bleed processes rely heavily on aeration, backwashing and chemical cleaning to maintain membrane permeability. The cleaning methods all damage the membranes over time. In addition, backwashing with permeate or chemical cleaner interrupts permeation and reduces the yield of the process. Aeration requires energy which add to the operating costs of a reactor and the resulting circulation of tank water requires significant open space in the tank. Processes that involve frequently draining the tank require less cleaning in some cases. Particularly in large systems, however, loss in production time can be high because it is difficult to drain a large municipal or industrial tank quickly. In some cases, the tank is raised and fitted with a large number of drains to promote rapid draining but these techniques increase the cost of an installation.
- It is an object of the present invention to provide a process and apparatus which uses immersed filtering membranes as part of a substantially continuous process for filtering water containing low concentrations of solids, for example to produce potable water.
- In one aspect, the invention provides an improvement to a process for filtering water using membranes immersed in an open tank. The improvement includes reducing the concentration of solids in the water in the tank from time to time through deconcentrations. The deconcentrations are performed by withdrawing retentate rich in solids and simultaneously replacing it with a similar volume of feed water such that the membranes remain immersed during the deconcentration and permeation is not interrupted. The volume of retentate removed in a deconcentration is between 40% and 300% of the volume of water normally in the tank. At the end of a deconcentration, the water in the tank has 40% or less of the average concentration of solids in the tank before the deconcentration. Preferably, one or more of aeration or backwashing are biased towards a later part of a period between deconcentrations.
- In another aspect, the invention provides an immersed membrane filter. One or more membrane modules are placed in an open tank spaced consecutively along a general flow path between an inlet and an outlet. The distance between membrane modules (measured along the flow path) is less than one half of the length of each membrane module (measured along the flow path). The total length of all of the membrane modules (measured along the flow path) excluding the distance between them (along the flow path) is at least twice the width of the membrane modules (measured perpendicular to the flow path). A similar flux of permeate is collected from the various membrane modules. Agitators, preferably aerators, are provided below the membrane modules and operated substantially throughout permeation to entrain tank water around the membrane modules and flow the water containing solids upwards through the modules. Tank water flows through a plurality of membrane modules sequentially in relation to the flow path before leaving the tank at the outlet. Preferably, one or more of aeration, backwashing and packing density are biased towards the outlet end of the tank. The tank may be deconcentrated from time to time as described above.
- In another aspect, the invention provides an open tank divided into a plurality of sequential filtration zones. Partitions between the filtration zones substantially prevent mixing between the filtration zones but for permitting water containing solids to flow from the first filtration zone to the last filtration zone through the filtration zones in sequence. One or more membrane modules are placed in each filtration zone and a similar permeate flux is withdrawn from each filtration zone. A non-porous casing around the one or more membrane modules in each filtration zone provides a vertical flow channel through the one or more membrane modules. Tank water flows downwards through the one or more membrane modules in each filtration zone. A plurality of passages connect the bottom of the vertical flow channel in one filtration zone to the top of the vertical flow channel of another filtration zone and permit the tank water to flow from the first filtration zone to the last filtration zone through the filtration zones consecutively. The passages include a weir at the tops of the partitions. Preferably, packing density, aeration and backwashing are biased towards an outlet end of the tank. The tank may be deconcentrated from time to time as described above. Alternatively, the last filtration zone may be deconcentrated by draining and refilling it while permeation from the last filtration zone is stopped.
- Preferred embodiments of the invention will now be described below with reference to the following figures:
-
FIG. 1 is a schematic representation of a general immersed membrane reactor. -
FIGS. 2, 3 and 4 are representations of various membrane modules. -
FIG. 5A is a schematic representation of an embodiment of the invention with a long aerated filtration train. -
FIG. 5B is a schematic cross section of the embodiment ofFIG. 5A . -
FIG. 6 is an elevation view of a membrane module adapted for use with a filtering reactor having membrane modules in series. -
FIG. 7 is a plan view of the membrane module ofFIG. 2 . -
FIG. 8 is a schematic representation of a filtering reactor having membrane modules in series. -
FIGS. 9 and 10 show tanks with alternate shapes. -
FIGS. 11 through 16 are charts showing the results of modelling experiments performed according to an embodiment similar to that ofFIG. 5 . -
FIG. 17 is a chart showing the results of an experiment performed with an embodiment similar to that ofFIG. 5 . - General Filtration Process
- The following description of a filtration process applies generally to the embodiments which are described further below unless inconsistent with the description of any particular embodiment.
- Referring now to
FIG. 1 , afirst reactor 10 is shown for treating a liquid feed having solids to produce a filtered permeate substantially free of solids and a consolidated retentate rich in solids. Such areactor 10 has many potential applications such as separating clean water from mixed liquor in a wastewater treatment plant or concentrating fruit juices etc., but will be described below as used for creating potable water from a natural supply of water such as a lake, well, or reservoir. Such a water supply typically contains colloids, suspended solids, bacteria and other particles which must be filtered out and will be collectively referred to as solids. - The
first reactor 10 includes afeed pump 12 which pumps feedwater 14 to be treated from awater supply 16 through aninlet 18 to atank 20 where it becomestank water 22. Alternatively, a gravity feed may be used withfeed pump 12 replaced by a feed valve. During permeation, thetank water 22 is maintained at a level which covers a plurality ofmembranes 24. Eachmembrane 24 has a permeate side which does not contact thetank water 22 and a retentate side which does contact thetank water 22. Preferably, themembranes 24 are hollow fibre membranes for which the outer surface of themembranes 24 is preferably the retentate side and thelumens 25 of themembranes 24 are preferably the permeate side. - Each
membrane 24 is attached to at least one but preferably twoheaders 26 such that the ends of themembranes 24 are surrounded by potting resin to produce a watertight connection between the outside of themembranes 24 and theheaders 26 while keeping thelumens 25 of themembranes 24 in fluid communication with a permeate channel in at least oneheader 26.Membranes 24 andheaders 26 together form of amembrane module 28. The permeate channels of theheaders 26 are connected to apermeate collector 30 and apermeate pump 32 through apermeate valve 34. When permeate pump 32 is operated and permeatevalve 34 opened, a negative pressure is created in thelumens 25 of themembranes 24 relative to thetank water 22 surrounding themembranes 24. The resulting transmembrane pressure is typically between 1 kPa and 150 kPa and more typically between 10 kPa and 70 kPa and draws tank water 22 (then referred to as permeate 36) throughmembranes 24 while themembranes 24 reject solids which remain in thetank water 22. Thus, filteredpermeate 36 is produced for use at apermeate outlet 38 through anoutlet valve 39. Periodically, astorage tank valve 64 is opened to admitpermeate 36 to astorage tank 62. The filteredpermeate 36 may require post treatment before being used as drinking water, but should have acceptable levels of colloids and other suspended solids. - In a municipal or
industrial reactor 10, discrete units each having a plurality ofmembranes 24 are assembled together into larger units calledmembrane modules 28 which may also be referred to as a cassette. Examples ofsuch membrane modules 28 are shown inFIGS. 2, 3 and 4 in which the discrete units arerectangular skeins 8. Eachrectangular skein 8 typically has a bunch between 2 cm and 10 cm wide ofhollow fibre membranes 24. Thehollow fibre membranes 24 typically have an outside diameter between 0.4 mm and 4.0 mm and are potted at a packing density between 10% and 40%. Thehollow fibre membranes 24 are typically between 400 mm and 1,800 mm long and mounted with between 0.1% and 5% slack. Themembranes 24 have an average pore size in the microfiltration or ultrafiltration range, preferably between 0.003 microns and 10 microns and more preferably between 0.02 microns and 1 micron. The preferred number ofmembrane modules 28 varies for different applications depending on factors such as the amount of filteredpermeate 36 required and the condition of thefeed water 14. - Referring to
FIG. 2 , for example, a plurality ofrectangular skeins 8 are connected to acommon permeate collector 30. Depending on the length of themembranes 24 and the depth of thetank 20, themembrane modules 28 shown inFIG. 2 may also be stacked one above the other. Referring toFIGS. 3 and 4 , therectangular skeins 8 are shown in alternate orientations. InFIG. 3 , themembranes 24 are oriented in a horizontal plane and thepermeate collector 30 is attached to a plurality ofrectangular skeins 8 stacked one above the other. InFIG. 4 , themembranes 24 are oriented horizontally in a vertical plane. Depending on the depth of theheaders 26 inFIG. 4 , thepermeate collector 30 may also be attached to a plurality of thesemembrane modules 28 stacked one above the other. The representations of themembrane modules 28 inFIGS. 2, 3 , and 4 have been simplified for clarity,actual membrane modules 28 typically havingrectangular skeins 8 much closer together and a large cassette often having many morerectangular skeins 8. -
Membrane modules 28 can be created with skeins of different shapes, particularly cylindrical, and with skeins of looped fibres attached to a single header. Similar modules or cassettes can also be created with tubular membranes in place of thehollow fibre membranes 24. For flat sheet membranes, pairs of membranes are typically attached to headers or casings that create an enclosed surface between the membranes and allow appropriate piping to be connected to the interior of the enclosed surface. Several of these units can be attached together to form a cassette of flat sheet membranes. - Commercially
available membrane modules 28 include those based on ZW 500 units made by ZENON Environmental Inc. and referred to in the examples further below. Each ZW 500 unit has two rectangular skeins of hollow fibre membranes having a pore size of approximately 0.1 microns oriented as shown inFIG. 2 with a total membrane surface area of approximately 47 square metres. In plan view, each ZW 500 unit is about 700 mm long and about 210 mm wide. Typically, several ZW 500 units are joined together into a cassette to provide a plurality of parallelrectangular skeins 8. For example, amembrane module 28 of 8 ZW 500 units is about 1830 mm by 710 mm, some additional space being required for frames, connections and other related apparatus. - Referring again to
FIG. 1 ,tank water 22 which does not flow out of thetank 20 through thepermeate outlet 38 flows out of thetank 20 through adrain valve 40 and aretentate outlet 42 to adrain 44 asretentate 46 with the assistance of aretentate pump 48 if necessary. Theretentate 46 is rich in the solids rejected by themembranes 24. - To provide aeration, an
air supply pump 50 blows ambient air, nitrogen or other suitable gases from anair intake 52 throughair distribution pipes 54 to aerator 56 which disperses scouring bubbles 58. Thebubbles 58 rise through themembrane module 28 and discourage solids from depositing on themembranes 24. In addition, where the design of thereactor 10 allows thetank water 22 to be entrained in the flow of risingbubbles 58, thebubbles 58 also create an air lift effect which in turn circulates thelocal tank water 22. - To provide backwashing, permeate
valve 34 andoutlet valve 39 are closed andbackwash valves 60 are opened.Permeate pump 32 is operated to push filteredpermeate 36 fromretentate tank 62 throughbackwash pipes 61 and then in a reverse direction throughpermeate collectors 30 and the walls of themembranes 24 thus pushing away solids. At the end of the backwash,backwash valves 60 are closed, permeatevalve 34 andoutlet valve 39 are re-opened andpressure tank valve 64 opened from time to time to re-fillretentate tank 62. - To provide chemical cleaning, a cleaning chemical such as sodium hypochlorite, sodium hydroxide or citric acid are provided in a
chemical tank 68.Permeate valve 34,outlet valve 39 andbackwash valves 60 are all closed while achemical backwash valve 66 is opened. Achemical pump 67 is operated to push the cleaning chemical through achemical backwash pipe 69 and then in a reverse direction throughpermeate collectors 30 and the walls of themembranes 24. At the end of the chemical cleaning,chemical pump 67 is turned off andchemical pump 66 is closed. Preferably, the chemical cleaning is followed by a permeate backwash to clear thepermeate collectors 30 andmembranes 24 of cleaning chemical before permeation resumes. - Preferably, aeration and backwashing clean the membranes sufficiently so that permeation can continue over extended periods of time. Permeate backwashes typically last for between 5 seconds and two minutes and are typically performed between once every 5 minutes and once every 3 hours. If such permeate backwashes are performed between more intensive restorative cleaning events, the filtering process is still considered continuous since permeation is only stopped momentarily. Similarly, if chemical cleaning is performed in short duration chemical backwashes while the
tank 20 remains full oftank water 22, the process is still considered continuous. In the cases, however, flow rates ofpermeate 36,retentate 46 and feedwater 14 are calculated as average flow rates over a day or such longer period of time as appropriate. In the description of the embodiments and examples which follow, flow rates of processes that are periodically interrupted as described above are measured as average flow rates unless they are described otherwise. - Rapid Flush Deconcentration
- Referring still to
FIG. 1 , in rapid flush deconcentration the filtration process proceeds as a number of repeated cycles which end with a procedure to deconcentrate thetank water 22, the procedure being referred to as a deconcentration. The cycles usually begin at the end of the preceding deconcentration. Some cycles, however, begin when anew reactor 10 is first put into operation or after intensive restorative cleaning or other maintenance procedures which require thetank 20 to be emptied. Regardless, the cycle begins with thetank 20 filled withmembranes 24 submerged intank water 22 with an initial concentration of solids similar to that of thefeed water 14. - At the start of a cycle, permeate pump 32 is turned on and sucks
tank water 22 through the walls of themembranes 24 which is discharged as filteredpermeate 36.Drain valves 40 initially remain closed and the concentration of solids in thetank water 22 rises. Whiledrain valves 40 are closed, thefeed pump 12 continues to pumpfeed water 14 into thetank 20 at about the same rate that filteredpermeate 36 leaves the tank such that the level of thetank water 22 is essentially constant during permeation. Aeration and backwashing are provided as required. - After a desired period of time, the
tank water 22 is deconcentrated. The desired period of time between deconcentrations may be based on the concentration of solids in thetank water 22 but preferably is chosen to achieve a desired recovery rate. For ZW 500 membrane modules used with typical feed water supplies operating with constant aeration and periodic backwashing between deconcentrations, a recovery rate of 95% (ie. 95% of the feed water becomes filtered permeate) or more can be maintained and is preferred when an operator wishes to discharge minimal amounts ofconsolidated retentate 46. This recovery rate results in a concentration of solids in thetank water 22 at the start of the deconcentrations approximately 20 times that of the feed water. However, the inventors have observed in tests performed with continuous membrane filtration processes and feed water having turbidity of 0.5 to 0.6 ntu and apparent colour of 33 Pt. Co. units that the rate at which the permeability of membranes decreases over time rises dramatically when the recovery rate is increased to over 93%. Accordingly, if the volume of wasted retentate is a minor factor, then the period between deconcentrations may be chosen to yield a 90% to 95% recovery rate or less. Typical cycle times when using ZW 500 units range from about 2 to 3 hours at a recovery rate of 90% and 4 to 5 hours at a recovery rate of 95% although cycle times will vary for other membrane modules. - The deconcentrations comprise a rapid flush of the
tank water 22 while maintaining the level oftank water 22 above the level of themembranes 24 and continuing permeation. To perform the rapid flush deconcentration, thedrain valves 40 are opened and retentate pump 48 rapidly drawsretentate 46 rich in solids out of thetank 20 if gravity flow alone is insufficient. Simultaneously, feedpump 12 increases the flow rate offeed water 14 into thetank 20 by an amount corresponding to the flow rate ofretentate 46 out of thetank 20. Preferably, theretentate 46 is removed at a sufficient rate, assisted byretentate pump 48 if necessary, such that thetank water 22 is not diluted significantly by mixing withincoming feed water 14 before it is flushed out of thetank 20. Some dilution necessarily occurs, and it is preferable to stop the flow ofconsolidated retentate 46 while thetank water 22 still has a concentration of solids greater than the concentration of solids in thefeed water 14 to avoid withdrawing an unacceptably high volume ofconsolidated retentate 46. However, the volume ofconsolidated retentate 46 withdrawn may exceed the volume of water in thetank 20. Preferably, aeration and any other source of mixing are turned off to minimize dilution of theretentate 46 and between 100% to 150% of the average volume of thetank water 22 is discharged during the rapid flush deconcentration. If aeration must be left on to provide continued cleaning, a higher volume oftank water 22 is discharged. More preferably, between 100% and 130% of the volume of the average volume of thetank water 22 is discharged. The total discharge time is typically less than 20 minutes and preferably less than 10 minutes. If there is aeration or other mixing at the time of the rapid flush, then between 150% and 300%, more preferably between 150% and 200%, of the average volume of thetank water 22 is discharged and the total discharge time is less than 25 minutes. After the deconcentration, thetank water 22 preferably has less than 40% of the concentration of solids that was present in thetank water 22 prior to the deconcentration. Where thefeed water 14 has high turbidity or where high recovery rates are used, however, thetank water 22 after a deconcentration preferably has less than 20% of the concentration of solids that was present in thetank water 22 prior to the deconcentration.Retentate 46 is typically disposed of down adrain 44 to a sewer or to the source of water where it initially came from. - Like a process without deconcentrations, there must still be a balance of solids and water between the
feed water 14,retentate 46 and filteredpermeate 36 over repeated cycles. Thus for a selected recovery rate, the average amount of solids in theretentate 46 in a process with deconcentrations will be the same as for a process without deconcentrations. Since theretentate 46 is typically diluted in rapid flush deconcentrations, however, thetank water 22 must have a higher concentration of solids immediately before a deconcentration compared to the constant concentration of solids in a fully mixed continuous bleed process. By replacing at least a substantial portion of the existingtank water 22 withfresh feed water 14, however, permeation continues in the next cycle with relativelyclean tank water 22 until solids again build up in thetank water 22 and another deconcentration is performed. Thus the average concentration of solids in thetank water 22 over time is an intermediate value between that of thefeed water 14 and theconsolidated retentate 46 and less than the constant concentration of solids in a fully mixed continuous bleed process at the same recovery rate. While thetank water 22 has a lower concentration of solids the membranes foul less rapidly. Accordingly, increased flux ofpermeate 36 is observed at a set transmembrane pressure or a higher transmembrane pressure can be used at the beginning of a cycle without excessive fouling of themembranes 24. - Preferably, a reduced flow rate of air bubbles 58 is initially supplied to the
tank 20 when the concentration of solids is low and themembranes 24 foul more slowly. As the concentration of solids rises in thetank water 22, the flow rate of air is also increased. Alternately, aeration is only provided directly before the deconcentration. In this way, excess air is not supplied while the concentration of solids is low in thetank water 22. Similarly, the frequency or duration of backwashing may be decreased when the concentration of solids in thetank water 22 is low to minimize loss in production due to backwashing. To the extent that aeration can be made to coincide with backwashing, the effectiveness of the aeration is increased since is does not have to work against the transmembrane pressure. - Despite the aeration, periodic backwashing, and periodic deconcentrations of the
tank water 22, long term fouling of the membranes may still occur, although more slowly than in a process without deconcentrations. As long term fouling occurs, power to thepermeate pump 32 may be increased to increase the transmembrane pressure across the walls of themembranes 24 to compensate for the reduced permeability. Eventually, a specified maximum transmembrane pressure for the system or a minimum tolerable permeability of themembranes 24 will be reached. At this time, intensive restorative cleaning is done. For ZeeWeed (a trade mark)brand membranes 24, intensive cleaning is preferably done when the transmembrane pressure exceeds 54 kPa or the permeability drops below 200 litres per square metre per hour per bar (L/m2/h/bar) at normal operating temperatures. The tank is typically emptied during the intensive maintenance cleaning, but this is independent of the periodic deconcentrations and occurs only infrequently, between once every two weeks to once every two months. - Long Aerated Filter Trains
- Referring now to
FIGS. 5A and 5B , a portion of another embodiment is shown. Components not illustrated inFIG. 5A or 5B are similar to those ofFIG. 1 and reference may be had toFIG. 1 to understand the general operation of the present embodiment. In this embodiment, asecond reactor 70 has a rectangular (in plan view)second tank 120 with aninlet end 72 and an outlet end 74. Preferably, theinlet end 72 is at one short end (as seen in plan view) of thesecond tank 120 and has aninlet 18 and the outlet end 74 is at the opposite short end of thesecond tank 120 and has aretentate outlet 42. During permeation, thesecond tank 120 is filled withtank water 22 which moves generally in ageneral flow path 76 between theinlet 18 and theretentate outlet 42, the word general meaning that the actual flow path of a volume oftank water 22 may deviate substantially from theflow path 76 as will be described below, but the average flow oftank water 22 has at least a component in the direction of theflow path 76. -
Membrane modules 28 are arranged in thesecond tank 120 in series along theflow path 76. Typically themembrane modules 28 are spaced apart horizontally along theflow path 76 to allow room for associated apparatus, installation and maintenance, and to provide a small movable volume oftank water 22 between eachmembrane module 28. This space is preferably less than one half of the length (measured along the flow path 76) of themembrane module 28 and for ZW 500 units is typically about 20 cm. Referring toFIGS. 5A and 5B , greater space is provided above, below and beside themembrane modules 28. For example, the distance between themembrane modules 28 and the long walls of thesecond tank 120 is typically about one half of the width of the membrane modules 28 (measured perpendicular to the flow path 76). Preferably, 6 ormore membrane modules 28 in series are used. More preferably, long trains of 12 or 16 ormore membrane modules 28 in series are used. Where a large system is required, eachmembrane module 28 is typically of the size of a cassette of 6 to 12 ZW 500 units. The total length of all of the membrane modules 28 (measured along the flow path 76) excluding the space between them (also measured along the flow path 76) is at least twice, and preferably at least four times, the width of the membrane modules 28 (measured perpendicular to the flow path 76). -
Feed water 14 continuously enters thesecond tank 120 at theinlet 18.Permeate pump 32 continuously withdraws filteredpermeate 36 throughmembranes 24 of eachmembrane module 28 andconsolidated retentate 46 continuously leaves thesecond tank 120 throughretentate outlet 42. The path of a volume oftank water 22, however, passes in series through some or all of themembrane modules 28. However, since solids are rejected by themembranes 24, the concentration of solids in the volume oftank water 22 increases downstream of eachmembrane module 28 it passes through. Thus the concentration of solids in the volume oftank water 22 increases from theinlet 18 to theretentate outlet 42 along its flow path. Downstream of themembrane module 28 nearest to theretentate outlet 42, thetank water 22 has a high concentration of solids of at least five times that of thefeed water 14, preferably at least 14 times that of thefeed water 14 and more preferably at least 20 times that of thefeed water 14. Conversely,tank water 22 near theinlet 18 has a concentration of solids similar to that of thefeed water 14. In long trains ofmembrane modules 28 in which the length of the membrane modules 28 (excluding the spaces between them) is four or more times their width, up to 75% of themembrane modules 28 operate in tank water with minimal solids concentration, the concentration of solids rising sharply only near theoutlet 42. - Since the concentration of solids in the
tank water 22 rises from theinlet 18 to theretentate outlet 42,membrane modules 28 near theinlet 18 operate in water that has a substantially lower concentration of solids than theconsolidated retentate 46 which flows out of theretentate outlet 42. The last membrane modules 28 (in the direction of the flow path 76) have a higher concentration of solids in thetank water 22 around them and are therefore likely to have reduced permeabilities. Thepermeate pump 32 may be placed near theoutlet 42 so that thelast membrane modules 28 will receive higher transmembrane pressures (relative to more distant membrane modules 28) to overcome their reduced permeability and provide more nearly even permeate flux from the set of membrane modules. The average concentration of solids in thetank water 22 is an intermediate concentration in relation to the concentration of solids in thefeed water 14 andconsolidated retentate 46. If the length of all of the membrane modules 28 (excluding spaces between them) is more than twice their width, the area of significantly reduced concentration can include more than half of thesecond tank 120. Thusconsolidated retentate 46 can be withdrawn having a high concentration of solids but the average concentration of solids in thetank water 22 is significantly less than the concentration of solids in theconsolidated retentate 46. The average permeability of themembrane modules 28 is increased as fouling occurs less rapidly. Since the permeability of themembranes 24 decreases rapidly when the concentration of solids is high, it is preferable ifmost membrane modules 28 operate intank water 22 having a concentration of solids less than 14 times that of the feed water and more preferably less than 10 times that of the feed water. - As mentioned above, the path of a volume of
tank water 22 passes in series through some or all of themembrane modules 28. This effect would not occur if thesecond reactor 70 operated like a completely stirred tank reactor. To counter this possibility, aeration is provided during the entire permeation cycle. While aeration is normally considered to be a mixing agent, in thesecond reactor 70 the inventors believe that the aeration (or alternately an agitator such as a rotating propeller) provided substantially throughout permeation encouragestank water 22 to flow through a plurality ofmembrane modules 28 sequentially in relation to the flow path when as will be explained below. - With the
inlet 18 andoutlet 42 at opposite ends of the tank, thetank flow 76 must have an average substantially horizontal flow frominlet 18 tooutlet 42. Themembrane modules 28, however, significantly resist such horizontal flow. Accordingly, the bulk of the horizontal flow has a tendency to by-pass the membrane modules by flowing beneath, over or beside them. The inventors believe that iftank water 22 readily by-passed themembrane modules 28, it would be difficult to avoid substantial mixing in thetank 20. - Assuming negligible horizontal flow through the
membrane modules 28, the horizontal velocity of by-pass flow typically ranges from about 0.05 to 0.3 m/s, decreasing towards theoutlet 42. Typical vertical velocities oftank water 22 upwards through themembrane module 28 are of a comparable magnitude, typically 0.05 to 0.2 m/s. Referring toFIGS. 5A and 5B , acassettes flow 78 is created in whichtank water 22 is drawn up into the bottom of amembrane module 28 released from the top of the membrane module, flows towards theoutlet 42 while descending to the bottom of thetank 20 where it is entrained in asecond membrane module 28 and so on. Thecassette flow 78 has a component flowing downwards besides the membrane modules 28 (as shown inFIG. 5B ) and a component flowing downwards between the membrane modules 28 (as shown inFIG. 5A ). The inventors have observed that the component flowing downwards besides themembrane modules 28 is about 90% of thecassette flow 78. The inventors believe that the flow component flowing downwards between themembrane modules 28 is much smaller than the flow downwards besides themembrane modules 28 because distance to the walls of thesecond tank 120 is greater than the distance betweenmembrane modules 28 and eachmembrane module 28 is surrounded by an upwards flow oftank water 22. Together, these factors result in a higher shear force inhibitingtank water 22 from flowing downwards betweenmembrane modules 28. -
Cassette flow 78 created within afirst membrane module 28 and flowing downwards betweenmembrane modules 28 likely mixes in part withtank water 22 similarly flowing downwards in thecassette flow 78 of anadjacent membrane module 28 and becomes part of thecassette flow 78 of theadjacent membrane module 28. Thus, a mixingflow 80 oftank water 22 circulating around amembrane module 28 may be drawn towards theinlet 18 by anupstream membrane module 28 or towards theretentate outlet 42 by adownstream membrane module 28. The degree of mixing in thesecond tank 120 may be expressed in relation to a recirculation rate defined as the flow rate of thecassette flow 78 through the centre of themembrane modules 28 divided by the flow rate of feedwater. Surprisingly, in modelling experiments to be described below, if thecassette flow 78 produces no net flow towards theinlet 18 or retentate outlet 42 (i.e. it is symmetrical about the membrane module 28) then the concentration of solids in thetank water 22 still increases along theflow path 76 even at unusually high recirculation rates and even under the assumption that the component ofcassette flow 78 downwards betweenadjacent membrane modules 28 is unusually high. - Although it is usually unnecessary, an operator may minimize mixing between
adjacent membrane modules 28 so that the concentration of solids in thesecond tank 120 will rise only near theretentate outlet 42 of thesecond tank 120 resulting in increased permeability in a greater number ofmembrane modules 28. Alternately, thesecond tank 120 can be made of a plurality of filtering zones wherein the outlet of a first filtering zone is connected to the inlet of a downstream filtering zone. The filtering zones may be created by breaking thesecond tank 120 into a plurality of containers or withbaffles 82 at the upper upstream edge or lower downstream edge of amembrane module 28 to restrictbackflows 80 flowing towards theinlet 18. Preferably, baffles are installed only onmembrane modules 28 located near theretentate outlet 42 where the rate of flow in theflow path 76 is reduced. - Membrane Modules in Series
- Referring now to
FIGS. 6 and 7 , anothersecond membrane module 110 havinghollow fibre membranes 24 is shown in elevation and plan view respectively. Themembranes module 110 is similar to that shown inFIG. 4 but the perimeter of thesecond membrane module 110 is surrounded by anon-porous casing 124 which defines a vertically orientedflow channel 126 through thesecond membrane module 110. Similar modules can be created withmembrane modules 28 as shown inFIGS. 2, 3 and 4 or with tubular or flat sheet membranes as described above. - Referring now to
FIG. 8 , athird reactor 128 has a plurality ofsecond membrane modules 110 in a plurality offiltration zones 130. Thethird reactor 128 has afeed pump 12 which pumps feedwater 14 to be treated from awater supply 16 through aninlet 18 to athird tank 140 where it becomestank water 22. During permeation, thefeed pump 12 is operated to keeptank water 22 at a level which covers themembranes 24. Thepermeate collector 30 of eachsecond membrane module 110 is connected to a set of pipes and valves as shown including a pair ofpermeate valves 144 and a pair ofbackwash valves 60. To withdraw permeate from asecond membrane module 110, its associatedpermeate valves 144 are opened while itsbackwash valves 60 are closed and an associatedpermeate pump 32 is turned on. The resulting suction creates a transmembrane pressure (“TMP”) from the outside of themembranes 24 to theirlumens 25. Themembranes 24 admit a flow of filteredpermeate 36 which is produced for use or further treatment at apermeate outlet 38. From time to time, apermeate storage valve 64 is opened to maintain a supply ofpermeate 36 in apermeate storage tank 62. Such an arrangement allowspermeate 36 to be withdrawn from eachfiltration zone 130 individually. Preferably, the permeate pumps 32 are operated to produce a similar flux ofpermeate 36 from eachfiltration zone 130. Since solids concentration in eachfiltration zone 130 differs, as will be explained further below, this typically requires each permeate pump 32 to be operated at a different speed. Alternatively, thesecond membrane modules 110 indifferent filtration zones 130 can be connected to acommon permeate pump 32. This will result in some variation in flux between the filtration zones 130 (because the downstreamsecond membrane modules 110 are likely to foul faster), but the amount of variation can be minimized by locating thepermeate pump 32 near theoutlet 42 as described above or by variations in aeration, backwashing and packing density to be described below. With any of these techniques, thesecond membrane modules 110 can be made to have similar permeate fluxes. -
Tank water 22 which does not flow out of thethird tank 140 through thepermeate outlet 38 flows out of thethird tank 140 through adrain valve 40 andretentate outlet 160 to adrain 44 asconsolidated retentate 46. Additional drains in each filtration zone 130 (not shown) are also provided to allow thethird tank 140 to be drained completely for testing or maintenance procedures. Theconsolidated retentate 46 is rich in the solids rejected by themembranes 24. Flow of theconsolidated retentate 46 may be assisted by aretentate pump 48 if required. Theinlet 18 andretentate outlet 160, however, are separated by thefiltration zones 130.Partitions 176 at the edges of thefiltration zones 130 force thetank water 22 to flow sequentially through thefiltration zones 130 in atank flow pattern 178. Thepartitions 176 have decreasing heights in the direction of thetank flow pattern 178 such that a difference in depth from onefiltration zone 130 to the next drives thetank flow pattern 178. The difference in depth betweenpartitions 176 varies with different applications, but is unlikely to be more than 1 m between the first andlast partition 176. Alternatively, flow from onefiltration zone 130 to the next could be through conduits and driven by differences in depth from onefiltration zone 130 to the next or driven by pumps. - While in normal operation, feed
pump 12 substantially continuously addsfeed water 14 to thethird tank 140 while one or more permeate pumps 32 substantially continuously withdrawpermeate 36. The process is typically operated to achieve a selected recovery rate defined as the portion offeed water 14 removed as permeate 36 (not includingpermeate 36 returned to thethird tank 140 during backwashing to be described further below) expressed as a percentage. The selected recovery rates is typically 90% or more and preferably 95% or more. - As the
tank water 22 moves from onefiltration zone 130 to the next, the solids concentration increases as solidslean permeate 36 is removed. This effect may be illustrated by a simplified example in which thethird reactor 128 shown inFIG. 8 is operated at an overall recovery rate of 95%. 100 flow units offeed water 14 having a concentration of 1 enters thethird tank 140 at theinlet 18. According to the recovery rate, 95 flow units leave thethird tank 140 aspermeate 36 while 5 flow units leave thethird tank 140 asconsolidated retentate 46. Assuming equal production from eachsecond membrane module 110, 19 flow units leave thethird tank 140 aspermeate 36 in each filtration zone. Assuming further (a) that all solids are rejected by themembranes 24 and (b) that the concentration of solids in afiltration zone 130 equals the concentration of solids in the flow to thenext filtration zone 130, the following chart is generated by applying a mass balance of fluid and solids to eachfiltration zone 130.Maximum Fil- Concen- Concen- tration tration Permeate Flow to tration Zone Flow In in inflow Flow out Next Zone in Zone 1 100 1 19 81 1.2 2 81 1.2 19 62 1.6 3 62 1.6 19 43 2.3 4 43 2.3 19 24 4.2 5 24 4.2 19 5 20 (to drain) - In comparison, if there were no
filtration zones 130 and the entirethird tank 140 was fully mixed, thetank water 22 would have aconcentration 20 times that of thefeed water 14 throughout. By providing a series ofsequential filtration zones 130 between theinlet 18 andretentate outlet 160, the concentration of solids in thetank water 22 in most of thefiltration zones 130 is significantly reduced. The reduced concentration of solids results in significantly reduced fouling of thesecond membrane modules 110 in theapplicable filtration zones 130. Among other benefits, less chemical cleaning is required for thesesecond membrane modules 110. Further, reduced aeration and backwashing routines are sufficient forindividual filtration zone 130 or groups offiltration zones 130 with reduced concentrations of solids. Unlike the embodiment above withoutseparate filtration zones 130, aeration is not required to preventtank water 22 from by passing the membrane modules and so less or even no aeration can be provided during substantial periods. Further, by forcingtank water 22 to flow through thecasings 124, aeration is not required to create local circulation oftank water 22 aroundsecond membrane modules 110. Accordingly, space in thethird tank 140 is not required for downcomers and thesecond membrane modules 110 can occupy 80% or more of the plan area or footprint of thetank 140. - Aeration is provided, nevertheless, to scour the
membranes 24 which can occur without creating an air lift effect in thetank water 22. To provide aeration, anair supply 50 associated with eachfiltration zone 130 is operable to blow air, nitrogen or other suitable gases throughair distribution pipes 54 to aheader 170 attached to a plurality ofaerators 56 below thesecond membrane module 110. During aeration, theaerators 56 emit scouringbubbles 58 below thesecond membrane module 110 which rise through themembranes 24. Thus aeration can be provided to eachfiltration zone 130 individually. - The
second membrane module 110 in eachfiltration zone 130 can also be backwashed individually by closing its associatedpermeate valves 144 and opening its associatedbackwash valves 60. The associated permeate pump 32 (or alternatively, a separate pump) is then operated to drawpermeate 36 from thepermeate storage tank 62 and pump it through thepermeate collector 30 and, ultimately, through themembranes 24 in reverse direction relative to permeation. Preferably thesecond membrane modules 110 inadjacent filtration zones 130 are not backwashed at the same time. The backwash typically lasts for between 15 seconds and one minute and involves a flux one to three times the permeate flux but in a reverse direction. Accordingly, the level of thetank water 22 in the backwashedfiltration zone 130 rise temporarily causingmore tank water 22 to flow to thenext filtration zone 130. Preferably, thedownstream partition 176 in each filtration zone is sufficiently lower than theupstream partition 176 such thattank water 22 does not flow over anupstream partition 176 during backwashing. - To achieve a higher density of
membranes 24 in thethird tank 140, thesecond membrane modules 110 are sized to nearly fill each filtration zone. Further, thesecond membrane modules 110 are positioned such thattank water 22 or feedwater 14 flowing into afiltration zone 130 must flow first through theflow channel 126 of thesecond membrane module 110. Thetank flow 178 thus generally flows downwards through eachsecond membrane module 110 then upwards outside of eachsecond membrane module 110 and over thedownstream partition 176. Accordingly, thetank flow 178 is transverse to themembranes 24 and generally inhibits solids-rich zones oftank water 22 from forming near themembranes 24. During backwashing, thetank flow 178 may temporarily flow upwards through thesecond membrane module 110 if the top of thecasing 24 around thesecond membrane module 110 is located near the normal level of thetank water 22. Such reverse flow does not significantly effect thegeneral tank flow 178 but it is preferred if during backwashing thetank water 22 does not overflow thesecond membrane module 110. In this way, after backwashing stops, there is a momentarily increasedtank flow 178 which assists in moving solids from near the bottom of thesecond membrane module 110 to thenext filtration zone 130. Forsecond membrane modules 110 with minimal aeration, the tank flow through asecond membrane module 110 approaches a plug flow and there is an increase in concentration of solids as thetank water 22 descends through thesecond membrane module 110. Accordingly,membranes 24 near the top of thesecond membrane module 110 experience a concentration of solids even lower than that predicted by the chart above, and comparatively more solids attach to thelower membranes 24. During aeration, thebubbles 56 rise upwards against thetank flow 178 and no space for downcomers is required in thefiltration zones 130. - Combining Long Aerated Filter Trains and Membrane Modules in Series with Rapid Flush Deconcentration.
- In another embodiment of the invention, the embodiments described with reference to
FIGS. 5 and 8 are operated in cycles including rapid flush deconcentrations. The resulting temporal reduction in concentration of solids produced by the deconcentrations works to further the effect of the spatial reductions in concentration of solids. With reference toFIGS. 5A and 5B or 8, at the start of a cycle, thesecond tank 120 orthird tank 140 is filled withtank water 22. Filteredpermeate 36 is withdrawn from thesecond tank 120 orthird tank 140 whiledrain valves 40 remain at least partially and preferably completely closed so that thetank water 22 becomes more concentrated with solids until a deconcentration is indicated as described above. - Permeation continues while the
second tank 120 orthird tank 140 is deconcentrated by simultaneously withdrawingconsolidated retentate 46 from thesecond tank 120 orthird tank 140 and increasing the rate that feedwater 14 enters thesecond tank 120 orthird tank 140 to maintain the level oftank water 22 above themembranes 24 during the flushing operation. When thetank water 22 is deconcentrated by a rapid flush while permeation continues, the volumes of water removed from thesecond tank 120 orthird tank 140 can be the same as those described above. Preferably, however, since only the downstream portion of thesecond tank 120 orthird tank 140 containstank water 22 at a high concentration of solids, lower flush volumes may be used since only the downstream part of thetank water 22 requires deconcentration. With the apparatus ofFIG. 8 or with the apparatus ofFIGS. 5A and 5B in which aeration is turned of during the deconcentration, between 20% and 75% of the volume of thetank water 22 is preferably removed and more preferably between 20% and 50%. If there is aeration at the time of the deconcentration with the apparatus ofFIGS. 5A and 5B , between 40% and 150% of the volume of thetank water 22 is preferably flushed, and more preferably between 40% and 75%. With the apparatus ofFIG. 8 , deconcentrations are preferably performed directly after backwashing events so that the increased flux of thetank flow 178 will entrain more solids. - Deconcentrations can also be performed by stopping permeation and the flow of
feed water 14 into thesecond tank 120 orthird tank 140 whileretentate 46 is withdrawn. The level of thetank water 22 drops and so thesecond tank 120 orthird tank 140 must first be refilled before permeation can resume. As suggested above, this process avoids dilution of theretentate 46 withfeed water 14 but also interrupts permeation. In the apparatus ofFIG. 8 , however, thelast filtration zone 130 can be drained separately while permeation is stopped in thatfiltration zone 130 only. Compared to a process in which a tank is emptied, such deconcentrations are performed more frequently but involve less volume each which reduces the capacity of thedrain 44 required. In addition, this technique advantageously allowstank water 22 rich in solids to be withdrawn while permeating throughmost membrane modules 28 and without diluting theretentate 46. While the flow offeed water 14 can be stopped completely while thelast filtration zone 130 is emptied, the flow path over thelast partition 176 is preferably fitted with a closure such as agated weir 180 or a valved conduit. The closure is shut at the start of the deconcentration which preventstank water 22 from flowing over thepartition 176 after thedrain valve 40 is opened.Retentate pump 48 may be operated to speed the draining if desired.Feed water 14 continues to be added to thethird tank 140 during the deconcentration until the level of thetank water 22 rises in thedownstream filtration zones 130 to the point where appreciable reverse flow may occur across thepartitions 176. After thelast filtration zone 130 is emptied,retentate pump 48 is turned off (if it was on) anddrain valve 40 is closed. The closure is opened releasing an initially rapid flow oftank water 22 which fills a portion of thelast filtration zone 130. The flow offeed water 14 is increased until the remainder of thelast filtration zone 130 is filled. To avoid damage to themembranes 24 during rapid flows oftank water 22, baffles (not shown) are preferably installed above thesecond membrane modules 110 to direct the flow and dissipate its energy. - Tapered Aeration
- With the embodiments discussed with reference to
FIGS. 5 and 8 , additional advantage is achieved by varying the amount of aeration along thesecond tank 120 orthird tank 140. For this purpose, the apparatus inFIGS. 5A and 5B is fitted with a separate aeration system for eachmembrane module 28 as shown inFIG. 8 , the connection between theair distribution pipes 54 and selectedaerators 56 are fitted with restricting orifices or, preferably, each aerator 56 has a flow control valve associated with it.Membrane modules 28 orsecond membrane modules 110 operating intank water 22 with low concentration of solids are aerated less forcefully, preferably based on the concentration ofsolids 22 in the tank water surrounding eachmembrane module 28 orsecond membrane module 110. The furthestupstream membrane module 28 orsecond membrane module 110 is exposed to the lowest concentration of solids and thus receives the least amount of air, subject in the embodiment ofFIGS. 5A and 5B to the need to entraintank water 22 that would otherwise by-pass themembrane modules 28. The mostdownstream membrane module 28 orsecond membrane module 110 is exposed to the highest concentration of solids and receives the most aeration. - Typically, all
aerators 56 are built to the same design and are rated with the same maximum air flow that can be passed through them. The minimum amount of air flow is typically about one half of the rated maximum air flow, below which theaerator 56 may fail to aerate evenly. Preferably, the upstream one half or two thirds of themembrane modules 28 orsecond membrane modules 110 are aerated at 50% to 60% of the rated capacity of theaerators 56 and the remainingmembrane modules 28 orsecond modules 110 are aerated at 80% to 100% of the rated capacity, the increase being made either linearly or in a step form change. Such a variation approximately follows the increase in solids concentration in thetank water 22. - Tapered Backwashing
- Additionally or alternately, tapered backwashing may be employed.
Membrane modules 28 orsecond membrane modules 110 operating intank water 22 with low concentration of solids require less backwashing. The furthestupstream membrane module 28 orsecond membrane module 110 is exposed to the lowest concentration of solids and receives the least amount of backwashing whereas the mostdownstream membrane module 28 orsecond membrane module 110 is exposed to the highest concentration of solids and receives the most backwashing. The amount of backwashing is typically increased between these extremes using a lower amount of backwashing for the upstream one half or two thirds ofmembrane modules 28 orsecond membrane modules 110 and then increasing either linearly or in step form to a higher amount for the remainingmembrane modules 28 orsecond membrane modules 110. For this purpose, the apparatus inFIGS. 5A and 5B is fitted with a separate backwashing system for eachmembrane module 28 as shown inFIG. 8 . - Backwashing can be varied in both frequency or duration. Precise parameters depend on the
feed water 14 and other variables but typically range from a 10 second backwash once an hour to a 30 second backwash once every five minutes, the lower amount being near the former regime and the higher amount being near the latter. - Flow Reversal
- In addition or alternatively, to reduce excessive loss of permeability (because some long term fouling effects are irreversible) and to prevent uneven damage to
different membrane module 28 when tapered aeration is used, the direction oftank flow 78 may be reversed periodically by providing aninlet 18 andretentate outlet 46 at opposite ends of thesecond tank 120 orthird tank 140. Preferably the reversal is done after periodic chemical cleaning which is required approximately every two weeks to two six months and often requires draining thesecond tank 120 orthird tank 140. Such flow reversal allows themembranes 24 near the ends of thesecond tank 120 orthird tank 140 to be operated at times in solidslean tank water 22 which substantially increases their useful life. Such flow reversal can be accomplished in the embodiment ofFIG. 8 with some modification but is inconvenient, the method being more suited to the embodiment ofFIGS. 5A and 5B . - Variable Packing Density
- In general,
membrane modules 28 orsecond membrane modules 110 with lower packing density are preferred in solidsrich tank water 22. The reduced packing density allowsbubbles 58 to reach themembranes 24 more easily and increases the cleaning or fouling inhibiting effect of aeration. For solidslean tank water 22, higher packing density is desirable as more membrane surface area is provided for a given volume ofsecond tank 120 orthird tank 140. Alternatively or additionally, the packing density ofdownstream membrane modules 28 orsecond membrane modules 110 is reduced relative toupstream membrane modules 28 orsecond membrane modules 110 with a corresponding change in the size of thefiltration zones 130. Preferred upstream packing densities vary from 20% to 30%. Preferred downstream packing densities vary from 10% to 20%. - Alternate Tank Shapes
- Referring to
FIG. 9 , around tank 220 is used.Inlet 18 is located at one point on the circumference of thetank 220 and theretentate outlet 42 is located in the middle of thetank 220, or alternately (as shown in dashed lines) at another point on the circumference of thetank 220.Membrane modules 28 orsecond membrane modules 110 are placed in a ring around the centre of thetank 220 in a horizontally spaced apart relationship. Aninternal divider 222 in thetank 220 is used to create acircular flow path 276 between theinlet 18 and theretentate outlet 42. - Referring to
FIG. 10 , a low aspect ratio orsquare tank 320 is used.Inlet 18 is located at one point on thetank 320 and theretentate outlet 42 is located at another point on thetank 320. Aninternal divider 322 in thetank 320 is used to create aflow path 376 between theinlet 18 and theretentate outlet 42.Membrane modules 28 orsecond membrane modules 110 are placed in series along theflow path 376 in a horizontally spaced apart relationship. Alternately, in a variation shown in dashed lines, theinternal divider 322 is a wall between separate tanks joined in series byfluid connector 324. - Where the
round tank 220 or low aspect ratio orsquare tank 320 is used in place of thethird tank 128,partitions 176 are provided betweensecond membrane modules 110. - A submerged membrane reactor according to
FIGS. 5A and 5B was modelled using experimental data from tests under a continuous process and assuming that the local flow around the membrane modules is symmetrical in the upstream and downstream directions—ie. the overall tank flow towards the outlet was discounted. The system comprises a tank 16.4 metres long, 3.28 metres wide with an average depth of water of about 2.5 metres. The tank of the reactor contains 12 membrane modules each being a cassette of 8 ZW500 membrane modules. Each cassette is approximately 1.82 metres high, 1.83 metres wide and 0.71 metres long along the flow path and placed in the tank so as to leave approximately 0.75 m between the edge of the cassettes and the long walls of the tank. The cassettes are spaced evenly between the inlet end and outlet end of the tank. Transmembrane pressure is maintained at a constant 50 kPa throughout the model and the permeability of the membranes at any time is determined by a chart based on experimental data relating sustainable permeabilities to the concentration of solids in the water surrounding the membranes. The flow rate of feedwater and consolidated retentate were adjusted as necessary for a recovery rate of 95%. The feed water is assumed to have an initial concentration of solids of 10 mg/l. - In a first series of modelling experiments, the membrane modules were assumed to be continuously aerated at a constant rate that would result in a total cassette flow of about 3800 litres per minute (for a velocity of 0.05 m/s) upwards through the centre of each cassette and a downward flow of 1900 litres per minute down each of the upstream and downstream edges of each membrane module. The model assumes that all of this cassette flow flows downwards between adjacent membrane modules. The model also assumes that the water between adjacent membrane modules mixes completely such that 50% of the water flowing downwardly along the edge of a cassette, or 950 litres per minute is entrained in the flow moving upward through each adjacent membrane module. The model further assumes that any by-pass flow around the
membrane modules 28 along the sides of the tank is negligible. - In a first test, the test reactor was modelled in continuous bleed operation, that is filtered permeate, consolidated retentate and feed water are all flow continuously. The concentration of solids at each cassette is shown in
FIG. 11 and increases from approximately 20 mg/l to 200 mg/l. As shown, the average concentration of solids surrounding the cassettes is significantly reduced while the consolidated retentate has a concentration of solids of 200 mg/l. The expected permeability of the membrane modules is also shown inFIG. 11 which suggests that such a reactor will operate continuously with an average permeability of over 200 L/m2/h/bar with 8 of 12 membrane modules operating with permeabilities above that average. In comparison, in a conventional fully mixed process operating at the same 95% recovery rate, the concentration of solids throughout the tank would be 200 mg/l and all membrane modules would operate at a permeability of approximately 155 L/m2/h/bar which would exceed the recommended operating conditions of the ZW500 membrane modules. - In a second modelling experiment, the first modelling experiment was modified to assume that the tank was emptied every four hours while permeation stops but with other parameters as above. The results of this experiment are shown in
FIG. 12 which indicates that all cassettes can operate at a permeability above 200 L/m2/h/bar with this process. - In a third modelling experiment, the first modelling experiment was modified to assume that the tank was deconcentrated every four hours by withdrawing consolidated retentate while increasing the flow rate of feedwater while permeation continues but maintaining a 95% recovery rate. The results of this experiment are shown in
FIG. 13 which again indicates that all cassettes can operate at a permeability above 200 L/m2/h/bar with this process. - In a fourth modelling experiment, varying rates of aeration and thus varying recirculation rates were used. The results of this experiment are shown in
FIG. 14 and indicate that recirculation rates of 25% produce drastically lowered concentrations of solids in the water surrounding a majority of cassettes and that even at a generous recirculation rate such as 100% or 165%, a majority of cassettes are exposed to water having a significantly reduced concentration of solids. - In a fifth modelling experiment, the model of the first modelling experiment was repeated without deconcentrations assuming a varying number of cassettes between 1 and 16. As shown in
FIG. 15 , the average concentration in the tank is reduced with even 2 or 4 cassettes and that with 6 or more cassettes, the average concentration of solids in the tank is nearly half of the concentration (200 mg/l) that would occur in the model with a conventional fully mixed process. - In a sixth modelling experiment, the model of the first modelling experiment was repeated without deconcentrations but with the recovery rate varying from 90% to 99% and compared to a model of a conventional fully mixed continuous bleed process operating at the same recovery rates. As shown in
FIG. 16 , a conventional process operating at a 95% recovery rate will have an average concentration of solids in the tank of 200 mg/l. The process and apparatus modelled for a long aerated filter train could be operated at a recovery rate of approximately 97.5% with the same average concentration of solids which would result in 50% less consolidated retentate to be disposed of. - In this example, an actual experimental apparatus was constructed and operated similar to
FIGS. 5A and 5B . The dimensions of the tank were as described for the modelling experiments above, but 16 cassettes of 8 ZW500 membrane modules each were installed consecutively 20 cm apart from each other in the direction of the flow path and used with constant aeration. The apparatus was run continuously without deconcentrations at a recovery rate of 91%. The yield was maintained at a constant 93 litres/second with 9.4 litres/second of consolidated retentate continuously leaving the tank. Colour was monitored at each cassette along the tank as an indicator of the concentration of solids at each cassette. As shown inFIG. 17 , the concentration of solids increased significantly only in the downstream 20% of the tank with most cassettes operating in relatively clean water. - An experiment was conducted with a water filtration system similar to
FIGS. 5A and 5B comprising 12 cassettes of 8 ZW 500 modules each. The aeration was linearly increased from about 13.6 Nm3/h for each ZW 500 for the first cassette to about 22.1 Nm3/h for each ZW 500 for the last cassette. This resulted in a total reduction in system aeration from the usual 1989 Nm3/h to 1785 Nm3/h, more than a 10% reduction while the rate of membrane fouling remained the same and foaming was considerably reduced. In this experiment, the system recovery was 84% while the water temperature was at 22.5 degrees C. - It is to be understood that what has been described are preferred embodiments of the invention. The invention nonetheless is susceptible to certain changes and alternative embodiments without departing from the subject invention, the scope of which is defined in the following claims.
Claims (8)
1. A process for treating water comprising the steps of:
a) providing one or more membrane modules in a generally vertically oriented flow channel;
b) flowing feed water into the flow channel from above the one or more membrane modules to immerse the one or more membrane modules;
c) applying a suction to the one or more membrane modules to withdraw permeate; and,
d) removing unpermeated water from the flow channel from below the one or more membrane modules.
2. The process of claim 1 further comprising a step of providing gas bubbles to scour the one or more membrane modules.
3. The process of claim 1 wherein steps (b), (c) and (d) are performed simultaneously and continuously over a period of time.
4. The process of claim 3 further comprising a step of scouring the one or more membrane modules during the period of time.
5. The process of claim 3 wherein the rate of feed flow is between 1.3 to 5.2 times the rate of permeate flow during the period of time.
6. An apparatus for treating water comprising;
a) a tank;
b) one or more membrane modules in the tank;
c) a non-porous casing surrounding the one or more membrane modules and defining a generally vertical flow channel containing the one or more membrane modules;
d) a source of suction in communication with the one or more membrane modules;
e) an inlet to the tank; and,
f) an outlet from the tank,
wherein,
g) the apparatus is configured such that water in the tank flows into the top of the casing and out of the bottom of the casing.
7. The apparatus of claim 6 wherein 80% or more of the footprint of the tank is covered with membrane modules.
8. The apparatus of claim 6 having a plurality of non-porous casings each defining a vertical flow channel containing one or more membrane modules, the apparatus configured such that water in the tank flows into the top of each casing and out from the bottom of each casing.
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US11/302,271 US20060091075A1 (en) | 1998-11-23 | 2005-12-14 | Water filtration using immersed membranes |
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US09/444,414 US6375848B1 (en) | 1998-11-23 | 1999-11-22 | Water filtration using immersed membranes |
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US11/006,626 US7025885B2 (en) | 1998-11-23 | 2004-12-08 | Water filtration using immersed membranes |
US11/302,271 US20060091075A1 (en) | 1998-11-23 | 2005-12-14 | Water filtration using immersed membranes |
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US11/006,626 Continuation US7025885B2 (en) | 1998-11-23 | 2004-12-08 | Water filtration using immersed membranes |
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US11/006,626 Expired - Lifetime US7025885B2 (en) | 1998-11-23 | 2004-12-08 | Water filtration using immersed membranes |
US11/302,271 Abandoned US20060091075A1 (en) | 1998-11-23 | 2005-12-14 | Water filtration using immersed membranes |
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US09/444,414 Expired - Fee Related US6375848B1 (en) | 1998-11-23 | 1999-11-22 | Water filtration using immersed membranes |
US10/098,365 Expired - Lifetime US6899812B2 (en) | 1998-11-23 | 2002-03-18 | Water filtration using immersed membranes |
US11/006,626 Expired - Lifetime US7025885B2 (en) | 1998-11-23 | 2004-12-08 | Water filtration using immersed membranes |
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EP (1) | EP1140330B1 (en) |
JP (1) | JP2002530188A (en) |
AT (1) | ATE292511T1 (en) |
AU (1) | AU773233B2 (en) |
DE (1) | DE69924642T2 (en) |
WO (1) | WO2000030742A1 (en) |
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US20110079548A1 (en) * | 2007-09-18 | 2011-04-07 | Asahi Kasei Chemicals Corporation | Hollow yarn film filtering apparatus |
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Also Published As
Publication number | Publication date |
---|---|
AU1255800A (en) | 2000-06-13 |
US6375848B1 (en) | 2002-04-23 |
WO2000030742A1 (en) | 2000-06-02 |
JP2002530188A (en) | 2002-09-17 |
US7025885B2 (en) | 2006-04-11 |
EP1140330A1 (en) | 2001-10-10 |
US20050082227A1 (en) | 2005-04-21 |
US6899812B2 (en) | 2005-05-31 |
US20020130080A1 (en) | 2002-09-19 |
ATE292511T1 (en) | 2005-04-15 |
DE69924642D1 (en) | 2005-05-12 |
DE69924642T2 (en) | 2006-02-09 |
EP1140330B1 (en) | 2005-04-06 |
AU773233B2 (en) | 2004-05-20 |
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