CA2279087A1 - Maintenance cleaning for membranes - Google Patents
Maintenance cleaning for membranes Download PDFInfo
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- CA2279087A1 CA2279087A1 CA002279087A CA2279087A CA2279087A1 CA 2279087 A1 CA2279087 A1 CA 2279087A1 CA 002279087 A CA002279087 A CA 002279087A CA 2279087 A CA2279087 A CA 2279087A CA 2279087 A1 CA2279087 A1 CA 2279087A1
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- membranes
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- chemical cleaner
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- permeate
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- 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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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- 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
- B01D61/18—Apparatus therefor
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D65/00—Accessories or auxiliary operations, in general, for separation processes or apparatus using semi-permeable membranes
- 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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- 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
- B01D2321/04—Backflushing
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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
- B01D2321/08—Use of hot water or water vapor
-
- 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
- B01D2321/16—Use of chemical agents
- B01D2321/168—Use of other chemical agents
-
- 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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- 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
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- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Water Supply & Treatment (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Life Sciences & Earth Sciences (AREA)
- Hydrology & Water Resources (AREA)
- Environmental & Geological Engineering (AREA)
- Organic Chemistry (AREA)
- Biodiversity & Conservation Biology (AREA)
- Microbiology (AREA)
- Separation Using Semi-Permeable Membranes (AREA)
Abstract
Membranes immersed in water are cleaned by (a) stopping permeation, (b) flowing a chemical cleaner through the membranes in a direction opposite to the direction in which permeate passes through the membrane, and (c) resuming permeation, wherein chemical cleaner remains substantially undiluted in an area adjacent the membranes and no agitation is provided in this area. The chemical cleaner is preferably applied in pulses over a period of time. The total amount of chemical cleaner used, however, is small. In an embodiment, the chemical cleaner is heated water.
The cleaning events are repeated between twice per day and once a week to reduce the rate of loss of permeability in the membranes over time.
The cleaning events are repeated between twice per day and once a week to reduce the rate of loss of permeability in the membranes over time.
Description
B&P File No. 4320-76 BERESKIN & PARR CANADA
Title: Maintenance Cleaning for Membranes Inventors: Hamid R. Rabie, Hadi Husain and Henry Behmann Title: Maintenance Cleaning for Membranes FIELD OF THE INVENTION
This invention relates to cleaning ultrafiltration or microfiltration membranes with a cleaning chemical.
BACKGROUND OF THE INVENTION
Membranes are used for separating a permeate lean in solids from a feed water rich in solids. Typically, one or more membranes have a retentate side in fluid communication with the feed water and a permeate side at which permeate is collected. Filtered feed water permeates through the walls of the membranes under the influence of a transmembrane pressure differential between the retentate side of the membranes and the permeate side of the membranes. Solids in the feed water are rejected by the membranes and remain on the retentate side of the membranes. The solids may be present in the feed water in solution, in suspension or as precipitates and may further include a variety of substances, some not actually solid, including colloids, microorganisms, exopolymeric substances excreted by microorganisms, suspended solids, and poorly dissolved organic or inorganic compounds such as salts, emulsions, proteins, humic acids, and others.
Over time, the solids foul the membranes which decreases their permeability. As the permeability of membranes decreases, the yield of the process similarly decreases or a higher transmembrane pressure is required to sustain the same yield. Once the decreased yield of the process or the increased transmembrane pressure becomes unacceptable, the membranes must be cleaned.
T
Title: Maintenance Cleaning for Membranes Inventors: Hamid R. Rabie, Hadi Husain and Henry Behmann Title: Maintenance Cleaning for Membranes FIELD OF THE INVENTION
This invention relates to cleaning ultrafiltration or microfiltration membranes with a cleaning chemical.
BACKGROUND OF THE INVENTION
Membranes are used for separating a permeate lean in solids from a feed water rich in solids. Typically, one or more membranes have a retentate side in fluid communication with the feed water and a permeate side at which permeate is collected. Filtered feed water permeates through the walls of the membranes under the influence of a transmembrane pressure differential between the retentate side of the membranes and the permeate side of the membranes. Solids in the feed water are rejected by the membranes and remain on the retentate side of the membranes. The solids may be present in the feed water in solution, in suspension or as precipitates and may further include a variety of substances, some not actually solid, including colloids, microorganisms, exopolymeric substances excreted by microorganisms, suspended solids, and poorly dissolved organic or inorganic compounds such as salts, emulsions, proteins, humic acids, and others.
Over time, the solids foul the membranes which decreases their permeability. As the permeability of membranes decreases, the yield of the process similarly decreases or a higher transmembrane pressure is required to sustain the same yield. Once the decreased yield of the process or the increased transmembrane pressure becomes unacceptable, the membranes must be cleaned.
T
Any solid can contribute to fouling and reduced membrane permeability, and the fouling may occur in different ways.
Fouling can also occur at the membrane surface or inside of the pores of the membrane. To counter the different types of fouling, many different types of cleaning regimens have been proposed and two or more types of cleaning may be used. Such cleaning usually includes both periodic regular cleaning and intensive recovery cleaning.
For periodic regular cleaning, permeation through the membranes is typically stopped momentarily. Air or water are flowed through the membranes under pressure to backwash the membranes. The force of the backwash physically pushes solids off of the membranes.
Typically, the membranes are simultaneously agitated, for example by aerating the feed water around the membranes with large, scouring bubbles to assist in shearing solids from the surface of the membranes. Such back washing and agitation is partially effective in removing solids from the surface of the membranes, but is not very effective for removing solids deposited inside the membrane pores and is almost ineffective for removing any type of solid chemically or biologically attached to the membranes.
Accordingly, fouling continues despite regular back washing and agitation and the permeability of the membranes decreases over time. After a short time, typically in the range of a few days to a few weeks, the permeability of the membranes reaches an unacceptable value and a different type of cleaning, which may be referred to as intensive recovery cleaning, is preformed.
Although necessary, intensive recovery cleaning may disrupt permeation for an extended period of time and is harsh on the membranes. In a first group of methods, the tank is drained and the membranes are back washed with a solution of chemical cleaners while the outer surfaces of the membranes are physically scrubbed. In a second group of methods, the membranes are soaked in one or more cleaning solutions either in the process tank (after it has been drained and filled with chemical cleaners) or in a special cleaning tank. After such intensive recovery cleaning, the permeability of the membranes is partially restored, but the remaining useful life of the membranes will have been reduced.
A third group of methods of intensive recovery cleaning is described in U.S. Patent No. 5,403,479 and Japanese Patent Application No.
2-248,836. In these methods, intensive recovery cleaning is performed without draining the tank or removing the membranes from the tank.
Permeation is stopped and the membranes are cleaned by flowing a chemical cleaner in a reverse direction through the membranes while the membranes are simultaneously agitated. After the cleaning step, the permeability of the membranes is substantially restored.
Such a process avoids removing the membranes or tank water from the tank but the amount of chemical cleaner is large. For waste water applications, the amount of chemical used in each cleaning event may not destroy the biological processes occurring in the waste water, but it still shocks the microorganisms and disrupts the digestion of mixed liquor.
Significant spikes of pollutants are observed after each cleaning by such methods. For potable water applications, the amount of chemical cleaner remaining in the tank after such cleaning events makes such methods unusable. With chemical cleaners based on chlorine, for example, such methods produce unacceptable levels of residual chlorine and trihalomethanes in the permeate.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method of cleaning membranes with a reduced amount of chemical cleaner. Such a method reduces the rate of decline in the permeability of the membranes so that intensive recovery cleaning is required less frequently.
In one aspect, the invention provides a method for cleaning one or more membranes immersed in water containing solids and used to permeate a water lean in solids wherein each cleaning event comprises the steps of:
(a) stopping permeation;
(b) flowing a chemical cleaner through the membranes in a direction opposite to the direction in which permeate passes through the membrane; and, (c) resuming permeation;
and wherein no agitation is provided to the water containing solids adjacent the membranes while chemical cleaner is flowed through the membranes and wherein the membranes remain immersed during steps (a), (b) and (c).
In another aspect, the invention provides method for cleaning one or more membranes immersed in water containing solids and used to permeate a water lean in solids wherein each cleaning event comprises the steps of:
(a) stopping permeation and agitation of the water containing solids;
(b) flowing a chemical cleaner through the membranes in a direction opposite to the direction in which permeate passes through the membrane;
(c) resuming permeation; and, (d) resuming agitation, and wherein permeate collected before resuming agitation is wasted or recycled to the water containing solids and wherein the membranes and the membranes remain immersed during steps (a), (b), (c) and (d).
The cleaning events are performed between 1 and 7 times per week. The amount of chemical cleaner used per square metre of membrane surface area per week is equivalent in antimicrobial efficacy to between 1 mg and 10 mg of NaOCI. The product of the concentration of the chemical cleaner expressed as an equivalent concentration of NaOCI in antimicrobial efficacy and the duration of all cleaning events in a week is between 2,000 minutes*mg/1 and 20,000 minutes*mg/1.
5. The method of claim 1 wherein the chemical cleaner is applied in pulses which last for between 10 seconds and 100 seconds and there is a time between pulses between 50 seconds and 3 minutes. The pulses are applied at a pressure between 5 kPa and 55 kPa.
In another aspect, the chemical cleaner is water heated to more than 25 degrees Celsius. The heated water is applied in pulses at a pressure between 5 kPa and 55 kPa. The pulses last for between 10 seconds and 100 seconds and there is a time between pulses between 50 seconds and 3 minutes. The duration of each cleaning event is between 30 minutes and 90 minutes and the cleaning events are repeated between twice per day and once every two days.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the invention will now be described with reference to the following figure or figures.
Figure 1 is a schematic diagram of an embodiment of the invention.
Figure 2 is a chart of results of tests of an embodiment of the present invention used in a bioreactor for creating potable water.
Figure 3 is a chart of results of tests of an embodiment of the present invention used in a bioreactor for treating waste water.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to Figure 1, a 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 a reactor 10 has many applications but will be described below as used for creating potable water from a natural supply of water such as a lake, well or reservoir or for separating clean water from mixed liquor in a waste water treatment plant.
The 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. If the process is being used for waste water treatment, biological activity in the tank water 22 substantially alters the character and concentration of pollutants in the tank water 22 and the tank water 22 would typically be referred to as mixed liquor 23. In this description, however, tank water 22 refers to both tank water 22 intended to be filtered for drinking and mixed liquor 23. During permeation, the tank water 22 is maintained at a level which covers one or more membranes 24.
Each membrane 24 has a permeate side 25 which does not contact tank _7_ water 22 and a retentate side 27 which does contact the tank water 22.
Membranes 24 made of hollow fibres are preferred although the membranes 24 may be of various other types such as tubular, ceramic, or flat sheet membranes. Typically, headers 26 connect a plurality of hollow fibre or tubular membranes 24 together, the headers 26 sealing the ends of the membranes and connecting the permeate sides 25 of the membranes 24 to appropriate piping. Similarly, flat sheet membranes are typically attached to headers or casings that create an enclosed surface on one side of a membrane or membranes and allow appropriate piping to be connected to the interior of the enclosed surface. A header or casing holding one or more membrane may be referred to as a module. A
plurality of modules may also be joined together and may be referred to as a cassette. In this description, however, the words "membrane" and "membranes" both refer to one or more membranes whether or not they are connected in one or more modules or cassettes.
Referring still to Figure 1, for hollow fibre membranes 24, the retentate side 27 of the membranes 24 is preferably the outside of the membranes and the permeate side 25 of the membranes 24 is preferably their lumens. The permeate sides 25 of the membranes 24 are held in fluid communication with headers 26 and together form a membrane module 28 which is 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 permeate side 25 of the membranes 24 relative to the tank water 22 surrounding the membranes 24. The resulting transmembrane pressure draws tank water 22 through membranes 24 while the membranes 24 reject pollutants which remain in the tank water 22. Thus, filtered permeate 36 is produced for use at a permeate outlet 38 through an outlet valve 39. The transmembrane pressure could alternatively be created by pressurizing the tank water 22.
_8_ The filtered permeate 36 may require post treatment before being used as drinking water or discharged at the end of a waste treatment process, but should have acceptable levels of solids. Preferably, the membranes 24 have an average pore size between 0.003 microns and 10 microns and more preferably between 0.02 microns and 1 micron. Suitable membranes include those sold under the ZEEWEED trade mark and produced by Zenon Environmental Inc. The total size and number of membranes 24 required varies for different applications depending on factors such as the amount of filtered permeate 36 required and the condition of the feed water 14. Similarly, the preferred transmembrane pressure to be applied to the membranes 24 varies for different membranes and the desired yield but preferably is less than 67 kPa for ZEEWEED hollow fibre membranes 24.
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 in a retentate outlet 42 to a drain 44 as retentate 46. The retentate 46 is rich in the solids rejected by the membranes 24. When producing potable water, the retentate 46 is typically sent back to the source that the feed water 14 was originally drawn from. In waste water treatment applications, the retentate 46 is a waste sludge which is further processed or disposed of. The retentate 46 may be withdrawn from the tank 20 either continuously or periodically. The reactor 10, however, is usually operated continuously in that, although there may be short periodic interruptions, feed water 14 flows into the tank 20 and permeate 36 is withdrawn from the tank over extended periods of time and retentate 46 is withdrawn as needed to preserve the required level of tank water 22 in the tank 20.
During permeation, solids accumulate on the surface of the membranes 24 and in their pores, fouling the membranes 24. Various techniques may prevent some of this fouling. Firstly, the membranes 24 may be agitated, possibly by mechanically agitating the tank water 22 near the membranes 24 but preferably by aerating the tank water 22 near the membranes 24. For this, an aeration system 49 has an air supply pump 50 which blows ambient air from an air intake 52 through air distribution pipes 54 to an aerator 56 which disperses air bubbles 58 into the tank water 22 near the membranes 24. The air bubbles 58 discourage solids from depositing on the membranes 24. Secondly, periodic backwashing may be used. For this, the membranes 24 are backwashed by closing permeate valve 34 and outlet valve 39 and opening backwash valves 60. A pressure tank valve 64 is opened and permeate pump 32 pushes filtered permeate 36 from a pressure tank 62 through a backwash pipe 63 to the headers 26 and through the walls of the membranes 24 in a reversed direction thus pushing away some of the solids attached to the membranes 24. At the end of the backwash, backwash valves 60 are closed and permeate valve 34 re-opened. Permeate pump 32 flows permeate 36 into pressure tank 62 until pressure tank 62 is refilled. Pressure tank valve 64 is then closed and outlet valve 39 opened. Such backwashing may occur approximately every 15 minutes for a period of 15 seconds and, although permeation is temporarily disrupted, the process is still considered continuous. Permeate 36 may be stored in a permeate tank 37 to even out minor disruptions in the flow of permeate 36.
With backwashing and the use of air bubbles 58 to clean the membranes 24, permeation typically continues for 1 or 2 weeks before the permeability of the membranes 24 drops to tree point where an intensive recovery cleaning event is required.
Embodiments of the present invention, to be described below, are directed at reducing the rate of loss of permeability of the membranes 24 so that the time between intensive recovery cleanings can be lengthened. This strategy is referred to generally as maintenance cleaning.
In addition to regular periodic backwashing, cleaning events are performed generally periodically at a frequency preferably ranging from once a day to once a week. The cleaning events are started before permeability of the membranes 24 drops to unacceptable levels, preferably while permeability is still above 70% of the permeability of the membranes 24 when new, and more preferably within a week of when permeation is started with new membranes 24 or membranes 24 that have just been through intensive recovery cleaning.
In a first embodiment, each cleaning event involves flowing chemical cleaner through the walls of the membranes 24 while permeation is temporarily stopped in a direction opposite to the direction in which permeate 36 flows through the membranes 24 during permeation.
The chemical cleaner used may be any chemical appropriate for the application and not overly harmful to the membranes 24. Chemicals such as sodium hypochlorite, hydrochloric acid and sulphuric acid are frequently used but most biocidal oxidative electrolytes having an oxidizing anion and an associated cation may be used. The cation is typically chlorine. The chemical cleaner may be used in a non-liquid form such as by flowing chemical in a gaseous state to the headers 26 or introducing it as a solid into the backwash line 63. Liquid chemical cleaners are preferred, however, because they are easier to handle and inject in the proper amounts.
To flow chemical cleaner through the walls of the membranes 24 while permeation is temporarily stopped, permeate valve 34, outlet valve 39 and backwash valves 60 are all closed and permeate pump 32 turned off. Chemical valve 66 is opened and chemical pump 67 turned on pushing chemical cleaner from chemical tank 68 into backwash line 63 to the headers 26 and through the walls of the membranes 24. Alternately, permeate valve 34 and outlet valve 39 may be closed and backwash valves 60 opened. Permeate pump 32 then pushes filtered permeate 36 from pressure tank 62 through backwash line 63 to the headers 26 and through the walls of the membranes 24. Chemical valve 66 is opened and chemical pump 67 turned on mixing chemical cleaner from chemical tank 68 with permeate 36 flowing through backwash line 63. Further alternately, permeate pump 32 is stopped and chemical valve 66, permeate valve 34 and outlet valve 39 are closed while backwash valves 60 are opened. A cross flow valve 69 is also opened connecting the chemical tank 68 to the pressure tank 62. Chemical pump 67 delivers chemical cleaner to pressure tank 62.
Permeate pump 32 is then operated to deliver the chemical cleaner to the membranes 24. Chemical cleaners could also be introduced directly to the headers 26 or the permeate collector 30 which may reduce the total volume used or allow alternate delivery mechanisms.
With some of the methods of flowing chemical cleaner through the walls of the membranes 24 described above, the chemical cleaner may be diluted before reaching the membranes 24. Accordingly, in the subject method the concentration of the chemical cleaner is measured as the chemical cleaner meets the permeate side 25 of the membranes 24 unless stated otherwise. This concentration will be referred to as "C".
As the chemical cleaner flows towards and through the walls of the membranes 24, it displaces tank water 22 in an area adjacent to the membranes. The chemical cleaner surrounds the membranes 24 but is not encouraged to mix with the tank water 22. In particular, sources of agitation, such as the aeration system 49, are preferably turned off if they have a significant effect on the tank water 22 adjacent the membranes 24.
Without such mixing, some chemical cleaner leaves the area adjacent the membranes 24 but only slowly. In the area adjacent the membranes 24, the chemical cleaner reacts with the solids on or in the membranes 24 killing some microorganisms attached to the membranes 24 and dissolving some of the solids. Outside of this area, the concentration of chemical cleaner in the tank water 22 drops.
The effectiveness of the chemical cleaner is dependant on the concentration of the chemical cleaner and the time that the chemical cleaner remains substantially undiluted in the area adjacent the membranes 24. For process calculations, the concentration of the chemical cleaner i n the area adjacent the membranes 24 is assumed to be the same as the initial concentration of the chemical cleaners 24, C. The time during which the chemical cleaner remains substantially undiluted in the area adjacent the membranes 24 will be called "T". Preferably, permeation is stopped before the chemical cleaner starts to flow through the membranes 24 and is resumed after agitation is resumed to dilute the chemical cleaner in the area adjacent the membranes 24. In this case, T is assumed for calculations to be the time between when chemical cleaner starts to flow through the membranes 24 and when agitation is resumed. If necessary, the permeate side 25 of the membranes 24 and piping containing chemical may also be flushed with a backpulse of filtered permeate 36 before resuming permeation.
Alternatively, however, permeation may resumed before agitation. This method is advantageous in that chemical cleaner in the area adjacent the membranes 24 is not dispersed into the tank 20, but the permeate 36 collected before resuming agitation is preferably wasted or recycled to the tank water 22 to reduce the amount of chemical cleaner entering the permeate tank 37. In this case, T is assumed for calculations to be the time during which chemical cleaner flows through the membranes 24. The effectiveness of a cleaning event is approximated by multiplying these two parameters to create a third parameter "CT".
Since the cleaning events may be repeated with varying frequency for different applications or concentrations of solids in the feed water 14, a parameter called the weekly CT is used as a basis for some calculations. The weekly CT is the sum of the CT parameters for the cleaning events performed during a week. If cleaning events are performed less frequently than once a week, a monthly CT parameter can be used instead with appropriate modifications to the calculations which depend on the weekly CT parameter.
The desired weekly CT is preferably chosen to maintain acceptable permeability of the membranes 24 or to reduce the rate of decline in permeability of membranes 24 over extended periods of time, preferably between 15 days and three months, so as to reduce the frequency of intensive recovery cleanings rather than to provide recovery cleaning itself.
There may be a slight instantaneous increase in permeability of the membranes 24 after a cleaning event, but this permeability gain is typically lost before the next cleaning event and is not significant enough to be considered recovery cleaning.
The weekly CT is preferably in the range of 2,000 min*mg/1 to 20,000 min*mg/1 when NaOCI is the chemical cleaner. When other chemical cleaners are used, the concentration of the chemical cleaner should be expressed as an equivalent concentration of NaOCI that would have the same antimicrobial efficacy. The precise weekly CT to use in a given application is preferably chosen to achieve a gradual decline in permeability over an extended period of time.
For a given weekly CT, the weekly duration of cleaning events is calculated by dividing the weekly CT by the concentration, C, of chemical cleaner. Once the total weekly duration of cleaning events is known, the frequency of cleaning events is next determined. Frequent cleaning events may be more effective and provide less variation in permeability of the membranes 24 over time but require more frequent disruptions to permeation. Preferably, cleaning events are also not so frequent that, given the residence time of the tank 20 or permeate tank 37, residual chemical cleaner from a prior cleaning event is still present at the start of the next cleaning event in significant amounts. Cleaning events are performed preferably between 1 and 7 times per week and more preferably between 1 and 3 times per week. The duration, T, of each cleaning event is then determined by dividing the weekly duration of cleaning events by the number of times per week that cleaning events are performed.
Once the duration of each cleaning event is known, the flow rate of chemical cleaner during each cleaning event is determined.
The flow rate is chosen to maintain an area adjacent to the membranes 24 in which the chemical cleaner is substantially undiluted.
Chemical cleaner may be applied at a steady rate over a significant portion of the duration, T, of the cleaning event. The permeate pump 32 or chemical pump 67, whichever governs, is controlled to feed the cleaning chemical into the membranes 24 at a low pressure.
Preferably, however, the chemical cleaner is supplied to the membranes 24 in pulses rather than continuously. In the time between pulses, the chemical cleaner moves from the area adjacent the membranes 24 into the tank water 22 generally, but the average concentration of chemical cleaner in the area adjacent the membranes 24 over the duration T
of the cleaning event is still sufficient to consider the chemical cleaner to be substantially undiluted in this area.
With a pulsed delivery of chemical cleaner, a higher pressure is used to deliver the same volume of chemical cleaner compared to when the chemical cleaner is delivered under constant pressure. This assists in reducing the relative size of variations in head losses in the membranes 24 or the piping to the membranes 24. Further, membranes rarely foul evenly and the pulsed delivery of chemical cleaner assists in providing an even distribution of chemical cleaner across the surface of the membranes 24. With less variable flow of chemical cleaner from one part of the membranes 24, less chemical cleaner is required to achieve a minimum level of cleaning throughout the membranes 24. The pulsed chemical cleaner delivery is particularly beneficial for modern submerged outside-in hollow fibre membranes 24 which may be between 1 metre to 3 metres in length, resulting in significant pressure drop in the membranes 24, but having permeability near or greater than 1000 litres per cubic meter per hour per bar of transmembrane pressure (lmh/bar). With such membranes, a pulse pressure between 5 and 55 kPa is preferred.
Preferably, the pulses last for between 10 seconds and 100 seconds. Preferably, the permeate pump 32 or chemical pump 67, whichever is controlling, supplies the chemical cleaner to the membranes 24 with sufficient pressure to produce a flux of chemical through the membranes 24 between 5 g/sq.ft./day and 30 g/sq.ft./day. After each pulse, the flow of chemical cleaner is stopped for a waiting period preferably between 50 seconds and 3 minutes and more preferably between 50 seconds and 1 minute in length. After the waiting period, the pulse and waiting period may be repeated and preferably are repeated between 5 and 30 times.
Preferably, the amount of chemical cleaner used per square metre of surface area of the membranes 24 per week is between 1 and 10 mg of NaOCI. When other chemical cleaners are used, an amount of chemical cleaner is used which is equivalent to the amount of NaOCI specified above in antimicrobial efficacy.
Once the regime for the cleaning events is known, the total volume of chemical cleaner introduced into the tank water 22, called the cleaning event dosage, can be calculated.
The cleaning event dosage is preferably safely below an amount which would disrupt treatment or filtration of the water. For example, in waste water treatment applications, the cleaning event dosage preferably is low enough that it does not disrupt the population of microorganisms to the point where a spike of pollutants makes the effluent quality unsatisfactory. For potable water applications, the cleaning event dosage preferably does not exceed the most limiting regulatory or design limit on the concentration of chemical cleaner in the permeate at any point of use. For example, with chlorine based chemical cleaners, trihalomethane formation is likely to be the controlling factor and can be predicted using trihalomethane formation tables. In appropriate circumstances, the volume of the permeate tank 37 may be considered in calculating the cleaning event dosage. Similarly, any prechlorination or chemical cleaner remaining in the tank 20 from a preceding cleaning event should be accounted for in determining whether a cleaning event dosage is acceptable.
In many cases, the cleaning event dosage will be well below the maximum cleaning event dosage that could be used. However, if this does not occur in a particular application, the cleaning regime is altered to give acceptable cleaning event dosages. In some cases, altering the frequency of cleaning events may produce acceptable cleaning event dosages without reducing the weekly CT, but in other cases a higher fouling index and lower weekly CT may be required.
After the cleaning event and dilution described above, backwash valves 60 are closed, permeate valve 34 is re-opened, pressure tank 64 opened if and as necessary to refill pressure tank 62, and permeation continues. New chemical cleaner is added to the chemical tank 68 as needed.
In another preferred embodiment, the cleaning steps are performed as described above with the exception that the chemical valve 66, chemical pump 67 and chemical tank 68 are replaced with a hot water valve 70, hot water pump 72 and boiler 74, except as described differently below.
The boiler 74 delivers heated water, preferably above 25 degrees Celsius and more preferably between 40 and 60 degrees Celsius, or steam in place of the chemical cleaners provided by the chemical tank 68. The inventors believe that the hot water or steam solubilizes some of the solids, particularly organic matter, both on the surface of the membranes 24 and in the pores of the membranes 24. The solubilized solids travel through the membranes 24 by permeation or disperse into the tank water 22. Solids may or may not be completely removed, but removing part of the solids with each cleaning event slows the long term rate of loss of permeability of the membranes 24.
The hot water may also kill some microorganisms attached to the membranes although it is not necessary to kill the microorganisms to achieve the desired effect. However, temperatures above 60 or 70 degree degrees Celsius are known to kill the bacteria and may be used if a large portion of the solids are bacteria.
Since no chemical cleaner is used, the flux of hot water or steam, the number and duration of the pulses and the wait time between them, and the frequency of cleaning events are not limited by resulting chemical concentrations but rather excess heating the tank water 22. The heated water may be provided continuously over a cleaning event but is preferably provided in pulses. Process parameters are preferably chosen to provide heated water in an area adjacent the membranes 24 for a sufficient amount of time such that at least readily solubilizable solids, particularly exopolymeric substances and other organic compounds and some inorganic compounds, may be solubilized.
Preferably, the pulses last for between 10 seconds and 100 seconds and have sufficient pressure to produce a flux of heated water through the membranes 24 between 5 g/sq.ft./day and 30 g/sq.ft./day. After each pulse, the flow of heated water is stopped for a waiting period preferably between 50 seconds and 3 minutes and more preferably between 50 seconds and 1 minute in length. After the waiting period, the pulse and waiting period may be repeated and preferably are repeated so that the cleaning event is between 30 minutes and 90 minutes in duration. Such cleaning events are preferably repeated between twice a day and once every two days and more preferably once a day.
Alternative or modified embodiments of the invention may also be constructed or practiced without departing from the scope of the invention. For example, when chemical cleaners are used but the tank 20 or permeate tank 37 are small, cleaning according to the invention may be timed to coincide with times when the tank water 22 is dumped. In waste water applications, sludge must be periodically discharged as retentate 46. If waste sludge is removed in discrete events performed immediately after a cleaning event, then some of the chemical cleaner will also be removed. In potable water applications, retentate 46 is also discharged to remove solids which build up in the tank 20. Retentate 46 can be discharged by emptying the tank or by performing a rapid flush during which retentate 46 is rapidly withdrawn, possibly with the assistance of a retentate pump 48, while excess feed water 14 is rapidly provided to keep the level of the tank water 22 constant. If the cleaning events are performed before either method of withdrawing retentate 46, then the chemical cleaners will be at least partially flushed out with the retentate 46. This method could allow the use of very small tanks, as used with high pressure filters, since the concentration of residual chemical cleaner in the permeate 36 would be substantially independent of the volume of the tank 20.
When hot water is used in place of chemical cleaners, the heated water or steam could be applied to the retentate side 27 of the membranes 24 by injection into the tank 20 or by heating the feed water 14.
In this case, permeation could be stopped momentarily to allow the membrane surfaces to be heated and then restarted to draw the dissolved solids through the membrane.
It is to be understood that what has been described are preferred embodiments to the invention. The invention nonetheless is susceptible to certain changes and alternative embodiments fully comprehended by the spirit of the invention as described above, and the scope of the claims of this patent.
Example 1: Waste Water Treatment An experimental membrane bioreactor using a ZEEWEED
500 membrane module having 46 square metres of membrane surface area was built for treating waste water and, in particular, for carbon oxidation, nitrification and phosphorus removal. At all times, the flow rate of permeate through the membranes was maintained at 15 gfd and the solids concentration in the bioreactor averaged between 15 g/1 and 20 g/1. The average flow through the bioreactor was 1,000 cubic metres per day and the peak flow was 2,000 cubic metres/day.
The bioreactor was first operated without cleaning according to the invention for 90 days. Permeability was not sustainable and decreased continuously. At the end of this time, permeability of the membranes had dropped to less than 3.0 gfd/psi.
The bioreactor was then operated with a fresh membrane module for 90 days with maintenance cleaning according to the present invention. The cleaning was performed twice per week using 100-125 mg/1 NaOCI solution for one hour in pulses at a rate of 3.6 mg per square metre per week. The permeability of the membranes decreased slowly and eventually stabilised at about 7.5 gfd/psi.
On an average basis, no significant decrease in effluent quality in terms of ammonia-nitrogen or total phosphorous occurred when cleaning according to the present invention was instituted. Concentration of cBOD5 in the effluent both with and without cleaning according to the present invention averaged 1.0 mg/1.
Example 2: Potable Water An experimental membrane bioreactor using ZEEWEED 10 membrane modules having 0.9 square metres of membrane surface area each was built for treating lake water to produce potable water. All experiments were performed at constant flux in which the flow is kept constant and the transmembrane pressure (TMP) was allowed to increase as membranes fouled. The raw water conditions were as follows:
Temperature (C) 10-20 TOC (mg/1) 3.0-5.0 Turbidity (ntu) 4.0-9.0 Apparent Colour (Pt Co 10-50 units) True Colour (Pt Co units)5.0-20.0 Experiments were performed with and without maintenance cleaning and at different fluxes. Cleaning events were done three times per week with 100 mg/1 NaOCI for 30 minutes. The cleaning dosage was between 2.8 and 3.7 mg NaOCI per square metre of membrane per week.
Figure 4 summarises the results obtained with and without maintenance cleaning. Each test lasted about 45-60 days. After an initial increase in TMP, the TMP reached a relatively constant value which is referred to as the sustainable TMP. Sustainable TMP is plotted as a function of fixed operating flux. Permeability can be calculated dividing the operating flux by TMP. In this figure, the "control" condition refers to operation without maintenance cleaning. Substantial improvement in sustainable TMP was obtained using maintenance cleaning.
The residual chlorine concentration in the process tank after each cleaning event was less than 0.5 mg/1. This level of residual chlorine in the process tank was low enough to continue the filtration process to produce potable water.
Example 3: Heated Water as a Chemical cleaner An experimental membrane bioreactor was built for treating a typical municipal waste water. ZW10 membrane modules were used each having a surface area of 0.9 square metres. The concentration of biomass was between 15 to 20 gMLSS/1, corresponding to a volumetric loading of between 1.2 to 2.3 kg COD/m3/d. COD and TKN removal were better than 95% with dissolved oxygen residuals between 0.5 and 1.5 mg 02 / 1 in the tank.
Experiments were performed at a constant transmembrane pressure of 34 kPa and the permeate flux was allowed to decline as the membranes fouled. Two modules were tested under the same conditions, one with and one without heated water maintenance cleaning. For cleaning, heated water maintenance cleaning was performed with water heated to 40C for 1 hour every day. Figure 5 shows the net flux results as a function of time and indicates that the heated water maintenance cleaning resulted in an improvement in flux averaging between 5 and 10 gfd over the duration of the test.
Fouling can also occur at the membrane surface or inside of the pores of the membrane. To counter the different types of fouling, many different types of cleaning regimens have been proposed and two or more types of cleaning may be used. Such cleaning usually includes both periodic regular cleaning and intensive recovery cleaning.
For periodic regular cleaning, permeation through the membranes is typically stopped momentarily. Air or water are flowed through the membranes under pressure to backwash the membranes. The force of the backwash physically pushes solids off of the membranes.
Typically, the membranes are simultaneously agitated, for example by aerating the feed water around the membranes with large, scouring bubbles to assist in shearing solids from the surface of the membranes. Such back washing and agitation is partially effective in removing solids from the surface of the membranes, but is not very effective for removing solids deposited inside the membrane pores and is almost ineffective for removing any type of solid chemically or biologically attached to the membranes.
Accordingly, fouling continues despite regular back washing and agitation and the permeability of the membranes decreases over time. After a short time, typically in the range of a few days to a few weeks, the permeability of the membranes reaches an unacceptable value and a different type of cleaning, which may be referred to as intensive recovery cleaning, is preformed.
Although necessary, intensive recovery cleaning may disrupt permeation for an extended period of time and is harsh on the membranes. In a first group of methods, the tank is drained and the membranes are back washed with a solution of chemical cleaners while the outer surfaces of the membranes are physically scrubbed. In a second group of methods, the membranes are soaked in one or more cleaning solutions either in the process tank (after it has been drained and filled with chemical cleaners) or in a special cleaning tank. After such intensive recovery cleaning, the permeability of the membranes is partially restored, but the remaining useful life of the membranes will have been reduced.
A third group of methods of intensive recovery cleaning is described in U.S. Patent No. 5,403,479 and Japanese Patent Application No.
2-248,836. In these methods, intensive recovery cleaning is performed without draining the tank or removing the membranes from the tank.
Permeation is stopped and the membranes are cleaned by flowing a chemical cleaner in a reverse direction through the membranes while the membranes are simultaneously agitated. After the cleaning step, the permeability of the membranes is substantially restored.
Such a process avoids removing the membranes or tank water from the tank but the amount of chemical cleaner is large. For waste water applications, the amount of chemical used in each cleaning event may not destroy the biological processes occurring in the waste water, but it still shocks the microorganisms and disrupts the digestion of mixed liquor.
Significant spikes of pollutants are observed after each cleaning by such methods. For potable water applications, the amount of chemical cleaner remaining in the tank after such cleaning events makes such methods unusable. With chemical cleaners based on chlorine, for example, such methods produce unacceptable levels of residual chlorine and trihalomethanes in the permeate.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method of cleaning membranes with a reduced amount of chemical cleaner. Such a method reduces the rate of decline in the permeability of the membranes so that intensive recovery cleaning is required less frequently.
In one aspect, the invention provides a method for cleaning one or more membranes immersed in water containing solids and used to permeate a water lean in solids wherein each cleaning event comprises the steps of:
(a) stopping permeation;
(b) flowing a chemical cleaner through the membranes in a direction opposite to the direction in which permeate passes through the membrane; and, (c) resuming permeation;
and wherein no agitation is provided to the water containing solids adjacent the membranes while chemical cleaner is flowed through the membranes and wherein the membranes remain immersed during steps (a), (b) and (c).
In another aspect, the invention provides method for cleaning one or more membranes immersed in water containing solids and used to permeate a water lean in solids wherein each cleaning event comprises the steps of:
(a) stopping permeation and agitation of the water containing solids;
(b) flowing a chemical cleaner through the membranes in a direction opposite to the direction in which permeate passes through the membrane;
(c) resuming permeation; and, (d) resuming agitation, and wherein permeate collected before resuming agitation is wasted or recycled to the water containing solids and wherein the membranes and the membranes remain immersed during steps (a), (b), (c) and (d).
The cleaning events are performed between 1 and 7 times per week. The amount of chemical cleaner used per square metre of membrane surface area per week is equivalent in antimicrobial efficacy to between 1 mg and 10 mg of NaOCI. The product of the concentration of the chemical cleaner expressed as an equivalent concentration of NaOCI in antimicrobial efficacy and the duration of all cleaning events in a week is between 2,000 minutes*mg/1 and 20,000 minutes*mg/1.
5. The method of claim 1 wherein the chemical cleaner is applied in pulses which last for between 10 seconds and 100 seconds and there is a time between pulses between 50 seconds and 3 minutes. The pulses are applied at a pressure between 5 kPa and 55 kPa.
In another aspect, the chemical cleaner is water heated to more than 25 degrees Celsius. The heated water is applied in pulses at a pressure between 5 kPa and 55 kPa. The pulses last for between 10 seconds and 100 seconds and there is a time between pulses between 50 seconds and 3 minutes. The duration of each cleaning event is between 30 minutes and 90 minutes and the cleaning events are repeated between twice per day and once every two days.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the invention will now be described with reference to the following figure or figures.
Figure 1 is a schematic diagram of an embodiment of the invention.
Figure 2 is a chart of results of tests of an embodiment of the present invention used in a bioreactor for creating potable water.
Figure 3 is a chart of results of tests of an embodiment of the present invention used in a bioreactor for treating waste water.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to Figure 1, a 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 a reactor 10 has many applications but will be described below as used for creating potable water from a natural supply of water such as a lake, well or reservoir or for separating clean water from mixed liquor in a waste water treatment plant.
The 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. If the process is being used for waste water treatment, biological activity in the tank water 22 substantially alters the character and concentration of pollutants in the tank water 22 and the tank water 22 would typically be referred to as mixed liquor 23. In this description, however, tank water 22 refers to both tank water 22 intended to be filtered for drinking and mixed liquor 23. During permeation, the tank water 22 is maintained at a level which covers one or more membranes 24.
Each membrane 24 has a permeate side 25 which does not contact tank _7_ water 22 and a retentate side 27 which does contact the tank water 22.
Membranes 24 made of hollow fibres are preferred although the membranes 24 may be of various other types such as tubular, ceramic, or flat sheet membranes. Typically, headers 26 connect a plurality of hollow fibre or tubular membranes 24 together, the headers 26 sealing the ends of the membranes and connecting the permeate sides 25 of the membranes 24 to appropriate piping. Similarly, flat sheet membranes are typically attached to headers or casings that create an enclosed surface on one side of a membrane or membranes and allow appropriate piping to be connected to the interior of the enclosed surface. A header or casing holding one or more membrane may be referred to as a module. A
plurality of modules may also be joined together and may be referred to as a cassette. In this description, however, the words "membrane" and "membranes" both refer to one or more membranes whether or not they are connected in one or more modules or cassettes.
Referring still to Figure 1, for hollow fibre membranes 24, the retentate side 27 of the membranes 24 is preferably the outside of the membranes and the permeate side 25 of the membranes 24 is preferably their lumens. The permeate sides 25 of the membranes 24 are held in fluid communication with headers 26 and together form a membrane module 28 which is 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 permeate side 25 of the membranes 24 relative to the tank water 22 surrounding the membranes 24. The resulting transmembrane pressure draws tank water 22 through membranes 24 while the membranes 24 reject pollutants which remain in the tank water 22. Thus, filtered permeate 36 is produced for use at a permeate outlet 38 through an outlet valve 39. The transmembrane pressure could alternatively be created by pressurizing the tank water 22.
_8_ The filtered permeate 36 may require post treatment before being used as drinking water or discharged at the end of a waste treatment process, but should have acceptable levels of solids. Preferably, the membranes 24 have an average pore size between 0.003 microns and 10 microns and more preferably between 0.02 microns and 1 micron. Suitable membranes include those sold under the ZEEWEED trade mark and produced by Zenon Environmental Inc. The total size and number of membranes 24 required varies for different applications depending on factors such as the amount of filtered permeate 36 required and the condition of the feed water 14. Similarly, the preferred transmembrane pressure to be applied to the membranes 24 varies for different membranes and the desired yield but preferably is less than 67 kPa for ZEEWEED hollow fibre membranes 24.
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 in a retentate outlet 42 to a drain 44 as retentate 46. The retentate 46 is rich in the solids rejected by the membranes 24. When producing potable water, the retentate 46 is typically sent back to the source that the feed water 14 was originally drawn from. In waste water treatment applications, the retentate 46 is a waste sludge which is further processed or disposed of. The retentate 46 may be withdrawn from the tank 20 either continuously or periodically. The reactor 10, however, is usually operated continuously in that, although there may be short periodic interruptions, feed water 14 flows into the tank 20 and permeate 36 is withdrawn from the tank over extended periods of time and retentate 46 is withdrawn as needed to preserve the required level of tank water 22 in the tank 20.
During permeation, solids accumulate on the surface of the membranes 24 and in their pores, fouling the membranes 24. Various techniques may prevent some of this fouling. Firstly, the membranes 24 may be agitated, possibly by mechanically agitating the tank water 22 near the membranes 24 but preferably by aerating the tank water 22 near the membranes 24. For this, an aeration system 49 has an air supply pump 50 which blows ambient air from an air intake 52 through air distribution pipes 54 to an aerator 56 which disperses air bubbles 58 into the tank water 22 near the membranes 24. The air bubbles 58 discourage solids from depositing on the membranes 24. Secondly, periodic backwashing may be used. For this, the membranes 24 are backwashed by closing permeate valve 34 and outlet valve 39 and opening backwash valves 60. A pressure tank valve 64 is opened and permeate pump 32 pushes filtered permeate 36 from a pressure tank 62 through a backwash pipe 63 to the headers 26 and through the walls of the membranes 24 in a reversed direction thus pushing away some of the solids attached to the membranes 24. At the end of the backwash, backwash valves 60 are closed and permeate valve 34 re-opened. Permeate pump 32 flows permeate 36 into pressure tank 62 until pressure tank 62 is refilled. Pressure tank valve 64 is then closed and outlet valve 39 opened. Such backwashing may occur approximately every 15 minutes for a period of 15 seconds and, although permeation is temporarily disrupted, the process is still considered continuous. Permeate 36 may be stored in a permeate tank 37 to even out minor disruptions in the flow of permeate 36.
With backwashing and the use of air bubbles 58 to clean the membranes 24, permeation typically continues for 1 or 2 weeks before the permeability of the membranes 24 drops to tree point where an intensive recovery cleaning event is required.
Embodiments of the present invention, to be described below, are directed at reducing the rate of loss of permeability of the membranes 24 so that the time between intensive recovery cleanings can be lengthened. This strategy is referred to generally as maintenance cleaning.
In addition to regular periodic backwashing, cleaning events are performed generally periodically at a frequency preferably ranging from once a day to once a week. The cleaning events are started before permeability of the membranes 24 drops to unacceptable levels, preferably while permeability is still above 70% of the permeability of the membranes 24 when new, and more preferably within a week of when permeation is started with new membranes 24 or membranes 24 that have just been through intensive recovery cleaning.
In a first embodiment, each cleaning event involves flowing chemical cleaner through the walls of the membranes 24 while permeation is temporarily stopped in a direction opposite to the direction in which permeate 36 flows through the membranes 24 during permeation.
The chemical cleaner used may be any chemical appropriate for the application and not overly harmful to the membranes 24. Chemicals such as sodium hypochlorite, hydrochloric acid and sulphuric acid are frequently used but most biocidal oxidative electrolytes having an oxidizing anion and an associated cation may be used. The cation is typically chlorine. The chemical cleaner may be used in a non-liquid form such as by flowing chemical in a gaseous state to the headers 26 or introducing it as a solid into the backwash line 63. Liquid chemical cleaners are preferred, however, because they are easier to handle and inject in the proper amounts.
To flow chemical cleaner through the walls of the membranes 24 while permeation is temporarily stopped, permeate valve 34, outlet valve 39 and backwash valves 60 are all closed and permeate pump 32 turned off. Chemical valve 66 is opened and chemical pump 67 turned on pushing chemical cleaner from chemical tank 68 into backwash line 63 to the headers 26 and through the walls of the membranes 24. Alternately, permeate valve 34 and outlet valve 39 may be closed and backwash valves 60 opened. Permeate pump 32 then pushes filtered permeate 36 from pressure tank 62 through backwash line 63 to the headers 26 and through the walls of the membranes 24. Chemical valve 66 is opened and chemical pump 67 turned on mixing chemical cleaner from chemical tank 68 with permeate 36 flowing through backwash line 63. Further alternately, permeate pump 32 is stopped and chemical valve 66, permeate valve 34 and outlet valve 39 are closed while backwash valves 60 are opened. A cross flow valve 69 is also opened connecting the chemical tank 68 to the pressure tank 62. Chemical pump 67 delivers chemical cleaner to pressure tank 62.
Permeate pump 32 is then operated to deliver the chemical cleaner to the membranes 24. Chemical cleaners could also be introduced directly to the headers 26 or the permeate collector 30 which may reduce the total volume used or allow alternate delivery mechanisms.
With some of the methods of flowing chemical cleaner through the walls of the membranes 24 described above, the chemical cleaner may be diluted before reaching the membranes 24. Accordingly, in the subject method the concentration of the chemical cleaner is measured as the chemical cleaner meets the permeate side 25 of the membranes 24 unless stated otherwise. This concentration will be referred to as "C".
As the chemical cleaner flows towards and through the walls of the membranes 24, it displaces tank water 22 in an area adjacent to the membranes. The chemical cleaner surrounds the membranes 24 but is not encouraged to mix with the tank water 22. In particular, sources of agitation, such as the aeration system 49, are preferably turned off if they have a significant effect on the tank water 22 adjacent the membranes 24.
Without such mixing, some chemical cleaner leaves the area adjacent the membranes 24 but only slowly. In the area adjacent the membranes 24, the chemical cleaner reacts with the solids on or in the membranes 24 killing some microorganisms attached to the membranes 24 and dissolving some of the solids. Outside of this area, the concentration of chemical cleaner in the tank water 22 drops.
The effectiveness of the chemical cleaner is dependant on the concentration of the chemical cleaner and the time that the chemical cleaner remains substantially undiluted in the area adjacent the membranes 24. For process calculations, the concentration of the chemical cleaner i n the area adjacent the membranes 24 is assumed to be the same as the initial concentration of the chemical cleaners 24, C. The time during which the chemical cleaner remains substantially undiluted in the area adjacent the membranes 24 will be called "T". Preferably, permeation is stopped before the chemical cleaner starts to flow through the membranes 24 and is resumed after agitation is resumed to dilute the chemical cleaner in the area adjacent the membranes 24. In this case, T is assumed for calculations to be the time between when chemical cleaner starts to flow through the membranes 24 and when agitation is resumed. If necessary, the permeate side 25 of the membranes 24 and piping containing chemical may also be flushed with a backpulse of filtered permeate 36 before resuming permeation.
Alternatively, however, permeation may resumed before agitation. This method is advantageous in that chemical cleaner in the area adjacent the membranes 24 is not dispersed into the tank 20, but the permeate 36 collected before resuming agitation is preferably wasted or recycled to the tank water 22 to reduce the amount of chemical cleaner entering the permeate tank 37. In this case, T is assumed for calculations to be the time during which chemical cleaner flows through the membranes 24. The effectiveness of a cleaning event is approximated by multiplying these two parameters to create a third parameter "CT".
Since the cleaning events may be repeated with varying frequency for different applications or concentrations of solids in the feed water 14, a parameter called the weekly CT is used as a basis for some calculations. The weekly CT is the sum of the CT parameters for the cleaning events performed during a week. If cleaning events are performed less frequently than once a week, a monthly CT parameter can be used instead with appropriate modifications to the calculations which depend on the weekly CT parameter.
The desired weekly CT is preferably chosen to maintain acceptable permeability of the membranes 24 or to reduce the rate of decline in permeability of membranes 24 over extended periods of time, preferably between 15 days and three months, so as to reduce the frequency of intensive recovery cleanings rather than to provide recovery cleaning itself.
There may be a slight instantaneous increase in permeability of the membranes 24 after a cleaning event, but this permeability gain is typically lost before the next cleaning event and is not significant enough to be considered recovery cleaning.
The weekly CT is preferably in the range of 2,000 min*mg/1 to 20,000 min*mg/1 when NaOCI is the chemical cleaner. When other chemical cleaners are used, the concentration of the chemical cleaner should be expressed as an equivalent concentration of NaOCI that would have the same antimicrobial efficacy. The precise weekly CT to use in a given application is preferably chosen to achieve a gradual decline in permeability over an extended period of time.
For a given weekly CT, the weekly duration of cleaning events is calculated by dividing the weekly CT by the concentration, C, of chemical cleaner. Once the total weekly duration of cleaning events is known, the frequency of cleaning events is next determined. Frequent cleaning events may be more effective and provide less variation in permeability of the membranes 24 over time but require more frequent disruptions to permeation. Preferably, cleaning events are also not so frequent that, given the residence time of the tank 20 or permeate tank 37, residual chemical cleaner from a prior cleaning event is still present at the start of the next cleaning event in significant amounts. Cleaning events are performed preferably between 1 and 7 times per week and more preferably between 1 and 3 times per week. The duration, T, of each cleaning event is then determined by dividing the weekly duration of cleaning events by the number of times per week that cleaning events are performed.
Once the duration of each cleaning event is known, the flow rate of chemical cleaner during each cleaning event is determined.
The flow rate is chosen to maintain an area adjacent to the membranes 24 in which the chemical cleaner is substantially undiluted.
Chemical cleaner may be applied at a steady rate over a significant portion of the duration, T, of the cleaning event. The permeate pump 32 or chemical pump 67, whichever governs, is controlled to feed the cleaning chemical into the membranes 24 at a low pressure.
Preferably, however, the chemical cleaner is supplied to the membranes 24 in pulses rather than continuously. In the time between pulses, the chemical cleaner moves from the area adjacent the membranes 24 into the tank water 22 generally, but the average concentration of chemical cleaner in the area adjacent the membranes 24 over the duration T
of the cleaning event is still sufficient to consider the chemical cleaner to be substantially undiluted in this area.
With a pulsed delivery of chemical cleaner, a higher pressure is used to deliver the same volume of chemical cleaner compared to when the chemical cleaner is delivered under constant pressure. This assists in reducing the relative size of variations in head losses in the membranes 24 or the piping to the membranes 24. Further, membranes rarely foul evenly and the pulsed delivery of chemical cleaner assists in providing an even distribution of chemical cleaner across the surface of the membranes 24. With less variable flow of chemical cleaner from one part of the membranes 24, less chemical cleaner is required to achieve a minimum level of cleaning throughout the membranes 24. The pulsed chemical cleaner delivery is particularly beneficial for modern submerged outside-in hollow fibre membranes 24 which may be between 1 metre to 3 metres in length, resulting in significant pressure drop in the membranes 24, but having permeability near or greater than 1000 litres per cubic meter per hour per bar of transmembrane pressure (lmh/bar). With such membranes, a pulse pressure between 5 and 55 kPa is preferred.
Preferably, the pulses last for between 10 seconds and 100 seconds. Preferably, the permeate pump 32 or chemical pump 67, whichever is controlling, supplies the chemical cleaner to the membranes 24 with sufficient pressure to produce a flux of chemical through the membranes 24 between 5 g/sq.ft./day and 30 g/sq.ft./day. After each pulse, the flow of chemical cleaner is stopped for a waiting period preferably between 50 seconds and 3 minutes and more preferably between 50 seconds and 1 minute in length. After the waiting period, the pulse and waiting period may be repeated and preferably are repeated between 5 and 30 times.
Preferably, the amount of chemical cleaner used per square metre of surface area of the membranes 24 per week is between 1 and 10 mg of NaOCI. When other chemical cleaners are used, an amount of chemical cleaner is used which is equivalent to the amount of NaOCI specified above in antimicrobial efficacy.
Once the regime for the cleaning events is known, the total volume of chemical cleaner introduced into the tank water 22, called the cleaning event dosage, can be calculated.
The cleaning event dosage is preferably safely below an amount which would disrupt treatment or filtration of the water. For example, in waste water treatment applications, the cleaning event dosage preferably is low enough that it does not disrupt the population of microorganisms to the point where a spike of pollutants makes the effluent quality unsatisfactory. For potable water applications, the cleaning event dosage preferably does not exceed the most limiting regulatory or design limit on the concentration of chemical cleaner in the permeate at any point of use. For example, with chlorine based chemical cleaners, trihalomethane formation is likely to be the controlling factor and can be predicted using trihalomethane formation tables. In appropriate circumstances, the volume of the permeate tank 37 may be considered in calculating the cleaning event dosage. Similarly, any prechlorination or chemical cleaner remaining in the tank 20 from a preceding cleaning event should be accounted for in determining whether a cleaning event dosage is acceptable.
In many cases, the cleaning event dosage will be well below the maximum cleaning event dosage that could be used. However, if this does not occur in a particular application, the cleaning regime is altered to give acceptable cleaning event dosages. In some cases, altering the frequency of cleaning events may produce acceptable cleaning event dosages without reducing the weekly CT, but in other cases a higher fouling index and lower weekly CT may be required.
After the cleaning event and dilution described above, backwash valves 60 are closed, permeate valve 34 is re-opened, pressure tank 64 opened if and as necessary to refill pressure tank 62, and permeation continues. New chemical cleaner is added to the chemical tank 68 as needed.
In another preferred embodiment, the cleaning steps are performed as described above with the exception that the chemical valve 66, chemical pump 67 and chemical tank 68 are replaced with a hot water valve 70, hot water pump 72 and boiler 74, except as described differently below.
The boiler 74 delivers heated water, preferably above 25 degrees Celsius and more preferably between 40 and 60 degrees Celsius, or steam in place of the chemical cleaners provided by the chemical tank 68. The inventors believe that the hot water or steam solubilizes some of the solids, particularly organic matter, both on the surface of the membranes 24 and in the pores of the membranes 24. The solubilized solids travel through the membranes 24 by permeation or disperse into the tank water 22. Solids may or may not be completely removed, but removing part of the solids with each cleaning event slows the long term rate of loss of permeability of the membranes 24.
The hot water may also kill some microorganisms attached to the membranes although it is not necessary to kill the microorganisms to achieve the desired effect. However, temperatures above 60 or 70 degree degrees Celsius are known to kill the bacteria and may be used if a large portion of the solids are bacteria.
Since no chemical cleaner is used, the flux of hot water or steam, the number and duration of the pulses and the wait time between them, and the frequency of cleaning events are not limited by resulting chemical concentrations but rather excess heating the tank water 22. The heated water may be provided continuously over a cleaning event but is preferably provided in pulses. Process parameters are preferably chosen to provide heated water in an area adjacent the membranes 24 for a sufficient amount of time such that at least readily solubilizable solids, particularly exopolymeric substances and other organic compounds and some inorganic compounds, may be solubilized.
Preferably, the pulses last for between 10 seconds and 100 seconds and have sufficient pressure to produce a flux of heated water through the membranes 24 between 5 g/sq.ft./day and 30 g/sq.ft./day. After each pulse, the flow of heated water is stopped for a waiting period preferably between 50 seconds and 3 minutes and more preferably between 50 seconds and 1 minute in length. After the waiting period, the pulse and waiting period may be repeated and preferably are repeated so that the cleaning event is between 30 minutes and 90 minutes in duration. Such cleaning events are preferably repeated between twice a day and once every two days and more preferably once a day.
Alternative or modified embodiments of the invention may also be constructed or practiced without departing from the scope of the invention. For example, when chemical cleaners are used but the tank 20 or permeate tank 37 are small, cleaning according to the invention may be timed to coincide with times when the tank water 22 is dumped. In waste water applications, sludge must be periodically discharged as retentate 46. If waste sludge is removed in discrete events performed immediately after a cleaning event, then some of the chemical cleaner will also be removed. In potable water applications, retentate 46 is also discharged to remove solids which build up in the tank 20. Retentate 46 can be discharged by emptying the tank or by performing a rapid flush during which retentate 46 is rapidly withdrawn, possibly with the assistance of a retentate pump 48, while excess feed water 14 is rapidly provided to keep the level of the tank water 22 constant. If the cleaning events are performed before either method of withdrawing retentate 46, then the chemical cleaners will be at least partially flushed out with the retentate 46. This method could allow the use of very small tanks, as used with high pressure filters, since the concentration of residual chemical cleaner in the permeate 36 would be substantially independent of the volume of the tank 20.
When hot water is used in place of chemical cleaners, the heated water or steam could be applied to the retentate side 27 of the membranes 24 by injection into the tank 20 or by heating the feed water 14.
In this case, permeation could be stopped momentarily to allow the membrane surfaces to be heated and then restarted to draw the dissolved solids through the membrane.
It is to be understood that what has been described are preferred embodiments to the invention. The invention nonetheless is susceptible to certain changes and alternative embodiments fully comprehended by the spirit of the invention as described above, and the scope of the claims of this patent.
Example 1: Waste Water Treatment An experimental membrane bioreactor using a ZEEWEED
500 membrane module having 46 square metres of membrane surface area was built for treating waste water and, in particular, for carbon oxidation, nitrification and phosphorus removal. At all times, the flow rate of permeate through the membranes was maintained at 15 gfd and the solids concentration in the bioreactor averaged between 15 g/1 and 20 g/1. The average flow through the bioreactor was 1,000 cubic metres per day and the peak flow was 2,000 cubic metres/day.
The bioreactor was first operated without cleaning according to the invention for 90 days. Permeability was not sustainable and decreased continuously. At the end of this time, permeability of the membranes had dropped to less than 3.0 gfd/psi.
The bioreactor was then operated with a fresh membrane module for 90 days with maintenance cleaning according to the present invention. The cleaning was performed twice per week using 100-125 mg/1 NaOCI solution for one hour in pulses at a rate of 3.6 mg per square metre per week. The permeability of the membranes decreased slowly and eventually stabilised at about 7.5 gfd/psi.
On an average basis, no significant decrease in effluent quality in terms of ammonia-nitrogen or total phosphorous occurred when cleaning according to the present invention was instituted. Concentration of cBOD5 in the effluent both with and without cleaning according to the present invention averaged 1.0 mg/1.
Example 2: Potable Water An experimental membrane bioreactor using ZEEWEED 10 membrane modules having 0.9 square metres of membrane surface area each was built for treating lake water to produce potable water. All experiments were performed at constant flux in which the flow is kept constant and the transmembrane pressure (TMP) was allowed to increase as membranes fouled. The raw water conditions were as follows:
Temperature (C) 10-20 TOC (mg/1) 3.0-5.0 Turbidity (ntu) 4.0-9.0 Apparent Colour (Pt Co 10-50 units) True Colour (Pt Co units)5.0-20.0 Experiments were performed with and without maintenance cleaning and at different fluxes. Cleaning events were done three times per week with 100 mg/1 NaOCI for 30 minutes. The cleaning dosage was between 2.8 and 3.7 mg NaOCI per square metre of membrane per week.
Figure 4 summarises the results obtained with and without maintenance cleaning. Each test lasted about 45-60 days. After an initial increase in TMP, the TMP reached a relatively constant value which is referred to as the sustainable TMP. Sustainable TMP is plotted as a function of fixed operating flux. Permeability can be calculated dividing the operating flux by TMP. In this figure, the "control" condition refers to operation without maintenance cleaning. Substantial improvement in sustainable TMP was obtained using maintenance cleaning.
The residual chlorine concentration in the process tank after each cleaning event was less than 0.5 mg/1. This level of residual chlorine in the process tank was low enough to continue the filtration process to produce potable water.
Example 3: Heated Water as a Chemical cleaner An experimental membrane bioreactor was built for treating a typical municipal waste water. ZW10 membrane modules were used each having a surface area of 0.9 square metres. The concentration of biomass was between 15 to 20 gMLSS/1, corresponding to a volumetric loading of between 1.2 to 2.3 kg COD/m3/d. COD and TKN removal were better than 95% with dissolved oxygen residuals between 0.5 and 1.5 mg 02 / 1 in the tank.
Experiments were performed at a constant transmembrane pressure of 34 kPa and the permeate flux was allowed to decline as the membranes fouled. Two modules were tested under the same conditions, one with and one without heated water maintenance cleaning. For cleaning, heated water maintenance cleaning was performed with water heated to 40C for 1 hour every day. Figure 5 shows the net flux results as a function of time and indicates that the heated water maintenance cleaning resulted in an improvement in flux averaging between 5 and 10 gfd over the duration of the test.
Claims (19)
1. A method for cleaning one or more membranes immersed in water containing solids and used to permeate a water lean in solids wherein each cleaning event comprises the steps of:
(a) stopping permeation;
(b) flowing a chemical cleaner through the membranes in a direction opposite to the direction in which permeate passes through the membrane; and, (c) resuming permeation;
and wherein no agitation is provided to the water containing solids adjacent the membranes while chemical cleaner is flowed through the membranes and wherein the membranes remain immersed during steps (a), (b) and (c).
(a) stopping permeation;
(b) flowing a chemical cleaner through the membranes in a direction opposite to the direction in which permeate passes through the membrane; and, (c) resuming permeation;
and wherein no agitation is provided to the water containing solids adjacent the membranes while chemical cleaner is flowed through the membranes and wherein the membranes remain immersed during steps (a), (b) and (c).
2. The method of claim 1 wherein cleaning events are performed between 1 and 7 times per week.
3. The method of claim 1 wherein the amount of chemical cleaner used per square metre of membrane surface area per week is equivalent in antimicrobial efficacy to between 1 mg and 10 mg of NaOCl.
4. The method of claim 3 wherein the product of the concentration of the chemical cleaner expressed as an equivalent concentration of NaOCl in antimicrobial efficacy and the duration of all cleaning events in a week is between 2,000 minutes*mg/1 and 20,000 minutes*mg/1.
5. The method of claim 1 wherein the chemical cleaner is applied in pulses.
6. The method of claim 5 wherein the pulses last for between seconds and 100 seconds and there is a time between pulses between 50 seconds and 3 minutes.
7. The method of claim 5 wherein cleaning events are performed between 1 and 7 times per week.
8. The method of claim 5 wherein the amount of chemical cleaner used per square metre of membrane surface area per week is equivalent in antimicrobial efficacy to between 1 mg and 10 mg of NaOCl.
9. The method of claim 8 wherein the product of the concentration of the chemical cleaner expressed as an equivalent concentration of NaOCl in antimicrobial efficacy and the duration of all cleaning events in a week is between 2,000 minutes*mg/l and 20,000 minutes*mg/l.
10. A method for cleaning one or more membranes immersed in water containing solids and used to permeate a water lean in solids wherein each cleaning event comprises the steps of:
(a) stopping permeation and agitation of the water containing solids;
(b) flowing a chemical cleaner through the membranes in a direction opposite to the direction in which permeate passes through the membrane;
(c) resuming permeation; and, (d) resuming agitation, and wherein permeate collected before resuming agitation is wasted or recycled to the water containing solids and wherein the membranes and the membranes remain immersed during steps (a), (b), (c) and (d).
(a) stopping permeation and agitation of the water containing solids;
(b) flowing a chemical cleaner through the membranes in a direction opposite to the direction in which permeate passes through the membrane;
(c) resuming permeation; and, (d) resuming agitation, and wherein permeate collected before resuming agitation is wasted or recycled to the water containing solids and wherein the membranes and the membranes remain immersed during steps (a), (b), (c) and (d).
11. The method of claim 5 wherein the pulses are applied at a pressure between 5 kPa and 55 kPa.
12. The method of claim 1 wherein the chemical cleaner is heated water.
13. The method of claim 12 wherein the chemical cleaner is heated to more than 25 degrees celcius.
14. The method of claim 13 wherein the chemical cleaner is heated to between 40 degrees celcius and 60 degrees celcius.
15. The method of claim 12 wherein the chemical cleaner is applied in pulses.
16. The method of claim 15 wherein the pulses are applied at a pressure between 5 kPa and 55 kPa.
17. The method of claim 15 wherein the pulses last for between 10 seconds and 100 seconds and there is a time between pulses between 50 seconds and 3 minutes.
18. The method of claim 13 wherein the duration of each cleaning event is between 30 minutes and 90 minutes.
19. The method of claim 18 wherein the cleaning events are repeated between twice per day and once every two days.
Priority Applications (5)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA002279087A CA2279087A1 (en) | 1999-07-29 | 1999-07-29 | Maintenance cleaning for membranes |
| PCT/CA2000/000875 WO2001008790A1 (en) | 1999-07-29 | 2000-07-27 | Chemical cleaning backwash for immersed filtering membranes |
| AU62570/00A AU6257000A (en) | 1999-07-29 | 2000-07-27 | Chemical cleaning backwash for immersed filtering membranes |
| PCT/CA2000/000876 WO2001008789A2 (en) | 1999-07-29 | 2000-07-27 | Maintenance cleaning for membranes |
| AU64190/00A AU6419000A (en) | 1999-07-29 | 2000-07-27 | Maintenance cleaning for membranes |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA002279087A CA2279087A1 (en) | 1999-07-29 | 1999-07-29 | Maintenance cleaning for membranes |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2279087A1 true CA2279087A1 (en) | 2001-01-29 |
Family
ID=4163874
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002279087A Abandoned CA2279087A1 (en) | 1999-07-29 | 1999-07-29 | Maintenance cleaning for membranes |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2279087A1 (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN114272760A (en) * | 2021-12-10 | 2022-04-05 | 上海市政工程设计研究总院（集团）有限公司 | Flat membrane scrubbing device |
-
1999
- 1999-07-29 CA CA002279087A patent/CA2279087A1/en not_active Abandoned
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN114272760A (en) * | 2021-12-10 | 2022-04-05 | 上海市政工程设计研究总院（集团）有限公司 | Flat membrane scrubbing device |
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