The present invention relates to the backwashing of hollow permeable membranes used in membrane filtration systems and, in particular, to an improved method of backwashing and cleaning the hollow permeable membranes.
Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
Known backwash systems include those described in our earlier International Application No. WO93/02779 the subject matter of which is incorporated herein by cross-reference.
During cleaning of membranes, cleaning solutions are often flowed through the membranes and their permeable walls to clean foulants from the membranes. Applying the cleaning solution under pressure assists the removal of foulants from the surface.
The typical known cleaning procedure for membranes involves cleaning the membranes in-situ. This procedure adds a set concentration of chemical, commonly 2% citric acid followed by 200-1000 ppm sodium hypochlorite, to the membrane in a solution of filtrate. This usually occurs at the start of the two hour cleaning period, after which the cleaning solution is filtered through the membrane and left to soak.
As the nature of membrane fouling varies according to feed quality and type, flux through the membrane and hours of operation, the amount and length of chemical cleaning required in each situation also varies. This often results in a one process fits all approach where a standard chemical cleaning stage is applied regardless of the amount of fouling. This can lead to large amounts of cleaning solution being used unnecessarily with the effect of additional cost and environmental impact in disposing of the waste solution after cleaning is complete.
It is an object of the invention to overcome or at least ameliorate one or more of the disadvantages of the prior art or at least provide a useful alternative.
According to a first aspect, the present invention provides a method of cleaning permeable, hollow membranes in an arrangement of the type wherein a pressure differential is applied across the walls of the permeable, hollow membranes immersed in a liquid suspension, said liquid suspension being applied to the outer surface of the porous hollow membranes to induce and sustain filtration through the membrane walls wherein:
(a) some of the liquid suspension passes through the walls of the membranes to be drawn off as clarified liquid or permeate from the hollow membrane lumens, and
(b) at least some of the solids are retained on or in the hollow membranes or otherwise as suspended solids within the liquid surrounding the membranes, the method of cleaning comprising the steps of;
viii) releasing the pressure applied by said pressurised gas to said filtrate side of the membrane wall to cause flow of said cleaning solution through said membrane from the outer side to the filtrate side under the effect of said accumulated pressure on the outer side of the membrane wall.
For preference the cleaning solution is a chemical cleaning solution.
Preferably, in step vi) gas, usually air, is applied such that the membrane lumen is substantially drained of cleaning solution. Preferably the pressure is accumulated in step vii) in a gas space provided on the outer side of the membrane wall or alternatively in a bladder arrangement.
The differential pressure of step iv) may be provided by applying gas pressure to the outer side of the membrane wall or by applying a vacuum to the filtrate side.
According to a further aspect the present invention provides a method of cleaning permeable, hollow membranes in an arrangement of the type wherein a pressure differential is applied across the walls of the permeable, hollow membranes immersed in a liquid suspension, said liquid suspension being applied to the outer surface of the porous hollow membranes to induce and sustain filtration through the membrane walls wherein:
(a) some of the liquid suspension passes through the walls of the membranes to be drawn off as clarified liquid or permeate from the hollow membrane lumens, and
(b) at least some of the solids are retained on or in the hollow membranes or otherwise as suspended solids within the liquid surrounding the membranes, the method of cleaning comprising the steps of;
According to yet a further aspect the present invention provides a method of cleaning permeable, hollow membranes in an arrangement of the type wherein a pressure differential is applied across the walls of the permeable, hollow membranes immersed in a liquid suspension, said liquid suspension being applied to the outer surface of the porous hollow membranes to induce and sustain filtration through the membrane walls wherein:
(a) some of the liquid suspension passes through the walls of the membranes to be drawn off as clarified liquid or permeate from the hollow membrane lumens, and
(b) at least some of the solids are retained on or in the hollow membranes or otherwise as suspended solids within the liquid surrounding the membranes, the method of cleaning comprising the steps of;
Preferably the pressure differential in step v) is produced by applying a pressurized gas to the filtrate side of the membrane wall to cause flow of the cleaning solution back to the outer side through the membrane wall.
The cleaning process can be repeated in cycles such that the cleaning solution is alternately moved from one side of the membrane to the other through the membrane wall.
The process can be applied to membranes submerged in an open vessel as well as pressurized membrane filtration systems.
According to another aspect of the present invention there is provided a method of controlling a chemical clean of a membrane comprising:
measuring pH and/or membrane resistance of a membrane for at least a portion of said clean; and
ceasing said chemical clean when pH and/or membrane resistance attains a predetermined value.
According to another aspect of the present invention there is provided a method of controlling a chemical clean of a membrane comprising:
measuring pH and/or membrane resistance of a membrane for at least a portion of said clean;
measuring elapsed time of the clean;
calculating a rate of change of pH with respect to time (dpH/dt) and/or a rate of change of membrane resistance (dR/dt) with respect to time; and
ceasing said chemical clean when dpH/dt and/or dR/dt attains a predetermined value.
According to another aspect of the present invention there is provided a method of controlling the chemical cleaning of a filtration system comprising the steps of measuring membrane resistance of a membrane for at least a portion of said clean;
measuring elapsed time of the clean;
calculating a rate of change of membrane resistance (dR/dt) with respect to time; and
using dR/dt to calculate a duration for completion of the clean.
According to another aspect of the present invention there is provided a method of controlling a chemical clean of a membrane comprising:
increasing the amount of chemical cleaning agent present during the clean;
measuring membrane resistance of a membrane for at least a portion of said clean;
ceasing the increase in chemical cleaning agent when membrane resistance attains a predetermined value.
Preferably, the amount of cleaning agent is increased incrementally.
For preference, the predetermined value approximates a steady-state value of membrane resistance. Preferably, the membranes are microfiltration or ultrafiltration type membranes.
Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:
a shows a simplified sectional side elevation of a membrane module with a lower portion of the module immersed in a chemical cleaning solution and suction applied to the membrane lumens;
b shows an enlarged sectional view of the membranes in the indicated region of
c shows an enlarged sectional view of the membranes in the indicated region of
a shows a simplified sectional side elevation of a membrane module of
b shows an enlarged sectional view of the membranes in the indicated region of
a shows a simplified sectional side elevation of a membrane module of
b shows an enlarged sectional view of the membranes in the indicated region of
c shows an enlarged sectional view of the membranes in the indicated region of
a shows a simplified sectional side elevation of another embodiment of a membrane module with a lower portion of the module immersed in a chemical cleaning solution and suction applied to the membrane lumens;
b shows an enlarged sectional view of the membranes in the indicated region of
c shows an enlarged sectional view of the membranes in the indicated region of
a shows a simplified sectional side elevation of the membrane module of
b shows an enlarged sectional view of the membranes in the indicated region of
a shows a simplified sectional side elevation of another embodiment of a membrane module with a lower portion of the module immersed in a chemical cleaning solution and suction applied to the membrane lumens;
b shows an enlarged sectional view of the membranes in the indicated region of
c shows an enlarged sectional view of the membranes in the indicated region of
a shows a simplified sectional side elevation of the membrane module of
b shows an enlarged sectional view of the membranes in the indicated region of
a shows a simplified sectional side elevation of an embodiment of a membrane module in an open vessel with a lower portion of the module immersed in a chemical cleaning solution and suction applied to the membrane lumens;
b shows an enlarged sectional view of the membranes in the indicated region of
c shows an enlarged sectional view of the membranes in the indicated region of
a shows a simplified sectional side elevation of a membrane module of the embodiment of
b shows an enlarged sectional view of the membranes in the indicated region of
Referring to the FIGS. 1 to 7, there is shown a membrane module 5 having a plurality of hollow fibre membranes 6. The fibre membranes 6 have their lumens 7 closed at the lower end in a lower pot 8 and open at the upper end through upper pot 9. The module 5 is contained in a vessel 10 having a controllable valve 11 for opening/closing the vessel 10 to atmosphere. Upper pot 8 is connected to a filtrate collection chamber 12 having a port 13.
One embodiment of the cleaning process according to the invention will now be described with reference to FIGS. 1 to 3. As best shown in
As shown in
As shown in
a to 7b illustrate embodiments of the invention where a bladder arrangement may be used instead of the gas space 16 to accumulate pressure.
Referring to
Referring to
a to 6c show a similar arrangement to
As shown in
Referring to
The process illustrated in the embodiments can be repeated in cycles such that cleaning solution is alternatively moved from one side of the membrane wall 15 to the other. This flow of cleaning solution to and from the membrane lumens 7 and well as along their length results in an effective chemical clean of the membrane module 5.
1. The membrane vessel was filled with filtrate via backfilling from the lumen side to the shell side, with simultaneous chlorine dosing into the filtrate line. The vessel filtrate level was about 30%, with a target volume of cleaning solution (NaOCl) of 30 mL.
2. The filtrate was then recirculated briefly through the system in order to ensure a well-mixed cleaning solution.
3. The lumens were then drained of liquid by 100 kPa air being applied to the filtrate line. This allowed the cleaning solution to diffuse through the pores and down the fibre length, which raised the filtrate vessel level. This step may be ended when the liquid level stops rising.
4. The lumens were then filled with the cleaning solution by using vacuum air applied to the lumen side of the membranes. During this step the level in the filtrate tank dropped as the liquid was pulled into the fibre lumens. This step may be ended when the liquid level stops falling.
5. The lumen fill and drain steps were repeated until contact time had reached 1800 seconds.
6. After 1800 seconds of cleaning solution contact, the vessel was topped up with feed. This allowed the remaining free chlorine in the cleaning solution to contact with the part of the module that was exposed during the clean.
7. The system was then aerated to maximise contact of solution with module.
8. The tank was then drained and flushed with filtrate before returning to service.
The data graphed in
The data shown in
One embodiment of the invention seeks to minimise the amount of chemical required by adding it incrementally to the membrane tank, whilst monitoring resistance through the membrane during a recirculation stage in the cleaning process. Chemical additions can cease when the further addition of chemical leads to change in the membrane resistance below a predetermined level, hence minimising the amount of excess chemical agent used in the cleaning process.
The resistance value can be monitored during filtrate recirculation. Typically, during a standard cleaning procedure, the chemical cleaning solution is recirculated at the start of the clean only, followed by up to 48 hours of soaking of the membranes. In the present embodiment, the chemical cleaning solution is recirculated for several minutes (for example −3 minutes) every 15-30 minutes during the soak/aeration steps, allowing the membrane resistance to be measured periodically throughout the cleaning process.
When the change in resistance per 3 recirculations drops below a predetermined value (for example −0.1) the cleaning process has recovered the maximum performance at that chemical concentration and further chemical agent is added. When the addition of further chemical agent effects the change in resistance by less than the predetermined value per 3 recirculations (for example −0.1), no further recovery can be achieved and the cleaning process can therefore be terminated immediately. Conversely, the cleaning potential can be maximized by extending the cleaning process so that the change in resistance per 3 circulations is below a certain predetermined value.
Referring to the graph shown in
It will be appreciated that using the above measurements it is possible to determine a resistance profile during the cleaning process for a particular membrane arrangement or configuration. The resistance profile can then be used to predict the end of cleaning process time, half-life and reduce chemical use in simultaneous cleans of similar systems. The resistance profile may be further used to determine whether chemicals are required to be added during the cleaning process with the type and amount of chemical being dependent on feed and foulant quality.
Typical cleaning solutions which may be used include acids, caustic solutions and oxidizing solutions (e.g. chlorine).
The invention may be embodied in a similar apparatus to that described in the aforementioned International Application No. WO93/02779 appropriately modified to operate in accordance with the inventive method.
It will be appreciated that further embodiments and exemplifications of the invention are possible without departing from the spirit or scope of the invention described.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCTAU0501919 | 12/19/2005 | WO | 6/21/2007 |
Number | Date | Country | |
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20080203016 A1 | Aug 2008 | US |