AU2004273542A1

AU2004273542A1 - Backwash and cleaning method

Info

Publication number: AU2004273542A1
Application number: AU2004273542A
Authority: AU
Inventors: Warren Thomas Johnson
Original assignee: US Filter Wastewater Group Inc
Current assignee: Evoqua Water Technologies LLC
Priority date: 2003-09-22
Filing date: 2004-09-22
Publication date: 2005-03-31
Anticipated expiration: 2024-09-22
Also published as: AU2004273542B2

Description

WO 2005/028086 PCT/AU2004/001292 Backwash and Cleaning Method FIELD OF THE INVENTION The present invention relates to concentration of solids in a suspension 5 using hollow fibre membranes and, in particular, to an improved method of backwashing and chemically cleaning the hollow fibre membranes. BACKGROUND ART 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 o10 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. A pressurized liquid backwash of hollow fibre membranes has been found 15 to be uneven along the length of the fibre membranes due to the frictional losses along the lumen. In membranes with the fibres closed at one end, the pressure of liquid is highest at the point of application of the pressurized flow to the fibres lumens and tapering off along the length of the membrane. This results in uneven backwashing and poor recovery of TMP at portions of the 20 fibres remote from the backwash application point. In fibres open at both ends, the backwash flow is a minimum towards the centre of the fibre. During chemical cleaning of membranes, cleaning solutions are often backflushed from the lumen side of the membrane to distribute the cleaning solution within the membrane fibre bundle. Applying the cleaning solution under 25 pressure assists the removal of foulants from the surface. However, the WO 2005/028086 PCT/AU2004/001292 -2 limitations of pressure drop down the lumen during this step mean that achieving the same applied transmembrane pressure (TMP) to all areas of the membrane cannot be readily achieved, especially for small diameter fibres where the pressure loss is greatest. This impacts on the efficiency of cleaning. 5 DISCLOSURE OF THE INVENTION 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 concentrating the solids of a liquid suspension comprising: 10 (i) providing a pressure differential across the walls of permeable, hollow membranes immersed in the 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 15 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, 20 (ii) periodically backwashing the membrane pores using the permeate by applying a gas at a pressure below the bubble point to the membrane lumens to progressively displace at least some of the liquid permeate within the lumens through the membrane pores resulting in removal the solids retained on or in the hollow membranes into the bulk liquid surrounding the membranes.

WO 2005/028086 PCT/AU2004/001292 -3 This process ensures that the differential pressure applied during backwash is close to the gas pressure at the liquid interface as it travels down the lumen thereby ensuring that the maximum differential pressure is applied across the membrane wall at all points, although not simultaneously. 5 According to a second aspect, the present invention provides a method of concentrating the solids of a liquid suspension comprising: (i) providing a pressure differential across the walls of permeable, hollow membranes immersed in the liquid suspension, said liquid suspension being applied to the outer surface of the porous hollow membranes to induce io 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 filtrate from the hollow membrane lumens, and (b) at least some of the solids are retained on or in the hollow 15 membranes or otherwise as suspended solids within the liquid surrounding the membranes, (ii) dislodging the retained solids from the membranes by applying a dislodging medium through the lumens of said membranes while concurrently draining liquid from said lumens, wherein the application of the dislodging 20 medium initially displaces liquid within the hollow membrane lumens through the hollow membrane with gas, to effect firstly a discharge of liquid in the lumens through the membrane walls, and secondly a transmembrane cleaning of the membranes by applying the gas at sufficient pressure onto the liquid to overcome the bubble point of the membrane, and ensure that the gas will 25 displace liquid and follow it through the larger pores of the membranes to WO 2005/028086 PCT/AU2004/001292 -4 dislodge any solids retained therein; and for the emerging gas to scour the external walls of the membranes and displace the removed solids into the bulk liquid surrounding the membranes. Preferably, said method is carried out as a continuous process utilising a 5 repetitive cycle of solid accumulation and solid discharge. According to a third aspect the present invention provides a concentrator for recovering fine solids from a liquid feed suspension comprising: (i) a vessel for containing said feed suspension; (ii) a plurality of permeable, hollow membranes within the vessel; 10 (iii) means for providing a pressure differential across walls of said membranes; (iv) means for withdrawing clarified liquid from the membrane; and (v) means for applying gas at a pressure below the bubble point to the liquid permeate in the membrane lumens to effect a discharge of at least some 15 of the liquid permeate in the lumens through the membrane walls to dislodge any solids retained therein and displace the removed solids into the bulk liquid surrounding the membranes. According to a fourth aspect the present invention provides a concentrator for recovering fine solids from a liquid feed suspension comprising: 20 (i) a vessel or tank for containing said feed suspension; (ii) a plurality of permeable, hollow membranes within the vessel or tank; (iii) means for providing a pressure differential across walls of said membranes; 25 (iv) means for withdrawing clarified liquid from the membrane; and WO 2005/028086 PCT/AU2004/001292 -5 (v) means for applying gas pressure to the liquid in the membrane lumens and walls while the vessel or tank is exposed to atmospheric pressure and while concurrently draining liquid from said lumens, to effect firstly a discharge of liquid in the lumens through the membrane walls, and secondly a 5 transmembrane cleaning of the membranes by applying the gas at sufficient pressure onto the liquid to overcome the bubble point of the membrane, and ensure that the gas will displace liquid and follow it through the larger pores of the membranes to dislodge any solids retained therein; and for the emerging gas to scour the external walls of the membranes and displace the removed to solids into the bulk liquid in the vessel or tank. Preferably, the backwash includes use of clean-in-place (CIP) chemical solutions as well as or instead of the filtrate. This may be employed in a number of different backwash methods. One such backwash method includes filtering the chemical cleaning 15 solution from the shell side, that is, from the outer surface or vessel side of the membrane into the membrane lumens. The normal backwash is then performed and the chemical solution forced back through the membrane pores in an even fashion by applying a gas as described above. Another alternate form of chemical backwash includes backwashing 20 initially with filtrate, that is, pushing the filtrate in a reverse direction through the membrane pores while injecting chemical cleaning solution into the filtrate. The filtrate/chemical solution mixture is then backwashed through the membrane by applying a gas as described above. Yet another alternate form of chemical backwash includes applying 25 chemical cleaning solution under pressure to the outer side of the membranes WO 2005/028086 PCT/AU2004/001292 -6 to force chemical cleaning solution through the membrane pores and fill the membrane lumens with the chemical cleaning solution. This is followed by the normal gas backwash described above. In order to minimise the volume of chemical cleaning solution used all (or 5 most) of the liquid in the system may be removed or drained from one side of the membrane, typically the filtrate side (or inside of the hollow membrane), then the outer side of the membrane is at least partially filled with chemical cleaning solution and a vacuum (or reduced pressure) applied to the filtrate side to cause the chemical cleaning solution to be drawn from the outer side of the membrane io to the filtrate side, then gas pressure is applied to the filtrate side to force the chemical cleaning solution in the reverse direction from the filtrate side through the membrane wall back to the outer side of the membrane. In another method, the filtrate side of the membrane(s) is drained or emptied of liquid and liquid on the outer side of the membranes is also partially 15 drained or emptied. The outer side of the membrane lumen is then at least partially filled chemical cleaning solution. The chemical cleaning solution applied to the outer side of the membranes is then pushed through with gas (for a pressurized system) or drawn through under suction (for a submerged non pressurized system) to fill the lumen with chemical cleaning solution and the 20 volume of chemical cleaning solution used is less than the hold-up volume of liquid on the outer side of the membranes. Only enough volume of chemical cleaning solution on the outer side of the membranes to fill the membrane lumens is required. Pressure can then be applied to the lumen side to drain the chemical cleaning solution from the lumen by pushing it back through the 25 membrane wall. This cycle can be repeated multiple times so that the chemical WO 2005/028086 PCT/AU2004/001292 -7 cleaning solution is alternately moved from one side of the membrane to the other through the membrane wall. Each of the above chemical cleaning methods has been found to provide a more efficient chemical backwash. The methods allow for a minimal use of 5 chemical cleaning solution while also enabling an enhanced washing process by providing a more efficient distribution of the chemical cleaning solution within the system. Desirably, these backwashes or cleans are performed on an intermittent basis. Using the methods described the reverse flow cleaning step can be io accomplished in such a way as to allow the transmembrane pressure (TMP) to be controlled by the gas pressure and to apply this TMP evenly along the membrane, even at the extremities from the lumen inlet. This ensures all areas of the membrane are contacted with chemical cleaning solution and that they are back-flushed with the same applied TMP. It also allows the chemical in the 15 lumens to be fully drained by the end of the reverse flow step, which aids in recovery of chemical cleaning solution, reduces flushing requirements, and reduces cleaning downtime. In one preferred form, the gas may be pulsed in its application to the membrane lumens. In one alternate form of the chemical solution backwash 20 described above, the backwash is performed with the vessel empty. The process can be applied to membranes submerged in an open vessel as well as pressurized membrane filtration systems. BRIEF DESCRIPTION OF THE DRAWINGS Preferred embodiments of the invention will now be described, by way of 25 example only, with reference to the accompanying drawings in which: WO 2005/028086 PCT/AU2004/001292 -8 Figure la shows a graph of transmembrane pressure (TMP) vs position along the membrane bundle of the membrane module configuration shown in Figure 1b; Figure lb shows a simplified sectional side elevation of a membrane 5 module immersed in a feed liquid with pressurized liquid applied to the membrane lumens; Figure 2a shows a graph of transmembrane pressure (TMP) vs position along the membrane bundle of the membrane module configuration shown in Figure 2b; 10 Figure 2b shows a simplified sectional side elevation of a membrane module immersed in a feed liquid with pressurized gas applied to the membrane lumens; Figure 3a shows a graph of transmembrane pressure (TMP) vs position along the membrane bundle of the membrane module configuration shown in 15 Figure 3b; Figure 3b shows a simplified sectional side elevation of a membrane module immersed in a feed liquid with pressurized gas applied to liquid filled membrane lumens; Figure 3c shows an enlarged sectional view of the membranes in the 20 indicated region of Figure 3b; Figure 4a shows a simplified sectional side elevation of a membrane module with the feed liquid drained from around the module; Figure 4b shows an enlarged sectional view of the membranes in the indicated region of Figure 4b; WO 2005/028086 PCT/AU2004/001292 -9 Figure 5a 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; Figure 5b shows an enlarged sectional view of the membranes in the 5 indicated region of Figure 5a; Figure 5c shows an enlarged sectional view of the membranes in the indicated region of Figure 5a; Figure 6a shows a simplified sectional side elevation of a membrane module with a lower portion of the module immersed in a chemical cleaning o10 solution and pressurized gas applied to the membrane lumens; and Figure 6b shows an enlarged sectional view of the membranes in the indicated region of Figure 6a. DESCRIPTION OF PREFERRED EMBODIMENTS Referring to Figures la and lb, the graph shown in Figure la illustrates 15 the change in transmembrane pressure (TMP) as the distance from the application of pressure flow increases. Figure lb shows a membrane module 5 having a plurality of hollow fibre membranes 6. The fibre membranes 6 are closed at the lower end in a lower pot 7 and open at the upper end through upper pot 8. The module is immersed in liquid 9 contained in a vessel 10. In 20 the case illustrated, pressurized liquid is applied to the open end of the fibre lumens 11 resulting in the TMP profile shown in Figure 1 a. As noted above, in membranes with the fibre membranes 6 closed at one end, the pressure of liquid is highest at the point of application of the pressurized flow to the fibres lumens 11 and tapers off along the length of the WO 2005/028086 PCT/AU2004/001292 -10 membrane 6. This results in uneven backwashing and poor recovery of TMP at portions of the fibre membranes 6 remote from the backwash application point. Figures 2a and 2b show a similar arrangement to Figure 1 but in this case pressurized gas is applied to the fibre membrane lumens 11 resulting in an even 5 distribution of TMP along the length of the fibre membranes 6. Figures 3a to 3c illustrate one embodiment of the invention where pressurized gas is applied at a pressure below the bubble point to liquid filled fibre membrane -lumens 11. As best shown in Figure 3c as the liquid is displaced through the membrane wall 12, the lumen 11 becomes filled with gas io resulting in a maximum TMP being applied along the length of the fibre membrane 6 as the liquid level within the fibre membrane lumen 11 drops. Figure 4a and 4b illustrate a further embodiment of the invention where liquid is drained from around the membrane module 5 before the backwashing process is commenced. The backwashing process is similar to that described 15 above for Figure 3. Referring to Figures 5 and 6, one embodiment of the cleaning process according to the invention is illustrated. The membrane module 5 is immersed at least partially in chemical cleaning solution 13 and suction is applied to the open ends of the fibre membrane lumens 11. As best shown in figure 5b, the 20 cleaning solution 13 is drawn through the membrane wall 12 and into the fibre membrane lumen 11. The cleaning solution 13 is then drawn up through the lumen 11 until it is completely filled as shown in Figure 5c. As shown in Figures 6a and 6b, pressurized gas is then applied to the cleaning solution filling the membrane lumen 11 and displaced through the membrane wall 12 as previously 25 described. This flow of cleaning solution to and from the membrane lumens 11 WO 2005/028086 PCT/AU2004/001292 -11 as well as along their length results in an effective chemical clean of the membrane module 5. The invention may be embodied in a similar apparatus to that described in the aforementioned International Application No. WO93/02779 appropriately 5 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.

Claims

1. A method of concentrating the solids of a liquid suspension comprising: (i) providing a pressure differential across the walls of permeable, hollow membranes immersed in the liquid suspension, said liquid suspension 5 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 filtrate from the hollow membrane lumens, and 10 (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, (ii) periodically backwashing the membrane pores using the filtrate by applying a gas at a pressure below the bubble point to the membrane lumens to 15is progressively displace at least some of the liquid filtrate within the lumens through the membrane pores resulting in removal the solids retained on or in the hollow membranes into the bulk liquid surrounding the membranes.

2. A method of concentrating the solids of a liquid suspension comprising: (i) providing a pressure differential across the walls of permeable, 20 hollow membranes immersed in the 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 filtrate from the 25 hollow membrane lumens, and WO 2005/028086 PCT/AU2004/001292 - 13 (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, (ii) dislodging the retained solids from the membranes by applying a 5 dislodging medium through the lumens of said membranes while concurrently draining liquid from said lumens, wherein the application of the dislodging medium initially displaces liquid within the hollow membrane lumens through the hollow membrane with gas, to effect firstly a discharge of liquid in the lumens through the membrane walls, and secondly a transmembrane cleaning of the 0to membranes by applying the gas at sufficient pressure onto the liquid to overcome the bubble point of the membrane, and ensure that the gas will displace liquid and follow it through the larger pores of the membranes to dislodge any solids retained therein; and for the emerging gas to scour the external walls of the membranes and displace the removed solids into the bulk 15 liquid surrounding the membranes.

3. A method of concentrating the solids of a liquid suspension according to claim 1 or 2 wherein said method is carried out as a continuous process utilising a repetitive cycle of solid accumulation and solid displacement or removal.

4. A method of concentrating the solids of a liquid suspension according to 20 any one of the preceding claims wherein the dislodging or backwashing step includes use of a chemical cleaning solution.

5. A method of concentrating the solids of a liquid suspension according to claim 4 including the step of filtering the chemical cleaning solution from the outer surface of the porous hollow membranes into the membrane lumens. and WO 2005/028086 PCT/AU2004/001292 -14 then displacing said chemical cleaning solution back through the membrane pores by application of said gas.

6. A method of concentrating the solids of a liquid suspension according to claim 4 wherein the backwashing or dislodging step includes displacing the 5 filtrate in a reverse direction through the membrane pores while injecting chemical cleaning solution into the filtrate.

7. A method of concentrating the solids of a liquid suspension according to claim 4 including the step of applying chemical cleaning solution under pressure to the outer surface of the porous hollow membranes to displace chemical io cleaning solution through the membrane pores into the membrane lumens and then displacing said chemical cleaning solution back through the membrane pores by application of said gas.

8. A method according to any one of claims 5 to 7 when appended to claim 1 including the step of removing at least part of the liquid remaining in the 15 membrane lumens prior to displacing chemical cleaning solution into the membrane lumens.

9. A method according to any one of claims 5 to 7 when appended to claim 2 wherein the displacing of chemical solution into the membrane lumens takes place before said transmembrane cleaning. 20

10. A method of concentrating the solids of a liquid suspension according to any one of the preceding claims wherein the gas is pulsed in its application to the membrane lumens.

11. A method of concentrating the solids of a liquid suspension according to any one of the preceding claims including the step of removing at least part of WO 2005/028086 PCT/AU2004/001292 - 15 the bulk liquid surrounding the membranes prior to the backwashing or dislodging step.

12. A concentrator for recovering fine solids from a liquid feed suspension comprising: 5 (i) a vessel for containing said feed suspension; (ii) a plurality of permeable, hollow membranes within the vessel; (iii) means for providing a pressure differential across walls of said membranes; (iv) means for withdrawing clarified liquid from the membrane; and 10 (v) means for applying gas at a pressure below the bubble point to the liquid permeate in the membrane lumens to effect a discharge of at least some of the liquid permeate in the lumens through the membrane walls to dislodge any solids retained therein and displace the removed solids into the bulk liquid surrounding the membranes. 15

13. A concentrator for recovering fine solids from a liquid feed suspension comprising: (i) a vessel or tank for containing said feed suspension; (ii) a plurality of permeable, hollow membranes within the vessel or tank; 20 (iii) means for providing a pressure differential across walls of said membranes; (iv) means for withdrawing clarified liquid from the membrane; and (v) means for applying gas pressure to the liquid in the membrane lumens and walls while the vessel or tank is exposed to atmospheric pressure 25 and while concurrently draining liquid from said lumens, to effect firstly a WO 2005/028086 PCT/AU2004/001292 -16 discharge of liquid in the lumens through the membrane walls, and secondly a transmembrane cleaning of the membranes by applying the gas at sufficient pressure onto the liquid to overcome the bubble point of the membrane, and ensure that the gas will displace liquid and follow it through the larger pores of 5 the membranes to dislodge any solids retained therein; and for the emerging gas to scour the external walls of the membranes and displace the removed solids into the bulk liquid in the vessel or tank.