AERATED BIOFILM REACTOR HOLLOW FIBRE MEMBRANE

Abstract

The present invention is concerned with a fibre membrane for use in a Membrane Supported Biofilm Reactor (MSBR) or the like, the fibre membrane comprising a substantially cylindrical sidewall defining an internal lumen from which gas can permeate through the sidewall, and characterised in that at least a part of an outer surface of the fibre membrane is engineered to define at least one biofilm retaining region which acts to retain a quantity of biofilm therein, in particular when the fibre membrane is subjected to a high sheer biofilm control event, such as experienced during a reactor cleaning cycle, for removing excess biofilm in order to prevent clogging of the reactor.

Claims

1. An aerated biofilm reactor fibre membrane comprising an internal lumen from which gas can permeate through the membrane; characterised in that at least a part of an outer surface of the fibre membrane comprises a pair of space apart protrusions which define at least one engineered biofilm retaining region therebetween, each protrusion having a height of between 10 μm and 500 μm above the nadir of the defined biofilm retaining region.

2. The fibre membrane according to claim 1, wherein the outer surface of the fibre membrane defines an array of the engineered biofilm retaining regions.

3. The fibre membrane according to claim 1, wherein the engineered biofilm retaining region of the outer surface comprises one or more concave regions.

4. The fibre membrane according to claim 1, wherein the outer surface comprises two or more substantially radially extending protrusions.

5. The fibre membrane according to claim 1, wherein the engineered biofilm retaining region of the outer surface comprises one or more substantially longitudinally extending corrugations.

6. The fibre membrane according to claim 1, wherein the outer surface of the fibre membrane is multilateral.

7. The fibre membrane according to claim 1, wherein an inner surface of the fibre membrane, which defines the lumen, is shaped to optimise gas transfer through the membrane.

8. The fibre membrane according to claim 1, wherein the fibre membrane is formed as a polymer extrusion.

9. The fibre membrane according to claim 1, wherein the lumen comprises an open end through which gas may be supplied to the lumen.

10. The fibre membrane according to claim 1, wherein the outer surface defines a cylindrical sidewall surrounding the lumen.

11. The fibre membrane according to claim 10, wherein the fibre membrane has an external diameter in the range of between 150 μm and 1500 μm.

12. The fibre membrane according to claim 1, wherein the fibre membrane comprises a gas permeable polymer.

13. The fibre membrane according to claim 1, wherein the fibre membrane comprises polydimethyl siloxane (PDMS).

14. A membrane aerated biofilm reactor comprising: a reactor vessel; a plurality of fibre membranes according to claim 1 located in the reactor vessel; a liquid inlet arranged to feed a liquid to be treated into the reactor vessel; and a liquid outlet from which treated liquid can be withdrawn from the reactor vessel.

15. The membrane aerated biofilm reactor according to claim 14, further comprising a process gas inlet for supplying a gas to the lumen of one or more of the fibre membranes.

16. The membrane aerated biofilm reactor according to claim 14, further comprising a scour gas inlet for introducing a scouring gas into the vessel to effect biofilm removal from the membranes.

17. The membrane aerated biofilm reactor according to claim 14, wherein the scour gas inlet is adapted to generate bubbles of the scouring gas.

18. The membrane aerated biofilm reactor according to claim 14, wherein the bubbles are dimensioned to have a diameter which prevents the bubbles from contacting the nadir of the biofilm retaining region of the membranes.

19. The membrane aerated biofilm reactor according to claim 14, wherein the fibre membranes are arranged in groups within the vessel.

20. A method of controlling biofilm thickness in a membrane aerated biofilm reactor which has an array of fibre membranes each comprising an internal lumen from which gas can permeate through the membrane, at least a part of an outer surface of each fibre membrane comprising a pair of space apart protrusions which define at least one engineered biofilm retaining region therebetween, the method comprising scouring excess biofilm from the external surface of the membrane using gas bubbles dimensioned to prevent contact with a nadir of the defined biofilm retaining region.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0051] A more complete understanding of the present invention, and the attendant advantages and features thereof, will be more readily understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein:

[0052] FIG. 1 illustrates a cross section of a conventional prior art hollow fibre for use in a membrane aerated biofilm reactor;

[0053] FIG. 2 illustrates a cross section of an aerated biofilm reactor membrane fibre according to a preferred embodiment of the present invention;

[0054] FIG. 3 illustrates a cross section of an alternative aerated biofilm reactor membrane fibre according to an aspect of the present invention;

[0055] FIG. 4 illustrates a cross section of another alternative aerated biofilm reactor membrane fibre according to the present invention;

[0056] FIG. 5 illustrates a reactor according to a further aspect of the present invention;

[0057] FIG. 6 illustrates the membrane fibre of FIG. 1 with an excess accumulation of biofilm on an exterior surface thereof;

[0058] FIG. 7 illustrates the membrane fibre of FIG. 6 following removal of a quantity of biofilm from the exterior surface thereof;

[0059] FIG. 8 illustrates the membrane fibre of FIG. 6 with biofilm remaining only in a plurality of engineered biofilm retaining regions;

[0060] FIG. 9 illustrates a schematic representation of a large diameter bubble of scouring gas in contact with a pair of membrane fibres according to the invention; and

[0061] FIG. 10 illustrate a schematic representation of a smaller diameter bubble of scouring gas in contact with the pair of membrane fibres according to the invention.

DETAILED DESCRIPTION

[0062] Referring now to FIG. 1 there is shown a cross-section of a conventional prior art hollow fibre F for use in a conventional membrane aerated biofilm reactor (not shown). The hollow fibre F is substantially cylindrical in cross-section and defines an interior lumen L through which gas such as oxygen, air, oxygen enriched air, hydrogen or any other suitable gas, may be supplied, and which then permeates through the sidewall of the hollow fibre F in order to, in use, oxygenate a biofilm colonising the outer surface of the hollow fibre F. It can be seen that the outer surface of the hollow fibre F is a substantially smooth and continuous surface.

[0063] Turning then to FIG. 2 there is illustrated a cross-section of a fibre membrane for use in a membrane aerated biofilm reactor 50 (FIG. 5), the fibre membrane being generally indicated as 10. The fibre membrane 10 comprises a substantially cylindrical side wall 12 which is generally annular in form, and thus defines an interior lumen 14 which extends longitudinally of the fibre membrane 10. In use a gas such as oxygen or the like is pumped into the lumen 14 and, by providing the sidewall 12 as a gas permeable material, the gas can permeate through the sidewall 12 to be supplied to a biofilm (not shown in FIG. 1) colonizing an outer surface 16 of the fibre membrane 10. Unlike prior art hollow fibres, the fibre membrane 10 of the present invention defines one or more, and preferably a plurality of, engineered biofilm retaining regions 18 which, as described in detail hereinafter, act to retain a quantity of biofilm therein, in particular when the fibre membrane 10 is subjected to a high sheer biofilm control event, such as experienced during a reactor cleaning cycle, for removing excess biofilm in order to prevent the issues discussed above. As a result, once such an event has been completed, the biofilm held in the retaining regions 18 ensure expedient regrowth of the biofilm to full operational levels, thus significantly reducing the lead time between such a cleaning event and a return to full operation of the reactor 50. Conventionally this would be a significantly longer period in order to facilitate reseeding of the biofilm and regrowth on the outer surface of the fibre to an operational level.

[0064] Unlike prior art fibres, in the FIG. 2 embodiment the outer surface 16 is multilateral, and includes a plurality of concave sides each of which defines a single biofilm retaining region 18 located between an adjacent pair of protrusions 20. It can be seen that an inner surface 22 of the sidewall 12 is circular but could also be multilateral, for example corresponding in number of sides to that of the outer surface 16. It will of course be appreciated that the shape of both the outer surface 16 and inner surface 22 may be varied as required. For example it may be preferable that the outer surface 16 and inner surface 22 are substantially parallel in order to provide the side wall 12 with a substantially uniform thickness, thereby ensuring an equal transfer of gas at all points around the side wall 12, in order to establish an equal growth rate of biofilm about the outer surface 16. Equally however it will understood that it may be desirable to encourage regions of increased or decreased biofilm thickness on the outer surface 16, by suitably altering the gas permeability of that region of the sidewall 12, for example by varying the thickness of the sidewall 12 at localised regions. The fibre membrane 12 preferably has a external diameter in the range of between 0.2 mm to 5 mm, more preferably between 0.35 mm and 0.9 mm, and most preferably 0.5 mm, which diameter is measured at the radially outmost extremity of the fibre membrane 12.

[0065] The engineering biofilm retaining regions 18 are preferably substantially concave in shape, although as described below other forms are also envisaged. Regardless of the profile of the retaining region 18, the effective depth, that is the distance from the tip or radially outermost point of the respective protrusions 20 to the nadir or lowermost point in the concave or otherwise depressed retaining region, is in the range of between 10 μm and 500 μm. This depth will vary depending on various operating parameters, in particular the amount of biofilm to be retained following a high shear event, and/or the mechanism by which the high shear or cleaning event is achieved, for example by gas scouring or the like, as detailed below.

[0066] The fibre membrane 12 is preferably produced by extruding a polymer through a suitably shaped die (not shown) to provide the desired external and internal profiles to the fibre membrane 10. It will however be immediately understood that any other suitable method of manufacturing the fibre membrane 10 may be employed, and the material or combination of materials selected to form the fibre membrane 10 may be varied. The fibre membrane 12 is preferably comprised of silicone (polydimethyl siloxane (PDMS)) Or a modified version of PDMS, although other suitable materials may be employed.

[0067] Referring to FIGS. 3 and 4 there are illustrated alternative embodiments of a fibre membrane according to the present invention and for use in a MABR, each variant providing an alternative sidewall profile, as dictated by the shape of an outer surface and/or an inner surface of the respective fibre membrane.

[0068] In particular, referring to FIG. 3 there is illustrated a fibre membrane 110 similar in cross-section to the fibre membrane 10 of the first embodiment, comprising a star shaped sidewall 112 surrounding an inner lumen 114. A plurality of engineered biofilm retaining regions 118 are defined between pairs of protrusions 120 on the side wall 112, the retaining regions having sloping sidewalls terminating at a central apex defining the nadir of the retaining region 118.

[0069] FIG. 4 illustrates a fibre membrane 210 which is again multi-lateral in form, defining six convex sides forming protrusions 420 and a substantially circular inner surface 422 defining a lumen 214. A plurality of engineered biofilm retaining regions 218 are again defined between adjacent protrusions 220.

[0070] In each of the above fibre membranes at least one, and preferably an array of, biofilm retaining regions are defined about an outer surface of the fibre membrane, such that during a high sheer event such as a biofilm control event in order to prevent clogging of a reactor, some level of biofilm is retained in the retaining regions on the outer surface of each fibres membrane, in order to facilitate a speedy regrowth of the biofilm following the high shear event, in order to allow the reactor to be fully operational in a reduced period of time.

[0071] Turing then to FIG. 5 there is schematically illustrated a MABR reactor according to an aspect of the present invention, and generally indicated as 50. The reactor 50 comprises a reactor vessel 52 which may be of any suitable size and shape, contained within which are multiple arrays or groups of the fibre membranes 10, only three of which groups or bunches are shown for illustrative purposes. In the preferred embodiment illustrates the fibre membranes 10 are arranged in a substantially vertical orientation in use, that is a longitudinal axis of each fibre membrane 10 extends substantially vertically through the vessel 52. A liquid inlet 54 supplies wastewater to be treated to the interior of the vessel 52 while a liquid outlet 56 is provided for removing treated wastewater.

[0072] A process gas inlet 58 supplies gas to the lumen (not shown) of each of the fibre membranes 10, which process gas may be air, and which then passes through the sidewall of the fibre membranes to feed the biofilm growing on the exterior surface of each fibre membrane 10. A process gas outlet 60 is provided to exhaust gas from the interior of the vessel 52.

[0073] As detailed above, following a period of operation of the reactor 50, excess biofilm A may develop on the exterior surface of the fibre membranes 10, as for example illustrated in FIG. 6, which may be detrimental to the operation of the reactor 50. It is therefore necessary to carry out a controlled stripping of the excess biofilm, and the reactor 50 is therefore provided with a scour air inlet 62 which supplies scour air (or other gas) to the interior of the vessel 52, preferably at a location below or adjacent the lower end of the fibre membranes 10. In the embodiment illustrated the scour air inlet 62 terminates in a manifold 64 which is operable to generate a high volume of bubbles which pass rapidly upwardly through the vessel 52 around the membranes 10, stripping excess biofilm therefrom, a partially stripped membrane 10 being illustrated in FIG. 7. Continued passage of the scouring air bubbles will remove most of the biofilm, but as illustrated in FIG. 8, the biofilm retaining regions 18 will ensure that a relatively thin layer of biofilm will be shielded within the retaining regions 18 and will therefore not be stripped from the membrane 10. This remaining thin layer will then allow a sufficient quantity of biofilm to quickly recolonise the membranes 10 in order to allow the reactor 50 to be back to full operational capacity in a significantly reduced period of time.

[0074] In order to ensure that the scouring air does not fully strip the biofilm from the engineered retaining regions 18 the scout air inlet, and in particular the manifold, is adapted to generated bubbles whose diameter is sufficiently large, relative to the dimensions of the biofilm retaining region 18, 218 to ensure that the bubble cannot contact the nadir of the biofilm retaining regions 18, 218. This is illustrated in FIG. 9, in which a first bubble B1 is illustrated having a relatively large diameter and which cannot therefore contact the nadir of the biofilm retaining region 18, 218, ensuring that a sufficient quantity of biofilm will be protected in the region 18, 218 during the passage of the bubbles B1. Conversely FIG. 10 shows a second bubble B2 having a relatively small diameter and which can therefore contact the nadir of the retaining region 18, 218 and which would thus remove essentially all of the biofilm, significantly reducing the regrowth rate of same following the high shear cleaning of the reactor 50.

[0075] The size of the bubbles generated to scour the excess biofilm is important to the overall performance of the reactor 50. In general bubbles can be defined as being “fine”, having a diameter of less than 3 mm, or “course” having a diameter greater than 6 mm. “Micro” bubbles have a diameter less than 1 mm. Larger bubbles produce large shear stress, but there have been many studies suggesting that small bubbles can effectively control fouling in a membrane biofilm reactor, and use significantly less air flow rate and therefore energy to do so. While the effect of bubble size on fouling control in a submerged membrane reactor can vary, larger bubbles have a stronger wake and the turbulence is beneficial in promoting mixing and suppressing concentration polarization, the same being true when the dissolved pollutants in the wastewater are being consumed by the biofilm. The use of fine bubbles to mix can therefore save energy, however course bubbles are generally regarded to be better for scouring or fouling removal. In the use of a MABR both types of bubbles could be employed to achieve different results, for example to promote micro-mixing (mixing near the surface of the biofilm), scour of external layers of biofilm, and create movement of the hollow fibre membranes in the liquid, also promoting mixing.

[0076] In an exemplary embodiment of the reactor 50, the membranes should have an inner “lumen” diameter of between 100 μm and 800 μm and more preferably between 300 μm and 500 μm, with an outer diameter which encompasses the protrusions 20 of between 150 μm and 1500 μm. The protrusions 20 preferably have a height between 10 μm and 500 μm above the nadir of the biofilm retaining region 18, more preferably between 100 μm and 300 μm. The angle of separation between adjacent protrusions 20 should be no more than 120° preferably less than 90° and most preferably less than 60°. The distance between the tips of protrusions 20 should be less than 1500 μm, preferably less than 1000 μm and most preferably less than 600 μm, so that even when fine bubbles which are in the range of 1-3 mm in diameter are used for mixing or the transfer of gas to the bulk liquid, biofilm is not scoured completely from the biofilm retaining regions 18.

[0077] Bubbles used for scouring of the biofilm should be greater than 6 mm in diameter in the category of coarse bubbles, however due to the different process requirements, which bubbles can be used, i.e. biofilm control, mixing, and gas transfer, the engineered biofilm retaining regions 18 will prevent complete biofilm removal with bubbles greater than 1 mm in diameter

[0078] The present invention therefore provides a novel means by which a quantity of biofilm can be retained on a fibre membrane during a shearing or cleaning event, in order to ensure that a reactor in which the membranes are treating wastewater can be quickly operational following such an event. Using membranes with engineered protrusions extending from the surface, as well as providing engineered biofilm retaining regions therebetween, provides additional surface area for the anchoring of the biofilm. The additional surface area per unit volume of biofilm does not change the strength or the attachment properties of the biofilm, but does reduce the risk of complete removal of the biofilm from the surface of the membrane during operation, and biofilm control events. The present invention therefore addressed the problems of the prior art to provide a novel membrane, reactor incorporating the membrane, and method of treating fouling of a biofilm reactor.