WO2015089675A1

WO2015089675A1 - A novel membrane bioreactor suitable for retaining specialized microorganisms

Info

Publication number: WO2015089675A1
Application number: PCT/CA2014/051242
Authority: WO
Inventors: Andrew Benedek; Juan Carlos Josse; Michael David Theodoulou
Original assignee: Anaergia Inc.
Priority date: 2013-12-20
Filing date: 2014-12-19
Publication date: 2015-06-25

Abstract

In a bioreactor, microorganisms are kept in a circulation loop in communication with the inside of a membrane. The outside the membrane communicates with a fluid substrate. One or more nutrients pass from the substrate through the membrane to the microorganisms in the circulation loop. A product of the microorganisms may be collected from the substrate or from the circulation loop. Make up fluid can be added into the circulation loop or permeate through the membrane. Optionally, replacement microorganisms or one or more additional nutrients may be added to the circulation loop. Optionally, there may be an attached growth on the outside of the membrane, or a suspended growth of microorganisms in the substrate applied to the outside of the membranes.

Description

A NOVEL MEMBRANE BIOREACTOR SUITABLE FOR RETAINING SPECIALIZED

MICROORGANISMS

FIELD

[0001] This specification relates to bioreactors for harvesting a product made by microorganisms.

BACKGROUND

[0002] In some membrane bioreactors used to treat wastewater, an ultrafiltration membrane is used to retain a suspended growth of bacteria in a process tank while withdrawing filtered water from the tank through the membrane. In the typical case of an asymmetric membrane, the separation layer of the membrane is in contact with the suspended growth.

[0003] EP 0761608 describes a process in which a filamentous fungus was cultured as a biofilm on the outer surface of a membrane. Nutrients were supplied to the biofilm from the insides of the membranes by convective flow through the membrane walls.

INTRODUCTION

[0004] This specification describes a bioreactor in which one or more types of bacteria or other microorganisms are kept in a circulation loop in communication with the inside of a membrane. A fluid substrate is applied on the outside the membrane. One or more nutrients pass from the substrate, through the membrane, to the microorganisms in the circulation loop. As a product of the microorganisms is formed, it may pass through the membrane back out to the substrate and be recovered from there. Alternatively, the product may be recovered from the circulation loop either by withdrawing some of the circulating fluid, or by withdrawing one or more components separated from the circulating fluid. Make up fluid can be added into the circulation loop directly or by permeating through the membrane. Optionally, replacement microorganisms or one or more additional nutrients may be added through the circulation loop. Optionally, there may be an attached growth on the outside of the membrane, or a suspended growth of microorganisms in the substrate applied to the outside of the membranes, or both. BRIEF DESCRIPTION OF THE FIGURES

[0005] Figure 1 is a schematic drawing of a bioreactor.

[0006] Figure 2 is a schematic drawing of another bioreactor.

[0007] Figure 3 is a schematic drawing of a bioreactor as in Figure 2 but with membrane solids separation.

DETAILED DESCRIPTION

[0008] Figure 1 shows a bioreactor 10 having a membrane tank 12 and a circulation loop 14. The membrane tank 12 shown is an open tank filled with a substrate 16. The substrate 12 is a typically an aqueous solution or mixture kept under aerobic, anoxic or anaerobic conditions. Optionally, the tank 12 may have a cover 18 to encourage anaerobic conditions, or to maintain a desirable partial pressure of one or more gasses over the substrate 16. A cover 18 may have a vent 20 to release excess pressure or collect a product gas. Alternatively, the tank 12 may be replaced with a closed vessel. In some cases the substrate 12 may be predominately a liquid other than water, or a gas. Make up substrate 12, or one or more nutrients, or microorganisms, may be added to the tank 12 through a tank inlet 22. Excess substrate may be removed from through a tank outlet 24

[0009] The tank 12 contains one or more membrane modules 26 immersed in the substrate 16. In the bioreactor 10 shown, each membrane module contains a plurality of ultrafiltration membranes. The membranes are made by first forming sheets of non-woven fabric into a pair of ribbed sheets. The ribbed sheets are glued together such that the ribs form tubular channels. A membrane forming dope is then cast on the outside of this assembly and quenched to from a porous membrane by non-solvent induced phase separation. The resulting membrane is asymmetric with a separation layer on its outer surface. Membranes of this type are available commercially from Fibracast Ltd. These modules are typically installed with the ribbed sheets oriented in vertical planes with a vertically extending header on each side of the sheets. Pumped cross flow through the tank 12, or an impeller or gas sparger underneath the membrane modules 26, can be used to increase the flow of substrate 12 past the membranes.

[0010] Optionally, flat sheet, hollow fiber, tubular or another type of membrane may be used. The membranes preferably have pores in the ultrafiltration or microfiltration range. In another option, a symmetrical membrane may be used. However, symmetrical membranes tend to have lower permeability than asymmetrical membranes with the same selectivity. The membranes should also be configured such that a cross flow can be provided through the inside of the membranes. For example, the ribbed sheet membranes described above have two headers connected to opposite ends of the tubular channels formed within the sheets. Similarly, a hollow fiber membrane module preferably has a header on each end of a bundle of hollow fibers. A fluid can be circulated through the inside of the membrane module by passing the fluid into one header and collecting it from the other header. The pore size of the membranes is chosen to at least selectively contain a desired strain of microorganisms in the circulation loop 14. Preferably, this desired strain of microorganisms is completely prevented from passing through the membrane.

[0011] The circulation loop 14 has a collection pipe 30 and a distribution pipe 32 connected to the one or more membrane modules 26. A pump 34 connected to the collection pipe 30 or the distribution pipe 32 or both causes a fluid, typically an aqueous solution or mixture, to recirculate through the membrane modules 26. The internal volume of the membrane modules 26, collection pipe 30 and distribution pipe 32 may be sufficient to support a desired population of microorganisms in the circulation loop 14. Optionally, a circulation vessel 36 may be added to increase the total volume of the circulation loop 14. The circulation vessel 36 may be an open tank as shown, or a closed vessel.

[0012] In the configuration shown in Figure 1 , the suction side of the pump 34 is connected to the collection pipe 30. The circulation vessel 36 is an open tank or otherwise maintained at ambient pressure. With the water surface in the circulation vessel 36 located at a sufficient height above the water surface in the tank 12, the insides of the membrane modules 26 have a higher pressure than the substrate 16. Alternatively, the circulation vessel 36 may be lowered, or a control valve 38 may be throttled, or the diameter of distribution pipe 32 may be reduced, or the outlet side of the pump 34 may be connected directly to the distribution pipe 32. By any of these or other methods, the pressure inside the membrane modules 26 may be made to be lower than the pressure of the substrate 16. Alternatively, the pressure inside the membrane modules 26 may be made to be similar to, for example within 5 kPa of, the pressure of the substrate 16. Accordingly, the circulation loop 14 may be arranged and operated to encourage a bulk flow of fluid in either direction between the circulation loop 14 and the tank 12, or to minimize such bulk flow. Preferably, when using an asymmetric membrane with its separation layer on the outside of the membranes, any bulk flow of fluid is from the tank 12 to the circulation loop 14. [0013] Regardless of the flow of bulk fluid, if any, one or more soluble substances may diffuse through the membranes according to a concentration gradient across the membranes. In particular, by bulk flow or by diffusion, one or more nutrients in the substrate 16 are transferred to microorganisms growing in the circulation loop. The nutrients may be provided in the substrate 16 directly through the tank inlet 22 or by another means, for example a gas sparger in the substrate 16. Alternatively, the nutrients may form in the substrate by chemical or biological reaction. In one option, the substrate 16 contains a suspended growth of microorganisms, or microorganisms form a biofilm on the outsides of the membrane modules 26, such that a nutrient produced by these tank side microorganisms can travel directly into the substrate 16.

[0014] The membrane modules 26 encourage the preferential transfer of nutrients from a substrate 16 having multiple components. The term "nutrient" is used to refer to any substance consumed by microorganisms in the circulation loop 14. Since microorganisms in the circulation loop 14 continuously consume the nutrient, the concentration of the nutrient is lower on the circulation loop 14 side of the membrane modules 26 and a concentration gradient causes diffusion of the nutrient to the circulation loop 14. Conversely, substances produced by the microorganisms in the circulation loop 14 have a high concentration in the circulation loop 14 and tend to diffuse across the membranes into the substrate 16. In some cases, these substances are waste products that would inhibit the growth of the

microorganisms in the circulation loop, and it is therefore beneficial to have these substances flow into the substrate 16. In some cases, a substance produced in the circulation loop 14 that diffuses into the substrate 16 may be a nutrient for microorganisms in the substrate 16, or may be a commercially valuable product that can be extracted from the substrate 16. Because of the beneficial effects of diffusion across the membranes in both directions, bulk flow of fluid across the membranes is typically minimized, for example by keeping the transmembrane pressure differential at 5 kPa or less.

[0015] Since diffusion, rather than permeation, is of primary interest, the pump 34 may be operated intermittently or otherwise so as to provide a low flow rate in the circulation loop 14. Many microorganisms are able to survive being moved through the circulation loop 14 and can be allowed to pass through the insides of the membrane modules 26 and the pump 34. Optionally, more delicate microorganisms may be confined to the circulation vessel 36. For example, an outlet screen or membrane 40 may be placed in line between the circulation vessel 36 and the distribution pipe 32. Alternatively, microorganisms may be retained as an attached growth in the circulation vessel 36. Optionally, a combination of methods may be used. For example, the microorganisms in the circulation vessel 36 may form an attached growth on a media such as resin beads or plastic balls which are kept in the circulation vessel 36 by an outlet screen 40.

[0016] In a case in which there are microorganisms both in the substrate 16 and the circulation loop 14, the membrane modules 26 serve to isolate these populations from each other. The microorganisms on opposite sides of the membrane modules 26 are preferably of different strains, or have different concentrations of one or more strains. Physically separating different strains of microorganisms can beneficially reduce competition between them and improve the growth rate or vitality of one or both strains. Although different strains of microorganisms might compete against each other if they were growing in the same tank, this competition is reduced in the bioreactor 10 by the lack of physical mixing of the different strains of microorganisms. Further, the inhibited flow of substances across the membranes may reduce competition for nutrients or competition by way of inhibitory compounds. Yet in some cases there may be a symbiotic relationship between different strains of

microorganism if their nutrients and products of metabolism are selectively transferred through the membranes that physically separate them.

[0017] Even with a transfer of at least one nutrient through the membranes, the microorganisms in the circulation loop 14 may require one or more additional nutrients.

These may be added through a feed line 42 in communication with the circulation vessel 36 or any other part of the circulation loop 14. In other cases, additional microorganisms may be added to the circulation loop 14 through the feed line 42. An optional bleed line 44 may be used to control the accumulation of dead microorganisms or inhibitory compounds in the circulation loop 14 if necessary.

[0018] The bioreactor 10 may be used to extract a useful product produced by microorganisms in the circulation loop 14. In particular, the bioreactor 10 may be used to extract a product that is selectively or absolutely rejected by the membrane, or a product that stays within, or is attached to, the microorganisms.

[0019] In the bioreactor 10, microorganisms or their products are removed through a harvesting loop 50. The harvesting loop 50 may be separated from the circulation loop 14 through one way valves 52. In the example shown, a second pump 54 removes a portion of the fluid flowing in the circulation pump 14 and passes it through a microsieve 56 or another solid-liquid separation device such as a membrane, flotation device or centrifuge. Filtrate 60 is returned to the circulation loop 14. A concentrate 58, rich in microorganisms, is removed. The concentrate 58 can then be processed to extract a product from the microorganisms. Alternatively or additionally, a liquid-liquid separation device may be provided in the harvesting loop 50, such as a decanting vessel, to separate a liquid product (such as an oil or alcohol) from the circulating fluid and microorganism mixture.

[0020] Optionally, fresh microorganisms may be added through the feed line 42 to make up for some or all of any microorganisms removed in the concentrate 58. Alternatively, population growth in the circulation loop 14 may make up for the microorganisms removed. In a situation in which the product is not attached to the microorganisms, a filtrate 60 or separated liquid fraction with the product can be removed from the circulation loop 50 with the concentrate 58 either returned to the circulation loop 14, or removed in whole or in part to control the average age of the population, with or without replenishment with new

microorganisms.

[0021] The removal, and optional replacement, of microorganisms in the circulation loop 14 may be beneficial to some strains of microorganisms that benefit from periodic refreshment or control of the average age of the population. Microorganisms can be removed, and optionally replaced, in a continuous or batch wise process. Removed microorganisms can be processed to extract products from them. For example, in a case where the microorganisms are being cultivated to produce lipids or other oils, removed microorganisms may be lysed or crushed to extract the oils. In other cases, the

microorganisms may release the product, for example a hormone or other pharmaceutical compound, when exposed to different environmental conditions in or the harvesting loop 50. In these cases, the microorganisms may be returned to the circulation loop 14 after they release the product. The harvesting loop 50 may be sterile or not as required for processing and, optionally, replenishment. The original charging of the recirculation loop 14 may also occur through the harvesting loop 50, or through the feed line 42. The harvesting loop 50 may be advantageous for removing products that diffuse slowly, or even products that diffuse easily and could possibly be extracted from the substrate 16, where product removal can be combined with maintenance of the microorganism population.

[0022] Figures 2 and 3 show alternative bioreactors 10.2 and 10.3 designed for use in two applications that will be described further below. Alternatively, the bioreactor 10 of Figure 1 may also be used in these applications or the bioreactors 10.2 or 10.3 may be used for other applications. Any additional or alternative features of the bioreactors 10.2, 10.3 in Figures 2 or 3 may be used with the bioreactor 10 of Figure 1.

[0023] Referring to Figures 2 and 3, influent A flows into reactor tank 1 which contains mixed liquor or another type of suspended growth. Reactor tank 1 also contains submerged membranes 2, which are preferably hollow fiber or formed sheet asymmetric microfiltration or ultrafiltration membranes having an outside separation layer and a lumen side. Entrained microorganisms, typically bacteria, are present on the lumen side of the submerged membranes. The submerged membranes 2 are preferably coupled with gas diffusers 3. Bubbles of gas D produced from gas diffusers 3 provide membrane scouring and mixed liquor (or other suspended growth) circulation. The bubbles may be made of air in the case of a process with mostly aerobic microorganisms suspended in the reactor tank 1. Alternatively, the bubbles may be made of biogas, oxygen reduced air or another anoxic gas, or air in controlled quantities, in the case of a process with mostly anoxic or anaerobic microorganisms suspended in the reactor tank 1.

[0024] In Figure 2, treated effluent B flows directly out of reactor tank 1 , for example by way of an overflow, weir or pipe. In Figure 3, treated effluent B flows out of reactor tank 1 as permeate extracted through a submerged solids separation membrane 4 by way of permeate pump 7. Optionally, the solids separation membrane 4 may be the same type of equipment as submerged membranes 2. Optionally, a wall 10 with one or more ports or an overflow may be added to separate the submerged membranes 2 from any solids separation membrane 4. Further optionally, the effluent B may be treated in another type or solid-liquid separation device, with or without a return of the solids portion, or by any other appropriate post treatment device or devices. The use of a submerged solids separation membrane 4 is illustrated in particular since in some cases it may be desirable to maintain a high solids concentration or retention time in the reactor tank 1 as a separate, or additional,

consideration from post-treating the treated effluent B.

[0025] A storage tank 5 contains a population of the microorganisms that are entrained in water on the lumen side of the submerged membranes 2. A recirculation pump 6 pumps a mixture of the entrained microorganisms through a feed line H leading to the lumen side of the submerged membranes. After flowing through the submerged membranes 2, the entrained microorganisms return to the storage tank 5 by way of a return line G.

Optionally, one or more nutrients E may be added to the storage tank 5 to enhance the growth of the entrained microorganisms. Further optionally, one or more qualities of the recirculating water, for example its pH or concentration of oxygen or another component, may be altered in the storage tank 5, feed line H or return line G.

[0026] Inside (or lumen side) waste sludge F is withdrawn from the storage tank 5 by a first waste sludge pump 9 or by gravity. Outside waste sludge C is withdrawn from the reactor tank 1 by way of a second waste sludge pump 8 or by gravity.

[0027] In one example, the bioreactors 10.2, 10.3 are used to provide nitrogen removal with anammox (anaerobic ammonium oxidation) bacteria. Reactor tank 1 is an open tank at ambient pressure. Influent A is wastewater containing ammonia. Gas D is process air which moves mixed liquor in the reactor tank 1 through the membrane sheets or fibers. Gas D also provides the oxygen required to biologically oxidize a portion, for example about half, of the ammonia in the influent A to nitrite using nitrosomonas bacteria. The conversion of all the ammonia to nitrite, and a further conversion of nitrite to nitrate, is avoided by maintaining a low dissolved oxygen concentration in the water (mixed liquor) in the reactor tank 1.

[0028] The storage tank 5 contains a culture of anammox bacteria. These bacteria are slow growing and so their loss with effluent or sludge is preferably minimized. In storage tank 5, the anammox bacteria exist in a suspended growth. The anammox bacteria may be kept in suspension with mechanical mixing. Nutrients E can be supplied to the storage tank 5 if and as required. A suspension of anammox bacteria entrained in water is circulated through the storage tank 5, feed line H, return line G and the lumen side of the submerged membranes 2 by recirculation pump 6.

[0029] The anammox bacteria do not exit through the lumen side of the submerged membranes 2, at least not in significant numbers, because they are generally larger than the membrane pores. The combination of soluble nitrite and ammonia in the mixed liquor of the reactor tank 1 comes in contact with the circulating (or entrained) anammox bacteria in the membrane lumens through the membrane pores. The anammox bacteria convert nitrite and ammonia into nitrogen gas. Some nitrogen gas exits through the membrane pores into the reactor tank 1. Remaining nitrogen gas is released to the atmosphere from the storage tank 5.

[0030] The outside waste sludge C contains excess suspended growth nitrosomonas bacteria. Excess Annamox bacteria, if any, are removed with inside waste sludge F. The flow rate of inside waste sludge F, if any, is much less than the flow rate of outside waste sludge C. [0031] The treated effluent B in the case of bioreactor 10.2 flows to a clarifier (not shown). Settled solids are returned to the reactor tank 1. The clarifier, or another solid liquid separation process, may be designed as for ordinary suspended growth processes. The clarifier produces the final effluent. In the case of bioreactor 10.3, submerged solids separation membranes 4 are used to produce permeate as treated effluent B. This permeate may be polished or may be the final effluent. The submerged solids separation membranes 4 leave almost all suspended solids in the reactor tank 1. Alternatively, ultrafiltration or microfiltration membranes, whether suction or positive pressure driven, can be located in a separate dedicated membrane tank or cartridge array. Mixed liquor flows to the separate tank or array and a concentrate flows back to the reactor tank 1 while permeate withdrawn to provide treated effluent B.

[0032] This process at least provides a useful alternative to a typical anammox process. Further, the process may provide improved control over the withdrawal of anammox containing sludge. In particular, the retention time and average age of the anammox bacteria can be increased relative to a conventional process. The surface area of the submerged membranes 2 can be selected to provide a large area for exchange of metabolites (nitrate and ammonia) in the mixed liquor with anammox bacteria. The reaction time between these metabolites and the anammox bacteria, and the oxygen concentration in water containing the anammox bacteria, can be controlled by selecting the recirculation rate and submerged membrane 2 surface area.

[0033] In another example, the bioreactors 10.2, 10.3 are used to provide two phase anaerobic digestion. In this case hydrolysis and acidification occur in the reactor tank 1 by way of a suspended anaerobic growth. Acetate forming and acetate degrading bacteria circulate on the lumen side of the submerged membranes 2 and provide methanogenesis in the storage tank 5. Volatile fatty acids (VFAs) are provided from the suspended anaerobic growth to the acetate forming and acetate consuming bacteria through the pores of the submerged membranes 2. Bacteria circulating in the lumen side of the submerged membranes 2 convert the VFAs to acetate and then to methane and carbon dioxide. In this example the two tanks 1 , 5 are both covered and the membrane scouring and mixed liquor movement is done with recirculated gas D from the reactor tank 1 headspace or by way of a mechanical mixer. Biogas is recovered primarily from the headspace of the storage tank 5. Optionally, biogas may also be recovered from the headspace of the reactor tank 1. In this example, the storage tank 5 is typically larger than as shown in the Figures. In particular, the storage tank 5 is likely to be larger than the reactor tank 1.

[0034] In these examples, active microorganisms are contained both inside the submerged membranes 2 and in the reactor tank 1 outside of the submerged membranes 2. The storage tank 5 provides a reservoir of lumen side microorganism and, optionally, a location to add nutrients or to make other adjustments, for example in pH or oxygen concentration. The system is like an attached growth system in the sense that the lumen side microorganisms communicate with a suspended growth in the reactor tank 1 without being mixed with the suspended growth in the reactor tank 1. However, the lumen side bacteria are themselves suspended rather than attached. Biological activity occurs on both sides of the submerged membranes 2. On the lumen side, circulating bacteria process metabolites produced outside the membrane in the suspended growth of the reactor 1. Metabolites and lumen side products that soluble or a gas can pass through the membrane.

[0035] The discussion above is meant to help provide an enabling disclosure of one or more inventions and not to limit any invention defined in the following claims.

Claims

CLAIMS:

We claim: 1. A bioreactor comprising,

a) a membrane having a first side in communication with a substrate and a second side; b) one or more types of microorganisms growing in a circulation loop in communication with the second side of the membrane; and,

c) a harvesting loop in communication with the circulation loop.

2. The bioreactor of claim 1 wherein the harvesting loop comprises a solid-liquid or liquid-liquid separation device.

3. A process comprising steps of,

a) providing a membrane having a first side and a second side;

b) circulating a fluid comprising or in communication with a culture of one or more types of microorganisms past the second side of the membrane;

c) supplying a nutrient to the circulating fluid from a substrate applied to the first side of the membrane; and,

d) extracting a product of the microorganisms, or replacing a portion of the microorganisms, or both, from the circulating fluid.

4. The process of claim 3 wherein step d) comprises a solid-liquid or liquid-liquid separation.

5. The process of claim 3 or 4 comprising a step of adding make-up fluid to the circulating fluid directly or by permeation through the membrane.

6. The process of any of claims 3 to 5 further comprising a step of adding replacement microorganisms to the circulating fluid.

7. The process of any of claims 3 to 6 further comprising a step of maintaining an attached growth on the first side of the membrane or a suspended growth in the substrate.

8. The process of any of claims 3 to 7 used to provide two stage anaerobic digestion.

9. The process of any of claims 3 to 8 wherein the microorganisms comprise anammox bacteria.