GB2155949A - Fluid bed biological reactor - Google Patents

Abstract

In an intensified process for the fermentation of microbes to produce biomass in a liquid medium in a reactor containing particles, the liquid medium has a specific gravity that is lower than the specific gravity of the biomass and which is higher than the specific gravity of the particles. In the process the particles are maintained as a fluidised bed by downward fluidisation by upflow of gas and/or mechanical agitation. The biomass grows on the surface of the particles during germentation and is knocked off the particles in the bed. The biomass and liquid medium are separated, generally within the reactor, and removed from the reactor. The process is usually a continuous process and is useful for reducing the biochemical oxygen demand of waste water. Novel apparatus for carrying out the continuous process comprises a reactor having inlet means for liquid medium and outlet means for liquid medium and biomass in which the inlet means is higher than the outlet means and the reactor contains gas diffusers above its base.

Description

SPECIFICATION Fluid bed biological reactor There are many processes in which biomass is produced in a reactor by fermentation in a liquid medium under aerobic or anaerobic conditions.

The biomass is the solid product obtained as a re sult of the fermentation.

To optimise the efficiency of these processes it is necessary to maximise the so-called "biomass hold-up". This is a measure of the mass of micro biological growth that can be supported per unit volume of reactor vessel. In conventional biological reactors, e.g. in activated sludge processes, the bi omass hold-up is often in the region 2-6 grams (dry mass) per litre of reactor volume. It is known that the biomass hold-up of a biological reactor is increased by the presence of support particles on or in which the microbes grow, even though the volume of liquid present in the reactor may be re duced. Proceses including particles in the liquid are often known as "intensified processes" and in these the biomass hold-up may be in the region 5 to 15 grams (dry mass) per litre of reactor volume.

There have been proposed various designs of apparatus for carrying out these intensified proc esses. Many of these processes utilise a bed of sand, or other high specific gravity material, flui dised by up flow of liquid and air or oxygen (for instance US Patent Specification 4009098 and Eu ropean Patent Specification 0072093). The use of sand requires powerful up-flow of water through complex diffusers to provide uniform fluidisation of the bed. The high flow rate of the liquid results in a low residence time in the reactor and so the liquid should be recycled several times and the ap paratus for this can involve complex and expensive engineering. High energy costs are required to generate the fluidisation.

The nature of the fluidisation achieved with high specific gravity particles can result in excessive at trition of biomass from the particles. Conversely because the particles are generally very small the coated particles can, unless precautions are taken, agglomerate into gelatinous masses.

Separation of the biomass from the small, heavy particles can be difficult and inconvenient to con duct. In US Patent Specification 4009098 the bio mass is removed from the top of the reactor and it is warned that agglomerates of the particles and biomass are liable to be carried from the reactor as the specific gravity of the particles decreases. In practice it is normal to separate the biomass from the particles in a vessel separate from the reactor, often batchwise, and this is inconvenient.

Removal of liquid medium from the reactor is normally from above the fluidised bed, the liquid medium normally flowing upwardly through the bed, to promote upward fluidisation, and out from the top of the reactor. The particles are normally prevented from escaping with the liquid medium as a result of having a specific gravity sufficiently high that they are not carried out of the outlet with the liquid medium. Thus in US Patent Specification 4009098 it is warned that the particles must have a specific gravity no less than about 1.1 in order to ensure that the particles are not carried out of the reactor.Although particles having specific gravities lower than sand, for instance plastic beads, activated carbon and volcanic cinders, are mentioned in US Patent Specification 4009098 the particles that are normally used when biomass is to be grown on their external surfaces are sand or other high density particles.

In all these processes the biomass grows on the outside of non macro-porous particles. In another method, for instance as described British Patent Specification 2 111 039, porous particles are utilised with the intention that the biomass will grow internally, with substantially no growth externally, with the result that the dimensions of the particles control the volume of biomass on each particle.

The particles can be of synthetic foamed material such as polyurethane or of, for instance, stainless steel wire spheres, polypropylene toroids, reticulated polyester foams or matted reticulated polypropylene sheets. As described in British Patent Specification 2111039 the liquid medium is introduced at the base of the reactor and passes upflow through the reactor and is taken off from the top through a sieve.

The particles have to be removed from the upper part of the reactor in order to permit extraction of the biomass from inside the particles. This removal is inconvenient, as is the fact that it has to be conducted frequently, for instance each particle being removed approximately once every two days. Another problem with the aerobic process is that there is a risk of the biomass that is growing at the centre of blocks becoming anaerobic and sour, and it may become mineralised and therefore impossible to remove. Removal of the particles requires complicated machinery involving belts, draining vessels, squeezers and returning means. The process does not achieve very high biomass hold-up values, usually 8 g/l or less. The reticular blocks are expensive and short-lived.Another problem is that the process is difficult to control and therefore to make efficient since there is a large variance in the mass of the particles, some of which have a large amount of entrapped air or gas, especially just after cleaning, and some of which are full of biomass.

It would be desirable to be able to operate an intensified process that avoids the disadvantages of the existing processes using non-porous particles on which biomass grows or porous particles in which it grows.

In the invention biomass is produced in a reactor by fermentation in a liquid medium containing non-porous particles on which biomass may grow and become coated, and in this process the liqui i medium has a specific gravity less than the specie gravity of the biomass but more than the specific gravity of the particles coated with biomass, the particles are formed into and maintained as an expanded or fluidised bed in the medium by downward fluidisation, biomass that grows on the particles is rubbed from the particles whilst they are contained in the bed in the liquid medium and, after fermentation, liquid medium and biomass are separated from the particles by flotation of the particles in the liquid medium and are removed from the reactor For convenience we refer below only to "fluidised" beds, but it should be realised that the beds need not be truly fluidised but may be merely pseudo-fluidised or expanded beds.

In conventional intensified processes using nonporous particles, the particles are formed into a fluidised bed by upward fluidisation, that is to say the particles adopt, on average, a position higher in the liquid medium during fluidisation than they adopt under quiescent conditions. It is important in the invention that the fluidised bed is formed by downward fluidisation, that is to say the particles in the fluidised bed adopt, on average, a position in the liquid medium lower than they would adopt under quiescent conditions. Thus if the particles are initially introduced into the liquid medium under quiescent conditions they will float but as a result of downward fluidisation they sink.

A necessary requirement of the process of the invention is that there should not be strong upflow of liquid medium through the reactor, since such an upflow would prevent downward fluidisation. It would also have the undesirable effect of holding biomass in the fluidised bed and tending to form agglomerates on the surface. By eliminating this strong upflow it is possible to have much gentler conditions in the bed than has been possible in previous systems, especially systems having nonporous particles, and so separation of the particles from the liquid medium, and the separation of the liquid medium from the biomass, can, if desired, be effected quite efficiently in one or more quiescent zones in the reactor or adjacent to the reactor merely by flotation and sedimentation.It is no longer essential or desirable to take the liquid medium from the top of the reactor, as in all prior processes, and so the risk of agglomerated biomass forming a layer on the top of the reactor is reduced and it is no longer necessary to remove the particles physically from the reactor in order to extract the biomass from them.

An important advantage of the invention is that the fluid flow necessary to generate downward fluidisation is very much less than the fluid flow necessary to generate upward fluidisation of, for instance, sand. Accordingly the total volume of fluid, and in particular the total volume of liquid medium, introduced to the reactor is independent of the volume needed for fluidisation. Thus whereas in prior processes using sand and other non-porous particles the volume of liquid being used was controlled by the amount required for fluidisation (with the result that very high flow rates and short residence times were required, this leading to the need for recycling) in the invention the liquid medium generally need not be recycled and can be maintained in the reactor for a time that is optimum for the fermentation process.

The process can be conducted as a batch process but generally is conducted continuously. In continuous processes there can be some upflow of liquid medium through the reactor provided this is not sufficiently strong to inhibit significantly the desired downward fluidisation but generally the flow of liquid medium through the reactor is substantially horizontal or downwards. Preferably the flow of liquid medium is downwards, with the liquid medium being introduced at the top of the reactor and removed from the reactor at a lower position. Such downflow may generate the desired downward fluidisation but any convenient means of achieving downward fluidisation may be used.

For instance mechanical agitation, for instance by stirrers, may be used alone, or in conjunction with gas-mixing and which may induce, directly or indirectly a downward movement of liquid through the bed. Generally some or all of the downward fluidisation is achieved by upflow of gas, the rate of flow and the manner of introduction (and in partic uiar the bubble size) being such that the upflowing gas reduces the effective density of the liquid medium and thus induces downward fluidisation, sometimes called pseudo fluidisation.

The fluidising gas may be introduced into the liquid medium through a plurality of diffusers that result in the formation of fine bubbles that will have the desired downward fluidisation effect. The diffusers or other means for introducing the gas are preferably positioned a substantial distance above the base of the reactor, and so the quiescent zone can form beneath the gas inlet means. Typically the gas inlet means are positioned above the base of the reactor by a distance of from one quarter to one half of the depth of the reactor.

The particles should not be too small and preferably they are larger than the conventional sand particles that have been used previously. Thus they will normally have an effective diameter of at least 1 mm and preferably at least 2, and usually more than 3, mm. The particles generally have a diameter of below 50 mm, and often below 10 mm. They generally have a specific gravity of not more than 95%, and preferably not more than 85%, of the specific gravity of the liquid medium. Generally the uncoated particles have a specific gravity of from 5% to 90% of the specific gravity of the medium.

For instance if the liquid medium is water the specific gravity of the particles typically is from 0.05 to 0.3 for closed cell foam or 0.7 to 0.9 for nonfoamed plastics. The particles must be sufficiently non-porous (i.e. not macro-porous) that there is substantially no growth of biomass within them.

They may be solid beads, hollow beads or closed cell expanded foam beads, they may be regularly or irregularly shaped. Preferably their shape would provide hollow/concave spaces which offer protection to the attached growth from attrition. Preferably, they are solid low density plastics materials, for instance low density polyethylene or closed cell expanded polystyrene or other plastics foam.

The biomass generally will have a specific gravity of from 102 to 130% of the specific gravity of the liquid medium, for instance often around 1.1 to 1.2 when the liquid medium is water. The specific gravity of the particles, and the rate of removal of biomass from the particles, must be such that most at least of the coated particles have a specific gravity less than that of the liquid medium. Any particles that are coated with so much biomass that they fall out of the fluidised bed will rise back into it when the coating falls off the particles.

Biomass will grow on the surfaces of the nonporous particles within the fluidised bed and as the thickness of the coating on a particle increases there will be an increasing tendency for the coating, or part of it, to be rubbed off the particle. This rubbing action will be generated partly as a result of movement of the particles through the liquid medium but especially as a result of collisions of the particles. It is often promoted by a change in the nature of the biomass with increasing depth of coating. For instance as the depth of the biomass coating on a particle increases the nature of the underlying biomass may change as a result of which the adhesion of the biomass to the particle may reduce, increasing the tendency for biomass to be rubbed off the particles.

After the liquid medium has been subjected to fermentation for the desired time it is separated from the particles by flotation of the particles, this separation being batchwise or, more usually, continuous. The separation can be achieved by, for instance, removing the liquid medium from a position distant from the fluidised bed. For instance there may be a horizontal quiescent zone between the fluidised bed and the outlet or the outlet may be below the base of the fluidised bed.

Thus wheresas in conventional reactors for intensified processes the outlet is at a position at the top of the reactor in the invention it is preferred for it to be a substantial distance from the top, generally being below the top by a distance of from 1/3 to 3/ 4 of the depth of the reactor.

A baffle system may be provided adjacent the outlet to minimise the escape of particles. For instance the baffles may define a quiescent zone around the outlet, with the outlet leading from the base of the zone, such that particles rise away from the outlet. In another construction the baffles may deflect particles from the outlet or they may provide a physical barrier, for instance a mesh, preventing the particles approaching the outlet.

The outlet should be distant from the inlet and typically the reactor is elongated, with the inlet being at one end and the outlet at the opposite end.

The biomass is also separated from the particles by flotation, as a result of the particles tending to float upwards and, under the conditions prevailing in the downwardly fluidised bed, the biomass that has been rubbed off the particles and the liquid medium both tending to move downwards relative to the particles. Preferably the effective density of the liquid medium in the fluidised bed due to upflowing gas varies across the horizontal area of the bed between values in which the biomass will tend to float and values in which biomass will tend to sink. For instance there may be areas within the bed or around the sides of the bed where the effective density of the liquid medium is higher, due to less fluidising gas, and the biomass will then tend to settle through these parts of the bed.

The separated biomass may be removed from the reactor entrained in the liquid medium ahd then separated from the liquid medium by conventional means such as sedimentation (optionally after flocculation) or by filtration but preferably some at least of the separation of biomass from liquid medium occurs within a quiescent zone in the reactor. If any of the particles enter the quiescent zone then separation of them from the liquid medium and the biomass should also occur in this quescent zone. Often the quiescent zone is positioned such that the liquid medium and any particles that separate from the biomass in the quiescent zone are returned from that zone to the fluidised bed.

The quiescent zone can be provided by the entire reactor, in which event all the particles and all the liquid medium will be in the quiescent zone. Thus the zone may be provided by terminating fluidisation and allowing quiescent conditions to prevail, and the particles and liquid medium may subsequently be returned from the quiescent zone to the fluidised bed by resuming fluidisation of the particles in the reactor.

Generally however the quiescent zone contains some only of the particles and some only of the liquid in the reactor, at any particular time. Thus the separation is normally conducted intermittently or continuously. It may be conducted in a separate vessel, with removed biomass, some of the liquid medium and optionally some of the particles being taken from the reactor to a separate sedimentation vessel and liquid medium and any particles subsequently being returned to the reactor, but preferably the quiescent zone is in the reactor. Preferably the zone in which the fluidised bed is maintained interconnects with the quiescent zone, generally through baffles for promoting quiescent conditions in the quiescent zone despite the turbulent conditions in the fluidised zone.Preferably the interconnection is such that liquid medium and particles may rise from the quiescent zone into the zone containing the fluidised bed, thereby being returned to the fluidised bed by flotation.

There may be baffles beneath the gas inlet means that provide the desired interconnection between the quiescent zone and the fluidised bed but which will promote quiescent conditions in the quiescent zone. Typically there are a plurality of inclined baffle plates positioned such that removed biomass, particles and liquid medium can pass down between the baffles and liquid medium and particles can rise up between the baffles. The baffles may be plates which may be corrugated.

Instead of or in addition to providing a quiescent zone beneath the fluidised bed and interconnecting with the bed there may be a quiescent zone between the fluidised bed and the outlet. There may be a substantially vertical baffle separating the fluidised bed and the quiescent zone adjacent the outlet, so as to define a sedimentation tank in the reactor.

In the aerobic process the fluidising gas is generally air and the liquid medium is generally water, the fermentation being an aerobic fermentation. It is often desirable to provide in an aerobic sewage treatment process an anoxic zone, that is to say a zone in which fermentation may continue free of externally supplied oxygen. For instance if a liquid medium contains nitrates it is desirable to provide an anoxic zone since denitrification proceeds in this zone causing removal of nitrates (which should be minimised in the outlet liquid). Also the provision of an anoxic zone can result in an equivalent amount of aerobic fermentation but reduced input of air, and thus reduced energy consumption.

The anoxic zone may be created by providing insufficient air to meet biological oxygen demand at, for instance, the inlet. Since conditions within the anoxic zone will be relatively quiescent compared to the fluidised bed, settlement of biomass may occur within the anoxic zone.

Preferably the apparatus includes, in sequence, an inlet zone into which liquid medium is downwardly introduced, a fluidising zone in which the particles are formed into a fluid bed partially by upward aeration, preferably a quiescent zone below the fluid bed, and the outlet. The invention includes novel apparatus, including for instance apparatus having zones arranged sequentially in this manner. Preferably the fluid flow through the reactor has plug flow characteristics.

The biomass that sediments as a sludge to the bottom of the apparatus may be left for weeks or months whilst the sludge undergoes further degradation. The sludge is preferably held in contact with gasified liquor. The structure of the sludge is slightly permeable to the diffusion of gas so that the microbes are in continuous contact with gas that promotes the digestion processes. For instance sewage sludge is held in contact with aerated water which helps prevent it becoming anaerobic and sour. After a suitable period the sludge may be removed via a sludge pipe. Sledge produced in apparatus according to the invention has a high solids content generally up to 2% by weight. Alternatively the effluent may be removed from the vessel whilst it still contains suspended biomass and placed in external settlement tanks.

Sometimes it may be advantageous to provide gentle stirring and/or aeration of the sludge.

The process according to the invention is suitable for many biochemical processes. They may be non-aqueous, with the liquid medium being an organic liquid, but usually are aqueous. They may be anaerobic, for instance using recycled methane that is formed in the fermentation as fluidising gas with or without mechanical mixing, or aerobic.

Typical processes include yeast fermentations, fermentations of pharmaceutical-producing bacteria, animal or plant cell cultures or other immobilised catalyst reactors. The process is especially useful for the aerobic treatment of waste water, for example sewage effluent. The biomass hold-up achieved in processes according to the invention may be in the range 6 to 16 g biomass (dry weight) per litre, and reduction of biochemical oxygen demand may be up to about 95%. The residence time of waste water in the apparatus is generally in the range 2 to 24 hours.

One novel form of apparatus suitable for use in the invention, and a process according to the invention, are now described with reference to the accompanying drawing which is a diagrammatic view of the reactor in use.

The reactor comprises a vessel 1 having a cover 13 and containing low density support particles 2.

The cover 13 has a sealed inlet 3 through which liquid medium 11 is introduced and a valved outlet 14 for gas. There is an output pipe 6 for removing treated effluent. At a level above the base of the vessel 1 there is a plurality of diffusers 5 attached to an air supply 4 for feeding air bubbles into the growth medium and reducing the effective density of the medium above the level of the diffusers. Below the level of the diffusers is a plurality of baffles 8 which create a zone of low turbulence at the bottom of the vessel and which are arranged to allow the upward passage of support particles which have entered the zone and the downward passage of sludge.

The outlet 6 is surround by a system of baffles 10 to prevent the escape of support particles from the vessel. A sludge pipe 7 removes accumulated sludge 12 from the bottom of the vessel.

In use, a bed of particles 2 floats on the surface of the growth medium 11. On introduction of air into the medium through the diffusers 5, the bed expands downwards into the less dense mixture of air and liquid medium and becomes fluidised. Microbes in the medium attach themselves to support particles and replicate, using up the biochemical oxygen demand of the sewage effluent. As the layers of microbes attached to the surface of the particles grow, the microbes on the innermost layers-become anaerobic and less adherent and so tend to slough off. Biomass is also knocked off the particles by attrition caused by the movement of the particles in the tubulence caused by the upflow of air bubbles and the downflow of sewage waste influent from the input pipe 3. Biomass 9 having a density greater than that of the growth medium, sinks to the bottom of the vessel below the baffles 8 where it settles as a layer 12. It is advantageous to leave the sludge in the vessel for a long period of time, even up to a few months.

In that period the sludge, because of its slightly permeable structure, remains in contact with oxy gen-containing liquid and therefore remains essentially sweet and, although the microbes are not resplicating at a great rate, they tend to digest most pthogenic micro-organisms in the sludge. The sltjdge does not, therefore, have to be removed to other settling tanks for further processing.

Claims (19)

1. A process for producing biomass by fermentat ion of microbes in a liquid medium in a reactor in which the medium contains non-porous particles on which the microbes may grow and which may become coated with biomass, and in which the liquid medium has a specific gravity less than the specific gravity of the biomass and more than the specific gravity of the particles coated with biomas s, the particles are formed into and maintained as an expanded or fluidised bed in the medium by downward fluidisation, biomass grown on the particles is rubbed from the particles in the bed and liquid medium and biomass are seprated from the particles by flotation of the particles in the liquid medium and liquid medium and biomass are removed from the reactor.

2. A process according to claim 1 in which downward fluidisation of the bed is achieved by upflow of gas and/or mechanical agitation.

3. A process according to claim 1 or claim 2 in which the biomass is separated from the liquid medium after fermentation.

4. A process according to claim 3 in which the separation of biomass from the liquid medium takes place in the reactor.

5. A process according to claim 4 in which downward fluidisation of the bed is achieved by the introduction of gas through diffusers above the base of the reactor to produce an upflow of gas bubbles through the liquid medium and in which the separation of biomass from liquid medium takes place below the level at which gas is introduced.

6. A process according to any preceding claim in which the liquid medium is aqueous.

7. A process according to any preceding claim which is a continuous process in which liquid medium is continuously introduced into the reactor through an inlet and liquid medium and biomass are removed from the reactor at one or more positions which are lower than the inlet.

8. A process according to claim 7 in which the biomass is removed through an outlet which is lower than the level of the bottom of the fluidised bed.

9. A process according to any preceding claim in which the uncoated particles have a specific gravity of below 90% of the specific gravity of the liquid medium.

10. A process according to any preceding claims in which the particles have hollow spaces in their surfaces.

11. A process according to any preceding claim in which downward fluidisation is by upflow of gas and the effective density of the liquid medium is varied across the horizontal plane of the bed by variation in the upflow of gas across the bed.

12. A process according to any preceding claim in which the fermentation includes aerobic fermentation and fluidisation is by upflow of a gas comprising oxygen.

13. A process according to any preceding claim in which the fermentation includes anoxic fermentation.

14. A process in which the liquid medium comprises waste water and which reduces the biochemical oxygen demand of the waste water.

15. Apparatus suitable for carrying out a process according to any preceding claim comprising a reactor vessel containing a fermentation zone and having inlet means for introducing liquid medium into the fermentation zone, outlet means for removing biomass and liquid medium from the fermentation zone, in which the outlet means are below the level of the inlet means, and diffusers for the introduction of gas into the reactor above the base of the reactor.

16. Apparatus according to claim 15 which comperises baffle means below the diffusers capable of providing a quiescent zone below the level of the baffle means in liquid present in the reactor.

17. Apparatus according to claim 15 or claim 16 in which the diffusers extend across part only of the cross sectional area of the reactor.

18. Apparatus according to any one of claims 15 to 17 having separate outlets for liquid medium and biomass, the outlet for biomass being adjacent the base of the reactor and the outlet for liquid medium being above the base of the reactor.

19. Apparatus according to any one of claims 15 to 18 in which the outlet for the liquid medium is surrounded by baffles for minimising the escape of solid particulate matter from the reactor.