AU661598B2 - Bioreactor system - Google Patents

Description

VERSIN

PCr quality of all Page% improved k= )0tTIMIt L% 'Alt 41tuott(,J'NIZATION INTERNATIONAL APPLICATION PUBLISHED UTLNDER~ Ti I PATENT COOPr RATION TREATY (PCI) (51) International Patent ClasslflcAtlon, 5 V 1) International Publication Number: WO 93/22418 CU2M 1/04, 3/02, C02F3122 Al l,)nternatlonal Publication Date: Ii November 1993(11.11.93) (21) International Application Number: PCT/G019Y00875 (81)Dlesignated States:. AU, CA, JP, KR, US, European patent (AT, 111, CHi, DE, DK, US, FR, GB, 0R, IE, IT, LU, (22) Internional Filng Date: 27 April 1993 (27,04,93) SIC -E PT. S U).

Priority data: 9209 175.0 28 April 1992 (28.04.92) (71) Applicant (for all desIgnatedl Stater except VS): PIRTFERM LIMITED 10GG); 50 Chartfield Avenue, Putney, London SWVl5 611G (Gflj.

(72) Inventor; and (for LAS onltqJ: PIRT, Stanley, John IGO 0BI; 50 C.hartlield Avenuei, Putney, London SWIS 6110

(GB),

(74) Agents: HOLMES, Michael, John et aL.; Frank B, Dchn Co., Imperial House, 15-19 Kingsway, London WVC2B3 6UZ (GO).

Published ith international search report.

Before the expiration of the daze limit for antending the clalmr and to be republihedl in the evn: of the receipt of amendowet.

6615r"98A (54)T1111. BIOREACTOR SYSTEM (57 1bstnact227 Theinenio povdesfemetaio vsses 10 cmpisngat eat nefemetatonzoeeah r sidzoesbengdi vie nt w o oecoprmet ytrnvrs afes(6 ec onann bps mas(7,8 o emtpasg oLei umfomte ne (9 o hezn t n ule 2) ogiuinlyspcd hrerm asbsatilyvetca ariink1) en aberrsau.The invention fute provides fermentation vesl 1)c m ethdsi sing suct eseest wheremetai n se tir i zoes bicyclia um fomt the edium) o therzoei t n ote 2)lniuial pcdteerm usatal etclpriin(1 en p Rrod to wich divde O the sa zo ne from94 Sc the ei onosi ne otergino adote noto hnescn ySp WO 93/22418 PC/Gl93/00875 This invention relates to bioreactors (fermentation vessels) and to asneinblies of such vessels.

in general, bioreactors are vessels designed to permit growth of microorganisms or other cells therein and to digest the substrate medium in which the cells are suspended. in continuous fermentation, such vessels permit passage of medium containing the cells through the vessel and continuous collection of the digested medium.

in one type of continuous fermentation, the vessel is elongated and the medium passes in plug flow or an approximation thereto, so that back and forward mixing are minimized and at any point along the length of the vessel, the medium is essentially under batch fermentation conditions and undergoes the required fermentation in the time taken to traverse the length 'of the tank.

Hlowever, it is highly beneficial for the suspended cells to maintain constant contact with the medium and to avoid settling out under gravity. Furthermore, it has been found that traditional cylindrical fermentation vessels, such as tanks or vats are dif ficult to scale up to capacities of several thousand cubic metrels since the change in geometry affects the biological environment wi'thin the vessel. Also it is frequently necessary to aerate the system with a stream of air bubbles which generates turbulence and causes mixing that are in conflict with ideal plug flow. These problems can be overcome by use of a long split channel bioreactor, for example of the type described in our International patent application No. PCT/EP9l/01323, published under number W092/017791 in which a tank is longitudinally divided by a vertical panel in which medium moving longitudinally through the tank is also caused to move cyclically around the panel under influence of a stream of gas in an approximation of plug flow.

We have now surprisingly found that the incorporation of transverse baffles into such a vessel such that bypass means are provided to permit the medium to pass longitudinally through the vessel whilst also cyclically moving around the panel not only improves the plug flow but permits efficient scale up.

According to the present invention we provide a fermentation vessel comprising at least one fermentation zone, each of said zones being divided into two or more compartments by transverse baffles each containing bypass means to permit passage of medium from the inlet of the zone to an outlet longitudinally spaced therefrom, a substantially vertical partition being provided which divides the said zone from the region of said inlet to the region of said outlet into two channels connected by gaps above and below said partition along its length and -;s inlet means being situated at the bottom of the said zone and on one side of the partition so that, in operation, influx of gas causes fermentation medium to flow transversely around the partition in the flow path defined by the two channels and the gaps above and below the partition while also moving longitudinally through the compartments of the said zone, means being provided to permit recycling of at least part of the effluent.

The present system, termed a baffled split channel bioreactor (BSCB), makes possible scale-up of the process while retaining unaltered conditions for fermentation.

On the large scale the vessels used in accordance with the invention will be about 3m in height and 0.3m in breadth with the length adjusted to accommodate the desired volume, for example 20m long for a tank with a volume of about 18m 3 These dimensions can be scaled down for 2 AMENDED SHEET smaller vessls which may have a volume less than 201.

The efficiency of the plug flow and the flexibility of the reactor design are greatly increased by the introduction of the transverse baffles, which divide the reactor into compartments. In order to increase the scale of operation, it is possible to connect an assembly of such vessels in parallel. It will be seen hereinafter, that by appropriate siting of the baffles and bypass means it is possible to arrange the flow of the liquid medium so that the inlet and outlet of the bioreactor are side by side or one above the other, which facilitates both recycling of the biomass (cells) and scale up in parallel assemblies.

In one convenient embodiment, a dividing wall separates the vessel into two fermentation zones one on each side of the wall so that longitudinal flow is through the compartments on one side, then after passage through a port in the dividing wall the horizontal liquid flow is reversed in the compartments on the other side of the dividing wall and inlet and outlet ends of the reactor are brought into juxtaposition.

In cases where flow reversal occurs, bypass means may be provided such that medium may pass between compartments separated by dividing walls arranged substantially parallel to the vertical partition. However, as further explained hereinafter, it may be advantageous to join two or more tanks in series; a number of such seriescombinations may also be connected in parallel.

Any suitable bypass means may be used to permit passage of medium in a direction along the longitudinal axis of the vessel, however apertures within the baffles, for example ports, are preferred. In such a case, appropriate siting of said ports may be used to control the movement of medium. For example, positioning of ports alternately towards the top or bottom of adjacent baffles ensures the 3 AMENDED SHEET WO 93/22418 P~r/GB03/00875 medium cycles vertically.

According to a further feature of the invention, we provide a method of fermentation in which a fermentation medium is introduced into the inlet of a fermentation vessel as herein described and passes to the outlet thereof while gas from the gas inlet means of said fermentation vessel causes the medium to move cyclically in a substantially perpendicular plane with respect to the base of the fermentor.

It is further desirable that the bioreactors are provided with means for controlling their temperature, for example, by means of a heat exchanger installed in each compartment, either heating or cooling may be required.

The design of BSCB modules is illustrated in Figures 1-13.

A key to the figures is given below: Figure 1 shows in perspective*a straight BSCB module with four compartments.

Figure 2 shows in perspective two baffles and the partition in one compartment.

Figure 3 shows-an elevation of a transverse cross section (entry end) of a BSCB.

Figure 4 shows a longitudinal elevation of a straight BSCB with four compartments.

Figure 5 shows a plan (top view) of a rectangular BSCB with four compartments.

Figure 6 shows a BSCB with four compartments arranged for reversal of horizontal liquid flow.

Figure 7 shows an elevation of the inlet and outlet end of a BSCB with reversal of liquid flow.

Figure 8 shows a longitudinal elevation of a BSCB with four compartments arranged for reversal of liquid flow.

Figure 9 shows in perspective a BSCB with four compartments arranged for reversal of liquid flow.

WO 93/22418 PC/GB3/00875 External dimensions are approximately to scale for a total capacity of about 18.81 with length breadth 25cm and height, Figure 10 shows in perspective a BSCB module with four compartments arranged for reversal of liquid flow drawn approximately to scale for a module with eoout 18m 3 capacity with length 10m, breadth 0.6m and height 3m.

Figure 11 shows a plan of the top of a BSCB module with four compartments arranged for reversal of liquid flow, drawn approximately to scale for an 18m 3 module with breadth 0.6m and length Figure 12 shows in perspective a two tier BSCB module with four compartments, two above and two below, and reversal of liquid flow.

Figure 13 shows in perspective a multimodule BSCB consisting of four two-tier modules .each with the first two compartments in the upper tier and the second two compartments (with reversal of liquid flow) in the lower tier.

The key to the Figures 1-14 is as follows: tank; 11, partition; 12, gas supply manifold; 13, liquid upflow stream- (riser); 14, liquid downflow stream (downcomer); 15, liquid level; 16, baffle; 17, low port; 18, high port; 19, inlet for substrate; 20, effluent culture stream; 21, flange for lid seal; 22, vent for gas; 23, recycled gas; 24, effluent gas; 25, gas sparger; 26, sedimentor; 27, recycled biomass; 28, excess sludge (biomass) discharge; 29, effluent supernatant; 30, direction of horizontal liquid flow; 31, ring or other type of seal; 32, lid; 33, heat exchanger; 34, inlet for air or other gas; 35, dividing wall; 36, culture feed from upper to lower tier; 37, base of upper tier of compartments.

WO 93/22418 PCf/GB93/00875 The straight BSCB module shown in Figure 1 consists of a tank (10) divided into four compartments by three transverse baffles (16) and the compartments are split into two channels by the partition (11) either central or offset, which has a gap both above and below it. The gaps allow the liquid contents of the module to be cycled vertically around the partition by means of an air or gas lift provided by means of gas sparger (25) situated on the riser (upflow) side (13) of the partition. The riser liquid stream (13) and the downflow liquid stream (downcomer)(14) are shown in Figure 3. The size of the gap below the partition is about half the breadth of the vessel. The transverse baffles (16) extend from the base of the tank to above the liquid level (15) but leave a gap below the lid (32) to unify the gas space. A perspective view of the baffles and partition in one compartment is shown in Figure 2. The baffles prevent horizontal flow of the liquid except for passage through at least one port in each baffle, alternately low and high The alternating levels of these ports prevents the liquid from by passing the vertical liquid cycling. The diameter of the ports should be sufficiently large to permit turbulent flow through it and offer no obstruction to solids suspended in the medium.

A side elevation and a plan of the top of a straight BSCB are shown in Figures 4 and 5 respectively.

Air or other gas is supplied through the inlet (34) and mixed if necessary with the recycled gas the gas mixture passes via the manifold (12) to the spargers situated in each compartment. The tank is normally sealed with a lid (32) which rests on a flange As shown in Figure 3 the lid is sealed on the flange by an ring or other suitable sealing device. A gas vent (22) is provided in the lid. Means may be provided for withdrawing effluent gas; a part of the effluent gas can WO 93/22418 PGIGW31008751 be recycled in steam (23) and the excess gas exits at (24)* Temperature control is achieved by means of a heat exchanger (33) shown located on the partition in Figure 4 but other sites could be used.

The substrate stream (19) is fed into the riser side of the first compartment. After passage through the compartments the digested medium exits in effluent stream An overflow weir or other means is provided to keep the liquid level (15) constant. If required, the effluent stream is passed to a sedimentor (26) or other device to concentrate the biomass (cells). Part of the biomass concentrate is recycled to the module in stream (27), excess biomass exits from the system in stream the supernatant liquor leaves the sedimentor in stream (29).

The BSCB is constructed of stainless steel or other non toxic metal or plastic including glass fibre plastic or other resistant and non-toxic material. The structure must be reinforced with ribs and possibly transverse struts between the walls to maintain the rigidity of the walls and the partition and baffles. The bottom of each longitudinal channel is preferably dished and corners are rounded to avoid angles where suspended solids could accumulate.

The number of compartments and their volume and length can be varied in order to isolate the various stages in the metabolic process. Usually the number of compartments will be in the range of 2-6.

In the alternative form of the BSCB shown in Figure 6 the compartments are arranged to achieve reversal of the horizontal liquid flow and so bring the entry and outlet ends of the reactor into juxtaposition as shown in Figure WO 3/22418 PCr/GB93/00875 7. The plan of the top of a BSCB with flow reversal given in Figure 6 shows that the dividing wall (35) and the baffles (16) divide the BSCB into four compartments each of which is split into channels by the partitions The dividing wall (35) rises from the base to above the liquid level and leaves a gap below the lid, as shown in the transverse section, Figure 7 and the longitudinal elevation, Figure 8. Substrate and recycled biomass entering the BSCB at pass into the riser (13) of the first compartment where vertical cycling around the partition (11) is induced by the gas sparger The riser (13) and downcomer (14) on each side of the dividing wall (35) are depicted in the cross section shown in Figure 7. The liquid passes into the second compartment through the low port cycles vertically around the central partition there, then crosses the dividing wall through the high port into the third compartment, cycles vertically, then passes into the fourth compartment through low port and finally, after vertical cycling, exits from the BSCB into the effluent stream It can be seen that the compartmented reactor facilitates reversal of liquid flow with consequent juxtaposition of inlet and outlet streams. A heat exchanger (33) may be either attached to the partition or inserted below it as shown in Figures 7 &nd 8. There is a common gas space above the baffles and dividing wall and gas leaves through vent (22).

Figure 9 shows in perspective a BSCB with four compartments and reversal of liquid flow. This figure is drawn with external dimensions of the reactor approximately to scale for a 18.81 total volume and a length of 30cm. The direction of the horizontal liquid flow on each side of the dividing wall (35) is shown by the horizontal arrows Biomass is concentrated in WO 93/22418 PCT/G93/00875 the sedimentor (26) and partially recycled in stream (27).

Supernatant leaves in stream (29) and excess biomass/sludge is removed in stream (28).

A larger scale BSCB with four compartments and reversal uf flow is shown in perspective in Figure 10 and a plan of the top in Figure 11. These figures are drawn approximately to scale for a reactor of total capacity 18m 3 and a length of 10m. The narrow cross section favours high rate vertical cycling and oxygen transfer. The arrows (30) indicate the direction of horizontal flow.

For flow reversal, the compartments may be arranged in a number of tiers. In a two-tier system, medium enters at one end of the top tier and flows longitudinally cycling vertically around the vertical panel and upon reaching the opposite end of the tier, the medium overflows, for example via a weir, into the compartment vertically below whereupon longitudinal flow is reversed.

Figure 12 shows in perspective a BSCB with four .compartments constructed in two tiers. The first two compartments form the top tier. Horizontal flow directions are indicated by the arrows The culture medium after passing through the top two compartments overflows through a weir and passes via stream (36) to the lower tier where the flow of the medium is reversed and the culture emerges in stream In Figure 12 the base (37) of the upper tier is the lid of the bottom tier, however it may be convenient to have a gap between the two tiers. Gas vents (22) are provided for each tier. This facility for vertical stacking of the compartments reduces the horizontal area required, brings the entry and exit ends of thr reactor into juxtaposition, also it facilitates recycling of the biomass and scale up of the plant.

WO 93/22418 ICT/GB93/00875 Scale-up of the bloreaction process is achieved by means of a multimodule assembly of BSCB. Figure 13 shows in perspective a multimodule consisting of four modules of the two tier type with reversal of liquid flow. The dividing walls (35) between the modules do not extend to the lid of the module so that there is a common gas space above the liquid level. Each tier has its common gas vent The liquid level in each module is set by a weir at the outlet. A common sedimentor (26) provides recycled biomass for the entire multimodule.

The multimodule concept (advanced in our patent application No PCT/EP91/01323) permits scale-up without disturbance of the conditions in the bioreaction and provides economies of scale. Thus it is possible to adapt the BSCB both to large centralized plants and down to single modules.

For multistage processes which require different conditions or different organisms in each stage modules are connected in series with appropriate changes in the conditions, for ,xample of substrate and temperature from module to module in the series.

The BSCB lends itself well to installation off shore, which is of great interest for the activated sludge treatment of sewage in coastal areas where convenient land sites are often not available. For this purpose the BSCB in multimodular form can be installed in a floating or shipboard plant or on a platform anchored offshore. In many cases such offshore plants may be coupled up to existing sewage outfalls ending in the sea. The BSCB can also be.installed in mobile plants mounted on lorries or trailers, which is of interest where the substrate availability is seasonal or temporary.

The BSCB may be utilised for the fermentation of a variety of organisms, for example microorganisms, plant or animal cells.

The BSCB may additionally be [provided with means to enable a vacuum or partial vacuum, to be applied to the tank.

As speci.ic embodiments of a fermenter module two continuous fermentation protesses are described below by way of example and with reference to the above description of the module. The first process is the aerobic activated sludge process for sewage purification. The second process is the anaerobic fermentation of sugar to produce ethanol.

In each case the process makes use of a BSCB module of the type shown in Figure 10. The module is divided into four compartments with reversal of liquid flow. The module has a height of 3m and breadth of 0.6m so that the distance between the partition and the wall is 0.15m. The gap between the partition and the base of the module is 0.15m.

The length of the module is 10m. The total volume of the module is about 18m 3 and the working culture volume is 3 ACTIVATED SLUDGE TREATMENT OF SEWAGE In the single stage activated sludge process the module is filled with sewage with a B.O.D. (biochemical oxygen demand) of about 250 mg 1

I

The temperature of the sewage is set as high as possible in the range 15-30'C. The contents of the module are inoculated with activated sludge then aerated by means of the sparger. The gas flow rate thrcugh the sparger is fixed between about 1.5 to 3 min to generate a liquid velocity in the downcomer and riser streams of about Sm mil. The vertical cycling keeps in suspension particles of matter present in the 11 AMENDED SHEET WO 93/22418 PCr/GB93/00875 medium. The aeration gas in either air or part recycled gas with air. The dissolved oxygen concentration as measured by an oxygen electrode plae'- near the base of the downcomer channel is maintained about 2mg 1* 1 by control of the rate of air flow from the manIfold.

After the initial batch culture during which the activated sludge is propagated, sewage together with recycled biomass sludge is continuously fed into the module through the inlet The temperature of this feed should be adjusted to the reaction temperature before it enters into the module. The feed rate of the sewage stream (19) is 72m 3 A sedimentor or other type of separator concentrates the biomass solids in the biomass recycle stream (27) to dry matter r 3 and the flow rate of this stream (27) is adjusted to be 2.23m 3 d'

I

During passage of the sewage through the module the volatile suspended solids (VSS) in the liquid increase by about 0.175 kg m" 3 The plug flow of the culture along the module facilitates the digestion of those substrates which are used in sequence. This process produces a liquid effluent (29) with a B.O.D. of 20 mg 1 1 and a volatile suspended solids (VSS) content of 30 mg 1 Modules in series are added if additional stages of purification such as anaerobic digestion, nitrification, denitrification and phosphate removal are required.

The process is scaled up by the use of a multimodule for example of the type shown in Figure 13. For instance, a sewaga flow rate of 1000m 3 d* 1 would require 14 mod'iles, each of 15m 3 working capacity, in the multimodule fermenter.

WYO 93/2241i8 PCT/GB9300875 The BSCBmay be utilised for the fermentation of a variety of organisms, for example microorganisms, plant or animal cells.

g BSCB may additionally be provided with means to enable aacuum or partial vacuum, to be applied to the tank.

As s ecific embodiments of a fermenter module two continous fermentation processes are described below by way of ekample and with reference to the above description f tho moule. The first process is the aerobic activated sludge process for sewage purification. The second process is the anaerobic fermentation of sugar to produce ethanol.

In each case the process makes use of a BSCB module of the type shown in Figure 10. The module is divided into four compartments with reversal of liquid flow. The module has a height of 3m and biradth of 0.6m so that the distance between the partition ad the wall is 0.15m. The gap between the partition and ,the base of the module is 0.15m.

The length of the module.is 10m. The total volume of the module is about 18m 3 and the working culture volume is 1S 3 ACTIVATED SLUDGE TREATMENT OF SEWAGE In the single stage activated sludge process the module is illed with sewage with a B.O.D. (biochemical oxygen demand) of about 250 mg 1 1. The temperature of the sewage is set as high as possible in the range 15-30". The contents of the module are inoculated with activated sludge then aerated by means of the sparger. The gas flow rate through the sparger is fixed between about 1.5 to 3 min*' to generate a liquid velocity in the downcomer and riser streams of about 5m min 1 The vertical cycling keeps in suspension particles of matter present in the WO 93/22418 I, CTtGB93/00875 ETHANOL PERMENTATION In the ethanol fermentation, thp 18m 3 rectangular module after cleaning and disinfecting is charged with disinfected culture medium, 13.5m. The culture contains: glucose, 185g 1 1 together with sources of B vitamins, nitrogen, such as ammonia or urea, phosphate, sulphate, magnesium, iron and trace elements as required to produce a yeast concentration of 20.9g dry weight 1i. The pH value of the medium is adjusted to pH 4.5. Before use the medium is pasteurized if necessary. The temperature of the medium is adjusted to 30-35"C. The inoculum of Saccharomyces uvaum is grown in the complete medium described above with 185g glucose 1 The module is inoculated with l.5m 3 of the inoculum culture which has just about reached its maximum gas production rate. Gas generation in the module is allowed to reach its peak then the effluent gas is recycled to the sparger at a rate of about 1.0m 3 min. Complete culturq medium is fed into the module through stream (19) at a flow rate of 1.0m 3 h"

I

A bleed of air from (12) at about 1.0m 3 min' into the spargers is required to avoid a sterol deficiency in the yeast.

After passage through the sedimentor, biomass concentrated to 150g dry weight 1'1 in the recycle stream '27) is fed back to the module at a flow rate of 0.5m 3 The liquid effluent from the culture contains about 80g ethanol 1" and the productivity of the culture is about 5.4 kg ethanol m* 3 h-1 Appli'cation of a vacuum to the culture in the fermenter module through the gas vent (22) causes the culture to boil at 350C when the gas pressure is 50 mm Hg.

Evaporation of the culture by vacuum fermentation with the temperature at the boiling point in the upcomer stream provides a means of cooling the contents of the module, WO 93/22418 PCT/0193/00875 removing ethanol from the culture, decreasing the carbon dioxide partial pressure and concentrating the biomass, the effects of which permit increase in the substrate feed rate and the medium strength, in particular the glucose concentration, and increase the ethanol productivity ot the module (kg m" 3 h up to four fold.

Claims (17)

1. A fermentation vessel comprising at least one fermentation zone, each of said zones being divided into two or more compartments by transverse baffles each containing bypass means to permit passage of medium from the inlet of the zone to an outlet longitudinally spaced therefrom, a substantially vertical partition being provided which divides the said zone from the region of said inlet to the region of said outlet into two channels connected by gaps above and below said partition along its length and gas inlet means being situated at the bottom of the said zone and on one side of the partition so that, in operation, influx of gas causes fermentation medium to flow transversely around the partition in the flow path defined by the two channels and the gaps above and below the partition while also moving longitudinally through the compartments of the said zone, means being provided to permit recycling of at least part of the effluent.

2. A vessel as claimed in claim 1 wherein the bypass means comprise apertures within the baffles.

3. A vessel as claimed in claim 2 wherein the apertures are alternately positioned in adjacent baffles either tooards the top of the baffle, or towards the bottom of the baffle.

4. A vessel as claimed in any one of the previous claims wherein the ratio of the height to breadth is in the range 3:1 to 10:1. A vessel as claimed in any one of the preceding claims in which the gas inlet means is a sparger in each compartment located on one side of the longitudinal partition.

AMENDED SHEET

6. A vessel as claimed in any one of the preceding claims which is sealed and in which means are provided to withdraw any effluent gas.

7. A vessel as claimed in any one of the preceding claims in which means are provided to apply at least a partial vacuum.

8. A vessel as claimed in any one of the preceding claims provided with heating and/or cooling means.

9. A vesua'l as claimed in any one of the preceding claims in which the outlet is connected to separation means to concentrate solids in the effluent and/or clarify the effluent.

A vessel as claimed in any one of the preceding claims wherein a dividing wall having bypass means separates the compartments into two groups, one on each side of the wall so that longitudinal flow of medium is through the compartments on one side of said wall followed by passage through the bypass means in said dividing wall and longitudinal flow in the reverse direction through the compartments on the other side of the dividing wall, and wherein inlet and outlet are brought into juxtaposition.

11. A vessel as claimed in any one of claims 1 to 9 wherein the compartments are arranged in tiers wherein longitudinal flow of medium in an upper tier is followed by overflow to the tier below in which longitudinal flow is in the reverse direction.

12. An assembly of vessels as claimed in claim 1 connected in parallel. 3- An- a1 ri theem 'Jsiu-rl-n--harc diiidin; 3l lE .and having a common gas space- .AMENDED SHEET 'WO 93/22418 PCT/GB92 't01

13. An assembly as claimed in claim 12 wherein the vessels share dividing walls and having a common gas space between the liquid level and the lid.

14. An assembly of vessels as claimed in claim 1 connected in series.

A method of fermentation in which a fermentation medium is introduced into the inlet of a fermentation vessel as defined in claim 1 and passes to the outlet thereof while gas from the gas inlet means of said fermentation vessel causes the medium to move cyclically in a substantially perpendicular plane with respect to the base of the fermentor.

16. A method as claimed in claim 15 in which the medium comprises solid particles maintained in suspension by transverse cycling around the substantially vertical partition.

17. A method as claimed in claim 15 or claim 16 wherein the medium comprises microorganisms or plant or animal cells. At