This invention relates to fermentation vessels and to assemblies of such vessels.

In general, fermentation vessels (fermentors) are 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 mixing is minimised 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.

However, it is highly beneficial for the suspended cells to maintain constant contact with the medium and to avoid settling out under gravity. The present invention provides an apparatus and ^" method in which medium passing along an elongated fermentation vessel can be caused to move cyclically in a lateral direction to effect stirring while moving longitudinally in an approximation to plug flow.

Furthermore, it has been found that traditional cylindrical fermentation vessels such as tanks or vats are difficult to scale up to capacities of several thousand cubic metres ( ³ ) such as are required, for example, to treat effluents such as human sewage since the change in geometiy affects the biological environment within the vessel.

According to the present invention we provide a fermentation vessel comprising an elongated horizontal

tank of narrow vertical cross-section having an inlet and an outlet longitudinally spaced therefrom, a substantially vertical panel being provided which divides the tank from the region of said inlet to the region of said outlet into two channels connected by gaps above and below said panel along its length and gas inlet means being situated at the bottom of the tank so that, in operation, influx of gas causes fermentation medium to flow transversely around the panel in the flow path defined by the two channels and the gaps above and below the panel while also moving longitudinally through the tank, means being provided to permit recycling of at least part of the effluent from the outlet back to the inlet.

The present system lends itself to scaling up while retaining suitable conditions for fermentation. In general, the tanks used in accordance with the invention will be about 3m in height and about 0.6m in breadth. In order to increase the scale of operation, it is possible to connect an assembly of such vessels in parallel. As will be seen hereinafter, the tanks are advantageously U-shaped, so that their inlets and outlets are relatively close to each other and this facilitates such parallel assemblies. However, as further explained hereinafter, it may be advantageous to join two or more tanks in series; a number of such series-combinations may also be connected in parallel.

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 claimed 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 the plane substantially perpendicular to the length of the fermentation vessel.

It is further desirable that the tanks are provided with means for controlling their temperature, eg.

heating and/or refrigeration means.

The design of the fermentor module is illustrated in Figures 1-11. A key to the figures is given below.

Figure 1 shows in perspective a straight fermentor module;

Figure 2 shows an elevation of the outlet end of a module;

Figure 3 shows an elevation of the inlet end of a module;

Figure 4 shows a plan of the top of a straight module;

Figure 5 shows a longitudinal elevation of a straight module;

Figure 6 shows in perspective a U-shaped fermentor module;

Figure 7 shows a plan of the top of a U-shaped fermentor module;

Figure 8 shows in perspective and to scale U- shaped fermentor module; length from inlet to b 1, 10m; nominal volume 18m ³ ;

Figure 9 shows in perspective and to scale a multimodule fermentor consisting of five U-shaped modules assembled in two dimensions; length from inlet to bend, 10m; nomi al volume of each module, 18m ³ ;

Figure 10 shows in perspective and to scale a multimodule fermentor consisting of ten U-shaped modules assembled in three dimensions; length from inlet to bend, 10m; nominal volume of each module, 18m ³ ;

Figure 11 shows an exploded diagram in perspective of a U-shaped fermentor module;

The key to the Figures 1-11 is as follows.

10, central panel; 1 ^' . gas sparger; 12, upcomer stream; 13, downcomer strt im; 14, lid; 15, flange; 16, gas outlet; 17, recycled gas stream; 18, gas compressor; 19, gas inlet; 20, heat exchange tube; 21, heat exchanger; 22, substrate feed stream; 23, inlet for substrate and biomass recycle; 24, outlet for culture;

25, culture overflow weir; 26, effluent culture stream; 27, separator for biomass and solids; 28, liquid effluent stream; 29, biomass recycle stream; 30, excess biomass effluent stream; 31, strut; 32, body section; 33, direction of horizontal flow of culture; 34, inner wall.

The straight fermentor module shown in Figure 1 consists of an elongated tank divided into two channels by the central panel 10 which has a gap both above and below it. The gaps allow the liquid contents of the module to be cycled vertically around the central panel by means of an air or gas lift provided by means of gas sparger 11 situated on the upcomer side of the central panel. The upcomer stream 12 and the downcomer stream

13 are shown in Figures 1-3. The gas may be fed in at both ends of the sparger 11.

The top of the module is normally sealed by a lid

14 fitted on a flange 15 as shown in Figure 4. The gas exits through the lid at outlet 16. A part of the effluent gas may be recycled in the stream 17. Fresh air or other gas is admitted through the gas outlet 19. The gas stream passes to the module through the compressor 18.

Temperature control is achieved by means of a heat exchange tube 20 connected to a heat exchanger 21.

The substrate stream 22 together with the recycled biomass 29 are fed into the module through the inlet 23. Plug flow of the culture occurs in the direction 33 as shown in Figures 1 and 4. The culture exits through the outlet 24 at the base of the module. The outlet 25 is an overflow weir connected to the effluent stream 26.

As shown in Figure 1 the effluent culture stream 26 passes into a separator 27 such as a hydrocyclone or sedimenter which concentrates the biomass and other solids into a sludge and separates the liquid which issues from the separator in the stream 28. Part of the biomass concentrate is recycled to the module in the

stream 29 and excess sludge exits from the system in stream 30.

In the U-shaped form of the fermentor module depicted in Figures 6-11 the channels are bent round into the U-shape so that the outlet and inlet ends are juxtaposed and the length of the module is halved. The outlet end and the feed end shown in Figures 2 and 3 respectively are identical to those used in the straight module. In the U-shaped module shown in Figures 6 and 7, the central panel 10, the sparger 11 and the heat exchange tube 20 are continuous round the bend of the module. The inner wall 34 which extends from the lid to the base has a gap at the bend in the module to permit the culture stream to turn. The biomass recycle, gas sparger and heat exchange systems associated with the U- shaped module are similar to those shown for the straight module in Figure 1.

The elongated form of the module and its relatively narrow cross section are illustrated in Figure 8 where, in outline, a U-shaped module is drawn to scale. The height and width of the module are kept constant to conserve the essential geometry of the module. The length of the module from end to end can be varied to alter the volume of the module as required.

The exploded diagram of a U-shaped module in Figure 11 shows the body section 32 separated from the ends of the module. Additional body sections may be inserted as required.

The module is fabricated of steel or other metal plate or plastic including glass fibre plastic or other suitable material. Struts 31 are inserted, as shown in Figures 6 and 7, or as required between the walls of the module to reinforce the structure and maintain its rigidity.

In order to scale up the bioreaction process a number of individual modules are connected in parallel to the feed and effluent streams, as shown in Figures 9

and 10, in either two dimensional or three dimensional multimodule fermentors. The individual modules can be joined together with their walls in common.

For multistage processes a number of modules are connected in series with changes in the appropriate conditions, for example, of substrate and temperature from module to module in the series.

As specific embodiments of a fermentor module two continuous fermentation processes 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 U-shaped fermentor modules of the type shown in Figures 6 and 8 may be used. The module has a height of 3m and width of 0.6m so that the distance between the central panel and the wall is 0.15m. The gap between the central panel and the base of the module is 0.15m. The length from the feed end of the module to the bend is 10m. The total volume of the module is about 18m ³ and the working culture volume is 15m ³ .

Single-stage processing 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 l ^"1 . The temperature of the sewage is set at 30°C. 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 3.0m ³ min ^"1 to generate a liquid velocity in the downcomer and upcomer streams between about 1.5 to 3.0 cms ^"1 . The vertical cycling keeps in suspension particles of matter present in the medium and forms a fluidised bed of the particles. The aeration gas is either air or part

recycled gas with air. The dissolved oxygen concentration as measured by an oxygen electrode placed near the base of the downcomer channel is maintained at about 2mg l ^"1 by control of the rate of air flow through the inlet 19.

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 23. The temperature of this feed is adjusted to about 30°C before it enters into the module. The feed rate of the sewage stream 22 is 72m ³ d ^'1 .

A hydrocyclone or other type of separator concentrates the biomass solids in the biomass recycle stream 29 to 50 kg dry matter m ^"3 and the flow rate of this stream 29 is adjusted to be 2.23m ³ d ^"1 .

During passage of the sewage through the module the volatile suspended solids (VSS) in the liquid increase by about 0.175 kg ^"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 28 with a B.O.D. of 20 mg l' ¹ and a volatile suspended solids (VSS) content of 30 mg l ^"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 t. _ use of a multimodule of either the two dimensional type shown in Figure 9 or the three dimensional type shown in Figure 10. For instance, a sewage flow rate of 1000m ³ d ^"1 would require 14 modules, each of 15m ³ working capacity, in the multimodule fermentor.

Ethanol fermentation

In the ethanol fermentation, the 18m ³ module after cleaning and disinfecting is charged with disinfected culture medium, 13.5m ³ . The culture contains: glucose, 185g l ^"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 l ^"1 . 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 uvaru is grown in the complete medium described above with 185g glucose l ^"1 . The module is inoculated with 1.5m ^"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 ^"1 . Complete culture medium is fed into the module through stream 22 at a flow rate of 1.0m ³ h ^"1 . A bleed of air at about 1.0m ³ min ^"1 into the sparger through the inlet 19 is required to avoid a sterol deficiency in the yeast.

By means of a hydrocyclone or other type of separator, biomass is concentrated to 150g dry weight l ^"1 in the biomass recycle stream 29, which is fed back to the module at a flow rate of 0.5m ³ h ^"1 . The liquid effluent from the culture contains about 80g ethanol I ^"1 and the productivity of the culture is 5.4 kg ethanol m ^"3 h ^"1 .

Application of a vacuum to the culture in the fermentor module through the outlet 16 causes the culture to boil at 35°C 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 may be used as a means of cooling the contents of the module, removing ethanol from the culture,

decreasing the carbon dioxide partial pressure and concentrating the biomass.

These effects permit increase in the substrate feed rate and the medium strength, in particular the glucose concentration, which increases the ethanol productivity of the module up to four fold.