FR2540134A1 - Processes and fermenters for biologically purifying organic effluents with recovery of biomass - Google Patents

Abstract

The subject of the invention is processes and fermenters for biologically purifying organic effluents with recovery of the biomass. A fermenter 1 according to the invention comprises an inlet 3 for the continuous supply of liquid effluent, an inlet 4 for stirring gas, an inlet 5 for a nutrient liquid, an overflow outlet 6 for purified effluent and a second outlet 7 provided with a valve 8 which opens intermittently to discharge biomass floccules. The outlet 6 comprises a settling vessel 9 divided into superposed compartments by plates 13. Blades 18 are mounted on a rotating shaft 15 and drive towards the culture chamber 1 the microbial floccules which sediment on the plates 13. One application is the biological purification of liquid effluents of agri-foodstuffs industries with recovery of usable biomass.

Description

 Processes and fermenters for biologically purifying organic effluents with biomass recovery.

 The subject of the invention is fermentation processes and selective self-seeding fermenters for biologically purifying liquid effluents containing organic materials by recovering biomass which can be used, for example, as a cattle feed or as fertilizer.

 The technical sector of the invention is that of the biological purification of liquid effluents containing organic matter and the construction of fermenters or bio-reactors intended for this purification.

 To purify liquid effluents containing organic matter, for example effluents discharged from the food industry or sewage water, oxidation or biological reduction is usually carried out in a non-sterile medium by populations of microorganisms which are naturally found in the culture centre.

 Under the usual culture conditions, the microorganisms which proliferate are bacteria of small size which cannot be collected economically and which do not allow a recovery of biomass. In addition, the biological oxidation of carbon and of the hydrocarbon products contained in organic matter, allows the effluent to treat phosphates and a large part of organic nitrogen.

An objective of the present invention is to provide new methods and new fermenters which make it possible to culture microorganisms under conditions which prevent the exponential proliferation of small bacteria, which do not collect in flakes and # which are called dispersible bacteria, and under conditions which on the contrary favor the proliferation of natural micro-organisms called flocculants which give rise to biomass flakes, which makes it possible to collect these flakes, and thus to separate them from the liquid by sieving, drying them and thus obtaining a biomass rich in proteins, lipids and carbohydrates and which can also contain a large part of phosphorus and organic nitrogen in the case where the effluent contained it and this biomass is usable, especially as animal feed or as fertilizer,
The methods according to the invention are methods of continuous biological purification of liquid effluents containing organic materials in a bio-reactor or fermenter containing an agitated and non-sterile culture medium.

The objective of the invention is achieved by means of a process according to which:
- a liquid flow is taken from said bio-reactor;
- The flakes of microorganisms it contains in suspension are separated from this liquid;
- these flakes are returned to the bio-reactor;
- one evacuates by a first outlet #e purified liquid freed from the flakes which mainly contains dispersible micro-organisms.

sands;
- And periodically evacuated from said bi # o-reactor, by a second outlet located there a level below the maximum level, the liquid containing in suspension a high proportion of flakes of microorganisms which are recovered.

 According to a preferred embodiment, the liquid exiting through the first outlet passes through de-cantation chambers, which communicate with said bio-re-actor, in which the liquid is withdrawn from agitation and in the bottom of which micro flakes -organisms settle by sedimentation and we sweep. slowly the bottom of said vfhlibres to return said flakes to the bio-reactor.

 According to a characteristic of the methods according to the invention, the liquid effluent is admitted into the bio-re-actor with a flow rate which corresponds to an overall dilution rate greater than the growth rate of the dispersible microorganisms, so that these micro -organisms cannot proliferate and an average hourly flow rate of two deu) is chosen, which corresponds to a dilution rate which makes it possible to select strains of flocculating microorganisms which proliferate in the culture chamber.

 Fermenters. according to the invention are intended for the continuous biological purification of effluents containing. organic materials and are of the type comprising a culture chamber which receives a continuous flow of effluents which are cultivated in an agitated liquid medium and not 'sterile.

 The fermenters according to the invention are characterized by the fact that the culture chamber comprises two liquid outlets, a first outlet in overflow or ensiphon which is equipped with means for covering the flakes of microorganisms and for returning them in said culture chamber and a second outlet which is situated at a level below the maximum level - and which is equipped with a motorized valve whose openings are controlled periodically.

 According to a preferred embodiment, the first outlet comprises at least one settling enclosure which communicates by a lateral opening with an opening, which enclosure is divided into compartments superimposed by trays and comprises, in each compartment, blades which are driven in rotation, at very slow speed, which scan said trays and which return to said culture chamber the flakes which are deposited by sedimentation on said trays.

 The invention results in the biological purification of effluents containing organic matter with recovery of biomass, usable for example as animal feed or as fertilizer.

 In the case where the effluents contain nitrogen and / or phosphorus, a large part of these is recovered in the biomass and the purified liquid is rid of it.

 An essential advantage of the methods according to the invention lies in the fact that the volume of the culture chamber is very reduced compared to the volume of fermenters, digesters and other bioreactors using traditional continuous biological purification methods. Indeed, the concentration of recoverable active biomass can become more than a hundred times that which would be obtained in a conventional bio-reactor for the same medium and this effect is all the more marked when the effluent to be treated is more diluted.

 The essential result of the methods according to the invention is that, by acting only on the effluent arrival flow rates and on the output flow rate of the flake suspension, self-selection of certain strains of microorganisms present in the non-sterile medium which assemble in clusters or easily recoverable flakes and prevent the proliferation of all strains of. bacteria and other dispersible microorganisms.

 The following description refers to the appended drawings which represent, without any limiting character, an exemplary embodiment of a fermenter according to the invention.

 Figure I is an axial section of the fermenter.

 Figure 2 is a horizontal section along II-II of Figure 1.

 FIG. 3 is a partial axial section of the settling chamber 9.

Figures -I and 2 represent a fermenter or bioreactor which essentially comprises a culture vessel or chamber
I of volume V. This tank can be cylindrical with a vertical or cylindro-conical axis, or spherical bottom or any other usual shape.

Tank I contains a liquid 2 culture, agitated and non-sterile. The tank I has an inlet 3 for a liquid effluent containing organic matter which arrives in the tank with a continuous hourly flow W.

 The effluent is, for example, a liquid discharge from an agrifood industry or an industrial discharge containing, for example, phenols or cyanides or even sewage.

 The ratio D "- W between the hourly flow rate W of the effluent and the volume V of the tank, which corresponds to the number of hourly renewals, is called the overall dilution rate of the liquid contained in the tank.

 The problem to be solved is to obtain a biological # purification of organic compounds by the metabolism of microorganisms which are naturally found in the non-sterile culture medium or which can optionally be introduced into this medium, at the start of— culture, to accelerate the start of culture.

 Among these micro-organisms some have the particularity of gathering in clusters or flakes which are kept in suspension in the stirred liquid and which can reach dimensions of several millimeters, which allows them to be easily recovered by sieving and thus recovering usable biomass. These microorganisms are said to be flocculent. The other microorganisms which do not flocculate, and which are generally small bacteria, are said to be dispersible.

 FIGS. 1 and 2 represent an application in which a culture of aerobic microorganisms is used and the tank of the bio-reactor has, at its bottom, an inlet. 4 of air which fulfills a double function of oxygenation and agitation of the liquid # ide.

 Alternatively, a culture of anaerobic microorganisms could be used and, in this case, line 4 would be used to inject a neutral gas such as nitrogen into the tank to stir the liquid.

 Alternatively, mechanical stirring could be used, but stirring by forced circulation of gas or liquid arriving at the bottom of the tank is preferable to avoid dispersing the flakes.

 The tank 1 also comprises a pipe 5 through which nutrients can be injected into the tank, so much so that a complete culture medium can be formed.

 The objective of the methods and apparatuses according to the invention is to promote the development, in the culture medium, of microorganisms which flocculate, by avoiding the exponential proliferation of dispersible microorganisms.

 The tank 1 has two liquid discharge outlets.

A first outlet 6 constitutes an overflow or a siphon, which is located X at a maximum level Ni. A second outlet 7, provided with a motorized valve 8, makes it possible to periodically evacuate a determined volume of a mixture of liquid and flakes which corresponds to the volume between the level Ni and the level N2 which is that of the second outlet located below level N1.

 The first outlet 6 serves to continuously or discontinuously remove purified liquid which has passed through a settling chamber 9, in which the liquid has been freed from the flakes, which have been returned to your culture chamber 1.

 The settling chamber 9 is located on the side of the chamber 1 and it surrounds an opening 10 located in the side wall of. this room.

Enclosure 9 is, for example, a portion of cylinder of vertical axis z zl, which is placed against the cylindrical wall of chamber 1. Enclosure 9 has at its upper part an orifice il which communicates by a pipe 12 with the outlet 6
The liquid culture medium - 2 enters the enclosure 9 through the opening 10 and it flows continuously, through the green opening 11. And the pipe - 12, towards the outlet 6, after the flakes in suspension have been separated from the liquid in the traiwersee of enclosure 9, so that the liquid which leaves through the g # rt # 6 is purified liquid which contains only dispersible mi-cro-organastes.

 To separate the flakes from the liquid, the enclosure 9 has plates 13 which divide it into superimposed compartments, of very flat shape. The distance between trays is of the order of a centimeter. The plates 13 have the form of portions of discs whose outside diameter is equal to the inside diameter of the enclosure 9. they can be incorporated into the enclosure 9 which can be constructed in the form of a stack of flat cylinders or else can be mounted sliding in enclosure 9.

 If the plates 13 were whole disks, they would enter the chamber 1 and they would generate the mixing of the liquid 2. To avoid this, the disks 13 are truncated. They are cut along a circular line 14 # which is centered substantially on the axis x xi of the tank 1 and which extends the side wall of this tank. The truncation of the trays allows the passage of the flakes which return to the tank.

 The plates 13 and the bottom of the enclosure 9 are pierced in their center with an orifice through which passes a shaft 15, of square or polygonal section. The shaft 15 is driven in continuous or intermittent rotation at very slow speed, of the order of 12 revolutions per hour, by a motor-reduction unit 16. The motor-reduction unit 16 can drive the lower end of the shaft 14 as shown in the figure or the upper end. In the case of FIG. 1, the line 12 is coaxial with the shaft 15 but it is well specified that it might not enter it in the event that the geared motor group would cause it. the upper end of the shaft 15. On the shaft 15 are mounted hubs 17 carrying blades 18 which are located between the plates 13.

The hubs 17 have in their center a hole of square or polygonal section through which the drive shaft 15 passes. They have on their two upper and lower faces a groove 19 or a circular recess and these grooves communicate with each other by holes 20 parallel to the axis z zi. The cup
II-II passes through one of the gorges 19.

 Radial slots 21 are cut in the plates 13 and these slots protrude on either side of the hub 17, so that each slot 21 puts a groove 19 in communication with the central part of the enclosure 9 where it is located decanting liquid.

 The slots 21 cut in the trays occupy a fixed position which is located in the area where the decanted liquid is located.

 FIG. 3 represents a partial axial section on a larger scale of a hub 17 placed between two plates 13. We see on this section the grooves 19, the bores 20 and the slots 21 cut out in the plates 13.

 FIG. 2 represents a preferred embodiment of the blades 18 which have a curved or spiral shape the convex face of which is directed forward in the direction of rotation, so that the blades 18 sweep the flakes which deposit on the plates 13 by driving them towards the periphery and that the liquid which is located near the center of the enclosure is rid of flakes. Of course, the blades 18 can have other shapes. The height of the hubs 17 and of the blades 18 is slightly less than the spacing between the plates. The external diameter of the blades 18 is very slightly less than the internal diameter of the enclosure 9.

 It can be seen in FIG. 2 that the hub 17 is substantially tangent to the extension of the cylindrical wall # of the chamber 1, so that the blades 18 penetrate inside the chamber '1 in their rotating movement and are freed from the flakes which can adhere to the blades by agitation of the liquid.

 The liquid which is placed inside the enclosure 9, between the blades 18, is protected from agitation by the same blades which form a screen and the flakes of microorganisms can be deposited by sedimentation on the plates 13.

 It will be noted that the entry edges of the plates form a grid which stops the largest flakes and that the blades constitute a rotating screen. This grid effect is added to the decanting effect to separate the flakes from the liquid.

 The rotating movement of the blades is very slow, so that it does not create turbulence in the liquid. The blades sweep the flakes towards the periphery of the enclosure 9 and towards the tank 1. It forms in the center of. enclosure 9, a zone of liquid free of flakes and it is this liquid which flows through the cutouts 21, the grooves - t9 and the conduits 20, then through the pipe 12 and which flows through the outlet 6 with an hourly flow Wl, which is close to the hourly flow W.

 It can be seen in FIG. 1 that the outlet 6 communicates via an inclined conduit 22 with the top of the bio-reactor for the evacuation of the air and the gases which form in the chamber 1.

 The conduit 6 can be maintained under slight overpressure, for example by passing through a bubbler, in order to maintain in the bio-reactor a slight overpressure, of the order of a few tenths of a bar, to facilitate the evacuation of the flakes by the second exit 7.

 FIG. 1 represents a preferred embodiment of the second outlet 7 which starts from the tank 1 with an inclination upwards, so that when the valve 8 is closed, it forms, upstream of the valve, a trapped air bubble which prevents the formation in the duct 7 of a plug of flakes which could clog the latter.

 The valve 8 is periodically opened, which is controlled for example by a programmer, and each time there flows from the tank 1 a suspension of flakes in water which has the same composition as the suspension contained in the culture medium . The average hourly flow rate of outlet 7 is equal to W2 with W1 + W2 = W. The ratio -Df = W2 is the dilution rate of the microorganisms which flocculate.

 Before explaining how it works, we recall that a strain of microorganisms is characterized by its growth rate p, with B = Bo is, Bo being the biomass of this strain at time zero and B the biomass at time t.

it follows from this equation that the generation time gt which is the time necessary for doubling the population, is equal to L2
L2 L2 being the natural logarithm of 2 equal to 0.693.

 It is also recalled that #when the dilution rate of a culture medium is higher than the growth rate of a strain, the latter can no longer exponentially accelerate. Since the dispersible bacteria are evacuated by the two outlets, the dilution rate of these bacteria is equal to Wv.

 v.

The principle of the methods and apparatus according to the invention consists
choose flow rate W choose a flow rate W such that the dilution rate V is greater than the growth rate of the bacteria dispersible under the culture conditions. Thus, at a culture temperature of 200C, a flow rate W greater than V, that is to say a dilution rate greater than 1 / h, is sufficient to prevent the exponential proliferation of common dispersible bacteria.

On the other hand, the flocculating microorganisms> the sedimentation of which is rapid, are recycled into the culture chamber 1 and are evacuated by the second outlet with an hourly flow rate W2 which corresponds to a much lower dilution rate W2, so
V they can proliferate even if their growth rate is low.

 Thus, by choosing the flow rates W and W2, it is possible to arrive at a self-selection of the strains of microorganisms, which both assimilate the substrate and give rise to flakes and avoid the proliferation of all dispersible microorganisms which are practically impossible to recover economically.

 Among all the strains of flocculating microorganisms and using the substrate, there is always one whose growth rate is higher under the culture conditions. It is the one which assimilates the substrate the best and which therefore leads to the best purification. The biomass of this strain increases exponentially and would lead to clogging of the fermenter if a periodic evacuation of the flakes was not planned.

 Harvesting of the flakes is carried out periodically by regular sampling, so as to constantly maintain a certain density of the population of flocculating microorganisms in chamber 1.

The flakes are usually pseudo-spherical clusters whose diameter is of the order of a few millimeters and can reach ten mm. These flakes are suspended in a pure and clear liquid.

 The flake suspension could not be removed homogeneously by a pump because the crop flow rate is relatively low and this would cause the flakes to settle in the pipes. The evacuation is therefore done by opening the valve 8 for a sufficient time to evacuate a determined volume between the Ni and N2 levels. In order to obtain the evacuation of a relatively small and "nevertheless fairly precise liquid volume, the # top of the # fermenter preferably has the shape of a truncated cone extended upwards by a narrower cylindrical part. as shown in figure 1.

 Let B be the dry weight of recoverable biomass per unit of volume in chamber 1 and let S be the concentration per unit of volume in limiting substrate, that is to say in substrate which runs out first.

 The yield Y = x 100.

 Due to the agitation of the culture medium, the concentration B is the same in the liquid which leaves through the second outlet 7 as in chamber 1.

In stable continuous operating conditions, the biomass B produced per unit of time is equal to that which is discharged through the output 7. The input flow of the limiting substrate is equal WS The flow of evacuated biomass B.W2 is equal to the flow of biomass produced 100 hence B 100 WSY
IOOY.WS from where B = W2 Y varies very little and is around 30 t. This equation shows that the dry weight of recoverable biomass per unit of fermenter volume is proportional to the ratio
W which can be very high, which shows that one can operate W2 in rooms of reduced volume.

 The only upper limit of B is the risk of clogging of the device when the concentration prevents agitation of the liquid.

Laboratory tests have shown that it is possible to reach concentrations of the order of 15 g / liter in the fermenter. This equation shows that, m & with very dilute effluents it is possible to arrive at a significant biomass concentration.

In steady state, because the quantity of flakes in the tank remains constant, the growth rate U of the flakes is W2 equal to the dilution rate of these, that is to say p = and the
L2.VV generation time of the flakes g = W2
This equation shows that the methods according to the invention give the possibility of acting on the generation time which is
V proportional to the WZ ratio, which makes it possible, for a given medium, to modify the strains of microorganisms which proliferate and to select the one which metabolizes the substrate the best by flocculating and which leads to the best purification.

 When a homogeneous suspension of particles circulates over a horizontal surface, the number of particles which deposit on this surface, in a given time, is proportional to the cdncentration and the speed of fall in the liquid.

 It does not depend on the height of the liquid above the surface. It follows that, for a given type of flakes, the efficiency of the settling tank 9 depends only on the total surface of the trays located in the unstirred part and does not depend on the volume of the settling compartments. It is therefore advantageous to reduce as much as possible the spacing between plates which can be of the order of a centimeter, so that the plates constitute at the same time a grid which stops the largest flakes.

Tests were carried out in the laboratory at room temperature and at PH = 7 on culture media containing glucose at concentrations up to Ig / liter and with overall dilution rates equal to or greater than 1. h 1. able to obtain and maintain stable cultures of flakes with dilution rates of the order of 10.h 1. It has been observed that for ratios V
W2 which give generation times of less than 6 hours, which eliminates the mushrooms having a slow growth, we obtain flakes, from 1 to 2 mm in diameter, having under the microscope a granular structure V due to agglomerated bacteria. higher W2 ratios which correspond to generation times greater than 6 h, fibrous flakes of up to one centimeter in size are obtained which are composed of filamentous fungi.

 Other tests have been carried out with the sole carbon source of phenol which is known to constitute a substrate which inhibits the growth of many microorganisms. Provided that a concentration of 100 mg / liter is not exceeded, a culture giving rise to fibrous flakes was obtained and gave rise to continuous production. Above 100 mg / liter, the culture can remain stable if the concentration is gradually increased, but the residual concentration of phenol at the outlet then exceeds 100 mg / liter. It is possible to use several bio-reactors in series and to pass the treated effluent leaving through the overflow into a second bio-reactor in order to obtain a sufficiently purified treated effluent.

 The fact that phenol can be metabolized demonstrates the flexibility of adaptation of the process according to the invention to most organic compounds.

 Laboratory tests were carried out on a culture chamber having a volume V - 300 ml with an 18 cm platform surface 2 swept by a three-blade reel rotating at 12 revolutions / hour. This prototype has proven effective, on a glucose-based substrate, up to a flow rate of 900 ml / hour, which leads to an efficiency of 500 liters / h / m of plateau.

 As a quantified industrial example, a 27 m3 fermenter could treat, with an hourly dilution rate equal to 2, an effluent flow rate of 54 m3 / h and should be equipped with 48 trays of 2.10 m in diameter spaced 1 cm apart, which would occupy a total height of 0.5 m.

 Practical industrial applications are the biological purification of effluents discharged by the agro-food industries, (distilleries, canneries, sweets, paper mills, factories of glues, gelatin etc ...), sewage, wastewater and effluents liquids discharged by certain industries which may contain phenols or cyanides, for example water discharged from coking plants.

 In the case where the effluent to be treated contains fine particles in suspension which are not metabolizable, these are coated in the flakes and recovered with them.

 The embodiment shown in Figures 1 and 2 includes a settling chamber 9 punished with superimposed trays.

It is clear that this is a non-limiting embodiment.

 The originality of the methods according to the invention resides in the use of a fermenter which is equipped with two outlets, corresponding to two dilution rates and whose outlet of the purified liquid comprises means for retaining the flakes of microorganisms and to send them back to the culture chamber.

 Other means for separating equivalent flakes from the settling trays can be used. For example, the liquid can be decanted in a decantation chamber, the bottom of which comprises movable brushes mounted on two endless chains which repel the sediments and the flakes towards the culture chamber. It is also possible to use grids or sieves placed at the entrance to chamber 9 and provided with a cleaning system which returns to the culture chamber the flakes stopped by the grids or sieves.

 According to a variant, the dispersible bacteria can be discharged discontinuously in order to increase their dilution effect without changing either the volume V of the fermenter or the flow rate W of the effluent. This result can be obtained for example by connecting the outlet of the settling chamber 9 to an outlet in siphon è periodic priming, which makes it possible to periodically pass the volume V of the culture to "a much lower volume V '. same result can be obtained by using an outlet 6 in overflow or overflow at a mechanically variable level or a drain pump connected to the outlet of enclosure 9.

As the volume V ′ is clearly less than V, a dilution effect is obtained which is greater than that which corresponds to the dilution ratio D = W in the case of a continuous outlet. This variant
V is useful when, for any reason, a sufficiently high value of the dilution ratio D cannot be used.

 The foregoing description refers more particularly to the purification of effluents but it is specified that the methods and apparatus according to the invention can be used to produce biomass from any liquid substrate containing biodegradable organic products.

Claims (9)

R E V E N D I C A T I O N S  1. Process for the continuous biological purification of liquid effluents containing organic materials in a bio-re-actor containing an agitated and non-sterile culture medium, characterized in that  - Liquid is taken from said bio-reactor (1) at two levels;  - part of the liquid is continuously withdrawn through a side opening (10) and this liquid is passed through a separator (9) where the flakes of microorganisms suspended in this liquid are separated, these flakes are returned to the bio -reactor and the purified liquid which has been freed from the flakes and which mainly contains dispersible micro-organisms is discharged via a first outlet (6);  - And periodically evacuated from said bio-reactor by a second outlet (7), which is located at "a level below the maxisumF level, a liquid containing a high proportion of flakes of microorganisms which is recovered.  2. Method according to claim 1, characterized in that the liquid leaving by the first outlet (6) passes through settling chambers (9) in which the liquid is removed from "stirring and in the bottom of which micro flakes -organisms are deposited by sedimentation and the bottom of said chambers is slowly scanned to return said flakes to the bio-reactor.  3. Method according to any one of claims 1 and 2, characterized in that liquid effluent is admitted into the bioreactor with a continuous flow rate which corresponds to an overall dilution rate greater than the growth rate of dispersible microorganisms, so that these microorganisms cannot proliferate and an average hourly flow rate of the second outlet (7) is chosen which corresponds to a dilution rate which makes it possible to select strains of flocculating microorganisms which proliferate in the culture chamber.  4. Selective fermenter with # self-seeding intended for the biological purification of liquid effluents containing organic materials of the type comprising a culture chamber (!) Which receives a continuous flow of effluents which are cultivated in an agitated medium and non-sterile, characterized in that said culture chamber (1) has two liquid outlets, a first outlet (6) in overflow or in siphon, which is e-which- peed means (9) to separate the micro flakes -organisms and to return these to said culture chamber, and a second outlet (i) which is situated at a level (N2) lower than the maximum level (N1) of said overflow or siphon and which is equipped with a motorized valve ( 8) whose openings are ordered periodically.  5. Fermenter according to claim 4, characterized in that said first outlet comprises at least one settling chamber (9) which communicates through a side opening with an opening (10) located in the side wall of said culture chamber (1) , which enclosure is divided into compartments # superimposed by plates (13) and comprises, in each compartment, blades (18) which are rotated, at very slow speed, which sweep said plates (13) and which return into said culture chamber the flakes that settle. by sedimentation on said plates.  6. Fermenter according to claim 5, characterized in that said culture chamber (1) is cylindrical or cylindro-conical with vertical axis (x xl), that said settling chamber (9) is a portion of cylinder with vertical axis ( z zl) attached to said chamber and that said plates (13) are portions of discs which are cut by a circular line (14), centered on the axis (x xi) of the culture chamber (1) and extending the side wall of this one.  7. Fermenter according to any one of claims 5 and 6, characterized in that said blades (18) are mounted on a hub (17) which has grooves (19) and channels (20) parallel to the axis and which communicates with a drain pipe (12) which is connected to said overflow or siphon.  8. Fermenter according to any one of claims 6 and 7, characterized in that said blades (18) have a curved shape whose convex face is directed forward in the direction of rotation, have a slightly smaller external diameter to the internal diameter of said settling enclosure (9) and a height slightly less than the spacing between plates (13).  9. Fermenter according to claim 7, characterized in that each plate (13) has a radial slot (21) which communicates one of said grooves (19) with the interior of the # settling enclosure.