WO2018096296A1

WO2018096296A1 - System for filtering a solution comprising microalgae for the harvesting thereof

Info

Publication number: WO2018096296A1
Application number: PCT/FR2017/053253
Authority: WO
Inventors: Alexandre Yannick Robert BESSON
Original assignee: Sas Alg&You (Alg And You)
Priority date: 2016-11-25
Filing date: 2017-11-24
Publication date: 2018-05-31
Also published as: FR3059246A1

Abstract

System (12) for filtering a solution comprising microalgae, the system comprising: a vessel (22) with a substantially cylindrical internal cavity (24), the axis of revolution (A) of which is substantially vertical, the cavity being intended for receiving a solution to be filtered comprising microalgae; a filtering piston (28) mounted in said cavity (24) such that it slides in a direction substantially parallel to the axis of revolution (A), and a first cylindrical filtering wall (28) configured to retain the microalgae, said first wall being located at least in a lower part (22a) of the cavity (24) and being configured to be scraped by the piston (26) when the latter is moving, wherein the system is characterised in that it further comprises a second circular filtering wall (40) located at the lower end of the cavity and configured to retain the microalgae for the harvesting thereof.

Description

System of filtration of a solution containing microalgae for their harvest

TECHNICAL AREA

The present invention relates in particular to a filtration system of a solution comprising microalgae. It also relates to an assembly comprising such a system and a microalgae culture reactor as well as microalgae harvesting and rinsing processes.

STATE OF THE ART

The cultivation of microalgae has undergone significant development over the last thirty years. Initially used for animal nutrition in aquaculture, microalgae are now exploited in many sectors of activity such as energy, agri-food, cosmetics and pharmaceuticals. Their nutritional virtues and the low cost of their culture make microalgae, such as spirulina or chlorella, a food resource of interest for both human and animal food. According to the United Nations Food and Agriculture Organization (FAO), spirulina may even be involved in the fight against malnutrition.

The microalgae are generally produced in a culture reactor of the photobioreactor type, which comprises a medium that is suitable for culturing microalgae. In photo-autotrophic regime, the microalgae are subjected in the reactor to a light radiation which promotes their multiplication.

The microalgae are produced in suspension in a liquid solution and must be separated from their culture medium by filtration, with a view to their use and in particular their consumption.

The microalgae can be separated from their culture medium by simple filtration. However, the high concentrations of microalgae and their production of polysaccharides in the culture medium imply a rapid clogging of the filter during harvesting.

To combat this phenomenon and increase the harvesting capacity, it is possible to use a forced filtration system with a scraper piston. This type of filtration makes it possible to increase the available filtering surface and improves the harvest of the microalgae by scraping the filter with each passage of the piston.

The document WO-A1-2011 / 1155822 describes a filtration system of this type comprising a vessel in which a piston is able to move. The piston is surrounded by a cylindrical filter which is configured to retain the microalgae and let their culture medium pass. The microalgae are sequentially agglutinated at the lower end of the vessel by the piston and are recovered at this lower end by a specific port of the vessel. When the piston is actuated, filtration stops. The system described in this document therefore allows a discontinuous filtration including several filtration phases interrupted and separated from each other by piston displacement phases.

The present invention provides an improvement to this technology, which provides a simple, effective and economical solution to at least some of the disadvantages of the present art.

SUMMARY OF THE INVENTION

The invention proposes a filtration system for a solution comprising microalgae, the system comprising:

a vessel comprising a substantially cylindrical internal cavity whose axis of revolution is substantially vertical, this cavity being intended to receive a solution to be filtered comprising microalgae,

a filtration piston slidably mounted in said cavity, in a direction substantially parallel to said axis of revolution, and a first filtering cylindrical wall configured to retain the microalgae, said first wall being located at least in a lower part of said cavity and being configured to be scraped by the piston during its movements,

characterized in that it further comprises a second circular filter wall located at the lower end of the cavity and configured to retain and harvest the microalgae.

The piston is configured to scrape the first filter wall for regeneration. Indeed, the scraping of this wall makes it possible to remove the microalgae deposited by filtration on the wall and to move them to the bottom of the cavity, at the level of the second filtering wall. The presence of this second circular wall makes it possible to significantly improve the filtration of the solution by limiting the risk of clogging of the system. In addition to providing a filtration function, the circular wall has a function of harvesting microalgae, that is to say that it is intended to serve as a support microalgae recovery. The displacement of the piston causes agglutination and concentration of microalgae on the circular wall, which forms an algal paste. A succession of pumping allows for example to obtain a significant harvest of microalgae paste 20% dry matter.

Unlike the prior art, the system according to the invention can allow continuous filtration of the solution. Indeed, during movement of the piston, the second filter surface is unclogged, remains operational and can continue to perform its filtering function without interruption.

The system according to the invention may comprise one or more of the following features, taken separately from each other or in combination with each other:

the second filtering wall is removably mounted inside the cavity and / or at the lower end of the vessel; alternatively, this second wall could be integral with the first wall and form with the latter a bowl of microalgae harvest;

the second filtering wall is substantially flat;

the piston has a generally cylindrical shape and comprises a lower end of a shape substantially complementary to that of said second filtering wall;

- A scraper member is connected to the lower end of the piston so as to be driven by the latter during its movements, this scraper being configured to pre-scrape said first filter wall and having an outer diameter less than that of the piston;

at least one seal is mounted between the piston and the vessel;

the system comprises at least one solution supply port to be filtered and at least one port for discharging a permeate; in a mode of

- realization, these two ports could be integrated in the same organ T, that is to say, three channels (first and second channels define the power port and the first channel defines with a third channel the port of evacuation);

said supply port is located at the upper end of the tank or at the lower end of the piston;

- said discharge port is located at the lower end of the tank;

the piston delimits two chambers inside the tank respectively located above and below it, said supply port opening into the upper chamber which is in fluid communication with the lower chamber by at least one channel; extending in or along the piston;

- The piston is configured to be actuated manually or by an actuating means, for example electric.

The present invention also relates to an assembly comprising a filtration system of a solution comprising microalgae or a system as described above, a microalgae culture reactor, and means for fluid connection of the system to the reactor. Said fluidic connection means may comprise:

a first three-way valve including:

. a first channel is connected to a first port of the tank of the system,

. a second path is connected to a third port of the reactor, and. a third channel is configured to be connected to a suction or evacuation means,

a second three-way valve including:

. a first channel is connected to a second port of the tank of the system,

. a second path is connected to a fourth port of the reactor, and. a third path is configured to be connected to a source of rinsing fluid.

A non-return valve can be mounted:

between the second channel of the first valve and the third port of the reactor, and / or

- at the exit of the third channel of the first valve, and / or

between the second channel of the second valve and the fourth port of the reactor, and / or

- at the entrance of the third channel of the second valve.

A pressure sensor can be mounted:

between the first channel of the first valve and the first port of the system, and / or

- Between the first channel of the second valve and the second port of the system.

A filter may be mounted between the first channel of the second valve and the second port of the reactor.

The filtration system may comprise all or part of the characteristics of the system as described above. The present invention also relates to a method for harvesting microalgae for example from an assembly as described above. The method can include:

a step of supplying the tank of the system with a solution comprising microalgae contained in the reactor, said solution passing through the first and second channels of the first valve, and

a step of evacuating or recycling a permeate from the tank of the system to the reactor, said permeate passing through the first and second channels of the second valve.

The present invention finally relates to a method for rinsing microalgae. The method can include:

a step of supplying the tank of the system with a rinsing fluid, said fluid passing through the first and third channels of the second valve, and

a step of evacuating the rinsing fluid from the vat, said fluid passing through the first and third channels of the first valve.

DESCRIPTION OF THE FIGURES

The invention will be better understood and other details, characteristics and advantages of the invention will appear on reading the following description given by way of nonlimiting example and with reference to the appended drawings in which:

FIG. 1 is a schematic view of an assembly according to the invention comprising a microalgae culture reactor and a filtration system for a solution comprising microalgae,

FIG. 2 is a diagrammatic view on a larger scale of a lower part of the tank of the system of FIG. 1,

FIG. 3 is a schematic view of an alternative embodiment of an assembly according to the invention,

FIG. 4 is a schematic view of an alternative embodiment of a filtration system, FIG. 5 is a diagrammatic view on a larger scale of a lower part of the tank of the system of FIG. 4, during a method of using this system,

FIGS. 6 and 7 are diagrammatic perspective views of geometric shapes of the tank of the filtration system,

FIG. 8 is a schematic perspective view of an alternative embodiment of the filtration system,

FIG. 9 is a schematic view of an alternative embodiment of an assembly according to the invention and means for connecting the filtration system to the reactor, and

- Figures 10 and 11 are schematic views of alternative embodiments of an assembly according to the invention and connecting means of the filtration system to the reactor. DETAILED DESCRIPTION

Figure 1 shows an assembly comprising a microalgae culture reactor 10, in particular food, and a filtration system 12 for the separation of microalgae from their culture medium and their harvest. Microalgae may for example be spirulina and / or chlorella.

The reactor 10 is schematically represented by a rectangle and comprises for example a container 14 containing a solution 16 comprising the microalgae. This reactor can be equipped with a light source for the cultivation of microalgae in solution 16.

The reactor 10 comprises two ports, an output port 18 and an input port 20. These ports are for example located at a lower end of the reactor.

The filtration system 12 comprises:

a tank 22 comprising an internal cavity 24 intended to receive the solution 16 to be filtered,

a filtration piston 26 slidably mounted in the cavity, and - A cylindrical filter wall 28 configured to be scraped by the piston during its movements.

The tank 22 has a substantially cylindrical general shape whose axis of revolution A is substantially vertical. The tank 22 generally comprises two parts, lower 22a and upper 22b, respectively. In the example shown, the parts 22a, 22b preferably have the same height (or longitudinal dimension along the axis A).

The piston 26 is housed in the cavity 24 and can be moved therein in a direction substantially parallel to the axis A, that is to say from top to bottom, and from bottom to top. It is thus movable from an upper position, shown in Figure 1, to a lower position.

The piston 26 has for example a generally cylindrical shape.

In the example shown, the piston 26 preferably has a height close to half a height of the tank 22. In its upper position, the piston occupies substantially the entire internal volume of the upper part of the cavity and, in its lower position, it occupies substantially the entire internal volume of the lower part of the cavity.

Sealing means are preferably provided between the piston 26 and the tank 22. A first seal 29 is for example mounted between the outer periphery of the lower end of the piston and the inner cylindrical face of the tank 22, and another seal 30 is for example mounted between the outer periphery of the upper end of the piston and the inner cylindrical face of the vessel 22. The distance or height between the seals 29, 30 is preferably greater than the height of the wall 28.

The piston 26 is connected at its upper end to a rod 32 for the actuation of the piston, manually or by a suitable means for example electrical. The rod 32 here extends upwardly along the axis A, substantially from the middle of the piston. The tank 22 may comprise a lid, not shown, removable. This cover may comprise a central orifice for passage of the rod 32 of the piston 26.

The tank 22 may further comprise a removable bottom 34. The lid and the bottom 34 can be fixed to the rest of the tank by screwing, quarter turn, etc. Ideally, they must be manually disassembled without the aid of any particular tool.

The removable bottom 34 may for example comprise or define only the lower end of the tank. It may comprise a lower circular wall 36 of the vessel and possibly a cylindrical peripheral rim 37, extending upwardly from the wall 36, and comprising, for example, a thread for screwing the bottom onto the remainder of the vessel (see FIG. 2).

Alternatively, the bottom 34 could be formed by the lower part 22a of the tank, which could comprise at its upper end (for example in the zone Z shown in Figure 2) a screw thread on the upper part 22b of the tank .

Sealing means may be provided between the bottom 34 and the remainder of the tank 22.

The cylindrical filter wall 28 is housed inside the tank 22. It extends here over the entire height of the lower portion 22a of the cavity. Alternatively, it could extend over a greater height or a smaller height of the tank. The wall 28 is kept at a distance from the wall of the lower part 22a of the tank so that the microalgae culture medium can pass through the wall 28 and flow between it and the wall of the lower part, until bottom of the tank (arrows 38).

The system 12 further comprises a second filter wall 40, which is circular and is disposed at the lower end of the vessel. In the example shown, it lines the bottom 34 and is spaced from it so that the microalgae culture medium can pass through the wall 40 (arrows 41). The spaces between the filtering walls 28, 40 and the tank can be obtained by spacers extending between the wall of the tank and the filtering wall 28, and between the bottom 34 and the filtering wall 40. As a variant, simple oversizes of material 42 could be provided on the wall of the tank and the bottom 34, the filtering walls 28, 40 being intended to bear on the free ends of these extra thicknesses.

The filtering walls 28, 40 are configured to retain the microalgae. The filtering walls make it possible to separate the microalgae from their culture medium. To do this, the filtering walls 28, 40 preferably have a pore size of between 1 and 100μπι, more preferably between 1 and δθμΐτι, and for example between 10 and 40μητι with regard for example to the collection of Spirulina.

The filtering wall 40 here has a generally planar shape and has an outer diameter substantially equal to or slightly less than the internal diameter of the tank, and in particular of its lower part 22a. The lower end of the piston 26 preferably has a shape substantially complementary to that of the filtering wall 40, and is therefore flat in the example shown.

The tank 22 comprises two ports, respectively a supply port 44 and an evacuation port 46.

In the embodiment of FIGS. 1 and 2, the evacuation port

46 is located at the lower end of the tank 22, and opens into the aforementioned space extending between the filter walls 28, 40 and the tank. It is thus configured to collect the culture medium after filtration, called permeate (arrow 48 - Figure 2).

The supply port 44 is here located at a lower end of the piston 26. It opens into the chamber 50 delimited by the piston 26, between its lower end and the bottom of the tank.

A first connecting pipe 52 connects the inlet port 20 of the reactor 10 to the discharge port 46 of the system. This first conduit 52 is here equipped with a non-return valve 54 which allows the passage of fluid only from the system 12 to the reactor 10. A second connecting duct 56 connects the outlet port 18 of the reactor 10 to the supply port 44 of the system. This second duct is here equipped with a non-return valve 57 which allows the passage of fluid only from the reactor 10 to the system 12. This duct 56 is also equipped with a three-way valve 58 including a channel (forming an inlet or output) is connected to the output port 18, a channel (here forming input) is connected to the power port 44, and a last channel (here forming output) is intended to be connected to the atmosphere for air suction in end of harvest.

The connection of the conduits to the ports is advantageously carried out by means of quick couplings, for example with elastic snap-fastening.

All of Figures 1 and 2 operates as follows:

The microalgae are cultured in the reactor 10. At the beginning of the harvest cycle, the piston is in the low position. By moving it upwards, the piston creates a depression in the chamber 50, which causes the supply of the chamber with a volume of solution comprising microalgae from the reactor 10. When the piston back down, it forces the culture medium to filtered by the filtering walls 28, 40. The culture medium can not go back because of the non-return valve 57. The permeate passes through these walls and is reinjected via the conduit 52 into the reactor, and the microalgae are retained by the filtering walls. The descent of the piston further causes the scraping of the wall 28 by the piston, and the agglutination of microalgae deposited on this wall 28 towards the bottom of the tank and in particular on the wall 40. The scraping makes it possible to unclog the filtering wall cylindrical which will be available again for the next passage. The piston can be moved several times from top to bottom then from bottom to top to successively filter several volumes of solution and thus recover a maximum of microalgae grown in the reactor. The conduits 52 and 58 may be flexible and / or flexible to avoid deterioration during movement of the piston. The accumulation of microalgae on the wall 40 forms an algal paste called "cake". Once the desired filtered volume is reached, the valve 58 is tilted to the air suction position. The rise / fall sequence of the piston is repeated several times to dry the microalgae cake formed and allow total reinjection of the permeate into the reactor. The system 12 can then be disconnected from the reactor 10 and the microalgae cake recovered on the wall 40, for example by dismounting the bottom 34. Once the cake has been recovered, the system 12 can be rinsed by suction of clean water using the piston 26 .

Figure 3 shows an alternative embodiment of the assembly 10, 12 and the filtration system. This set includes the same characteristics as that described in the foregoing, insofar as these features are not in contradiction with the following.

The tank 22 comprises two ports, respectively a supply port 44 'and a discharge port 46. The discharge port 46 is similar to that described in the foregoing.

The supply port 44 'is here located at an upper end of the tank 22. The port 44' is connected to the valve 58 by a non-return valve 57. It opens into a chamber 60 delimited by the piston 26, between its upper end and the top of the tank. The piston 26 thus delimits two chambers, respectively upper 60 and lower 50. The two chambers 50, 60 communicate with each other by at least one channel 62 extending in the piston, passing through it, between the piston and the vessel, or at through a wall of the tank. This channel 62 is here equipped with a non-return valve 64 which allows the passage of fluid only from the chamber 60 to the chamber 50.

The first connecting pipe 52 is similar to that mentioned above. The second connecting duct 56 connects the outlet port 18 of the reactor 10 to the supply port 44 'of the system. This second duct 56 can be equipped with the non-return valve 57 which allows the passage of fluid only from the reactor 10 to the system 12. This duct 56 can also be equipped with a three-way valve 58, as in the previous case.

All of Figures 1 and 2 operates as follows:

At the beginning of the harvesting cycle, the system 12 is connected to the reactor 10 and the valve 58 is in the suction position in this tank. The piston is in the up position. During the descent of the piston, a volume of solution comprising microalgae contained in the reactor 10 is sucked into the upper part of the tank 22 thanks to the depression generated, that is to say in the chamber 60. When the piston goes up, the culture volume can not go back because of the non-return valve 57 and passes through the channel 62 in the chamber 50 to occupy the lower part of the tank. On the descent of the piston, unable to go up in the tank because of the non-return valve 64, the initial volume passes through the walls 28, 40. The microalgae are retained on the filter wall 28 and directly pushed and pressed by the piston on the lower filter wall 40. The permeate is simultaneously reinjected largely in the reactor 10. At the same time, a new culture volume is sucked into the upper part of the tank. The scraping makes it possible to unclog the wall 28 which will be available again for the next passage. During the new rise of the piston, the new volume passes into the lower chamber 50 to be filtered during the next descent which also allows the suction of a new volume of solution. The rise / fall cycle of the piston can be repeated several times to adjust the total volume of filtered culture. Note that the injection into the lower chamber 50 through the piston allows to limit the resuspension of the cake formed in previous passages and optimizes the harvest. Once the desired filtered volume is reached, the three-way valve 58 located upstream of the system 12 is tilted to the air suction position. The rise / fall sequence of the piston is repeated several times to dry the microalgae cake formed and allow the total reinjection of the permeate into the reactor 10. The system 12 can then be disconnected from the reactor and the cake of microalgae recovered on the removable wall 40. Once the cake is recovered, the system 12 can be rinsed by suction of clean water through the piston. Figures 4 and 5 show another alternative embodiment of the filtration system. This system 12 includes the same characteristics as the system of all of Figures 1 and 2 and further comprises the following features.

The piston 26 carries a scraper member 66 which is connected to its lower end, for example by spacers 68. The member 66 is thus integral with the piston 26 and intended to be driven by the latter during its movements. The spacers 68 axially separate the member 66 from the piston 26.

The member 66 may have a generally circular or cylindrical shape, and of small thickness or height. It preferably has an outer diameter smaller than the outer diameter of the piston. Thus, the member 66 is intended to perform a first scraping or pre-scraping of the wall 28, during the displacement of the piston, the piston ensuring it a second scraping of the wall just after the first scraping.

As is schematically represented in FIG. 5, it is understood that an inner peripheral portion 70a of the cylindrical deposit of microalgae on the wall 28 is scraped off and driven by the member 66 during its passage, and that the remainder namely a part external device 70b of the cylindrical deposit of microalgae on the wall 28 is scraped and driven by the piston during its passage. This makes it possible to preserve a filtering surface with little loss of load, which can facilitate the descent of the piston at the end of the race in particular. FIG. 5 also makes it possible to assess the microalgae cake on the wall 40. FIGS. 6 and 7 show possible geometrical shapes for the tank 22 of the filtration system 12 according to the invention. The tank 22 has For example, a capacity of between 200 ml and 10 l, preferably between 500 ml and 5 l, and for example of the order of 1 l. It may have a height greater than its diameter, as shown in FIG. 6, or a diameter higher than its height. It will be understood that these dimensions influence the filtering surfaces respectively of the walls 28, 40. The tank of FIG. 6 will have a filtering wall 40 of smaller surface area than that of the tank of FIG.

Figure 8 is a more concrete example of a filtration system 12 according to the invention. The system 12 comprises a horizontal platform 72 intended to be placed on a support such as a table or a work plan. A substantially vertical column 74 extends upwardly from the platform 72, this column carrying an arm 76 for supporting the tank 22. The tank may be detachably engaged in an orifice 76a of the arm 76. In the mounting position on the arm, the tank can be suspended and kept at a distance from the platform 72.

A lever 78 of elongated shape is articulated by one of its longitudinal ends to the upper end of the column 74. Its other longitudinal end is connected to a handle 80. The axis B of articulation of the lever 78 to the column 74 is substantially horizontal. The lever is further articulated, substantially in the middle, at the upper end of the rod 32 of the piston 26. The articulation axis C of the lever 78 to the rod 32 is substantially parallel to the axis B.

It is understood that a user can grasp the handle 80 and move the lever 78 up and down, and up and down, by rotation of the lever about the B axis, to move the piston up and down, and down at the top, inside the tank 22. The lever arm effect of this system reduces the effort provided by the user to move the piston and filter the microalgae solution. FIG. 9 illustrates an embodiment of the means for connecting a reactor 10 to a filtration system 12, which is for example a system of the aforementioned type and described with reference to the preceding figures.

References 52 and 56 designate the connection ducts from reactor 10 to system 12, already described in the foregoing. In addition to the check valve 54, the conduit 52 includes a pressure sensor 82 and a filter 84, such as an ultrafiltration device.

The conduit 52 further comprises a three-way valve 86 of which a channel (forming an outlet) is connected by the non-return valve 54 to the inlet port of the reactor 10, a channel (forming an inlet or an outlet) connected by the filter 84 and the sensor 82 at the port 20 of the system 12, and a last channel (inlet) is connected, preferably by another check valve 86, to a source 88 of rinsing fluid.

The duct 56 is similar to that described above and furthermore comprises, at the outlet of the last channel of the valve 58, a non-return valve 88 which thus connects this channel to the suction means 90. A pressure sensor 91 can be mounted between valve 58 and port 18.

A bypass duct 92 can connect the valves 58, 86.

The fluidic circuit shown in FIG. 9 allows both the filtration of a solution comprising microalgae by the system 12, and the rinsing of these microalgae and the system.

The arrows 94 represent the flow of the fluids during a filtration operation. As mentioned in the foregoing, the solution contained in the reactor 10 is conveyed via the conduit 52 into the system 12 for the purpose of separating the microalgae, which are deposited on the walls 28, 40, of the culture medium which is rerouted by the conduit 56 in the reactor 10.

During a rinsing operation, the fluids flow according to the arrows 98. The rinsing fluid is for example water that is sucked through the valve 86 and filtered by the filter 84 before entering the tank 22 of the system 12 for rinsing microalgae. The water is then discharged through the port 18 through the conduit 52 by suction through the valve 58. It is thus understood that the reactor 10 is not used during this operation and the valves 58, 86 can be interconnected. by the bypass duct 92, to avoid any passage of rinsing fluid in the reactor.

The assembly shown in FIG. 9 comprises a filtration system 12, a reactor 10, and means for fluidic connection of the system to the reactor. The system 12 is as described in the foregoing. Alternatively, it could be a filtration system of another type, that is to say by necessarily scraper piston.

FIGS. 10 and 11 illustrate alternative embodiments of the means for connecting a reactor 10 to a filtration system 12, which is for example a system of the aforementioned type and described with reference to FIGS. 1 to 8.

The variant embodiment of FIG. 10 differs from the embodiment of FIG. 9 essentially in that the reactor 10 is equipped with a single port 46 'serving for feeding and evacuation.

The three-way valve 86 has a path (forming an output) connected by the non-return valve 54 directly to the conduit 56, downstream of the port 46 'and upstream of the non-return valve 57. The valve 86 further comprises a channel ( forming an input or output) connected by the filter 84 and the sensor 82 to the port 20 of the system 12, and a last channel (forming an inlet) connected, preferably by another nonreturn valve 86, to a source 88 of rinsing fluid .

The variant embodiment of FIG. 11 represents a simplification of the connection means between the reactor 10 and the filtration system 2.

The conduit 56 includes the non-return valve 57 which is connected to the port 46 'of the reactor and the port 18 of the system. The conduit 52 includes the check valve 54 which is connected to port 46 'of the reactor and port 20 of the system. The variant of FIG. 11 allows the recycling of the culture medium. It does not include a filter 84 although it could understand one. Moreover, it does not allow a backwashing unlike previous embodiments.

Claims

1. System (12) for filtering a solution comprising microalgae, the system comprising:

a tank (22) comprising a substantially cylindrical internal cavity (24) whose axis of revolution (A) is substantially vertical, this cavity being intended to receive a solution to be filtered comprising microalgae,

a filtration piston (26) slidably mounted in said cavity (24), in a direction substantially parallel to said axis of revolution (A), and

a first filtering cylindrical wall (28) configured to retain the microalgae, said first wall being located at least in a lower part (22a) of said cavity (24) and being configured to be scraped by the piston (26) during its displacement

characterized in that it further comprises a second filtering circular wall (40) located at the lower end of the cavity and configured to retain and harvest microalgae.

2. System (12) according to the preceding claim, wherein the second filter wall (40) is removably mounted within the cavity (24) and / or at the lower end of the vessel (22).

3. System (12) according to one of the preceding claims, wherein the piston (26) has a generally cylindrical shape and comprises a lower end of substantially complementary shape of that of said second filter wall (40).

4. System (12) according to one of the preceding claims, wherein a scraper member (66) is connected to the lower end of the piston (26) so as to be driven by the latter during its movements, this scraper member being configured to pre-scrape said first filter wall (28) and having an outer diameter smaller than that of the piston.

5. System (12) according to one of the preceding claims, wherein it comprises at least one port (44, 44 ') for supplying solution to be filtered and at least one port (46) for discharging a permeate. .

6. System (12) according to one of the preceding claims, wherein the piston (26) defines two chambers (50, 60) inside the tank (22) respectively above and below it, said supply port (44 ') opening into the upper chamber (60) which is in fluid communication with the lower chamber (50) by at least one channel (62) extending in or along the piston (26). ).

7. An assembly comprising a system (12) according to one of the preceding claims, a reactor (10) of microalgae culture, and means for fluid connection of the system to the reactor, said fluidic coupling means comprising:

a first three-way valve (58) of which:

. a first channel is connected to a first port (18) of the tank (22) of the system (12),

. a second path is connected to a third port (44, 44 ') of the reactor (10), and

. a third channel is configured to be connected to a suction or evacuation means (90),

a second three-way valve (86) of which:

. a first channel is connected to a second port (20) of the tank of the system,

. a second path is connected to a fourth port (46) of the reactor, and

. a third path is configured to be connected to a source (88) of rinsing fluid.

8. Assembly according to the preceding claim, wherein a non-return valve (57, 88, 54, 86) is mounted:

between the second channel of the first valve (58) and the third port of the reactor (44, 44 '), and / or at the output of the third channel of the first valve (58), and / or

between the second channel of the second valve (86) and the fourth port (46) of the reactor (10), and / or

at the input of the third channel of the second valve (86).

9. A method for harvesting microalgae from an assembly according to claim 7 or 8, characterized in that it comprises:

a step of feeding the tank (22) of the system (12) with a solution comprising microalgae contained in the reactor (10), said solution passing through the first and second channels of the first valve (58), and

a step of evacuating or recycling a permeate from the tank of the system to the reactor, said permeate passing through the first and second channels of the second valve (86).

10. Process for rinsing microalgae from an assembly according to claim 7 or 8, characterized in that it comprises:

a step of feeding the tank (22) of the system (12) with a rinsing fluid, said fluid passing through the first and third channels of the second valve (86), and

- A step of evacuating the rinsing fluid from the tank, said fluid passing through the first and third channels of the first valve (58).