WO2019025670A1

WO2019025670A1 - System for harvesting microalgae and microalga exudates from culture water loaded with same, comprising tangential flow on a membrane filter

Info

Publication number: WO2019025670A1
Application number: PCT/FR2018/000203
Authority: WO
Inventors: René-Jean GUILLARD
Original assignee: Skyworld International Overseas Limited
Priority date: 2017-08-03
Filing date: 2018-08-03
Publication date: 2019-02-07
Also published as: FR3069785A1

Abstract

The present invention relates to a system for harvesting microalgae from culture water loaded with same, said system comprising at least one sub-assembly (12) having a plate (4) for distributing the loaded water, provided with a loaded water tangential flow channel (20), a first membrane filter (2A) filtering the water by retaining the microalgae, a plate (6) for the flow of the filtered water, having at least one filtered water flow channel (38), and a second membrane filter (2B), wherein the assembly consisting of the plate (4) and of the first membrane (2A) has a feed inlet (22) at one end of the channel (20) and a collecting outlet (24) at the other end of the channel (20). The system is characterised in that it comprises a means for blowing a pressurised air stream in order to clean the surface of each membrane (2A), and in that the channel (38) is disposed substantially opposite the channel (20).

Description

MICRO-ALGAE HARVESTING SYSTEM AND MICROALGAE EXUDATES IN CHARGED CULTURE WATER, COMPRISING A

TANGENTIAL CIRCULATION ON A FILTERING MEMBRANE TECHNICAL FIELD OF THE INVENTION

The present invention relates to a system for harvesting micro-algae contained in a charged culture water, as well as a method of harvesting these micro-algae. It also relates to a harvesting system and a process for harvesting exudates of microalgae which, when they are placed in very particular situations, in particular stress, have been released into the environment. Moreover, in this respect, the applicants of the present patent application file the same day a patent application which sets out the means they propose to precisely put certain microalgae in these very particular situations so that then they secrete a large quantity of exudates into the medium containing the microalgae. It is then the medium thus loaded which is treated in the aforementioned harvesting system, and this in order to simultaneously allow the recovery of exudates which are found to have been released into the medium.

BACKGROUND OF THE INVENTION

The cultivation of micro-algae is an activity more and more widespread around the world.

In this respect, it is known to produce so-called hydroponic, that is to say, above-ground, cultures. selected micro-algae that develop in transparent tubes called photo-bioreactors or "PBR" by abbreviation, these tubes receiving a liquid solution containing NPK nutrients as well as carbon dioxide dissolved in this liquid solution to synthesize fast biomass. Advantageously, the transparent tubes are superimposed in greenhouses so as to form batteries of rows arranged parallel to one another. The water contained in the culture tubes is kept at temperature by the greenhouse effect, and by an exchanger fed by a hot water circuit.

For these crops above ground, it is possible in particular to use organic residues from a methanization unit of agricultural waste or livestock effluents. The gas produced by this methanation unit can also be used to supply a cogeneration unit that delivers the heat and electricity required for the crops.

It is also known to cultivate micro-algae in open ponds, or closed under greenhouse, according to a mode of culture called "raceway".

The varieties of microalgae chosen have a rapid growth, which can give a doubling of biomass in a day, and they are also rich in oils and valuable elements. Very high yields per hectare are obtained, which can be more than a hundred times higher than the yields of plants grown in the soil.

The production of micro-algae is processed in a concentration and packaging station, to be delivered in liquid phase in drums. This produces a product presented in limited volumes and masses, which reduces at the same time: transport costs, energy consumption and pollution.

The type of microalgae is selected and the parameters of the hydroponic culture are adjusted to obtain specific growths rich in desired elements according to the applications, intended in particular for the pharmaceutical, cosmetic and food markets.

For example, concentrated micro-algae are used in particular in the fields of: cosmetics, with lipid extracts, protein extracts and so-called molecules anti-aging ; from the pharmacy, with omega 3/6; food, with human or animal nutritional supplements; from industry, with bio-plastics and dyes; and energy with oil extractions to replace fossil fuels or biomass methaniza- tion.

However, the harvesting of such a mature biomass, ie the separation of micro-algae from their culture medium, poses different technical and financial problems.

Indeed, the size of the cells can be very small, usually a few microns, and the structure of these cells is extremely fragile, which makes their harvest difficult. In addition, since the concentration of biomass in the culture water is generally very low, it is necessary to treat a large amount of water to, in fact, harvest only a few grams of biomass. Thus, for this meager harvest, it is nevertheless still necessary to recycle in parallel a significant amount of water, which implies significant costs.

Various water filtration systems for harvesting are known. However, these systems are still derived from processes used for other activities, and therefore require compromise. They are often poorly adapted. In addition, they can consume a large amount of energy to maintain the flow of water, which increases the costs of harvesting.

For example, the so-called reverse osmosis technique is not very satisfactory because, since, by transfer through a semi-selective membrane, it is necessary to separate under the effect of a pressure gradient a liquid product called permeate. , which passes through the filter membrane, and a solid product said concentrate or retentate, which does not pass through the membrane, it is understood that, in this case, the biomass, which is structurally very fragile, would be deteriorated. In addition, the clogging of the pores of the filters is often irreversible, thus requiring the rapid change of the latter.

Among the charged liquid fluid filtration systems indicated above, there is more particularly known, for example from US Pat. No. 3,560,377, a system for harvesting macromolecules by ultrafiltration in a spiral path. By extrapolation with the invention, it is possible to consider that the system in question comprises a filter membrane having porosities water filter loaded in holding micro-algae, a water distribution plate charge which ^is applied against the face of the filtering membrane situated on the side of the arrival of the charged water and which has at least one tangential circulation channel of charged water, and a circulation plate of the filtered water which is applied against the opposite face of the filter membrane and which comprises at least one filtered water circulation channel, the harvesting system comprising at least one subassembly, successively comprising a charged water distribution plate, a first filter membrane, a filter plate, flow of the filtered water and a second filter membrane which is applied against the face of the filtered water circulation plate which is located on the side opposite to that of the In the first diaphragm, the assembly consisting of the distribution plate and the first filter membrane has a feed inlet at one end of the channel or tangential circulation channels of charged water and a collection outlet at Another end of the channel or channels of charged water, wherein the or each subassembly has bores passing through the assembly and including a filled water supply bore and a filtered water outlet bore.

However, in this system, the filtered water circulation channel of the filtered water circulation plate consists of several concentric circular channels, some of which are connected by a radial channel. This system has the disadvantage that, because of the arrangement of the latter channel, the filtration efficiency through the membrane is not optimal.

Moreover, it is in no case provided the introduction of air pulses. The declogging possibly necessary is carried out by reverse scanning.

The aforementioned document relates to the filtration of very diverse fluids, in particular macromolecular solutions, without aiming more particularly at the culture of microalgae.

It is also known, for example by the patent application

US 201 / 0,238,235, a filtration system of the same type, but in which, even less effectively concerning the filtration through the membrane, both the tangential circulation channel of water loaded with the plate of The distribution and the filtered water circulation channel of the circulation plate are channels extending rectilinearly between their inlet and outlet.

This document mentions in particular the culture filtration of micro-algae.

Note that it is mentioned in this document the introduction of air pulses in the distribution channel of the distribution plate, but this is only expected during a test performed to fluctuate so the pressure in this channel for verification purposes, while it is also stated that the normal pressure is stable without introduction of air.

The declogging possibly necessary is therefore carried out by reverse scanning.

It is also known, for example from the publication of the German patent application DE 37.31.864, a system of the aforementioned type, but in which there is no provision for a subassembly comprising a second filter membrane, so that the filtration is even less effective than in the two cases mentioned above. It's just a test system, it's about a cell culture in general and no means of declogging is provided. The distribution plate channel has a general shape with a change of direction to reduce the wear of the membrane.

Finally, there is known, for example from British patent application GB 2,360,958, another harvesting system which does not have either a subassembly comprising a second membrane. Although having "meanders" to reduce clogging, the system is part of a very different harvesting arrangement.

GENERAL DESCRIPTION OF THE INVENTION

The present invention is intended in particular to avoid the various disadvantages mentioned above in connection with the prior art and it is more particularly aimed at improving the efficiency of membrane filtration.

It proposes for this purpose a harvesting system of the type taught in part by US Pat. No. 3,560,377, said system being characterized in that it further comprises means for blowing a flow of air under pressure through the channel of circulating filtered water from each filtered water collecting plate, through the filter membrane, in order to detach the micro-algae and to decolour the surface of each filter membrane on which said micro-algae have accumulated, in that at least one tangential circulation channel of charged water and at least the filtered water circulation channel which faces it each form a spiral, and in that the channel (s) for circulating filtered water are substantially arranged in front of the channel or, respectively, in front of the tangential flow channel or channels of charged water.

An advantage of this harvesting system is that the tangential flow channel or channels impose on the charged water a circulation which is tangent on the surface of the membrane. filter and which is carried out with a certain speed depending on the flow and the section of these channels, which has the effect of carrying micro-algae by distributing them along the channels, that is to say avoiding a local accumulation.

This results in a substantially constant removal of micro ^¬ algae along the length of the channels, which therefore limits the local obstruction of the filter membrane and gives a good flow through this membrane allowing to use with the best performance the all of its surface and to space the pick-ups in the harvesting system. Energy consumption is reduced as well as production costs.

In addition, since the system according to the invention operates under very low pressure, the biomass can be harvested without risk of deterioration of the cells.

Although the invention applies both to the harvest of microalgae and that of exudates of micro-algae, these two applications, throughout the remainder of the description as well as in the claims, will be in a concern. simplification under the term "micro-algae harvest". By "charged water," one will understand indifferently water loaded with micro-algae or water loaded with exudates of micro-algae.

The harvesting system according to the invention may additionally comprise one or more of the following characteristics, which may be combined with each other.

According to a preferred construction, the or each subassembly has bores passing through each subassembly and comprising a piercing of charged water supply practiced in isolation in each filtering membrane and in each circulation plate of the filtered water opposite from the supply inlet of the channel of each distribution plate, a microalgae harvest piercing connected to the collection outlet of the channel. each distribution plate and the hole corresponding thereto pierced opposite in the filter membrane glued under said plate and in each water circulation plate filtered, a filtered water bore connected to the collection outlet of each filtered water circulation plate and the corresponding orifice pierced opposite in the filter membrane located under said filtered water circulation plate and a through hole for injecting compressed air pierced in the filtered water circulation plate.

Advantageously, the upstream or inlet end of the filtered water circulation channel of the filtered water circulation plate is located close to but slightly away from the projection, on said filtered water circulation plate. , the supply of the distribution plate and that the other end of the filtered water circulation channel opens into an outlet which is distinct from the projection, on said filtered water circulation plate, of the collection exit.

According to a preferred embodiment, the first filter membrane is glued under the distribution plate.

According to another preferred construction, the pressurized air line crosses the subset from bottom to top, forming a succession of bends at the level of the filtered water collecting plates, the pressurized air duct opening into the filtered water circulation channel cut from each filtered water collection plate and, conversely, does not open into any charged water circulation channel cut in each distribution plate.

More preferably, at least one face of each filter membrane has a coating of a material having an extremely low coefficient of friction, the face thus coated being the upper face with respect to the filtering membrane bonded to the distribution plate and the underside for the filter membrane glued to the circulation plate of the filtered water.

Still advantageously, the arrival of charged water is connected to a water column having at its top a float. In this construction, the harvesting system comprises a return of a small portion of filtered water at the top of the column.

More preferably, the harvesting system according to the invention comprises means for treating the filtered water with ultraviolet rays.

According to yet another particularly advantageous embodiment, each membrane has a face having an extremely low coefficient of friction, such as polytetrafluoroethylene, and a rough face consisting of polyvinylidene fluoride.

The second object of the invention is a process for harvesting micro-algae and micro-algae exudates contained in charged culture water, said method using a filter membrane having porosities filtering the water loaded with water. retaining said micro-algae, said method being characterized in that it consists in circulating tangentially the water charged on the upper face of this filter membrane located on the side of the arrival of this charged water, in at least one channel of circulation, and to evacuate the collection of micro-algae through a collection outlet located at the other end of said circulation channel.

When the harvesting method according to the invention uses a filter membrane having pores filtering the charged water retaining these microalgae, at least one circulation channel, a charged water inlet located at one end of the circulation channel, a crop outlet located at the other end of the circulation channel and a filtered water outlet, such a method is remarkable in that it comprises a harvest cycle of circulating tangentially the water loaded on the the filter membrane located on the side of the charged water inlet, in the circulation channel, by closing the crop outlet and by opening the filtered water outlet, to automatically detect a rise in pressure of charged water due to a progressive clogging of the filtering membrane, to be closed the filtered water outlet and open the crop outlet, to inject compressed air from the filtered water outlet to the loaded water inlet so as to detach the micro-algae deposited and to return to the beginning cycle by closing the crop outlet and opening the filtered water outlet.

Preferably, at the same time as the injection of compressed air, a pure water flow rate is controlled, which flow of pure water causes, by tangential effect, the micro-algae detached from the channel thus providing a concentrate at the crop outlet. ..

Also preferably, the injection of compressed air is at very low pressure.

More preferably, the harvest cycle takes place under very low pressure. BRIEF DESCRIPTION OF THE DRAWINGS

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

- Figure 1 is an exploded view of a stack of a harvest system according to the invention, having a single subset of membranes and distribution and circulation plates disposed between two clamping plates;

FIGS. 2 and 3 are front views of the circulation plates of this stack, namely a distribution plate and a permeate circulation plate for, respectively, the charged water and the filtered water, comprising circulation each forming a spiral;

FIG. 4 is a front view of each filter membrane of this stack; and

- Figure 5 is a view of a harvest system forming a complete installation. DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

FIG. 1 illustrates, for the harvesting system according to the invention, a subassembly 12 comprising a stack of rectangular flat panels, having a first filter membrane 2 receiving, glued above, a plate 4 for distributing the water loaded microalgae and, glued below, a plate 6 for collecting the filtered water. The subassembly 12 also includes a second filter membrane 2, which is glued below the filtered water collection plate 6.

The complete stack of the harvesting system according to the invention comprises a succession of such superposed identical subassemblies 12, said superposition of subassemblies 12 being clamped between two thick and rigid end clamping plates 8.

Each clamping plate 8 comprises, in the vicinity of its periphery, a series of bores 10 fitted on corresponding bores made in the filtering membranes 2 and in the distribution and collecting plates 6, which bores 10 receive clamping screws passing through the together. The thick clamping plates distribute the pressure of the clamping screws evenly over the entire surface of the various components of the stack.

FIG. 2 shows a view from above of the plate 4 for dispensing charged water, which plate is cut in its thickness so as to produce a channel of constant width 20 drawn in a spiral having an external beginning or end 22 forming an inlet of charged water, disposed near an angle of said plate 4, then five turns tapering progressively towards the center of this plate and ending with an end or inner end 24 constituting a harvesting outlet concentrated microalgae schematically by the arrow 28. The free part in the center of the spiral formed by the channel 20 further comprises a circular bore 26 independent of this channel, this drilling being intended to allow the vertical passage of the filtered water which is shown schematically in Figure 1 by the arrow SI .

Figure 3 shows in top view the filtered water collection plate 6, which is similarly cut in its thickness so as to achieve a similar channel 38 for collecting filtered water forming the same spiral.

The beginning or outer end 30 of this collection channel

38 is slightly offset from that 22 of the distribution plate 4, a circular bore 32, adjacent but totally independent of this channel 38, intended to allow the vertical passage of the charged water, being on the other hand aligned with the outer end 22 of the channel 20 of said distribution plate.

The inner end 34 of the channel 38 is also slightly offset from that 24 of the distribution plate 4, and it is very precisely aligned with the circular bore 26 of said distribution plate to allow in turn the vertical passage SI of filtered water.

A circular bore 36, independent of the channel 38, is aligned with the inner end 24 constituting the crop outlet of the concentrated microalgae of the distribution plate 4, to form a means for the vertical passage of harvest.

On these two plates 4 and 6, the holes 10 of the clamping screws are arranged on the outer contour of these plates, as well as in their center and in the vicinity of this center, so as to obtain the best distribution of the clamping pressure. while avoiding crossing the channels 20, 38.

FIG. 4 shows a top view of the filtering membrane

2, which comprises three circular holes aligned with those of the distribution and collection plates 4 and 6, namely a first external bore 32 intended to allow the vertical passage of the charged water, a second internal bore 26 intended to allow the vertical passage of the filtered water and a third internal bore 36 for allowing the vertical passage of harvest.

The filter membrane 2 is formed of a porous material which retains particles whose size is greater than the diameters of its porosities in the channel 20 of the distribution plate 4, which size may be for example of the order of one micron, porous material which instead allows the filtered liquid to pass on the other side to the channel 38 of the filtered water collection plate 6.

The two distribution plates 4 and collection 6 are bonded to the porous membrane 2Ά, on both sides of it. Advantageously, the surface of the porous membrane 2Δ which faces the channel 20 of the distribution plate 4, that is to say the upper face, is made of a non-stick material such as "Teflon", in order to to obtain the best possible slip of the microalgae harvested on this upper surface.

A variable number of modular subassemblies 12 are arranged between the two clamping plates 8, thus forming for each subassembly a filtration circuit arranged in parallel.

Advantageously, as shown in FIG. 1, each subassembly 12 will comprise a second porous membrane 2B identical to the first, this second membrane being bonded under the collection plate 6. However, in this variant with two membranes , 2B which is glued under the collection plate 6 will be by its side does not include the release material.

In other words, in this construction, it is the roughest faces of the two membranes 2 which will be arranged facing each other, framing the collecting plate 6.

The upper clamping plate 8 comprises a common feed of charged water E which is aligned with the vertical feed bores 22 and 32, and which therefore feeds all the distribution plates 4 and passes only through all the collecting plates 6 and the filtering membranes 2. The lower clamping plate 8 has a common harvesting outlet S2 aligned with the vertical harvesting holes 24 and 36, which is fed by all, the distribution plates 4, and a common filtered water outlet SI which is aligned with the vertical bores of filtered water 26 and 34, and which is therefore fed by all the collection plates 6.

Finally, for a specific purpose which will be explained below, the water distribution plate 4 loaded with microalgae, the plate 6 for collecting the filtered water and the membrane 2, or both membranes 2 of each subassembly 12 is pierced in the vicinity of one of their angles with a through hole, respectively 27 for the plate 4, 37 for the plate 6 and 47 for each membrane 2. The holes 27 and 37 are substantially aligned on the entire height of the harvesting system, to the extent that the hole 37 passing through the plate 6 is tangent, or even opens slightly into the channel 38 while the hole 27 passing through the plate 4 is slightly offset towards the center of this plate so as to However, the holes 27 and 47 which pass through the plates 4 and respectively the membranes 2 are perfectly aligned.

By choosing the number of modular sub-assemblies 12, it is thus possible to produce a filtration circuit comprising a variable filter surface and a harvesting capacity which is thus easily adjustable.

The operation of the harvesting system described above is as follows.

The micro-algae-laden water is circulated in the stack of the subassemblies 12, from the upper subassembly, the crop outlet S2 being closed and the filtered water outlet SI being however open. All the channels 20 of the distribution plates 4 have identical flow rates, thanks to the parallel arrangement of all these plates and a construction carried out identically between all these plates 4.

With the charged water advancing in each channel 20 of distribution plate 4, a flow of water filtered through the filtering membranes 2 placed under each plate 4 and simultaneously a deposit of the micro-algae is obtained along these channels. on the surfaces of these membranes.

There is then gradually a clogging of the surfaces of the filtering membranes 2 which results in a reduction of the flow rate and a rise in pressure of the charged water. This rise in pressure is detected automatically, to trigger a cycle of harvesting biomass.

The biomass harvesting cycle is done by closing the filtered water outlet SI, by opening the crop outlet S2, and injecting compressed air at very low pressure, for example 50 gr / cm ² , into the through hole 47 of the membrane 2B (or in the through hole 37 of the collection plate 6) of the subset 12 of the lowest harvesting system. A passage of micro-bubbles of air is obtained through all the filtering membranes 2, in the opposite direction of the passage of the filtered water, that is to say from bottom to top, by circulating first in the channel 38 then in the channel 20, and these micro-bubbles repel and thus detach the micro-algae deposited on the walls of the channels 20 of the filter membranes 2 which are glued under the distribution plates 4.

Thus, at each plate 4, the microalgae have been detached from the filtering membranes 2 and are in suspension in the space between the two teflon surfaces facing each other on the said membranes 2 which surround the membrane. plate 4 concerned.

By simultaneously controlling for a short time a flow of pure water in the distribution plates 4, which flow of water then leads by a tangential effect all the microalgae that have been detached from the walls of the channels 20, and thus obtaining at harvest outlet S2 a concentrate comprising in a suspension a high density of micro-algae which are found in perfect condition since the treatment they have undergone has been constantly mild, and still under very low pressure.

An exemplary embodiment of the micro system ^¬ harvesting algae described above is to use plates 4 and 6 which are made of metal or plastics, for example Plexiglas or polypropylene.

The filter membranes may be of the type marketed under the name ML-2F by the Chinese company Minglie, which is headquartered in Shanghai. The face having an extremely low coefficient of friction is composed of polytetrafluoroethylene (PTFE) and the rough side is made of polyvinylidene fluoride (PVDF).

The porosity of such membranes can be chosen in a range from 0.05 μ to 1 μ.

FIG. 5 represents an example of an installation making it possible to implement the invention, this installation comprising a chassis 40 forming a cage made of aluminum profiles, placed on rollers 42 and having above four lifting rings 44 .

The stack of subassemblies of panels 12 arranged between the two clamping plates 8 is fixed inside the frame 40, on a tank 52 for recovering the filtered water which oozes outside this stack, in particular by the edges lateral filter membranes after crossing their thicknesses.

The chassis 40 comprises a mast 48 supporting a water column 46 connected to the inlet of charged water coming from the photo-bioreactors, comprising an equivalent level of water, column which is equipped at its top with a float 50 framed by two upper and lower level witnesses. By this water column 46 forming an expansion vessel, it firstly obtains a protection against any excess pressure that may damage the filter membranes 2.

When starting the flow of water loaded into the harvesting system, just after a harvest, the filter membranes 2 are not clogged, we obtain the maximum water flow which causes a loss of load and a drop in the supply pressure. The float 50 is then at its low level.

Depending on the progress of the harvest, the filter membranes 2 become clogged, the flow of charged water decreases and its pressure increases slightly. It then causes a rise of the float 50 which, arriving at the upper control, indicates that the clogging is sufficient to carry out the harvest of micro-algae. We then launch a harvest cycle.

Furthermore, a small portion of the filtered water is continuously returned to the top of the column 46, to continuously repel the rise of water loaded into the column without modifying its level, and therefore its operating principle. Filtered water is thus preserved in the column which avoids stagnation of charged water and soiling of the transparent walls.

Filtered water is continuously returned to the photo bioreactors for recycling and limiting outside water consumption. Advantageously, ultraviolet treatment of the stream of filtered water is carried out at the outlet of the harvesting system, which, with filtration by the membranes, maintains an acceptable sanitary level of the culture medium by reducing the concentration of bacteria or micro-organisms. -organisms that can colonize the fluid circuits.

This results in a simple and efficient harvesting system that requires little maintenance, is efficient and consumes little energy.

It goes without saying that the invention is not limited to the only preferred embodiments described above. On the contrary, it embraces all the possible variants of embodiment, insofar as the latter do not go beyond the scope delimited by the appended claims which define the scope of the present invention.

Claims

1 - microalgae harvesting system and micro algae exudates ^¬ content (s) in a charged culture water, said system comprising a filter membrane (2A) having filter pores contaminated water retaining these micro algae, a plate (4) for dispensing the charged water which is applied against the face of the filtering membrane (2A) located on the side of the arrival of the charged water (22) and which has at least one channel (20) tangential circulation of charged water, and a plate (6) for circulating the filtered water which is applied against the opposite face of the filter membrane (2A) and which comprises at least one circulation channel (38). of filtered water, the harvesting system comprising at least one subassembly (12), successively comprising a plate (4) for distributing the charged water, a first filtering membrane (2A), a circulation plate (6). filtered water and a second filter membrane (2B) which is applied against the face of the filtered water circulation plate (6) which is situated on the side opposite to that of the first membrane (2A), the assembly consisting of the distribution plate (4) and the first filter membrane ( 2A) has a supply inlet (22) at one end of the channel (20) or tangential circulation channels of charged water and a collection outlet (24) at the other end of the channel (20). ) or of these channels of charged water, the or each subassembly (12) having bores passing through said subassembly (12) and comprising a piercing hole (22, 32) charged with water, a piercing (24) , 36) microalgae harvesting and a filtered water outlet bore (26, 34)

characterized in that it further comprises means for blowing a stream of pressurized air through the filtered water circulation channel (38) of each filtered water collecting plate (6) through the filtering membrane (2A), in order to take off the micro-algae and to unclog the surface of each filter membrane (2A) on which said micro-algae have accumulated, in that at least one channel (20) of tangential circulation of charged water and at least the channel (38) of filtered water circulation which makes it face each form a spiral, and in that the channel or channels (38) of filtered water circulation are substantially arranged in front of the channel (20) or, respectively, in front of the channel or channels (20) of tangential flow of charged water. 2 - harvesting system according to claim 1, characterized in that the or each subassembly (12) has bores passing through each subassembly and comprising a filled water supply bore (32) made in isolation in each filter membrane (2) and in each plate (6) for circulating the filtered water opposite the supply inlet (22) of the channel (20) of each distribution plate (4), a crop piercing ( 36) of micro-algae connected to the collection outlet (24) of the channel (20) of each distribution plate (4) and to the orifice (36) which corresponds thereto pierced opposite in the adhered filtering membrane (2A) under said plate (4) and in each filtered water circulation plate (6), a filtered water bore (26) connected to the collection outlet (34) of each circulation plate (6) of the filtered water and the orifice (26) corresponding thereto pierced opposite in the filtering membrane (2B) located under said plate (6) d e circulation of the filtered water and a through hole of compressed air injection (37) pierced in the filtered water circulation plate (6).

3 - Harvesting system according to any one of claims 1 and 2, characterized in that the upstream end or inlet (30) of the channel (38) filtered water circulation of the plate (6) circulation of the filtered water is located nearby but slightly set back from the projection, on said filtered water circulation plate (6), from the feed 22 of the distribution plate (4) and in that the other end of the filtered water circulation channel (38) opens into an outlet (34) which is distinct from the projection (36) on said circulation plate (6) filtered water, from the collection outlet (24).

4 - Harvesting system according to any one of claims 1 to 3, characterized in that the first filter membrane (2A) is glued under the distribution plate (4).

5 - harvesting system according to any one of claims 1 to 4, characterized in that the pressurized air duct passes from below upwards the subassembly, forming a succession of bends (37) at the plates for collecting the filtered water (6), the pressurized air duct opening into the channel (38) for circulating filtered water cut from each plate (6) for collecting the filtered water and, at reverse, not opening into any channel (20) charged water circulation cut in each plate (4) distribution.

6 - harvesting system according to any one of claims 1 to 5, characterized in that at least one face of each filter membrane (2) has a coating of a material having an extremely low coefficient of friction, the face thus coated being the upper face with respect to the filter membrane (2A) adhered to the distribution plate (4) and the lower face with respect to the filter membrane (2B) adhered to the circulation plate (6) of the filtered water. 7 - harvesting system according to any one of claims 1 to 6, characterized in that the inflow of charged water (E) is connected to a water column (46) having at its top a float (50) .