WO2011013104A1

WO2011013104A1 - Low-cost photobioreactor for microalgae cultivation

Info

Publication number: WO2011013104A1
Application number: PCT/IB2010/053480
Authority: WO
Inventors: Mario Tredici; Liliana Rodolfi; Giacomo Sampietro; Niccolò BASSI
Original assignee: Fotosintetica & Microbiologica S.R.L.
Priority date: 2009-07-30
Filing date: 2010-07-30
Publication date: 2011-02-03
Also published as: BR112012002121A2; ITFI20090167A1; EP2459695A1; CL2012000210A1; IT1398499B1

Abstract

A reactor is described for the culture of photosynthetic microorganisms or plant cells comprising a culture chamber and a supporting structure made of a base and vertical uprights in which said vertical uprights are inserted in the said base.

Description

LOW-COST PHOTOBIOREACTOR FOR MICROALGAE CULTIVATION

Field of the invention

The present invention concerns the field of reactors (photobioreactors) for the cultivation of microalgae (but also cyanobacteria, anoxygenic photosynthetic bacteria, microalgae and plant cells) in particular those comprising culture chambers made of material transparent to the photosynthetically active radiation. State of the art

It is known that the industrial exploitation of oxygenic photosynthetic microorganisms, microalgae and cyanobacteria in particular, is limited by the high cost of the culture systems and by the difficulties encountered in their scaling-up, or in other words in increasing the reactors up to dimensions suitable to make them commercially profitable. With the exclusion of few special cases (production of high-value products such as labeled molecules), the industrial production of microalgae and derived products requires plants able to produce hundreds or thousands of kilograms of biomass per year. In particular fields, such as for^' example that of feed and biofuels, the required production level is hundreds or thousands of tons of biomass per year. Considering that the volumetric productivity of the culture systems for phototrophic microorganisms, or photobioreactors, under autotrophic conditions, rarely exceeds 2 grams per liter per day, industrial plants for microaigae culture must make use of culture systems of low cost per unit volume and able to contain tens or hundreds of cubic meters of the culture.

The difficulties in scaling-up photobioreactors, derive from technical and biological constraints, among which the need for:

a high illuminated surface to volume ratio (hereinafter referred to as "SW) in order to achieve high volumetric productivities and maintain high cell concentrations;

removal of photosynthetically generated oxygen;

efficient thermoregulation;

continuous culture mixing;

high transparency of the reactors walls to the photosynthetically active radiation (wavelength ranging between 400 and 700 nm for oxygenic phototrophs, and between 400 and more than 900 nm for anoxygenic phototrophs);

resistance of the culture chamber to mechanical stress and degradation by

UV radiation;

avoidance of biofouling (the adhesion of cells or particulate or pigmented matter to the reactor walls, which may reduce transmission of the radiation useful to the growth of the cultivated microorganisms);

CO₂ supply as carbon source required by the culture:

To comply with the above mentioned requirements several kinds of reactors have been designed and, in particular, in the patent application WO2004/074423 efficient reactors for algae biomass production have been described which comprise:

a culture chamber made of material transparent to the photosyntetically active radiation and suitable for holding the microorganisms or the cells to be cultivated suspended in a suitable culture medium;

a rigid framework comprising a base, a series of vertical uprights and (when necessary) a grid, suitable for containing the said culture chamber.

The culture chamber must necessarily be transparent so as to allow transmittance of the photosynthetically active radiation to the cells kept inside the chamber. For example the culture chamber can be made of transparent plastic thin sheets, films or tubes, preferably having a thickness lower than 1 mm in order to increase transmittance and reduce costs.

Preferably the culture chamber is made of flexible plastic film most preferably plastic sheets. Alternatively the chamber can be made from sheets of rigid transparent material as for example fiberglass, PVC, polymethyl methacrylate, polycarbonate and similar.

According to a particularly preferred embodiment of the invention the material of the chamber, either rigid or flexible, has anti-adhesive properties so as to limit biofouling. In the above mentioned patent application WO 2004/074423 no reference is made to the characteristics of the base or the uprights and, from the enclosed figures, it appears evident that the uprights are designed to be applied on the sides of the base by means of suitable devices. Despite their advantage in respect of the previous prior art these reactors present a drawback that, due to the liquid pressure, the vertical uprights tend to bend outwards. This phenomenon increases the culture volume and reduces the SiA/. The bending of the uprights increases with the culture height and distance between the uprights of the same row (adjacent uprights). Thus, to limit the bending it is necessary to use higher uprights and/or to place closer the adjacent uprights. These solutions bring to heavier and more expensive structures, and increase culture shading.

It is obvious, therefore, that improvements are necessary to design a low cost reactor suitable for commercial production of microalgae even in the case of low- value products (as for example algae biomass for feeding animals or biofuel or CO₂ biofixation) but also for the production of algal biomass for use at village level (as for example in sub-saharian Africa).

Summary of the invention

A reactor is described for the industrial culture of photosynthetic organisms or plant cells, comprising a culture chamber made of material transparent to the photosynthetically active radiation and a rigid framework consisting of a base and vertical uprights that have one end (the lower) inserted directly into the base for a certain length, while the upper ends are connected in pairs or joined by means of a unique horizontal connecting bar.

Brief description of the figures

Figure 1 (a-c) shows a top view, a frontal view, and a side view of the reactor according to the invention.

Figure 2 shows a detail of a particular embodiment of the reactor according to the invention.

Figure 3 shows a further particular embodiment of the invention.

Figure 4 shows a further particular embodiment of the invention.

Figure 5 shows a further particular embodiment of the invention

Figure 6 shows a further particular embodiment of the invention

Detailed description of the invention

The present invention allows to overcome the main limitations of photobioreactors thanks to a reactor similar to the one described in the patent application WO2004/074423 in which, however, the vertical uprights are directly inserted into the base or in ground.

Therefore, as shown in figure 1 a reactor 10 according to the invention comprises: a culture chamber 11 made of material transparent to photosynthetically active radiation and suitable for holding the microorganisms or the cells to be cultivated suspended in a suitable culture medium;

a rigid framework comprising a base 12 and a series of vertical uprights 13 directly inserted into the base 12.

The way by which the uprights 13 are inserted into the base 12 depends on the material constituting the base itself.

When the base is made of materials such as wood, metal, concrete, rigid resins or similar materials, the uprights can be introduced in suitable holes made in the base; in particular, when the base is in concrete or resin or similar material, the uprights can be inserted in the fresh material and they will remain blocked into the base when the material solidifies.

The above solutions are valid even when the uprights are not inserted in a specifically built base but on surfaces already available (as for example the ground, a concrete platform, a sandy shore, etc.).

Furthermore, when the uprights are inserted directly into the soil, those at the ends of the reactor can be better anchored to the ground by means of suitable ties connected to pegs driven into the ground as shown in figure 3.

The length of the portion 14 of the upright 13 inserted into the base or into the ground/substrate depends on the reactor size and the characteristics of the base, but preferably it will not be inferior to 5 cm.

Furthermore, the upper end of each upright will preferably be joined to the corresponding end of the opposite upright and, when possibly, connected to a sole horizontal bar 15 (for example a U bar) to increase stability of the whole structure. To further reduce the bending of the uprights, the upper ends of two opposite uprights will not be joined in a single point, but preferably the joining will involve a portion of the upright not inferior to 3 cm.

The solution envisaged in the present invention is extremely practical, greatly facilitates reactor assembly and reduces costs. Furthermore, according to a particular embodiment of the invention (very useful when the chamber is, for example, higher than 1 m), the chamber can be modified by welding the opposite walls in certain points in which a hole can be made that will allow the insertion of devices connecting two opposite vertical uprights so as that bending in that portion is prevented.

As shown in figure 2, in this case the walls of the chamber 11 present one or more welded areas 15 at the center of which a hole 16 is made.

Preferably, the welded areas 15 are located at the same distance along the chamber wall, and when preferred, externally around the hole 16, on one or both walls, a reinforcement 17 made of rigid or semi-rigid material is applied to strengthen the wall around the hole.

Most preferably, the holes will be made so as to be in front of two opposite vertical uprights 13.

Through the hole 16 a suitable connecting tool 18 is then introduced which allows connecting two opposite vertical uprights 13.

Connecting tools, according to the invention, are for example: ropes, rings, hooks and similar. Particularly preferred are devices of metal or other rigid material suitably bent, as for example to form an "S", as shown in figure 2.

The welding of the opposite chamber walls can be made by simple thermo-welding

(if the chamber material allows it) or by using suitable glues.

Also in this case, as in above mentioned patent application WO2004/074423, the reactor can be made of culture units, or modules, in any number according to the production needs and to the available land, and might be provided with one or several perforated tubes placed for example at the bottom of the culture chamber

(not shown in figure). In said tubes compressed air, or compressed air mixed with

CO₂ or with other gasses suitably chosen, is introduced. The air exits from the holes into the culture achieving mixing and removal of dissolved oxygen. In the typical case in which air is injected, it will provide the required oxygen for cell respiration during the dark period. Air bubbling achieves turbulent mixing of the culture and thus provides a suitable light-dark cycle to the cells and, at least partially, cleans the internal surface and reduces the risk of biofouling. When preferred, the culture chamber may be provided with sections or internal channels, made for example by welding the opposite reactor's walls, and suitable to guide the ascending gas bubbles along predetermined pathways.

Alternatively to air-bubbling, or additionally, the culture could be mixed by means of pumps that circulates the culture inside the chamber at suitable flow and speed. The reactor will be provided with suitable systems for temperature control.

Said systems can consist for example of one or more tubes (serpentines) made of metal or any other material having high thermal conductivity. The serpentine may cross the reactor chamber longitudinally at different heights. Inside the serpentine the thermoregulated liquid is circulated.

A temperature probe is connected to an actuator that opens a valve or activates a pump, which circulates the thermoregulated fluid in the serpentine according to the thermal needs of the culture.

Alternatively, temperature may be regulated by sprinklers that nebulise water or another fluid on the reactor walls so as to achieve evaporative cooling. The opening of the sprinklers is regulated as in the previous example by a temperature probe. According to a preferred embodiment of the invention, the liquid sprayed onto the walls is collected by a suitable drain and recycled. The base of the reactor may be provided with two lateral wings that form an open rectangular or trapezoidal trough that collects and recycle the sprayed water. The trough can also collect possible leaks of culture medium. The trough can also be used to contain water or another liquid for thermoregulation. The liquid will efficiently exchange heat with the culture thanks to the large portion of the culture chamber that is submerged and in contact with the liquid. The two systems here described can work in combination. Alternatively, when the culture is circulated by means of a pump, it can be sent to a heat exchanger or to a system that achieves cooling by heat exchange or evaporation.

If the culture is made from a plastic sheet, it will be open at the top, and might be closed, hermetically or not, by a suitable cover sheet provided with outlets for air and gasses and inlets for electrodes and probes. If a large flexible plastic tube is used, suitable holes for gas exit and probes will be provided in the upper part of the tube. At the bottom of the culture chamber suitable valves may be provided for harvesting the culture and/or emptying the reactor.

The reactors according to the invention may be placed vertically on the ground in parallel rows and with different orientations. Alternatively, the reactors may be placed on the ground with an inclination different from the vertical and, as an extreme case, they may also assume a horizontal inclination. Besides, also the orientation and the distance between the reactors may vary depending from the climatic and topographic conditions and the photochemical requirements of the culture.

The reactors according to the invention offer the possibility to be connected so as to have a continuity of the culture medium and realize modules of bigger size. Typically, the connection consists of a tube of suitable diameter inserted into the reactor at the bottom near the close extremities of the two reactors to be connected. In order to obtain a continuous flow from a reactor to the next, an internal zone of the reactor will be isolated, for example by welding the walls, at the level of the connecting tube. This zone is not bubbled. The culture in this non- bubbled zone has a higher specific weight and moves down and along the connection tube to a second reactor. This latter is provided with a second tube at the opposite side, again in a non-bubbled zone, which returns the culture to the first reactor or to a series of reactors similarly connected to each other.

The reactors can be fed with the culture medium in continuous, in which case, at the opposite end, a suitable overflow tube (or alternatively a pump) ensures the culture discharge to the harvesting device or into another reactor.

The advantages of directly inserting the uprights into the base or the soil that replace the single point connection described in application WO2004/074423 can be summarized as follows:

• direct insertion of the vertical uprights into the base or in the soil reduces their outward bending and allows a higher SiA/ of the culture chamber reducing the need for a large number of uprights or of uprights of larger size. This solution reduces the cost of the reactor and the shading of the culture chamber by the containment structure; • when the reactor is filled with the culture medium, the hydrostatic pressure causes the bulging of the culture chamber which, due to the adhesion of the film to the uprights, is lifted so that its bottom (differently from the above mentioned application) does not touch the base or the ground. This phenomenon allows some radiation to reach the bottom of the chamber; it may be increased by covering the top surface of the base with white or reflective paint or resin. Furthermore it allows to place the reactor on an un- leveled surface even in the presence of roughness that, if in direct contact, would damage some kinds of flexible plastic films;

• direct insertion of the vertical uprights in the soil leads to further reduction of costs;

• with the embodiment comprising welded buttons, cheaper and higher reactors can be built.

Furthermore, a structure made of uprights which are directly inserted into the base or into the soil, as described, allows a further embodiment of the invention as shown in figures 4-6.

As illustrated, it is possible to place all the uprights 13 along a single row so that the culture chamber runs alternatively at one side of an upright and at the opposite side of the next upright (following a serpentine pattern). In this way distribution of forces is more homogeneous and the reactor is simpler and less expensive.

The single chamber can be divided into two or more horizontal superimposed chambers (as shown in figure 5) that push on the upright with opposed forces that partially compensate each other. This minimizes the uprights bending and they may be of reduced size and cost. Furthermore, in this embodiment, the different superimposed chambers can be sustained by means of a grid or net 19 kept in tension between two uprights 13 placed at the opposite ends of the structure.

The uprights 13, according to these last particular embodiments of the invention, can be perfectly aligned (as shown in figures 5) or slightly non-aligned as shown in figure 4. In the first case, the uprights 13 may be provided with a suitable curvature 20 at the inferior end, near the base in which they are inserted, so as to have enough space in the chamber to allow the insertion of the tubes for air-bubbling or thermoregulation or other. In the case in which the chamber is divided in several different superimposed chambers, the use of pumps for culture circulation is particularly suitable. In this case, the chamber will not be closed at the lateral ends, but connected to suitable manifold collectors, which will be in turn connected to the circulating pump.

Claims

1. Photobioreactor (10) for the culture of microalgae comprising:

a culture chamber (11) consisting of material transparent to the photosynthetically active radiation and suitable for holding the microorganisms or the cells to be cultivated, suspended in a suitable culture medium;

- a rigid framework made of a base (12) and vertical uprights (13) suitable for containing the said culture chamber and characterized by the fact that said uprights (13) have one end (the lower) inserted directly into the base (12) or directly into the soil.

2 . Reactor according to claim 1 , wherein the surface facing the culture chamber of the said base may be covered with material able to reflect the impinging radiation.

3. Reactor according to claims 1 and 2, wherein the walls of the chamber (11 ) have been welded in one or more points (15) in which a hole (16) is made that allows the insertion of devices connecting two opposite vertical uprights (13) placed in front of said holes (16).

4. Reactor according to claim 3, wherein said welded points (15) are located at the same distance from each other.

5. Reactor according to claims 3 and 4, wherein externally around the hole (16), on one or both walls, a reinforcement (17) made of rigid or semi-rigid material is applied to strengthen the wall around the hole.

6. Reactor according to claim 5, wherein the holes (16) are made so as to be in front of two opposite vertical uprights (13) .

7. Reactor according to claim 6, wherein a suitable connection (18) is introduced which allows connecting two opposite vertical uprights (13) through the hole (16).

8. Reactor according to claim 7, wherein said connecting elements (18) are ropes, rings, hooks and similar able to bind the culture chamber (11) coupling two opposite uprights (13) through the holes (16).

9. Reactor according to claim 8, wherein said connections are made of metal or other rigid material suitably bent, as for example to form an "S".

10. Reactor according to claims 1-9, wherein the length of the portion (14) of the upright (13) inserted into the base or in the ground is longer or equal to 5 cm.

11. Reactor according to claim 10, wherein the upper ends of the uprights (13) are joined in pairs or connected by an horizontal bar.

12. Reactor according to the previous claims, wherein the reactor's base is provided with two lateral wings that form a rectangular or trapezoidal trough able to collect the sprayed water or possible leaks of culture medium.

13. Reactor according to the previous claims, wherein the culture chambers run alternatively at one side of an upright and at the opposite side of the next upright, following a serpentine pattern.

14. Reactor according to claim 13, wherein the culture chambers are divided in two or more horizontal superimposed chambers.

15. Reactor according to claim 14, wherein the superimposed chambers are sustained by means of a grid or net (19) kept in tension between two uprights (13) placed at the opposite ends of the structure.

16. Reactor according to claims 13-15, wherein the uprights (13) are perfectly aligned or slightly non-aligned.

17. Reactor according to claim 16, wherein the uprights (13) bear a curvature (20) at the inferior end, near the base in which they are inserted, so as to have enough space in the chamber to allow the insertion of the tubes for air-bubbling or thermoregulation or other.