WO2008010737A1

WO2008010737A1 - Photobioreactor for photosynthetic microorganism culture

Info

Publication number: WO2008010737A1
Application number: PCT/PT2006/000018
Authority: WO
Inventors: Antonio Manuel Cordoso Marques Ferreira
Original assignee: TECNIA PROCESSOS E EQUIPAMENTOS INDUSTRIAIS E AMBINTAIS
Current assignee: TECNIA PROCESSOS E EQUIPAMENTOS INDUSTRIAIS E AMBINTAIS
Priority date: 2006-07-21
Filing date: 2006-07-21
Publication date: 2008-01-24
Anticipated expiration: 2009-01-21

Abstract

The present invention relates to a photobioreactor for the cultivation of photosynthetic microorganisms. The production of biomass, for example algae, is carried out in a set of transparent tubular cells (5), through which the culture medium flows, the said tubes being exposed to sunlight in order to promote photosynthesis.

Description

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DESCRIPTION

"PHOTOBIOREACTOR FOR PHOTOSYNTHETIC MICROORGANISM CULTURE"

Scope of the invention

The invention relates to a new photobioreactor for producing biomass. Photobioreactors are fermenters in which phototrophic microorganisms such as algae and cyanobacteria are cultivated, where their growth and propagation are promoted at the same time as various substances are produced by the photosynthetic cells.

Prior art

The potential for producing photosynthetic products by photosynthesis techniques using simple matter, through microalgae, has been recognised in the last years. The ability of simple microorganisms, such as green algae, to use sunlight and carbon dioxide to produce complex organic matter with the release of molecular oxygen is of great interest and has been explored in many different ways, as will be explained hereunder.

Microalgae (blue-green algae and green algae) are photoautotrophic microorganisms able to use sunlight to metabolise carbon dioxide (CO₂) in energy-rich organic - 3 -

compounds (CH₂O) , with the consequent release of oxygen (O₂) . Organic compounds, represented by CH₂O, are used as building blocks for microalgae growth (when referring to microalgae we are also including cyanobacteria) . Water is also needed in the process as an additional substrate, as can be seen in equation 1.

CO₂ + H₂O + "light energy" -> CH₂O + O₂ (equation 1)

This process of light-driven biomass growth is called photosynthesis.

Microalgae, including cyanobacteria, have the capacity to absorb light energy (photons) and store it as chemical energy through the formation of chemical bonds . The basic unit of the photosynthetic apparatus is the photosystem. Light energy, i.e. photons, is absorbed by carotenoids and chlorophyll pigments of the photosystem antenna complex. Excitation energy passes to the pigment bed towards the reaction centre.

For microalgae cultivation only simple nutrients need to be provided, such as ammonium or nitrates, phosphates, trace amounts of certain metals and, most importantly, carbon dioxide. Since algae are autotrophic, they can grow on this simple and cheap medium. The only problem is that algae are phototrophic and that light energy is the growth limiting "substrate". However, this is also an advantage because the energy source can be cost- _ 4 -

free - sunlight. On the other hand, it is very difficult to expose the culture to a satisfactory amount of light energy and to utilise this energy efficiently for biomass production. For these reasons, the structure and operational conditions of the photoreactor have to be optimised.

Microalgae are rich in many specific and attractive compounds, some of which are very interesting as nutritional supplements, such as long-chain polyunsaturated fatty acids. Other nutriceuticals derived from microalgae are vitamins and antioxidants, such as /S-carotene and astaxanthin. As well as important applications in the food industry, microalgae are also used in the pharmaceutical market as they contain sterols, which can be used as building blocks for pharmaceuticals (hormones) . Furthermore, cyanobacteria are a potential source of compounds with biomedical applications, such as antimicrobial, antiviral and anticancer compounds.

These microorganisms can also be used in environmental engineering, in the treatment of waste water, as they help to purify water. As well as reducing the concentration of nutrients in wastewaters, such as nitrogen and phosphorus, microalgae can also be used to purify an effluent containing pollutants like metals and/or radionuclides. The metals in the effluent are in the form of free metal ions or they are combined in chemical bonds. The ionic form is generally the most toxic. The metals can - 5 -

complex with compounds that are negatively charged, which are good to trap the metal ions that are positively- charged. It has been demonstrated that microalgae are very powerful metal ion absorbers, particularly as a result of their large negatively charged cellular surface.

Various different systems for the production of microalgae are known in the art. The two major classes of photobioreactors are the open and enclosed systems, with each type having unique properties for the production of biomass .

Examples of the open systems are outdoor ponds which usually has the disadvantage of imprecise control over process parameters. These systems offer little or no control over temperature and incident light intensity, and they also have low CO₂ (carbon dioxide) utilisation efficiency due to the lack of turbulent flow and the escape of gases from the culture medium. Contamination by other microorganisms also occurs and influences the growth of the culture, as well as decreasing the quality of the product.

Since there are various problems associated with ponds, there is a growing interest in enclosed photoreactors . These enclosed systems offer better control over process variables, greater CO₂ utilisation efficiency and reduced contamination from the outside environment. The most common enclosed photoreactor is the rigid tubular reactor, although other configurations also exist. These - 6 -

systems can be exposed to sunlight or to artificial light . Although photoreactors with artificial light offer a better control over process variables, they become too expensive when constructed on an industrial scale.

Several photobioreactors have been developed for the culture of photosynthetic microorganisms. Among them are the systems of the patents US3955318, SE8903801, WO2005006838, JP2002262858 , US4438591.

Patent US3955318 refers to a method of producing an algae product and of purifying aqueous organic waste material to provide clean water. Starting algae and aqueous waste are admixed in sufficient amount to provide nutrients for the algae, which are grown in symbiotic relationship with aerobic bacteria present. The mixture is aerated and, if desired, carbon dioxide added, and is exposed to alternate periods of light (e.g. 1/2 second to ten seconds) and darkness (about ten times as long as the light period) to accelerate growth of the algae, harvesting the algae product to maintain the growth rate at a very high level . A device for effecting agitation and intermittent light exposure includes a reaction tank having a generally vertical wall in the form of a parabolic curve, injectors debouching into the bottom of the tank opposite the parabolic-curve wall and causing circulation so that injected and other material is forced rapidly upwardly, and a baffle transversely across the upper portion of the tank. The baffle supports and directs the flow of a selected - 7 -

proportion of an aqueous reaction mass across the upper portion of the tank, exposing the selected proportion to limited exposure to a source of light. A system for carrying out the method is provided.

Patent SE8903801 refers to an apparatus for microalgae cultivation in a fluid-based medium, which involves the medium being made to move so that objects with a solid structure and abrasive surface rest against vessel walls. The vessel involved is formed to create one or more closed vertical circulation circuits for the medium with the microalgae. One or more mouthpieces for gas or liquid are arranged at the bottom of each circuit, and the medium contains objects with a hard structure and abrasive surfaces, and with the same compactness as the medium and algae themselves. Two parallel vertical light -permeable tubes form the vessel, the tubes being connected at the top and bottom to form a closed circuit, and one or more light sources are arranged at least between the tubes. The vessel has a base and a light-permeable outer cylindrical container wall, and a similar inner wall. A cylindrical screen wall is fitted concentrically between the outer and inner walls with its edges located above the base and below the surface of the medium. One or more light sources are arranged at least inside the inner container wall .

In patent WO2005006838 the invention relates to a photobioreactor for cultivating algae comprising a disposable element for holding the cultivation medium and a - 8 -

cage-like component supporting the disposable element. The algae may comprise both marine and fresh water algae . The photobioreactor is especially advantageous for algae which are difficult to cultivate due to contamination problems.

Patent JP2002262858 intends to provide a method for industrially efficiently cultivating a large amount of blue-green algae at a low cost. This method comprises cultivating blue-green algae in a culture pond situated in an agricultural greenhouse kept at 15-42 deg. C, optimally 22-34 deg. C with 2,000-6,500 lux irradiation light quantity, optimally 4,000-5,000 lux, and >=0.03 vol . % carbon dioxide concentration using a culture solution comprising ocean deep water. The blue-green algae are, for example, Spirulina platensis, Spirulina maxima, Spirulina ienneri, Spirulina flavorience, Spirula laxima or Spirulina maiol . The culture solution is kept at >= pH 8, preferably 9-11. The agricultural greenhouse is made of metal frames and transparent resin films cover the frames.

In patent US4438591 the invention relates to algal cell growth, modification and harvesting, and more particularly to systems, apparatus and methods for growing, enhancing the growth of and harvesting of motile swimming microorganisms, especially unicellular algae, such as Dunaliella, which multiply by cell division. - 9 -

Objectives of the invention

The objective of the present invention is to develop an enclosed photobioreactor, for the production of biomass, which is able to overcome the problems associated with open systems such as ponds and raceways, and which at the same time is more economic than the artificial enclosed photoreactors that already exist.

Various photobioreactors have already been developed for the production of microalgae biomass, since these microorganisms have important applications. However, in the present invention a new structure is proposed for the photobioreactor, namely a set of transparent tubes aligned vertically to form units through which the culture medium flows in an upflow direction, the tubes being exposed to sunlight.

Summary of the invention

The present invention comprises a photobioreactor for biomass production, as defined in claim 1. It is essentially characterised in that it contains a set of transparent tubes (cells) grouped into units (modules) , each unit having a certain number of cells through which the culture medium flows vertically in an upflow direction. The total number of cells depends on the daily flow of culture medium to be processed. The culture medium is provided with the necessary nutrients, carbon dioxide and - 10 -

light in order to perform photosynthesis.

The invention also relates to a process for the production of photosynthetic microorganisms, according to claim 9.

Brief description of the drawings

The description that follows is based on the drawings attached hereto, which represent without any restrictive character:

FIG. 1 represents one unit of the photobioreactor (one module) comprising a set of cells (tubes) aligned vertically; and

FIG. 2 is a schematic drawing of the photobioreactor indicating the inlet, outlet and recirculation streams of the system (culture medium) . By way of example, four units of cells are represented.

Detailed description of the invention

The photobioreactor of the present invention is proposed for the culture of photosynthetic microorganisms and it can be used for the culture of any type of photosynthetic organism having light requirements.

A further objective of the invention is a process - 11 -

for the production of photosynthetic microorganisms.

As can be seen in figures 1 and 2, the unit (3) of the photobioreactor (figure 1) essentially comprises a set of tubular cells (5) . A collector (4) is connected to the bottom of all the cells (4) and another collector (6) is connected to the top of all the cells (5) .

The culture medium (1) enters the unit (3) by the action of the pump (2) and feeds the cells (5) from collector (4) in an upflow direction. The culture medium is collected in the collector (6) . The cells (5) are arranged vertically in parallel at a distance which allows light, either artificial or natural, to reach the whole surface of the cells. Each of the cells is manufactured in a transparent material permitting the passage of light and the material used must be resistant to external aggressions and tension forces.

Figure 2 illustrates the assembly of a possible embodiment of the present invention comprising four modules

(3, 12, 13 and 14) identical to the module (3) previously described. The culture medium (1) is pumped (2) to the first unit or module (3) of the photobioreactor through the collector (4) located at the base of the respective unit and is distributed simultaneously through all the cells (5) in an upflow direction, being collected in the collector (6) at the top of the module from where it is discharged through the tubular connection (7) to the next module (12) . - 12 -

The culture medium is always fed to each unit from the bottom and is collected at the top. From the moment the culture medium is fed to the first module (3) until it is discharged into the last module (14) in the present case, it has to pass through all the cells (5) in all the modules

(3, 12, 13 and 14), completing a time cycle during which it is exposed to sunlight. After the first cycle the culture medium is recirculated (11) by a pump (10) to the first module (3) of cells (5) and repeats the same path as before. The cycles are repeated as many times as needed for the culture medium to achieve the desired concentration of products (substances) . For example, if the purpose of the culture is to produce biomass of a certain microalgae, the culture medium is discharged when the desired concentration has been reached (maximum production possible) . On the other hand, if the photobioreactor is to be used for wastewater treatment, in order to reduce the concentration of nutrients, such as nitrogen and phosphorus, the culture medium (effluent to be treated) will be recirculated until the nutrient concentration has been reduced to a minimum amount .

The culture medium is pumped (2) through the cells (5) at a relatively high velocity, usually between 8 and 25 cm/s. The culture medium should be fed (1) to the photobioreactor with a flow permitting a velocity through the cells which makes it possible to generate the turbulence necessary to prevent sedimentation of the microalgae and to ensure that there is no self-shadowing by - 13 -

the microalgae. Attention must also be paid to the diameter of the cells, as turbulence has to be created (large diameters do not encourage turbulence) . Turbulence helps to prevent microalgae from adhering to the walls of the tubes and maximises the use of sunlight in photosynthesis, as it allows the cells to receive intermittent exposure to sunlight, which is known to improve photosynthesis efficiency.

The nutrients and carbon source necessary for microalgae growth, such as carbon dioxide (CO₂) , phosphorus

(P) and nitrogen (N) , need to be supplied. The supply of carbon dioxide in accordance with its actual consumption by the microalgae is of great importance. A possible way of supplying CO₂ (carbon dioxide) is in the form of gas, and a CO₂ dosage system can be provided that automatically responds to microalgae growth by introducing the necessary quantity of CO₂. If the culture medium is an effluent from a wastewater treatment plant, for example, there is no need to supply nutrients and/or carbon dioxide since they are

(usually) already present in the effluent. The light energy needed is exclusively solar, which makes the system much more economical. However, in an alternative embodiment of the proposed system, artificial light could be supplied to the system or simply means for improving luminosity, for example reflective means.

Of the various operational parameters, the dilution rate of the system must be controlled in order to - 14 -

reach an optimum point, which means that the microalgae concentration cannot exceed a specific value, otherwise part of the culture has to be discharged (9) and fresh medium will have to be fed to the system. If the microalgae concentration reaches a very high level, problems of self- shadowing may arise leading to the creation of dark zones where the light cannot penetrate, resulting in a low efficiency of the photosynthetic process.

During the photosynthetic process, as previously mentioned, molecular oxygen is released, which raises the pressure inside the tubular cells (5) . In order to overcome this problem, which could damage the system, there are gas escape valves (8) in the collectors (6) at the top of each of the cell (tube) modules (3, 12, 13, 14) . These valves purge gases from the system in order to remove excess air/oxygen. A possible embodiment of the system could involve the recovery of the oxygen generated, to be used in another process .

After discharge (9), it is necessary to separate the culture medium into a liquid phase and into a solid phase which contains the microorganisms (biomass) . The culture unit should preferably comprise means for performing this separation, which can be achieved by various processes. The biomass can be separated from the liquid phase by centrifugation or with a filter unit. For example, if the photobioreactor is applied in wastewater treatment, once the cycles in the photobioreactor have been - 15 -

completed and the culture medium has achieved the desired nutrient concentration (minimum or no nutrients) , the effluent is discharged to an ultrafiltration membrane module where the biomass is concentrated and the permeate is pure water, ready to be reused. The concentrated biomass can be reintroduced into the system depending on the concentration of the culture in suspension in the photobioreactor . If the culture medium is very dense the culture must be diluted and no biomass should be reintroduced into the system, but if the culture is very diluted and needs to be concentrated, more biomass must be introduced into the system.

The photobioreactor of the present invention is easy to assemble and since it consists of a set of modules, each with a set of cells, if a cell is damaged there is no need to change the whole structure but only the cell in the module involved. Therefore the photobiorector can be repaired without stopping. The type of structure proposed also makes it possible to introduce into or withdraw from the system nutrients or products that are lacking or are in excess.

The system proposed for the photobioreactor of the present invention can have automatic control, meaning that all the modules can be computer controlled, which can save time and labour. Another advantage of this multi-unit system is its flexibility, since it is possible to switch from the production of one product to another without many - 16 -

modifications .

Although preferred embodiments of the present invention have been described in detail herein and illustrated in the attached drawings, it must be stressed that the invention is not limited to these embodiments and that various alterations can be made.

As previously mentioned, a further objective of the invention is a process for the production of photosynthetic microorganisms which is characterised in that it comprises the following steps: a) cultivation of photosynthetic microorganisms in a photobioreactor as previously defined, in order to obtain a culture medium containing photosynthetic microorganisms; b) removal of the culture medium from the photobioreactor when the desired concentration of biomass is reached or, if necessary, for ensuring the optimum functioning of the system; c) recirculation of the remaining portion of the culture medium to the beginning of the process in order to guarantee maximum exposure to sunlight and thus increase the efficiency of the photosynthesis; d) separation of the culture medium of step b) into a solid phase containing photosynthetic microorganisms and into a liquid phase; e) feeding of the photobioreactor with the liquid phase obtained in step d) if the culture medium is too dense and needs to be diluted; - 17 -

f) discharge of the liquid obtained in step d) if the culture medium in the photobioreactor is correctly diluted; g) if the photoreactor is used in the treatment of wastewater effluents, in order to reduce the concentration of nutrients such as nitrogen and phosphorus, the liquid phase obtained in step d) can be reused for different purposes outside of the system and the solid phase obtained in step c) , which corresponds to concentrated biomass, can be recirculated into the photobioreactor in order to maintain the desired concentration level in the culture medium of the photobioreactor, or it can be discharged if the concentration in the biomass culture medium is above the necessary level .

All the stages of the process can be controlled through means for automatically controlling the variables which affect the photobioreactor, namely computer means.

Claims

- 18 -CLAIMS :

1. A photobioreactor for the cultivation of photosynthetic microorganisms which can be used for the treatment of effluents, of the type that uses units with transparent tubular elements (5) in a closed system, characterised in that it comprises:

- At least one module (3, 12, 13, 14,...) comprising a set of transparent tubular cells (5) aligned in parallel through which the culture medium flows in an upflow direction, for the cultivation of photosynthetic microorganisms ;

- A collector (4) connected to the bottom of the cells (5) of each module (3, 12, 13, 14,...), which receives the culture medium and distributes it through the said cells (5) ;

- A collector (6) connected to the top of the cells (5) of each module (3, 12, 13, 14,...), which collects the culture medium coming from the said cells (5) ; - Means (2) for feeding the set of cells (5) of the set of modules (3, 12, 13, 14,...) with the culture medium and nutrients necessary for the development of the microorganisms in the culture and the production of biomass by photosynthesis; - Means (9) for removing the biomass synthesis product after the last module;

- Means (10) for recirculating the culture medium from the last to the first module (3) ;

- Escape valves for molecular oxygen or other gases created - 19 -

during the process positioned in each top collector (6) ;

- Means for automatically controlling the variables which affect the photobioreactor;

- Means for separating a portion of the culture medium removed from the photobioreactor into a liquid phase and into a solid phase which contains the biomass, and for recirculating each of the phases if necessary.

2. A photobioreactor according to claim 1, characterised in that the culture medium passes from one module to another by means of a tubular connection (7) existing between top collector (6) and bottom collector (4) of the adjacent module.

3. A photobioreactor according to claim 1, characterised in that the culture medium is fed to the first module and has to pass through the cells of all the modules that constitute the photobioreactor in order to complete a cycle, during which the culture is exposed to sunlight and photosynthesis is promoted.

4. A photobioreactor according to claim 1, characterised in that the turbulence in the upflow in the cells (5) , which is necessary in order to prevent sedimentation of the microalgae, adherence thereof to the walls of the tubes and consequent shadowing, is guaranteed by the velocity of the flow, between 8 and 25 cm/s, and by the diameter of each of the cells (5) .

5. A photobioreactor according to claim 1, - 20 -

characterised in that the nutrients necessary for the microalgae to grow and the dilution rate, i.e. the concentration of microalgae in the culture medium, are controlled by control means.

6. A photobioreactor according to claim 5, characterised in that if the culture media are wastewater effluents, nutirents can be provided if necessary.

7. A photobioreactor according to claim 1, characterised in that the culture medium discharged into the last module is recirculated through as many cycles as necessary in order to increase the retention time of the culture in the cells, thus increasing the production of biomass.

8. A photobioreactor according to claim 1, characterised in that it also comprises means for transferring the liquid phase obtained by the phase separation means to the culture medium, if it is necessary to dilute the culture medium.

9. A photobioreactor according to claim 1, characterised in that the means (2) for feeding the set of cells (5) of the set of modules (3, 12, 13, 14,...) and the means (10) for recirculating the culture medium from the last to the first module (3) are pumps.

10. A process for the production of - 21 -

photosynthetic microorganisms, characterised in that the light provided is natural and/or artificial and means for improving luminosity can be used.

11. A process for the production of photosynthetic microorganisms, characterised in that it comprises the following steps: a) cultivation of photosynthetic microorganisms in a photobioreactor as defined in claims 1 to 10, in order to obtain a culture medium containing photosynthetic microorganisms ; b) removal of the culture medium from the photobioreactor when the desired concentration of biomass is reached or, if necessary, for ensuring the optimum functioning of the system; c) recirculation of the remaining portion of the culture medium to the beginning of the process in order to guarantee maximum exposure to sunlight and thus increasing the efficiency of the photosynthesis; d) separation of the culture medium of step b) into a solid phase containing photosynthetic microorganisms and into a liquid phase.

12. A process for the production of photosynthetic microorganisms according to claim 11, characterised in that it further comprises the following steps : e) feeding of the photobioreactor with the liquid phase obtained in step d) if the culture medium is too dense and - 22 -

needs to be diluted; f) discharge of the liquid obtained in step d) if the culture medium in the photobioreactor is correctly diluted; g) if the photoreactor is used in the treatment of wastewater effluents, in order to reduce the concentration of nutrients such as nitrogen and phosphorus, the liquid phase obtained in step d) can be reused for different purposes outside of the system and the solid phase obtained in step c) , which corresponds to concentrated biomass, can be recirculated into the photobioreactor in order to maintain the desired concentration level in the culture medium of the photobioreactor, or it can be discharged if the concentration in the biomass culture medium is above the necessary level .

13. A process for the production of photosynthetic microorganisms according to claim 12, characterised in that for the treatment of wastewater it includes, if necessary, the recirculation of the solid phase, which consists of the concentrated biomass obtained in step d) of claim 11, in order to maintain the concentration of photosynthetic microorganisms at a constant level .