Carbon capture

Abstract

The present invention refers to an open pond reactor for CO2 capture and biomass production, a system for CO2 capture and biomass production, a method for CO2 capture and biomass production and the use of the reactor or the system for producing biomass- based food, feed, biomaterials, biofertilizers or biofuels and for effluent treatment such as waste water, sewage, combustion gas or biogas treatment.

Description

[0001] CARBON CAPTURE

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to the area of direct air carbon capture (DAC). More specifically, the present invention relates to an open pond reactor for CO2 capture and biomass production, a system for CO2 capture and biomass production, a method for CO2 capture and biomass production and the use of the reactor or the system for producing biomass-based food, feed, biomaterials, biofertilizers or biofuels and for effluent treatment.

[0004] BACKGROUND

[0005] Production of biomass in the form of photosynthetic microorganisms, such as microalgae and cyanobacteria, has emerged in the last decades as an alternative to the commonly applied techniques for CO2 capture. Microalgae and cyanobacteria cultivation has higher productivity levels than superior plants, resulting in greater levels of CO2 fixation, up to 200 tons CCh / ha year. In addition, biomass has a large biotechnological potential for producing valuable substances for agriculture, feed, food, nutraceutical, and pharmaceutical industries. Furthermore, other applications can be attributed to the photosynthetic process performed by microalgae and cyanobacteria such as wastewater treatment and biofuel production.

[0006] Open reactors, in particular raceway reactors, have been used for more than 50 years for the industrial production of microalgae biomass. Today, most of the worldwide microalgae biomass production is carried out using raceway open reactors due to their simplicity and low construction cost. However, these type of reactors have certain problems related to their limited mass transfer, low productivity, high risk of contamination and poor control of culture conditions, making them not attractive enough for CO2 capture applications.

[0007] Therefore, there is a clear need for new optimized reactors with better mass transfer capacity, productivity, lower risk of contamination and better control of culture and biomass growing conditions for CO2 capture.

[0008] BRIEF DESCRIPTION OF THE INVENTION

[0009] The authors of the present invention have developed an open pond reactor for CO2 capture and / or sequestration and biomass production, a system for CO2 capture and biomass production and a method for CO2 capture and biomass production.

[0010] It has been observed that the open pond reactor of the present invention has an unexpected high CO2 capture and biomass production efficiency, a low risk of contamination and a good control of biomass growing conditions. The CO2 capture values and the biomass production are optimized thanks to the configuration of the pit of the open pond reactor. In particular, the authors have observed that when the volume of the pit of the reactor is in a certain range of the total volume of the reactor, such as equal to or between 4 and 22 % of the total volume of the reactor, an improvement of gas transfer between the aqueous culture medium and air is produced in said pit, leading to an increase in CO2 capture and thus, in biomass production. On the contrary, the authors experimentally observed that when the volume of the pit is below 4% of the total volume of the reactor, there is not enough CO2 mass transfer, and therefore, the production of biomass is reduced. In addition, if the volume of the pit is too large, for example over 22 % of the total volume of the reactor, there is too much dark volume (understood as a volume wherein only a reduced amount of light is able to reach) in the reactor and the production of biomass is significantly reduced. In addition, in the latest case there are also dead regions with uncontrolled accumulation of biomass. Moreover, the reactor of the invention allows reducing the cost and energy consumption of biomass production with respect to conventional reactors, since it makes not necessary the use of additional carbonation sources such as an additional injected flow of CO2, to obtain a high biomass yield. Thus, the reactor of the invention allows working with air as the only source of CO2, significantly reducing the cost of CO2 capture and biomass production by eliminating the need of additional carbonation sources.

[0011] Therefore, a first aspect of the invention is directed to an open pond reactor (1) for CO2 capture and biomass production, comprising:

[0012] - an open pond vessel (2) comprising at least one flow channel;

[0013] - a pit (3) located at the bottom of the at least one flow channel of the open pond vessel; wherein the open pond vessel (2) and the pit (3) are adapted for receiving, containing and circulating an aqueous culture medium; wherein the open pond vessel (2) and the pit (3) are in fluidic communication; wherein the volume of the pit is equal to or between 8 and 12% of the total volume of the reactor (1); and wherein the pit (3) comprises air inlets.

[0014] A second aspect of the invention is directed to a system for CO2 capture and biomass production, comprising:

[0015] • the open pond reactor (1) according to of the invention in any of its particular embodiments, or an open pond reactor (1) comprising: o an open pond vessel (2) comprising at least one flow channel; o a pit (3) located at the bottom of the at least one flow channel of the open pond vessel; wherein the open pond vessel (2) and the pit (3) are adapted for receiving, containing and circulating an aqueous culture medium; wherein the open pond vessel (2) and the pit (3) are in fluidic communication; wherein the volume of the pit is equal to or between 4 and 22% of the total volume of the reactor (1); and wherein the pit (3) comprises air inlets;

[0016] • an air injection device adapted for injecting air through the air inlets of the pit (3) of the open pond reactor (1); and

[0017] • an aqueous culture medium comprising microalgae and / or cyanobacteria.

[0018] An additional aspect of the invention is directed to a method for CO2 capture and biomass production comprising the following steps:

[0019] (i) providing the system according to the invention in any of its particular embodiments, comprising: a) the open pond reactor (1) according to the invention in any of its particular embodiments, or an open pond reactor (1) comprising:

[0020] • an open pond vessel (2) comprising at least one flow channel;

[0021] • a pit (3) located at the bottom of the at least one flow channel of the open pond vessel; wherein the open pond vessel (2) and the pit (3) are adapted for receiving, containing and circulating an aqueous culture medium; wherein the open pond vessel (2) and the pit (3) are in fluidic communication; wherein the volume of the pit is equal to or between 4 and 22% of the total volume of the reactor (1); and wherein the pit (3) comprises air inlets; b) an air injection device adapted for injecting air through the air inlets of the pit (3) of the open pond reactor (1) of (a); and c) an aqueous culture medium comprising microalgae and / or cyanobacteria;

[0022] (ii) adding the aqueous culture medium to the open pond reactor of (a) and circulating said aqueous culture medium inside said open pond reactor; and

[0023] (iii) during circulation:

[0024] - adding to the aqueous culture medium nutrients for microalgae and / or cyanobacteria growth;

[0025] - irradiating the aqueous culture medium with light; and

[0026] - providing an air flow below 5 L / m2 / min through the air inlets of the pit of the open pond reactor of step (i) by using the air injection device of step (i).

[0027] An additional aspect of the invention is directed to the use of the reactor of the invention in any of its particular embodiments, or the system of the invention in any of its particular embodiments for producing biomass-based food, feed, biomaterials, biofertilizers or biofuels.

[0028] An additional aspect of the invention is directed to the use of the reactor of the invention in any of its particular embodiments, or the system of the invention in any of its particular embodiments for effluent treatment; preferably wherein the effluent is selected from waste water, sewage, combustion gas, biogas or a mixture thereof.

[0029] FIGURES

[0030] Figure 1 shows (a) a top view and (b) a lateral view perspective of the open pond reactor according to an embodiment of the invention.

[0031] Figure 2 shows a top view of part of the channel wherein the pit is located of the open pond reactor according to an embodiment of the invention.

[0032] Figure 3 shows a diffuser of the open pond reactor according to an embodiment of the invention.

[0033] Figure 4 shows a scheme of the pit of the open pond reactor comprising two diffusers according to an embodiment of the invention. DETAILED DESCRIPTION OF THE INVENTION

[0034] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this disclosure belongs. As used herein, the singular forms “a” “an” and “the” include plural reference unless the context clearly dictates otherwise.

[0035] Reactor

[0036] As stated above, a first aspect of the invention is directed to an open pond reactor (1) for CO2 capture and biomass production, comprising:

[0037] - an open pond vessel (2) comprising at least one flow channel;

[0038] - a pit (3) located at the bottom of the at least one flow channel of the open pond vessel; wherein the open pond vessel (2) and the pit (3) are adapted for receiving, containing and circulating an aqueous culture medium; wherein the open pond vessel (2) and the pit (3) are in fluidic communication; wherein the volume of the pit is equal to or between 8 and 12% of the total volume of the reactor (1); and wherein the pit (3) comprises air inlets.

[0039] In a particular embodiment, the open pond reactor (1) consist of:

[0040] - an open pond vessel (2) comprising at least one flow channel;

[0041] - a pit (3) located at the bottom of the at least one flow channel of the open pond vessel; wherein the open pond vessel (2) and the pit (3) are adapted for receiving, containing and circulating an aqueous culture medium; wherein the open pond vessel (2) and the pit (3) are in fluidic communication; wherein the volume of the pit is equal to or between 8 and 12% of the total volume of the reactor (1); and wherein the pit (3) comprises air inlets.

[0042] Flow channel In the context of the present invention, the expression “open pond vessel” is understood as known in the art, as a basin or canal adapted for receiving, containing and circulating a liquid such as an aqueous culture medium, moreover, the top of the open pond vessel is “open”, i.e. it is not covered, thus open pond vessels are adapted for allowing a liquid such as an aqueous culture medium being in contact with air and receiving a radiation such as sunlight.

[0043] In an embodiment, the open pond reactor is adapted for outdoor biomass production.

[0044] In an embodiment, the open pond reactor is adapted for being exposed to light, in particular to sunlight such as direct sunlight and / or air.

[0045] In an embodiment, the flow channels of the open pond vessel (2) are open basins or canals adapted for receiving, containing and circulating an aqueous culture medium. In a particular embodiment, the flow channels of the open pond reactor are adapted for allowing the aqueous culture medium to flow and / or circulate in a continuous loop.

[0046] In an embodiment, the open pond vessel (2) consist of one flow channel.

[0047] In an embodiment, the open pond vessel (2) comprises two flow channels. In another embodiment, the open pond vessel (2) consist of two flow channels.

[0048] In an embodiment, each flow channel is adapted for outdoor biomass production; preferably is adapted for being exposed to sunlight and / or air.

[0049] In an embodiment each flow channel is connected by a bend at is end; preferably by a semicircular curved bend; more preferably by a semicircular 180° bend at its end.

[0050] In a more particular embodiment, the flow channel has the shape of a substantially rounded rectangle with four sections: two straight parallel segments, separated by an intermediate partition and joined at their ends with a semicircular curved bend; more preferably by a semicircular 180° bend. In an even more particular embodiment, the width of the straight segments of the flow channel is between 0.2 and 25 m, more preferably between 0.5 and 15 m; more preferably equal to or between 1 and 10 m.

[0051] In another embodiment, the flow channel has at least an aperture adapted for receive or expel an aqueous culture medium, preferably two apertures; more preferably an entry and an exit of aqueous culture medium.

[0052] In an embodiment, the open pond reactor (1) is a raceway reactor. In the context of the present invention a raceway reactor is a type of bioreactor as commonly known in the art. It preferably comprises a preferably shallow channel wherein a nutrient-rich medium circulates continuously, usually driven by a paddlewheel.

[0053] In another embodiment, the open pond reactor (1) has a surface area equal or between 20 and 1000 m2, preferably equal or between 50 and 400 m2, more preferably equal or between 60 and 300 m2, much more preferably equal or between 70 and 200 m2, even much more preferably an surface are equal or between 80 and 150 m2.

[0054] In another embodiment, the open pond reactor (1) has a surface area of about 20, 30, 40, 50, 60, 70, 80, 90, 100, 150, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 800, 850, 900, 950 or 1000 m2.

[0055] In another embodiment the flow channel is a raceway channel.

[0056] In another embodiment, the flow channel comprises a bottom, at least one peripheral wall and a top opening; wherein the at least one peripheral wall extends substantially vertically from the bottom to define the top opening of the channel. In a particular embodiment, the flow channel is open at its top or has a top opening (i.e. the flow channel does not have a top cover or lid).

[0057] In an embodiment, the bottom of the flow channel intersects the at least one peripheral wall; preferably the bottom of the flow channel is substantially flat.

[0058] In a more particular embodiment, the flow channel comprises a bottom, at least one peripheral wall and a top opening; wherein the at least one peripheral wall extends substantially vertically from the bottom to define the top opening of the channel; and wherein the flow channel is adapted for allowing the aqueous culture medium to flow or circulate in a continuous loop. In a more particular embodiment, the flow channel is adapted for allowing the aqueous culture medium being in contact with air and for receiving an external radiation such as sunlight.

[0059] In a more particular embodiment, the flow channel is adapted for receiving an external electromagnetic radiation such as light, in particular infrared, visible, ultraviolet light or a combination thereof; preferably sunlight. In particular, the flow channel is adapted for receiving an external electromagnetic radiation such as light, in particular infrared, visible, ultraviolet light or a combination thereof; preferably sunlight, reaching the aqueous culture medium. In a more particular embodiment, the flow channel has a top opening without a top cover or lid.

[0060] The open pond vessel (2) and the pit (3) of the open pond reactor (1) are in fluidic communication; preferably the aqueous culture medium circulates through the open pond vessel (2) and the pit (3) (i.e. to and from the at least one flow channel of the open pond vessel and the pit).

[0061] Pit

[0062] In an embodiment, the pit (3) comprises a bottom, at least one peripheral wall and a top opening; wherein the peripheral wall of the pit extends downwards from the bottom of the flow channel to the bottom of the pit; preferably, the pit upper portion, understood as the highest point of the at least one peripheral wall of the pit, is located at the bottom of the flow channel, understood as the greater depth of the flow channel.

[0063] In a particular embodiment, the pit (3) is adapted for receiving, containing and circulating an aqueous culture medium; particularly, the pit is adapted for being completely filled with an aqueous culture medium.

[0064] In a more particular embodiment, the bottom of the pit (3) is substantially flat.

[0065] In an embodiment, the pit has a cylindrical shape.

[0066] In an embodiment, the volume of the pit (3) is equal to or between 9 and 11% of the total volume of the reactor (1); even much more preferably is about 10% of the total volume of the reactor (1).

[0067] In an embodiment, the pit (3) comprises air inlets. In a particular embodiment, the air inlets of the pit are adapted for injecting air, in particular for injecting only air (i.e. air injection only); preferably for only allowing entering or injecting air.

[0068] In an embodiment the air inlets of the pit (3) are adapted for injecting air in the pit; preferably for injecting only air in the pit. In a more particular embodiment, the air inlets of the pit (3) are adapted for not injecting a gas different than air.

[0069] In an embodiment the air inlets of the pit (3) are located at the bottom of the pit. In another embodiment, the air is injected in an upwards direction, i.e. from the bottom to the top of the pit; more preferably the air is bubbled from the bottom to the top of the pit and against the direction of the flow; even much more preferably he air is bubbled from the bottom to the top of the pit and against the direction of the flow with a diffuser.

[0070] In a particular embodiment, the pit (3) does not comprise CO2 inlets, in particular CO2 gas inlets.

[0071] In another embodiment, each of the air inlets is in fluidic communication with at least one air diffuser; preferably with at least 4 diffusers; more preferably with at least 10 diffusers. In an embodiment, the at least one air diffuser is adapted for bubbling air. In an embodiment, each flow channel of the reactor has the shape of a rounded rectangle with four sections: two straight parallel segments, separated by an intermediate partition and joined at their ends with a semicircular curved bend and the pit is located at the bottom of one of the two straight parallel segments of the flow channel. In a more particular embodiment, the pit has a width equivalent or less than that of the flow channel.

[0072] In a particular embodiment, the pit has a depth equal to or less than 5 m; preferably equal to or less than 3 m; more preferably equal to or less than 2 m.

[0073] During biomass production, the pit is completely filled with aqueous culture medium without air pockets. The authors of the present invention have observed that the characteristics of the pit of the reactor lead to a lower risk of contamination, particularly if said pit has a cylindrical shape.

[0074] In an embodiment, the open pond reactor comprises a device (4) adapted for circulating liquid inside the reactor, such as an aqueous culture medium, preferably an agitator such as a paddle-wheel agitator, preferably a paddle-wheel agitator that propels the liquid when rotating around its axis. In an embodiment, the rotational speed of the paddle-wheel agitator is comprised between 10 and 80 rpm, preferably between 20 and 70 rpm; more preferably between 25 and 60 rpm.

[0075] In a particular embodiment, each flow channel of the reactor has the shape of a rounded rectangle with four sections: two straight parallel segments, separated by an intermediate partition and joined at their ends with a semicircular curved bend and the device (4) adapted for circulating liquid is located at the bottom of one of the two straight parallel segments of the flow channel, preferably before the pit in the direction of the circulation of the liquid; more preferably the device is located at the exit of one of the curves towards the contiguous straight segment of the flow channel.

[0076] System

[0077] A second aspect of the invention is directed to a system for CO2 capture and biomass production, comprising:

[0078] - the open pond reactor (1) according to of the invention in any of its particular embodiments, or an open pond reactor (1) comprising: o an open pond vessel (2) comprising at least one flow channel; o a pit (3) located at the bottom of the at least one flow channel of the open pond vessel; wherein the open pond vessel (2) and the pit (3) are adapted for receiving, containing and circulating an aqueous culture medium; wherein the open pond vessel (2) and the pit (3) are in fluidic communication; wherein the volume of the pit is equal to or between 4 and 22% of the total volume of the reactor (1), preferably equal to or between 4 and 21 , 20, 19, 18 or 17%, more preferably equal to or between 4 and 16%, even more preferably equal to or between 8 and 12%, even much more preferably equal to or between 9 and 11 %, more preferably about 10%; and wherein the pit (3) comprises air inlets;

[0079] - an air injection device adapted for injecting air through the air inlets of the pit (3) of the open pond reactor (1); and

[0080] - an aqueous culture medium comprising microalgae and / or cyanobacteria.

[0081] In the context of the present invention, the expression “air injection device” is understood as a device adapted for injecting a flow of air through the pit air inlets of the open pond reactor (1) of the present invention.

[0082] In an embodiment, the air injection device is an air pump or an air blower.

[0083] In an embodiment, the air injection device is in fluidic communication with the air inlets of the pit (3) of the open pond reactor (1) of the invention.

[0084] The system for CO2 capture only has air as carbonation source; particularly has only air as CO2 source.

[0085] In a particular embodiment, the system for CO2 capture does not have an additional carbonation source rather than air.

[0086] In a particular embodiment, the system for CO2 capture does not have a CO2 injection device.

[0087] In another particular embodiment, the system for CO2 capture does not have any other carbonation source than air; in particular it does not have any other CO2 source than air.

[0088] In an embodiment, the only carbonation or CO2 source of the system for CO2 capture is air.

[0089] In a particular embodiment, air is understood as having the composition as known in the art. In a particular embodiment, air comprises between 0.02 and 0.05 % by volume of CO2; preferably between 0.03 and 0.045%; more preferably about 0.04% or about 400 parts per million (ppm) as known in the art. In a particular embodiment, air, preferably dry air, comprises about 78% by volume of N2, about 21 % by volume of O2, about 1 % by volume of Ar, about 0.04% by volume of carbon dioxide; more preferably comprises about 78.08 % by volume of N2, about 20.94 % by volume of O2, about 0.93 % by volume of Ar, about 0.04% by volume of carbon dioxide.

[0090] In a particular embodiment, the system further comprises a device adapted for measuring the dissolved oxygen in the aqueous culture medium, particularly by any method known in the art such as polarography or optical dissolved oxygen concentration meters.

[0091] In a particular embodiment, the system further comprises a device adapted for measuring the pH in the aqueous culture medium, particularly by any method known in the art.

[0092] Aqueous culture medium

[0093] The system for CO2 capture and biomass production of the invention, comprises an aqueous culture medium comprising microalgae and / or cyanobacteria; preferably microalgae or cyanobacteria.

[0094] In the context of the present invention the expression “aqueous culture medium” is understood as known in the art, as a culture medium comprising nutrients and water adapted for culturing and growing microalgae and / or cyanobacteria.

[0095] In an embodiment, the aqueous culture medium has a basic pH; preferably has a pH equal to or over 9; even much more preferably equal to o over 9.5; more preferably of about 10.

[0096] In an embodiment, the aqueous culture medium has a basic pH; preferably has a pH equal to or between 8 and 11.5; preferably equal to or between 8.5 and 11 ; more preferably equal to or between 9 and 10.5; even much more preferably of about 10.

[0097] In another embodiment, the culture media has dissolved oxygen values below 20 mg / L; wherein the dissolved oxygen values have been measured with dissolved oxygen concentration meters known in the art such as polarography or optical dissolved oxygen concentration meters.

[0098] In an embodiment, the microalgae or cyanobacteria are adapted for a basic pH; preferably has a pH equal to or between 8 and 11.5; preferably equal to or between 8.5 and 11 ; more preferably equal to or between 9 and 10.5; even much more preferably of about 10.

[0099] In a particular embodiment, the aqueous culture medium comprises microalgae; preferably microalgae selected from Bacillariophyceae (diatoms), Chlorophyceae (green algae), Chrysophyceae (golden algae), Cyanophyceae (blue algae) and mixtures thereof

[0100] In an embodiment, the aqueous culture medium comprises freshwater cyanobacteria species; particularly photosynthetic freshwater cyanobacteria species.

[0101] In an embodiment, the aqueous culture medium comprises freshwater microalgae species; preferably freshwater microalgae species selected from Scenedesmus, Chlorella, Dunaliella, Haematococcus, Spirulina, and Anabaena microalgae; more preferably Scenedesmus or Chlorella; even much more preferably Scenedesmus sp.

[0102] In an embodiment, the system for CO2 capture and biomass production of the invention, comprises a sheet of aqueous culture medium on the open pond vessel flow channels and the pit of the reactor of the invention. In particular, the sheet of aqueous culture medium completely fills the pit of the reactor. In a more particular embodiment, the sheet of aqueous culture medium has a depth in the open pond vessel, preferably measured in the flow channels of the open pond vessel, of between 0.2 and 1.5 m; preferably between 0.4 and 1.2 m.

[0103] In a particular embodiment, the aqueous culture medium comprises an effluent that need to be treated; preferably an effluent such as waste water, sewage, combustion gas, biogas of a mixture thereof.

[0104] In an embodiment, the system for CO2 capture and biomass production, consists of:

[0105] • the open pond reactor (1) according to the invention in any of its particular embodiments or an open pond reactor comprising:

[0106] - an open pond vessel (2) comprising at least one flow channel;

[0107] - a pit (3) located at the bottom of the at least one flow channel of the open pond vessel; wherein the open pond vessel (2) and the pit (3) are adapted for receiving, containing and circulating an aqueous culture medium; wherein the open pond vessel (2) and the pit (3) are in fluidic communication; wherein the volume of the pit is equal to or between 4 and 22% of the total volume of the reactor (1); and wherein the pit (3) comprises air inlets;

[0108] • an air injection device adapted for injecting air through the air inlets of the pit (3) of the open pond reactor (1); and

[0109] • an aqueous culture medium comprising microalgae or cyanobacteria.

[0110] Method for CO2 capture

[0111] An additional aspect of the invention is directed to a method for CO2 capture and biomass production comprising the following steps:

[0112] (i) providing the system according to of the invention in any of its particular embodiments, comprising: a) the open pond reactor (1) according to of the invention in any of its particular embodiments, or an open pond reactor (1) comprising:

[0113] ■ an open pond vessel (2) comprising at least one flow channel;

[0114] ■ a pit (3) located at the bottom of the at least one flow channel of the open pond vessel; wherein the open pond vessel (2) and the pit (3) are adapted for receiving, containing and circulating an aqueous culture medium; wherein the open pond vessel (2) and the pit (3) are in fluidic communication; wherein the volume of the pit is equal to or between 4 and 22% of the total volume of the reactor (1), preferably between 4 and 16% of the total volume of the reactor (1), more preferably between 8 and 12% of the total volume of the reactor (1), even much more preferably between 9 and 11% of the total volume of the reactor (1), more preferably about 10% of the total volume of the reactor (1); and wherein the pit (3) comprises air inlets; b) an air injection device adapted for injecting air through the air inlets of the pit (3) of the open pond reactor (1) of (a); and c) an aqueous culture medium comprising microalgae and / or cyanobacteria; (ii) adding the aqueous culture medium comprising microalgae and / or cyanobacteria to the open pond reactor of (a) and circulating said aqueous culture medium inside said open pond reactor; and

[0115] (iii) during circulation:

[0116] - adding to the aqueous culture medium nutrients for microalgae or cyanobacteria growth;

[0117] - irradiating the aqueous culture medium with light; and

[0118] - providing an air flow below 5 L / m2 / min through the air inlets of the pit of the open pond reactor of step (i) (a) with the air injection device of step (i).

[0119] In a particular embodiment, step (ii) comprises circulating the aqueous culture medium inside the open pond reactor in a continuous loop; preferably in a closed continuous loop; more preferably at a constant speed; even much more preferably at a constant speed and continuously. In an embodiment, the circulation of step (ii) is performed with the device (4) adapted for circulating liquid of the reactor as has been defined in any of the particular embodiments above.

[0120] In a particular embodiment, step (iii) comprises irradiating the aqueous culture medium with an electromagnetic radiation such as light, in particular light selected from infrared, visible, ultraviolet light or a combination thereof; more preferably with sunlight as known in the art. In a more particular embodiment, the irradiation of step (iii) is not continuous, in particular is performed during at least 4 h per day; more preferably is performed during between 4 and 14 h per day; even more preferably is performed during the daylight hours of the day. In a more particular embodiment, the irradiation of step (iii) is performed on the aqueous culture medium through the top opening of the flow channel of the reactor of the present invention.

[0121] In a particular embodiment, step (iii) comprises measuring the dissolved oxygen in the aqueous culture medium, particularly by any method known in the art.

[0122] In a particular embodiment, step (iii) comprises measuring the pH of the aqueous culture medium, particularly by any method known in the art.

[0123] In an embodiment the air flow of step (iii) is provided in a not continuous way; preferably between one and three times per day. In a particular embodiment, the air flow of step (iii) is provided if the dissolved oxygen in the aqueous culture medium is over 20 mg / l and / or if the pH in the aqueous culture medium is over 11.5; preferably wherein the air flow is below 5 L / m2 / min.

[0124] In a particular embodiment, step (iii) comprises providing an air flow through the gas inlets of the pit of the open pond reactor with the air injection device of step (i) if the dissolved oxygen in the aqueous culture medium is over 20 mg / l; preferably wherein the air flow is below 5 L / m2 / min.

[0125] In a particular embodiment, step (iii) comprises providing an air flow through the gas inlets of the pit of the open pond reactor with the air injection device of step (i) if the pH in the aqueous culture medium is over 11.5; preferably over 11 , in particular wherein the air flow is up to 5 L / m2 / min.

[0126] In an embodiment, the only CO2 source of the method of the invention for CO2 capture and biomass production is the CO2 present in the air. In an embodiment, the CO2 source of the method of the invention consist of air.

[0127] In an embodiment, the method of the invention for CO2 capture and biomass production does not comprise an additional carbonation source rather than the air.

[0128] In another embodiment, the method of the invention for CO2 capture and biomass production does not comprise a CO2 injection device.

[0129] In a particular embodiment, the method further comprises a step (iv) of harvesting the microalgae and / or cyanobacteria; particularly the harvesting step was performed once per day.

[0130] In a more particular embodiment, during the harvesting step (iv) more than 5% of the total volume of the aqueous culture medium is removed and replaced by new aqueous culture medium; preferably more than 10%; more preferably about 20%.

[0131] In a more particular embodiment, during the harvesting step (iv) an amount equal or between 0.1 and 20% of the total volume of the aqueous culture medium is removed and replaced by new aqueous culture medium; preferably an amount equal or between 1 and 10 %; more preferably an amount equal or between 1.5 and 5 %.

[0132] In a particular embodiment, the harvesting step comprises a step of removing microalgae and / or cyanobacteria from the aqueous culture medium by sedimentation, filtration, or centrifugation.

[0133] Uses

[0134] An additional aspect of the invention is directed to the use of the reactor of the invention in any of its particular embodiments, or the system of the invention in any of its particular embodiments, for producing biomass, preferably for producing biomass-based food, feed, biomaterials, biofertilizers or biofuels.

[0135] An alternative aspect of the invention is directed to a method for producing biomassbased food, feed, biomaterials, biofertilizers or biofuels that comprises a step of producing biomass with the reactor or the system of the invention in any of its particular embodiments.

[0136] An additional aspect of the invention is directed to the use of the reactor of the invention in any of its particular embodiments, or the system of the invention in any of its particular embodiments for effluent treatment; preferably wherein the effluent is selected from waste water, sewage, combustion gas or biogas.

[0137] An alternative aspect of the invention is directed to a method for treating an effluent that comprises a step of producing biomass with the reactor or the system of the invention in any of its particular embodiments; preferably wherein the effluent is selected from waste water, sewage, combustion gas or biogas.

[0138] Throughout the description and claims the word “comprises" and variations of the word, are not intended to exclude other technical features, additives, components or steps. Furthermore, the word “comprise” encompasses the case of “consisting of’. In addition, all the particular features described for the open pond reactor of the present invention in any of its particular embodiment may apply to the system and method of the present invention. Additional objects, advantages and features of the invention will become apparent to those skilled in the art upon examination of the description or may be learned by practice of the invention. The following examples are provided by way of illustration, and they are not intended to be limiting of the present invention. Furthermore, the present invention covers all possible combinations of particular and preferred embodiments described herein.

[0139] EXAMPLES The invention is illustrated by means of the following examples that in no case limit the scope of the invention.

[0140] Example 1

[0141] A raceway open reactor was used on Example 1. The reactor comprised two 80 m length channels (4 m wide) connected by 180° bends at each end, and operated at an aqueous culture depth of 0.2 m (see figure 1). The reactor was made of 1.5 mm semi rigid polyethylene film. In addition, the reactor had a cylindrical 14 m3pit (thus contributing with 10% to the total volume of the reactor) which was 2.3 m deep and had a diameter of 2.8 m. Moreover, the reactor comprised a paddle-wheel to circulate the liquid inside the reactor.

[0142] As showed on figures 2, 3 and 4, air was automatically injected at the bottom of the pit of the reactor through diffusers to (i) provide CO2 to the system (the CO2 was part of the air composition), (ii) control the dissolved oxygen (the optimum DO value is below 20 mg / L) and (iii) control the pH of the culture (the optimum pH value is a basic pH with a value about 10). In order to do that, two sets of air diffusers connected with an air blower (P<300 mbar, 3 kW) were at the bottom of the pit: a first set with 4 diffusers and a second set with 8 diffusers. Each diffuser can provide an air flow up to 8 m3 / h.

[0143] Three experiments were performed with the reactor working at semi-continuous mode: (i) a first one providing only air through the diffusers of the pit (no additional CO2 was injected to the system such as pure CO2 or flue gases), (ii) a second one providing a pure CO2 gas flow and (iii) a third one not providing any gas flow through the diffusers. For this purpose, the reactor was filled with culture medium (142 m3volume) and it was inoculated with 7 m3of seed microalgae culture (Scenedesmus sp). Then, it was operated in batch mode for one week, following the semi-continuous operation start. For that, about a 20% of the culture was harvested daily and replaced with fresh medium, this operation being maintained for two months without problems.

[0144] When air was provided to the reactor at a 96 m3 / h flow through the diffusers at the bottom of the pit as a CO2 source, then 15 g / m2day of microalgae were produced. Under the same conditions, when pure CO2 was provided to the reactor at a 0.3 m3 / h flow through the diffusers at the bottom of the slump, then 20 g / m2day of microalgae were produced. Under the same conditions, when no air and no CO2 was provided to the reactor through the diffusers, then 5 g / m2day of microalgae were produced. The authors observed that the cost of providing air as a CO2 source was about 30% cheaper than providing CO2 while getting an optimal amount of biomass.

[0145] Example 2.

[0146] In another example, the same technology described on example 1 was evaluated at a smaller reactor as the one described on example 1 using a raceway reactor comprising two 1 m wide channels of 40 m length, thus, having a total 80 m2surface. The reactor has a 0.8 m3pit which was 2 m deep and had a diameter of 0.87 m, thus contributing with 5% to the total volume of the reactor. Moreover, the reactor comprised a paddle-wheel to circulate the liquid inside the reactor.

[0147] This reactor was used for the production of microalgae biomass only using air as CO2 source, providing biomass productivity values up to 15 g / m2day. This value confirm the applicability of the process and its scalability.

[0148] Example 3.

[0149] A similar experiment as in example 2 was performed using a reactor with a pit contributing with a 15% to the total volume of the reactor leading to biomass productivity values up to 15 g / m2day.

[0150] In addition, by testing different pit volumes, the authors observed that when the volume of the pit is below 4% of the total volume of the reactor, there is not enough CO2 mass transfer between the air source and the liquid medium, and therefore, the production of biomass is reduced. In addition, if the volume of the pit is too large, such as over 22 % of the total volume of the reactor, the dark volume (the area where only a reduced amount of light reaches) of the reactor is excessive and the production of microalgae is significantly reduced. In the latest case there are also dead regions with uncontrolled accumulation of biomass. Thus, pit volume values between 4 and 22% lead to acceptable production of biomass. Nevertheless, the authors have observed that the best results regarding mass transfer and reduction of dead regions and contamination were obtained when the volume of the pit was around a 10 % of the total volume of the reactor. In addition, results showed that when the aqueous culture medium had a basic pH, the CO2 mass transfer and thus, CO2 capture and biomass production was improved over other pH values. In addition, the authors have observed that the technology proposed is feasible for the production of different microalgae strains although the use of robust strains capable to growth outdoor in open systems such as Scenedesmus sp, is preferred.

[0151] Example 4.

[0152] In another example the microalgae biomass produced with the system and reactor described on example 1 is separated from the culture media, dried and used as human food, feed for animals and plant fertilizer.

[0153] Example 5.

[0154] The technology proposed is feasible for the production of high quality and low quality biomass, including that suitable for wastewater treatment. In another example, the system described on example 1 is utilized to provide oxygen removal and carbon capture necessities for a 80 m2reactor on which microalgae related wastewater treatment is performed.

[0155] Example 6.

[0156] The technology proposed is also feasible for the direct capture of carbon from flue gases. In another example of the technology, CO2 is provided to the system described on example 1 using flue gas from a biomass boiler. The flue gas contains about an 8% in volume of CO2. The authors have observed that despite the percentage of CO2 on flue gas is higher than on air, since the reactor and the system are able to provide CO2 on demand (for example, modifying the gas flows entering the pit of the reactor), it is not an issue to keep the pH at values above 10.

[0157] Example 7.

[0158] In another example, the same technology described in example 1 was evaluated using raceway reactors similar to those described in example 1 , but with different pit volumes and surface areas, as shown on Table 1 below. Moreover, the reactors comprised a paddlewheel to circulate the liquid inside the reactor.

[0159] Table 1.

[0160] Using only air as a CO2 source, these reactors were used to capture carbon and produce microalgae biomass. In particular, the airflow indicated on Table 1 was injected through the diffusers of the pit (no additional CO2 was injected to the system, such as pure CO2 or flue gases).

[0161] The results showed that the energy required to supply air to the system was significantly less than when pure CO2 was injected into the system. For example, an energy of 0.34 kWh / day was used when an airflow of 100 l / min was introduced to the system through the pit diffusers. But for introducing an equivalent amount of CO2 from flue gases (considering an energy demand of this capture process of 4 MJ / kg), between 1.5 and 4.8 kWh / day would be needed. The energy consumption of direct air injection is therefore significantly lower than the energy consumption of injection of CO2. Air injection is therefore much more cost effective.

[0162] In addition, results showed that the biomass production was optimized when the volume of the pit is equal to or between 4 and 22% of the total volume of the reactor; preferably equal to or between 8 and 12% of the total volume of the reactor.

Claims

CLAIMS1 . An open pond reactor (1) for CO2 capture and biomass production, comprising:- an open pond vessel (2) comprising at least one flow channel;- a pit (3) located at the bottom of the at least one flow channel of the open pond vessel; wherein the open pond vessel (2) and the pit (3) are adapted for receiving, containing and circulating an aqueous culture medium; wherein the open pond vessel (2) and the pit (3) are in fluidic communication; wherein the volume of the pit is equal to or between 8 and 12% of the total volume of the reactor (1); and wherein the pit (3) comprises air inlets.

2. The open pond reactor according to claim 1 , wherein the volume of the pit (3) is equal to or between 9 and 11 % of the total volume of the reactor (1).

3. The open pond reactor according to claim 1 or 2, wherein the pit (3) comprises gas diffusers (4), and wherein each of the air inlets is in fluidic communication with at least one air diffuser.

4. The open pond reactor according to any of claims 1 to 3, wherein the open pond vessel (2) is a raceway open pond reactor comprising two flow channels connected by bends at each end.

5. The open pond reactor according to any of claims 1 to 4, wherein the pit (3) has a cylindrical shape.

6. The open pond reactor according to any of claims 1 to 5, comprising a device (5) adapted for circulating liquid inside the reactor, preferably a paddle-wheel.

7. A system for CO2 capture and biomass production, comprising:• the open pond reactor (1) according to any of claims 1 to 6, or an open pond reactor comprising:- an open pond vessel (2) comprising at least one flow channel;- a pit (3) located at the bottom of the at least one flow channel of the open pond vessel; wherein the open pond vessel (2) and the pit (3) are adapted for receiving, containing and circulating an aqueous culture medium; wherein the open pond vessel (2) and the pit (3) are in fluidic communication; wherein the volume of the pit is equal to or between 4 and 22% of the total volume of the reactor (1), and wherein the pit (3) comprises air inlets;• an air injection device adapted for injecting air through the air inlets of the pit (3) of the open pond reactor (1); and• an aqueous culture medium comprising microalgae and / or cyanobacteria.

8. The system for CO2 capture according to claim 7, wherein the aqueous culture medium has a pH equal to or over 9; preferably equal to or over 10.

9. The system for CO2 capture according to any of claims 7 or 8, wherein the culture medium has dissolved oxygen values below 20 mg / L; wherein the dissolved oxygen values have been measured with dissolved oxygen concentration meters known in the art such as polarography or optical dissolved oxygen concentration meters.

10. The system for CO2 capture according to any of claims 7 to 9, wherein the microalgae of the aqueous culture medium is a Scenedesmus, Chlorella, Dunaliella, Haematococcus, Spirulina, or Anabaena microalgae.11 . The system for CO2 capture according to any of claims 7 to 10, wherein the only CO2 source of the system is air.

12. A method for CO2 capture and biomass production comprising the following steps:(i) providing the system according to any of claims 7 to 11 , comprising:(a) the open pond reactor (1) according to any of claims 1 to 6, or an open pond reactor comprising:- an open pond vessel (2) comprising at least one flow channel;- a pit (3) located at the bottom of the at least one flow channel of the open pond vessel;- wherein the open pond vessel (2) and the pit (3) are adaptedfor receiving, containing and circulating an aqueous culture medium; wherein the open pond vessel (2) and the pit (3) are in fluidic communication, wherein the volume of the pit is equal to or between 4 and 22% of the total volume of the reactor (1), and wherein the pit (3) comprises air inlets; b) an air injection device adapted for injecting air through the air inlets of the pit (3) of the open pond reactor (1) of (a); and c) an aqueous culture medium comprising microalgae and / or cyanobacteria;(ii) adding the aqueous culture medium comprising microalgae or cyanobacteria to the open pond reactor of (a) and circulating said aqueous culture medium inside said open pond reactor; and(iii) during circulation:- adding to the aqueous culture medium nutrients for microalgae and / or cyanobacteria growth;- irradiating the aqueous culture medium with light; and- providing an air flow below 5 L / m2 / min through the air inlets of the pit of the open pond reactor by using the air injection device of step (i).

13. The method according to claim 12; wherein the aqueous culture medium has a pH equal to or over 9; and / or wherein the only CO2 source of the system is air.

14. Use of the reactor according to any of claims 1-6, or the system according to any of claims 7-11 for producing biomass-based food, feed, biomaterials, biofertilizers or biofuels.

15. Use of the reactor according to any of claim 1-6, or the system according to any of claims 7-11 for effluent treatment; preferably wherein the effluent is selected from waste water, sewage, combustion gas, biogas or a mixture thereof.

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