TECHNICAL FIELD

The invention relates to a plant and method for the production of photosynthetic microorganisms, such as microalgae or cyanobacteria, which allow the transformation of CO ₂ into biomass usable as a source of nutrition, for the extraction of biomolecules of nutraceutical or pharmaceutical interest, or for the production of biofuels. The plant and the process can be managed ecologically and economically.

BACKGROUND ART

In recent years, problems related to global warming and pollution, combined with the growing themes of circular economy and energy savings, have prompted research to propose new solutions to lower carbon dioxide emissions, or transform it into other products.

Among the various technologies investigated is the biological sequestration of CO ₂ . If a hectare of forest, for example, has the capacity to sequester approximately 10-20 tonnes/year of CO ₂ depending on the geographical area and type of tree considered, the same area, used for photobioreactor greenhouses for the production of microalgae could sequester more than 200- 400 tonnes/year of CO ₂ , depending on the photobioreactor technology used, thus making this technology particularly competitive.

The subtraction of CO ₂ from the atmosphere can usefully be linked to algal biomass production. This is a biological sequestration of CO ₂ . In fact, these single-celled microorganisms are predominantly photoautotrophs, that is, they use light to obtain energy and CO ₂ as a source of carbon for biosynthesis.

In addition, in the coming years, it is expected that the production of microalgae will develop rapidly, justified by the increasing number of areas of use.

Microalgae are between 1 and 50 pm in size, and can grow and live in hostile environments thanks to their simple single-cell or multi-cellular structure.

Production of biomass from microalgae requires for photosynthesis: in addition to water and carbon dioxide macro- and micronutrients (minerals) and an energy source (such as sunlight). In return, the process provides oxygen and biomass from which high value products such as pigments, fatty acids and bioactive metabolites can be extracted. The biomass produced can also be used as a source of bioenergy (biodiesel, methane and hydrogen), for the production of bioplastics, as well as as biofertiliser, animal feed, aquaculture food or for human consumption (health food).

One of the most important characteristics of microalgae is that they are non-toxic (at least for commonly used strains) and, therefore, are expected to replace, as far as possible, synthetically obtained pharmacological intermediates.

A comparison of the productivity of traditional plant sources and microalgae for biofuel production shows the great advantage of algae, which have an oil yield of 10-20 tons per hectare per year compared to classic plants such as maize, soybean, sunflower, rapeseed, canola, jatropha, coconut or oil palm, which reach values in the range of 0.17 tons per hectare per year for maize, and of 5.9 tons per hectare per year for oil palm.

The management of plants for the production of microalgae addresses problems related to the energy consumption required, in particular, for heating photobioreactors and recirculating the culture solution. Energy spent on maintaining the required temperature in photobioreactors and recirculating the algal solution heavily affects the cost of producing microalgae and may make this technology financially unacceptable for all productions. Currently, the cost of producing biofuels exceeds the cost of producing oil, but the expectation is that, with the development of microalgae growth and treatment technology, the cost of cultivation may also decrease. Document US 10 287 643 B2 describes the use of waste gas from the reduction of ferrous minerals in blast furnaces to feed a bioreactor. Also from other areas, the use of industrial waste in the context of bioreactors (US 2010/0255458 Al, WO 2011/139804 Al, US 2012/0308989 Al) is known.

DISCLOSURE OF THE INVENTION The object of the invention is to overcome the aforementioned drawbacks and propose a plant and process for the production of photosynthetic microorganisms, in particular microalgae, that is fed and managed efficiently and economically to lower the costs for the production of microorganisms, and that also takes into account environmental issues.

The object of the invention is initially achieved by a combined plant for the exploitation of steel mill waste for the production of photosynthetic microorganisms, such as microalgae and/or cyanobacteria, which comprises:

(a) at least one photobioreactor with:

(a-1) a container suitable for containing said microorganisms in water;

(a-2) a light source adapted to provide light suitable for the growth of said microorganisms;

(a-3) a system for agitating and/or moving said microorganisms within said container; (a-4) a connection of said container to a water supply;

(a-5) a nutrient feed for introducing nutrients for said microorganisms into said water; (a-6) a connection to a source of carbon dioxide (CO ₂ );

(b) a heating device, in particular with a heat exchanger for heating said water; (c) a harvesting system equipped with separating devices for separating said microorganisms from the water at the end of the growth cycle;

(d) a control unit for coordinating and controlling the various components of the combined plant;

(e) a power supply to power the control unit and electrical components of the combined plant; and

(f) a metallurgical plant, preferably steel plant comprising

(f-1) at least one source producing carbon dioxide (CO ₂ ) as a waste product and corresponding to said carbon dioxide source; and

(f-2) at least one apparatus producing heat usable as thermal waste and corresponding to said heating device.

The term photobioreactor refers to open or closed culture systems for the growth of photosynthetic microorganisms; it thus refers both to closed reactors and open tanks.

The photobioreactor(s) is/are powered by equipment integrated in the steel mill whose thermal and CO ₂ waste are economic sources to manage the plant. The proposed solution leverages waste borrowed from other areas of production and ecologically and economically links steelmaking to biotechnology. In this perspective, the installation of photosynthetic microorganism plants has the potential to be a biological sequestration system of CO ₂ released as waste product in the steel mill.

The macrocategory of photosynthetic microorganisms includes not only microalgae but also prokaryotic cyanobacteria (Cyanophyceae). Eukaryotic microalgae are green algae (Chlorophyta) and diatomaceous algae (Bacillariophyta).

An example list of algae strains suitable for implementing the invention includes Ankistrodesmus sp., Botrycoccus braunii, Chaetocerus calcitrans, Chaetocerus muelleri, Chlorella, Dunaliella sp., Ellipsoidion, Isochrysis sp., Moncdlanthus salina, Nannochloris sp., Nannochloropsis sp., Neochloris oleabundans, Niizschia sp., Pavlova lutheri, Pavlova salina, Phaeodactylum tricomutum, Prymnesinm parvum, Scenedesmus dimorphus, Scenedesmus obliquus, Schizochytrium sp., Skeletonema, Spirulina and Stichococcus. Among these, algae with higher lipid content are of particular interest for the production of biodiesel. Specific mention should be made of Botrycoccus braunii, Chlorella, Dunaliella sp. Nannochloris sp., Nannochloropsis sp., Neochloris oleabundans, Phaeodactylum tricomutum, Scenedesmus obliquus, Schizochytrium sp., and Skeletonema, which can have a lipid content even greater than 50 % on dry matter.

Microalgae of various types have been made available on the market for a long time from a wide range of suppliers.

On an industrial scale, the plant subject of the invention comprises not only a photobioreactor, but ideally a plurality of photobioreactors. They can be fed with CO ₂ , water, light and nutrients, and operated from a single source whose connection to the photobioreactors branches off to power the individual photobioreactors or groups of photobioreactors. A skilled person with general knowledge easily identifies the possible and suitable connections between photobioreactors and feeding sources. Examples for water sources are sewage, brackish water, sea water and drinking water. The demands imposed by the use of the product will determine the usable water supply. The water used for the production of microalgae for nutraceuticals is preferably drinking water. However, the combined plant according to the invention can also be used for wastewater treatment; in this case, it will direct the product obtained to the production of biofuel (oil, Hz, CHt) or only to the cleaning of water, without any added market. Microorganisms can grow by natural light (sun), artificial light, or mixed systems. In an advantageous variant of the invention, the light source is sunlight. The market offers a wide choice of lamps suitable for use in the cultivation of photosynthetic microorganisms, for example LED (light-emitting diode) lighting is preferred. As already mentioned at the beginning, agitation and/or movement of the microalgal solution requires particular energy efforts. Agitation can obviously take place with classic agitators that move water mechanically. Alternatively, mixing is possible by exploiting the bubble of a gas being introduced, such as compressed air. The aim is always to avoid sedimentation of the algal mass that would result in its death. In addition, movement allows effective contact of microorganisms with nutrients.

The electrical energy required for lighting, recirculation of the solution, maintenance at temperature and, in general, for the electrical components of the plant, can be obtained besides from recovery systems inside the steel mills (TFV (= thermophotovoltaic) or TE (= thermoelectric) type systems), for example also from photovoltaic and/or wind recovery. When they have reached a certain amount, microorganisms can be collected from inside the photobioreactor via filters through which the water containing the microorganisms passes, with filtered water remaining in the container. The water with the microorganisms can also be discharged from the container, passed through an external collection system, for example one equipped with filters. The filtered water is then returned to the container after the microorganisms have been separated from it.

Depending on the size and type of the photobioreactor, the biomass can be collected continuously, semi-continuously or in batches.

A heat exchanger powered by the heating device may transfer heat directly to the water in the container, or may alternatively transfer its heat to the water before it enters the container. Advantageously, a water temperature control system regulates the heat input from the heating device to the heat exchanger and from it to the cultivation water.

Nutrient feeding can take place directly in the container, or upstream in the water pipes, or even earlier directly in the water supply. The skilled person readily identifies the type and concentrations of nutrients for the cultured microorganisms. For the non-nutraceutical production of algae and if the water is not (too) polluted, it is possible to use waste water sources from steel mills. Advantageously, the system according to the invention also provides a cooling device to be able to condition the temperature of the water even in hot climates, thus avoiding temperatures that are too high for the microorganisms.

Within a steel mill or metallurgical plant, and, in particular a siderurgical plant, there are usually multiple sources of CO ₂ . CO ₂ can be released from melting furnaces, heat treatment furnaces, annealing furnaces, ladle preheating, basket preheating and drying stations. Any device/machine with one or more burners, or where some form of fossil fuel or natural gas is combusted, releases CO ₂ . The selection of some fumes rather than others is dictated by the needs of microalgae that are sensitive to certain substances toxic to them. Fumes containing at least 8 to 10% by volume of CO 2 with a low powder content are preferred, thus de-dusted fumes. Some algal strains, however, also grow with 100% CO ₂ . Fumes introduced into photobioreactors also usually require cooling to 40°C, or lower temperatures.

The carbon dioxide source can feed the cultivation water in the container with CO ₂ , or it can be introduced with the gas flow necessary for the movement of the algal solution. Depending on the type of microalgae, this source will have characteristics of temperature, pressure, dust content and concentration well defined and be known to the expert to ensure that the microalgae have an ecosystem suitable for their growth and development. This source can be fed continuously or discontinuously, depending on the operating methods selected and the state of growth of the microalgae; in any case, only part of it is seized biologically. The culture water is preferably recirculated and only the water lost by evaporation and in the collection phase is replenished.

The control unit advantageously regulates the flows of water, carbon dioxide and nutrients, the degree of agitation, the temperature of the water, the concentration of nutrients, the moments of collection of the product, the pH, any additions with water or seed microorganisms, etc. To this end, the photobioreactor is advantageously equipped with corresponding sensors, such as absorbance or optical density sensors, sensors for pH, specific ions, temperature, CO ₂ , etc.

In addition to the primary metallurgy machinery suitable for melting steel, the siderurgical plant may also include parts of secondary metallurgy where the product is refined and cast. There are also machines for rolling, often including heating furnaces using heating elements or burners. Many of the machines that make up these areas provide CO ₂ or thermal waste, which can be used, respectively, as a carbon dioxide source and/or heat source to feed or heat the bioreactor. Different types of furnaces in the steel mill produce different mixtures of waste gases as a source of carbon dioxide, which may vary in their composition in terms of CO ₂ and other components, such as H2O, nitrogen, oxygen, oxides of nitrogen or sulphur, dusts, etc. The composition of the waste gas may be corrected by removing or reducing some components by methods known in the art or by diluting the gas with other gases, such as air. Systems that enrich CO ₂ waste gas can also be considered. Preferably, the combined plant according to the invention comprises for this purpose devices for gas purification (e.g., de-dusting), for changing its composition, for compressing it or also for cooling it.

The gas from the siderurgical plant production site is preferably cooled to 30°C - 40°C. Advantageously, the gas is also de-dusted. The heat separated from gas cooling could then serve as thermal waste to heat the culture water, but also to district heat the plant’s utilities if any or to fed energy recovery devices (ORC: Organic Rankine Cycle, absorbers, etc.).

The CO ₂ containing gas from the steel mill production sites can be used as is or, in order to obtain the CO ₂ concentration required by the algae strain, undergo, as illustrated above, dilution treatments (with N2 or with air, where dilution can also serve to cool the gas) to reduce the CO 2 concentration, or treatment in separators (membrane separators, absorbers, PSA (pressure swing adsorption), TSA (temperature swing adsorption), and VPSA (vacuum pressure swing adsorption)) to obtain a gas with increased CO ₂ concentration.

In this regard, advantageously, the combined plant according to the invention further comprises gas pre-treatment systems containing CO ₂ deriving from the carbon dioxide source, to dilute the gas with other gases, enrich the gas with CO ₂ and/or separate gas components. Other pre- treatments are gas temperature and/or pressure setting.

Preferably, the carbon dioxide source provides a gas having the following characteristics: CO ₂ content of 1 to 100 vol. %, a powder concentration of less than 5 mg/Nm ³ and an SOx concentration of less than 100 mg/Nm ³ , wherein the remaining components of the gas comprise in particular O2, N2 and/or H2O.

In a preferred embodiment of the invention, carbon dioxide, i.e. the gas containing it provided by the CO ₂ source, acts as a co-stirring system by its own gurgling when introduced into the water contained in the container. Portions of the gas could be stored for use when the furnaces of the steel mill do not release CO ₂ because they are idle. In two preferred embodiments of the invention, said photobioreactor is a closed photobioreactor or an open tank. Techniques for the mass cultivation of microalgae are essentially based on two options: Open Ponds and closed photobioreactors. The state of the art knows various embodiments for both alternatives. Growth in open ponds occurs by growing algae outdoors and under sunlight, inside large tanks. Cultures in open tanks have two main problems, the first is the excessive evaporation of the liquid, especially in hot climates, resulting in progressive change in salinity and the possible loss of essential components. Second, open ponds are more exposed to contamination by bacteria, protozoa and other microalgae that can proliferate by competing for growth with the strain of interest. This susceptibility to contamination makes this type of technique less preferable in a context where the quality control of the product used for food pmposes under conditions where health problems are already serious, must be to the maximum extent possible. To minimise such contamination, maximise and control biomass growth and optimise the growth parameters of selected algal strains, it is advisable to cultivate within closed and controlled photobioreactors.

Photobioreactors must be designed with techniques known to the skilled person in order to maximise productivity, minimise evaporative losses and ensure product quality.

In a particularly advantageous embodiment of the invention, the apparatus that produces heat in the form of thermal waste, furnishes the thermal waste at temperatures between 30°C and 40°C. Low-temperature thermal waste is usually considered waste from the machinery making up the plant, since its temperatures are too low to be used for other purposes inside the steel mill or elsewhere. In fact, in the state of the art, such waste is generally brought to cooling, thus effectively wasting the energy contained in it. This waste, in accordance with the present invention, is then used to minimise the costs incurred to heat the photobioreactor. Such low- temperature heat producing apparatuses are, for example, heating furnaces, indirect circuits for the cooling of semi-finished products, casting machines, cooling of smoke systems, etc., which, through heat exchanger systems, yield their energy to the algal solution or to the feedwater. According to the invention, in particular in the case of a heating device with a heat exchanger, the thermal waste is not only used to heat the culture water of the bioreactor, but - giving their heat to the bioreactor - are also cooled during this use, thus avoiding to cool them in another way wasting energy contained therein and saving energy for their cooling. Thermal waste cooled in this manner is also reusable, for example in the metallurgical or siderurgical plant for cooling purposes.

The cultivation of microalgae requires maintaining a reactor temperature ranging between 25°C and 35°C, which can be achieved by leveraging said low-temperature thermal waste present in metallurgical plants, and, in particular, siderurgical plants, and which generally go to waste. In case of refrigeration request, in the summer months, heat pumps or dedicated chillers can be used to maintain the required temperature range.

In a preferred embodiment of the invention, the combined plant further comprises a compressed air supply for supplementing or replacing agitation and/or movement, in case of insufficient stirring effect caused by the recirculation of the algal solution. If the steel mill comprises multiple sources of CO ₂ , they can replace each other to supply the photobioreactor continuously with carbon dioxide.

A second aspect of the invention relates to a process of producing photosynthetic microorganisms, and, in particular, microalgae and/or cyanobacteria, with the following steps: (I) provision of a photobioreactor containing microorganisms in water with nutrients for said micro-organisms,

(Π) heating of the water by thermal waste from a metallurgical, preferably siderurgical, plant,

(III) irradiation of said microorganisms with light of suitable wavelength(s),

(IV) introduction of a gas containing carbon dioxide (CO ₂ ) into said water wherein said gas comes from a metallurgical plant, preferably a siderurgical plant, and stirring said microorganisms in said water;

(V) growth of said microorganisms by photosynthesis; and

(VI) collection of said microorganisms produced.

Preferably, the process according to the invention is performed in a combined plant, according to the invention. The individual steps of the process employ, in embodiments of the invention, singularly or in combination, the features of the plant specified above. Specifically, particular components of the plant are used and particular steps of feeding the culture medium, purification/enrichment/dilution of the CO ₂ containing gas, collection of the microorganisms produced, recirculation of the culture water, etc., are performed as illustrated above, in relation to the combined plant according to the invention. A further aspect of the invention relates to a use of the microorganisms produced in the combined plant according to the invention, or obtained according to the process according to the invention for animal or human nutrition, for the extraction of biomolecules of nutraceutical or pharmaceutical interest, or for transformation into biofuel. In particular, the large-scale cultivation of algae is useful for their use in the field of:

- aquaculture as feed for mussels, crustaceans and larval stages of fish,

- nutraceutical for the high nutritional power of microorganisms,

- pharmaceutical for the continuous discovery of bioactive molecules in microorganisms for the treatment of diseases,

- bioenergy for high oil content and production of hydrogen and biodiesel, and

- environmental for their bioremedial properties of polluted water, soil and air.

The features described for one aspect of the invention may be transferred mutatis mutandis to the other aspects of the invention.

The advantages in exploiting the mass production of microalgae for integrated food purposes are: no competition with other food resources, reuse of non-cultivatable or contaminated land, high growth rate, high content of proteins, supplements and bioactive molecules, continuous production during the year, limited water demand, manipulation of the production of the substances of interest, do not require the use of pesticides or herbicides, and - use of biomass for the extraction of numerous active compounds and molecules. In summary, it can be seen that the production of algae in the steel mill offers different advantages and achieves the objectives initially mentioned; in particular, the objective of being able to manage the production of photosynthetic microorganisms in an effective and economic way while reducing the discharge of CO ₂ into the atmosphere by producing biomass for different uses. The invention proposes an integrated microalgae cultivation technology, which exploits the by-products of a steel mill or metallurgical plant, preferably siderurgical plant, such as fumes (waste gas), as raw material for photosynthesis and, possibly, as a stirring system and heat from thermal waste for photobioreactor heating, allowing, at the same time, microorganisms, and, in particular, quality microalgae, to grow.

For a skilled person with general knowledge, this proposal is adaptable to any microalgae growth plant, both in open tanks, suitably covered to avoid contamination, and in closed photobioreactors.

The industrial applicability begins from the moment waste is recovered from a metallurgical plant, and preferably a siderurgical plant, to produce a high-value product that finds application in a variety of industrial fields, such as the food, pharmaceutical and energy sectors.

The technology in question is not (only) a method of sequestering CO ₂ streams, but of reusing CO ₂ to produce high-quality material with high added value and satisfactory productivity.

For example, industrial scaling up on a surface of 1000 m ² is conceivable, for the production of about 15-20 tons/year of dry biomass, by obtaining the separation of 30-40 tons/year of CO ₂ from flue gases and 550 tons/year of CO ₂ equivalent from thermal waste recovery.

Said objects and advantages will be further highlighted during the description of preferred embodiment examples of the invention provided by non-limiting way of example.

Variant embodiments of the invention are the object of the dependent claims. The description of the preferred examples of execution of the combined plant and the process for the production of photosynthetic microorganisms, as well as their use, is provided by way of non-limiting example, with reference to the drawings appended hereto. In particular, unless specified otherwise, the number, shape, size and materials of the plant and of the single components may vary, and equivalent elements may be applied without deviating from the inventive concept.

DESCRIPTION OF PREFERRED EMBODIMENT EXAMPLES

Fig. 1 illustrates in a block diagram an embodiment example of the combined plant according to the invention.

Fig. 2 illustrates in a flow chart an embodiment example of the process according to the invention.

Fig. 1 illustrates in a block diagram an embodiment example of the combined plant according to the invention. Note a steel mill 10 and a battery 12 of a plurality of photobioreactors 14. Each bioreactor 14 is equipped with a LED light 16, a stirrer 18 and a sensor system 20 that detects parameters like optical density, temperature, CO ₂ concentration and nutrients.

The bioreactors 14 are fed with CO ₂ (arrow 44) from a carbon dioxide source, such as a heating furnace 48, which produces waste gases containing CO ₂ , whose composition, temperature and possibly pressure can be modified by a pre-treatment system 46.

An example with the use of gas from the billet reheating furnace involves a gas pre-treatment by gas quenching with dust separation at 30°C and a gas compression at 1.5 bar. For example, a suitable volumetric composition of the gas could be 3.3 vol. % O2, 9.5 vol. % CO ₂ , 82.1 vol. % N2 and 4.6 vol. % H2O, but may vary according to the needs of the algae produced. Other apparatuses present in the steel environment, such as, for example, closed type cooling circuits 38, produce thermal waste 39 that feeds heat exchange systems 34 to heat water from a water supply 40 before introducing it (arrow 36b) into photobioreactors. During operation of the photobioreactors, the algal solution contained therein can be recirculated in the heat exchanger 34 (arrow 36c) to maintain its temperature at the desired value. Alternatively, heating devices (not illustrated) may be provided inside the bioreactor 14.

Obviously, the combined plant may be provided with cooling devices (not illustrated) in order to enable cooling of the algal solution exposed to hot outdoor temperatures.

The values detected by the sensors 20 are transmitted (arrow 22) to a control unit 24 which manages, on the basis of the values received, lighting (arrow 26), algae collection parameters in a filter 28 (arrow 29), addition of nutrients from a tank with a nutrient solution 30 (arrow 32), integration with water, water heating with heat exchanger 34 (and upstream with the thermal waste producer of the steel mill 38) and supply with heated water and recirculation of water from the bioreactors 14 and from the algae collection system (filter) 28 (arrows 36a), and flow 44 of CO ₂ 42 or the pre-treatment unit 46, for which it sets flow rate, temperature, pressure and composition of the CO ₂ containing gas coming from the furnace 48 of the steel mill 10 (arrow 49).

The microalgae collection unit 28 allows the return of separated water during filtration from the microalgae, and its return to the photobioreactors 14 directly (arrow 50a) or through a heat treatment in the heat exchanger 34 (arrow 50b) to the photobioreactor battery 12. The collected microalgae are subsequently intended for various external units 52 for different uses. To simplify the representation, the contribution of COz, heated water and nutrients has been represented as a feeding supply of the battery 12 as a unit, but is intended as a feeding supply of each individual photobioreactor 14.

The stirrers 18 serve for mixing the culture medium, if it is not stirred by the stream of gas 44 or when this stream is not sufficient.

The water supply 40 is mains water for high product enhancement. Water is sent to the photobioreactors 14 in adequate quantity and at the appropriate temperature.

The use of LEDs 16 or other light sources for the lighting of bioreactors 14 is different in terms of productivity and energy costs. LEDs are preferred because of lower power consumption, longer lifespan and better algae growth performance and, thus, CO ₂ consumption.

The flue gas to be treated will be sent during production hours. In the case of discontinuous production, compressed air will be sent during idle hours to keep the algae under stirring. The suspended biomass will be continuously moved and kept in circulation, passing through a heat exchanger 34 (with energy recovery from thermal waste), so as to maintain the ideal temperature. The reactors 14 will ideally be installed into a greenhouse, but may also be kept outdoors. A basic system sizing rule recommends that for each kg of dry biomass 7-10 kg of CO ₂ must be fed. Approximately 15% of the CO ₂ supplied is separated from the system.

The costs for the plant itself, lighting, gas pre-treatment, photovoltaics, any accumulator, and collection are amortised with the value of the biomass within a short time. The use of steel mill waste instead of or in addition to heating energy from the electricity grid or from photovoltaic or wind plants, therefore, improves the efficiency of the technology, and is viewed from the perspective of the circular economy.

Fig. 2 illustrates in a flow chart an embodiment example of the process according to the invention. A photobioreactor 100 is prepared. The heated water from thermal waste from a steel mill 102 is introduced into the photobioreactor at the temperature required by the microalgae. Microalgae seeding, nutrients that microalgae need and carbon dioxide that comes from waste gas from the steel mill are poured into the water; the culture medium thus obtained is agitated and illuminated at the wavelength required by microalgae 104.

Algae begin growing 106. During growth, respective sensors monitor various key parameters of the culture medium 108. That is how water temperature, pH, salinity (i.e. nutrient concentrations), CO ₂ content dissolved in the water and optical density used to determine the amount of algae produced are monitored.

The values measured by the sensors are compared by a controller with nominal values and, in case of deviations, the control unit intervenes to adjust the parameters 110. The control unit may, for example, as illustrated above for the plant, add nutrients or modulate carbon dioxide intake, or may affect water heating by intervening on heating by thermal waste. During hot periods, the control unit may also provide for cooling of the algal solution and suspend heating by thermal waste. When, instead, the value of the optical density (or another parameter useful for this purpose) indicates a certain amount of microalgae produced, the control unit begins to harvest 112 them.

The microalgae collected are then destined for different uses 114, such as processing into biofuel, extraction of biomolecules of nutraceutical or pharmaceutical interest, use as food for animals or humans, etc.

Within the basic process illustrated above, the individual steps may comprise interventions already described with reference to the plant depicted in Figure 1: pre-treatment of the gas containing carbon dioxide prior to introduction into the culture medium, modification of the gas composition, introduction of fresh water, recirculation of bioreactor water or of the water separated during algae collection, etc.

As example, Table 2 below shows the average composition of a suitable gas from a billet- annealing furnace that continuously produces gas at about 800°C :

The gas collected from the annealing furnace, after cooling and filtration (de-dusting), can be sent to the photobioreactor greenhouse. The amount of gas to be sent per reactor depends on the geometry of the reactor itself and is an easily computable parameter. As mentioned above, cultivation is managed by a process control which is based, inter alia, on the measurements of various algal solution parameters.

For 8-10kg of CO ₂ fed, about 2kg of CO ₂ is absorbed, leading to the growth of about 1kg of dry biomass. In the implementing phase, the plant and the process for the production of photosynthetic microoiganisms and their use, per the invention, may be further modified or subj ect to executive variants not described herein. If such modifications or such variants should fall within the scope of the following claims, they should all be considered protected by the present patent. In practice, the materials used, as well as dimensions, numbers and shapes, provided that they are compatible with the specific use and where not otherwise specified, may be different, according to requirements.