WO2016185438A1

WO2016185438A1 - Plant and method for producing microorganisms in aquaculture

Info

Publication number: WO2016185438A1
Application number: PCT/IB2016/052973
Authority: WO
Inventors: Guido EMILIANI
Original assignee: Micoperi Blue Growth S.R.L.
Priority date: 2015-05-21
Filing date: 2016-05-20
Publication date: 2016-11-24
Also published as: EP3298123A1; ITUB20150681A1

Abstract

A plant for producing microorganisms in aquaculture, comprising at least one container for a liquid culture medium suitable for growing microorganisms. The container is placed inside a greenhouse equipped with a system for controlling the internal environmental conditions. Preferably, the greenhouse is associated with a plant producing biogas by means of anaerobic digestion, which produces electrical power that is supplied to the equipment fitted in the greenhouse, and produces thermal energy for heating the greenhouse.

Description

PLANT AND METHOD FOR PRODUCING MICROORGANISMS IN AQUACULTURE

Field of the invention

The present invention relates to the production of microorganisms in aquaculture. The invention was developed with particular regard, although not restrictively, to the intensive production in aquaculture of microorganisms such as cyanobacteria and microalgae.

Technological background

It is known that many microorganisms such as cyanobacteria and microalgae are the sources of substances with very useful properties, for which there is a great demand for food or pharmaceutical use, inter alia. Examples of microorganisms that have been cultured widely for quite some time are Arthrospira platensis and Arthrospira maxima, generally known as spirulina.

In the simplest form of production, spirulina is cultivated in tanks filled with water with features reproducing the natural growth environment of the microorganism. In particular, the culture environment consists of a solution of mineral salts in water capable of providing all the nutrients required for the microorganism to grow and multiply. The main nutrient for spirulina is carbon, the normal source of which is the carbon dioxide present dissolved in water. The processes of growth and multiplication of the microorganism occur through photosynthetic processes, favoured by the presence of light and by an optimal temperature in the range of from 25 to 37 °C. The depth of the water in the tanks therefore cannot exceed a certain value in order to ensure adequate penetration of light into the water. Normally the water level in the tanks for cultivating spirulina is around 20 cm. The water in the tanks must also be kept constantly agitated to allow uniform exposure to the light of all the spirulina contained in the tanks .

Large quantities of spirulina are intensively produced at sites where large or very large outdoor tanks are provided on extensive spaces. The large surface area of the tanks is a necessary consequence of the impossibility of increasing the depth of the cultivation water beyond a few tens of centimetres. This type of production is subject to several problems. Outdoor tanks are not protected, and therefore do not allow high product quality to be achieved. Furthermore, keeping the water agitated in such large tanks, where there are substantial load losses through friction, requires a not- insignificant expenditure of energy. Last but not least, outdoor tanks are subject to seasonal temperature variations and weather conditions allowing optimal production in only a few months of the year. In fact, high temperature, for example above 43 °C, can lead to the death of the microorganism, while the speed of multiplication is reduced when the temperature falls below the optimum temperature. At 20 °C, spirulina practically stops growing. When cultivating spirulina outdoors, the more the weather remains climatically within the above-mentioned temperature range, the longer the harvesting period will be. Clearly, continental, temperate, high-altitude and desert-like climate zones are disadvantaged for producing spirulina, either since the harvesting period is short because of the low temperature throughout a large part of the year, or since the high temperature causes the death of the microorganisms or, at least, fast evaporation of the water in the tanks, which therefore have to be continually topped up.

The problem of too cold a climate can be compensated for artificially, as is the case for all vegetable products, by constructing tanks and basins placed inside greenhouses, which provide protection not only against the cold, evaporation, insects and dust, but also against heavy rain, which can rapidly fill up outdoor tanks, making them overflow, thus incurring consequent losses of product.

Producing tanks or basins in a greenhouse can favour the production of higher-quality microorganisms. However, the known plants for cultivating microalgae in a greenhouse do not allow regular, repeatable growth, such as to guarantee predictable productivity. Greenhouses also have high energy requirements or expensive solutions for achieving and maintaining conditions inside the greenhouse that are ideal for the optimised and regular growth of the microalgae. Greenhouse cultivation of microorganisms has therefore been limited until now to small-medium production units, with no guarantee of continuity, quality, repeatability and optimisation of the culture.

US 2014/0113276 describes a plant for culturing microalgae used for producing biofuel. The plant comprises ring-shaped tanks produced in the floor of a covering structure having an aluminium frame comprising glass covering panels. The plant comprises a lighting system arranged over the ring-shaped tanks, to provide lighting for the microalgae cultures in the tanks. A heating system comprises pipes, through which hot water flows from a boiler. The pipes are arranged at the bottom of the ring-shaped tanks in order to heat the water for culturing the microalgae. A temperature sensor inside the structure makes it possible to automate the heating process, so as to keep the culture water temperature within a predetermined range of values.

The plant described in US 2014/0113276 presents serious problems of water evaporation from the tanks because of the difference in temperature between air and water, especially when the water heating starts because of too low a temperature recorded by the sensor in the structure. In addition to increasing water consumption, a high level of evaporation causes other negative effects, such as an increased concentration of the culture, consequently shading the cells and thereby slowing down the productivity or requiring greater luminous flux. Furthermore, the evaporation of the water ensures an artificial increase in the cell density of the culture, consequently leading to errors in assessing algal productivity. Moreover, the evaporation leads to an increased concentration of the nutrients, deviating from the efficient value for optimal growth of the microalgae. Trials conducted by the applicant have also demonstrated that cells in a culture in contact with surfaces that are too warm suffer great thermal stress, which can lead to their death, as suggested by the formation of scum. This involves a greater release of algal organic matter, which may favour an increase in the bacterial load. This drawback detracts substantially from the quality of the culture, which consequently becomes unusable, for example for food or pharmaceutical use, incurring substantial associated financial losses, too.

If the environment outside the covering structure is at a very low temperature, there may be a perceptible temperature difference between water and air, in the case of direct heating of the water at the bottom of the tank, which can therefore make it difficult if not impossible to maintain the temperature of the culture within the optimal range.

Other known plants for cultivating microorganisms are designed to create conditions of overheating inside the greenhouse, and only provide cooling systems for reducing the temperature reached inside the greenhouse.

The object of the present invention is to implement a microorganism production plant that resolves the above- mentioned problems of the prior art. Another object of the invention is to implement a microorganism production plant that is advantageous, simple to implement and economical to run. Another object of the invention is to implement a plant for the intensive production of microorganisms providing constant control and easy regulation of the cultivation parameters, for example, but not restrictively, the level of nutrients and the pH of the cultivation water, the water temperature, the lighting level, etc. Another object of the invention is to implement a plant in which the internal temperature can be maintained within a predetermined range of values without the risk of the microorganisms suffering from thermal shock and without the culture medium being subjected to too rapid a temperature gradient, so as to keep the temperature in the culture medium as constant and uniform as possible, even in the absence of or with minimal agitation and mixing of the culture medium.

The remainder of the present description, for simplicity of presentation, will refer to the cultivation of spirulina, but the considerations that are presented and the features of the invention can certainly be applied to the cultivation of other species of microorganisms and, in particular but not restrictively, cyanobacteria and microalgae.

Summary of the invention

In the trials conducted by the applicant, particularly but not exclusively for the intensive production of high- quality, high-reproducibility microorganisms, containers such as tanks and basins were used for cultivation in aquaculture in a greenhouse.

According to a particular aspect, it is provided that a plant for culturing microorganisms in aquaculture comprises at least one container, in which a liquid culture medium that is suitable for growing microorganisms is placed. The greenhouse in which the container is placed is equipped with a system for controlling the internal environmental conditions. This makes it possible to precisely regulate the growth process of the microorganisms and therefore to achieve the aforesaid high quality and reproducibility.

According to another aspect, the greenhouse comprises a piece of equipment for directly heating the air that is controllable by the system for controlling the internal environmental conditions. By regulating the temperature by means of direct heating of the air inside the greenhouse, it is possible to keep the culture medium, which is typically aqueous, at a practically constant temperature equal to the air temperature, and in particular to prevent large changes in temperature in the culture medium. The temperature gradient in the culture medium remains fairly low.

The applicant has identified a source of heat and electricity that, because of its features, is perfectly suited to the features of greenhouse cultivation of microorganisms. The energy source is a biogas-production plant producing biogas by means of anaerobic digestion, particularly but not exclusively by means of plant biomass, commonly known as a biodigester. It is common to find these types of plants on farms, which are themselves independent producers of the biomasses fed to the biodigesters .

According to a particular aspect, a description is given of the synergistic combination of a biodigester plant with greenhouse cultivation of microorganisms in aquaculture.

In greater detail, the at least one greenhouse is associated with a plant producing biogas by means of anaerobic digestion, which produces electrical power that is supplied to the equipment fitted in the greenhouse, and produces thermal energy for heating the greenhouse. Under particular conditions and with suitable quality controls, it is also possible to use the biogas-production plant for producing carbon dioxide suitable for use in the liquid culture medium to provide the nutrients required by the microorganisms and to regulate the pH. Advantageously, the biogas-production plant is positioned close to the at least one greenhouse, preferably but not exclusively within the territory of the same farm or consortium of farms.

The heating equipment can advantageously be a heat exchanger supplied with the thermal energy produced by the biogas-production plant.

According to a particular aspect, the greenhouse comprises shading members that are controllable by the system for controlling the internal environmental conditions. The greenhouse can comprise lamps for artificial lighting, these also being controllable by the system for controlling the internal environmental conditions, which control system can also control air conditioning equipment inside the greenhouse .

According to another particular aspect, the microorganisms produced are cyanobacteria and/or microalgae. These can comprise in particular Arthrospira platensis or Arthrospira maxima (spirulina) . Spirulina production using the plant described has proved particularly abundant because said spirulina is not affected (or is only minimally affected) by the environmental conditions outside the greenhouse. The plant described is therefore particularly useful for producing microorganisms in aquaculture, and in particular spirulina, in non-tropical countries where temperatures and the amount of light during the year are typically much lower than in the natural growth environment of spirulina.

According to a particular aspect, a description is given of a container for the liquid culture medium, which container is a tank having a capacity of over 1000 L. The tank is preferably shaped like a ring and comprises means for generating a flow of liquid circulating in the ring-shaped tank. The whole tank is advantageously supplied with the electrical power generated both at low cost and close to the greenhouses by the biogas-production plant.

A description is also given of a method for producing microorganisms in a plant of the above-mentioned type, the main steps of which comprise:

providing an inoculum of at least one microorganism; providing a liquid culture medium suitable for growing said at least one microorganism; inoculating the microorganism into the liquid culture medium inside the at least one container of the at least one greenhouse; waiting for a predetermined period of acclimatisation and growth of the microorganism, while controlling the environmental conditions within the at least one greenhouse; taking a sample quantity of microorganism from the at least one container; restoring at least part of the liquid culture medium in the at least one container.

Brief description of the figures

Further features and advantages will become apparent from the following detailed description of a preferred embodiment of the invention, given purely by way of non-restrictive example, with reference to the attached drawings, in which:

Fig. 1 is a diagrammatic illustration of a biodigester plant that can be used in the present invention, and

Fig. 2 is a diagrammatic illustration of a microorganism production plant comprising a biodigester plant according to Fig. 1 associated with a greenhouse, inside which greenhouse tanks for cultivating microorganisms in aquaculture are placed.

Detailed description

A plant for anaerobic digestion of refuse, also known simply as a biodigester, is characterised substantially by three main sections: a reception and pre-treatment section, a section for preparing the substrate, for anaerobic digestion and for energy production, and a section for dehydration, accelerated bioxidation, post-maturation, refinement, and storage of products and waste. There are many types of anaerobic digestion plants, and their specific features are known to experts in the field. The present detailed description is limited to pointing out the most relevant elements, beyond the features of each specific biodigester, contributing to the implementation of the plant for producing microorganisms in aquaculture according to the present invention .

With reference to Fig. 1, the reference numeral 10 indicates, as a whole, a biogas-product ion plant or biodigester, which implements a process of anaerobic digestion of biomass, generally plant biomass, preferably but not exclusively produced near the site where the microorganism production plant of the present invention is implemented. It is advantageous to produce biomass within the same company or consortium of companies that manage the biodigester, so as to have guaranteed independence in the supply chain of the substrates used for the operation of the biodigester. Even more advantageously, it is expedient to also implement the microorganism production plant of the present invention within the same company or consortium of companies, which can thereby independently exploit part of the production of the biodigester, as will become clearer below . The biodigester 10 comprises a fermenter 12, to which material is supplied in a known manner. For example, solid feed material can be fed in the direction of the arrow A using a hopper 14 that introduces the feed material into a feed reservoir 15. From there, the feed product is transferred to a mixer 16, optionally incorporated into the feed reservoir 15. The feed material is measured out continuously and is sent to the fermenter 12 by means of a transport system 18, such as a screw conveyor, a pump, a conveyor or other known means, regulated by a control system (not shown in the figure for the sake of simplicity) . The mixer 16 can optionally be combined with a crushing and/or compaction system, or other systems for pre-treating the feed solids. It is also possible to feed the fermenter 12 with liquids, for example animal faeces or other organic fluids.

The fermenter 12 can be produced by several methods known in the biodigester field. It can, for example, be formed of a container 20 provided with a hermetic seal 22. The biogas produced in the fermenter 12 can be sent to a storage reservoir 24, also known as a gas tank, after optional treatment in a scrubber 23. The storage reservoir is for example provided with a cover 26. The cover 26 can for example comprise a double polyethylene membrane, wherein the inner membrane ensures the storage of the biogas, while the outer membrane provides protection from atmospheric agents. The two membranes of the cover 26 are separated by a volume of compressed air, which has the function of keeping the pressure constant inside the storage reservoir 24. In essence, the storage reservoir 24 acts as a buffer that mitigates any discontinuity in production of biogas by the fermenter 12 and the biogas requirement of the user. Other configurations of both the fermenter 12 and the storage reservoir 24 are of course possible depending on the size of the plant, the quantity and quality of biogas produced, the requirements of the user, the plant site and so on. Simply by way of example, the fermenter 12 can be produced in such a way that the hermetic seal 22 of the container 20 is replaced by the double-membrane cover described above with reference to the cover 26. In this case, the container 20 acts as both a fermenter and a gas tank having an internal pressure that is kept practically constant over time.

The biogas produced by the fermenter 12 and optionally stored in the storage reservoir 24 is taken off for some of the users at the outlet B. Part of the biogas produced is retained and used for being supplied to one or more motors 28. It is possible, for example, to use internal combustion engines supplied with precisely the biogas taken from the storage reservoir 24, or in some cases directly from the fermenter 12. One or more alternators 30 are connected in a known manner to the one or more motors 28 for producing electrical power E. Alternatively, as known, the portion of biogas retained in the plant can be indirectly supplied to fuel cells. In this case, the best known technology is that of utilising the biogas in a reformer in order to produce syngas, which is in turn used in fuel cells to generate electrical power E. These and other methods for producing electrical power by means of a biodigester are known to experts in the field and will not be described in further detail .

In the production of electrical power, for example by means of the motor 28, thermal energy is also generated. A large part of the thermal energy generated is used for producing hot water. Part of the hot water is sent via a heating circuit 32 to a heat exchanger 34 contained in the fermenter 12. The heat exchanger 34 allows the digester to be kept at a temperature between around 37 and 40 °C, which is optimal for the anaerobic digestion of the biomasses in the fermenter 12. The heating circuit 32 can comprise additional coolers 38, of a type generally known in the field, for regulating the temperature of the intake water entering the fermenter 12 and the temperature of the return water sent back to the motor 28.

Another part of the hot water generated by the motor 28 can be taken from a hot water outlet HW and sent to an external heat exchanger unit 36, the function of which will be described in greater detail below. The water returning from the heat exchanger unit 36, at a lower temperature than the temperature of the water at the outlet HW, can re-enter the plant via the cold water inlet CW.

Having yielded its calorie content, the residual product, known as the digestate, of the fermentation in the fermenter 12 is harvested in a collection container 42 so as to give rise to a solid SF and/or liquid LF fertiliser.

To bring down the concentration of hydrogen sulfide (H₂S), which develops together with methane during anaerobic fermentation (most typically, the concentration of methane is around 53.0 % of the total volume), it is expedient to use a known process of biological desulfurisation, which can provide for the insufflation of oxygen into the fermenter 12.

Fig. 2 illustrates diagrammatically an example of a plant for producing microorganisms in aquaculture, wherein one or more greenhouses 40 are expediently connected to the biogas- production plant 10, shown diagrammatically here as a single block with the inlets and outlets A, B, LF, SF, E, HW and CW described above. One or more tanks 42 for producing microorganisms in aquaculture in a controlled environment are placed inside each greenhouse 40 for example. In addition or alternatively to the tanks 42, known photobioreactors of various kinds can be used.

The greatest advantage of the present invention lies in the adoption of techniques of aquaculture in containers that are open and exposed to the air, such as the tanks 42, since in this case their arrangement inside controlled-environment greenhouses permits the production of microorganisms on a vast scale, achieving particularly high quality. The tanks 42 are in fact protected from ordinary contamination by animals, for example bird faeces or colonisation by frogs or other amphibians, something that in any case is rather common in outdoor aquaculture plant.

Moreover, the production of microorganisms in aquaculture takes advantage of the possibility of precisely regulating the environmental conditions inside the greenhouse 40, and in particular light/shade cycles, temperature and ventilation, in such a way as to achieve the maximum yield in microorganism production.

The greenhouse 40 comprises an electronic control unit 50, which controls the activation of the equipment in the greenhouse 40 according to instructions set and conditions recorded by sensors (not shown) for the temperature, humidity, sunlight etc. In particular, the electronic control unit 50 can control the activation and regulation of shading and/or ventilation elements 52, such as moveable curtaining or panels, air-conditioning and/or heating and/or ventilation systems 54, such as fans, heat pumps, air conditioners, etc. The electronic control unit 50 can also control the programmed switching on of lighting elements 56. The electronic control unit can also control the activation and regulation of the heat exchanger 36 supplied, as stated above, with the hot water HW coming from the biodigester plant 10. The heat exchanger 36 can be regulated by controlling electrically controlled valves (not illustrated) placed at the outlet HW and/or the inlet CW.

The electronic control unit 50 also controls and regulates the operating parameters of the aquaculture tank, such as the speed of a stirrer 58 that controls the flow of the water inside the tank 42, which tank is preferably ring- shaped. The electronic control unit 50 can control, in addition to the supply of hot water to the tank 42, the input of nutrients for microorganism growth, for example the input of C0₂, by means of a supply plant (not shown in the figures) . The total or partial collection of microorganisms through the collection orifice M of the tank 42 can be controlled and regulated, for example in a highly automated production plant, by the electronic control unit 50.

Naturally, the electronic control unit 50 can be composed materially of a plurality of circuit boards and electronic components combined within a single casing, or separated into several casings. The electronic control unit 50 can be incorporated into or replaced by an electronic processor or data transmission system, for example controlled by remote electronic processors, or by means of smartphone applications, and other techniques known in the electronic control field. The whole electronic system can further, as known, be incorporated into data entry means, or reporting, signalling or alarm means, etc.

The association between the biodigester plant 10 and the greenhouse 42 is particularly advantageous because of the possibility of generating low-cost electrical power in areas already provided and suitable for the construction of greenhouses, such as farms or agricultural consortia, which typically use and manage biodigesters . The availability of large expanses of land on which to position the greenhouses, the predisposition of farmers towards greenhouse cultivation and the now well-established practice of using soil-less greenhouse cultivation, as well as the low-cost availability of biomass to be supplied to the biodigesters, makes it particularly useful and advantageous to connect on the same site one or more biodigesters 10 to one or more greenhouses 40 inside which one or more plants for producing microorganisms in aquaculture are arranged. According to a particularly advantageous aspect, the plant of the invention has dimensions such that the heat produced by the biodigester 10 is sufficient to ensure a constant temperature of around 27 °C inside the greenhouse 40 during the winter months. This represents a distinct advantage over the known types of systems for producing microorganisms, because one of the barriers to intensive production is precisely the difficulty or high cost of maintaining a temperature suitable for producing microorganisms in non-tropical areas of the world even in the winter months .

In a biodigester plant, simply as a non-restrictive example, it is possible for example to achieve recovered thermal production deriving from the cooling circuit of the motor 28 of the order of 2*10⁶ kWh, of which less than half can be reused for the purposes of maintaining the process conditions in the fermenter 12, while the remainder can be used for heating the greenhouse 40. At the same time it is possible to produce, with the same biodigester plant 10 net electrical power using the generator 30, i.e. net of consumption by the biodigester itself, of the order of 4*10⁶ kWh, which is amply sufficient to supply the electrical consumption of the greenhouse 40. This latter aspect too represents a distinct advantage over the known types of microorganism production, which are notoriously costly as regards the consumption of electrical power for supplying adequate artificial lighting in the darker winter months and/or for supplying air conditioning in warmer or more humid periods of the year. The energy produced and available at low cost from the biodigester allows even greenhouses with very large overall dimensions to be provided with lighting and air-conditioning and therefore to be capable of achieving high production rates. In a particularly advantageous, but not thereby restrictive example of the use of the present invention, the microorganisms produced by the plant of the present invention are strains of Arthrospira platensis or Arthrospira maxima, commonly known as spirulina.

The liquid used for producing spirulina is a solution of mineral salts dissolved in water, capable of providing all the chemical elements required by the algae. The pH of the culture medium is between 8.5 and 11.0, with an optimal value of 9.5. The temperature of the culture liquid has a direct influence on the productivity of spirulina; the algae can in fact survive up to a minimum temperature of around 3-5 °C, although appreciable growth takes place only above 20 °C. Maximum growth occurs between 30 and 35 °C. Exceeding 39-40 °C for a few hours causes serious stress to the cells, blocking the growth factor for several days. If the temperature goes outside the critical threshold for consecutive periods, the stress condition inevitably causes the culture to die. For this reason the cultivation of spirulina in tanks 42 placed inside temperature-controlled greenhouses 40 is particularly remarkable in the present invention compared with known cultivation in outdoor tanks.

Light is one of the main growth factors for the culture: this parameter is regulated on the basis of the state of well-being of the cells and is greatly influenced by latitude, climatic zone and time of year. Various studies have found that very strong brightness produced by maximum solar irradiance, with values of around 100,000 lux in midsummer and when the sun is at its zenith, can be dangerous particularly in the following cases:

for a culture at a low temperature (below 14-15 °C, typically in the early morning) , and particularly if exposed suddenly after a period of darkness ; for a culture at an already high temperature, since said temperature can increase further, exceeding the critical threshold; for a highly diluted culture (for example at a concentration below 0.4 g IT¹) , since the total incident radiation would be excessive relative to the number of cells present in the culture; for a culture already stressed by other factors, worsening the state of health of the culture; and for a culture not yet adapted to the external light conditions, for example in the case of a new inoculum of microorganisms in the tanks 42.

Furthermore, when the temperature and concentration of the cells in the culture are in the optimal range, the organisms can benefit from maximum exposure to sunlight, thereby maximising the production of biomass per unit of time .

For these reasons, the adoption of the microorganism culture within greenhouses 40 in a controlled environment proves particularly advantageous compared with the cultivation systems of the prior art. As indicated above, the greenhouses 40 are provided with a shading system 52, for example comprising blackout and/or insulating sheets capable of reducing the solar irradiance falling on the culture systems by up to 50 % and more, so as to be able to best control the amount of light during the phase of starting the newly inoculated cultures and during the middle of the day in summer . Stirring the culture liquid allows correct distribution of the nutrients and algal cells, and also permits uniform exposure of the culture to sunlight, thus avoiding phenomena of over- or under-exposure. Stirring can take place by means of compressed air (not shown in the figures) and/or the mechanical stirrer 58, so as to generate laminar flows of ~15 m/min (2025 cm/sec) . The power levels required for moving the cultures can be estimated at around 26 kW for plants measuring 1000 m².

In a typical plant for producing spirulina in greenhouse tanks, the culture medium is prepared first of all. The water used must be potable and free of any contaminants and/or chemical and biological pollutants, preferably with a low content of calcium ions, for example below 70 mg IT¹.

The inlet water can be filtered and softened; alternatively it is also possible to use brackish water (diluted seawater or water from deep levels) with a maximum sodium chloride concentration of 4-5 g IT¹, provided that the potability and purity conditions are ensured. Before being introduced into the tank, salts are dissolved in the water, the nature and proportion of said salts varying according to the microorganism cultivated.

To ensure cleanness and the absence of contamination, the tanks 42 accommodating the spirulina culture are washed beforehand with a sodium hypochlorite solution or other washing solutions. The tanks 42 are then rinsed twice with water and once with filtered water (of the order of 1 μπι) . In this step, it is also important to test the tightness and efficiency of all the systems.

The tanks 42 are then made operational starting from an inoculum (starter), for example an initial volume with a high concentration of cells (0.8-1 g IT¹) of the spirulina strain. The starter can be produced at or transported to the site, and optionally transferred initially into 150L photobioreactors or similar, for an initial acclimatisation period. Next, said starter is inoculated inside the tanks 42 which, in a non-restrictive embodiment, can have a volume of over 1000 L, and preferably of around 1200 L.

By using successive dilutions of the culture and growth phases, it is possible to double the total volume each time, while adding fresh liquid medium, and achieving the optimal concentration of 0.8 g L^D1 dry weight every time. During the first 2-3 dilutions, it is particularly important to shade the culture. This makes it possible to produce optimal, abundant, high-quality microorganisms, unlike what is achieved with the types of cultivation of the prior art.

In general, setting up a 100,000-L tank, starting from a 300L starter, requires a period of around six weeks for the culture to become operational. During this period it is important to rigorously check the parameters of pH, temperature and light. This is made possible by regulating the parameters using the electronic control system illustrated in Fig. 2, simply by way of example, by the electronic control unit 50. Furthermore, this process is made simple and economical by coupling the greenhouse 40 to the biodigester plant 10, as described above.

When the tanks 42 are operational and the concentration of the spirulina culture is above 0.8 g IT¹, it is possible to start the harvesting process. The applicant has estimated that in good light and temperature conditions, it is possible to harvest up to a third of each tank 42 every three days, around 10 % daily. These estimates may vary according to environmental factors, the volumes cultivated and the time of year in question. Harvesting ends before the concentration falls below 0.5 g/L, and more preferably before it falls below 0.6 g/L .

Cultivating microorganisms in a greenhouse also limits the evaporation of the water compared with cultivation in outdoor tanks of the prior art. Estimates made by the applicant indicate a maximum loss of water of around 5 % of the total volume every three to five days over the hottest periods. This is a notably lower percentage than the loss of water from the outdoor tanks of the prior art.

According to another advantageous aspect of the present invention, the biodigester plant 10 could also be used for supplying carbon dioxide C0₂ to the culture tanks, so as to increase the productivity of microorganisms. The release of C0₂ into the culture is regulated so as to keep the pH preferably around 10. Estimated C0₂ consumption is approximately 0.8 kg per kg of dry spirulina harvested. Since a C0₂ source such as the biodigester 10 is available, it is not necessary to completely renew the culture medium of the microorganisms by discharging the liquid; rather, it is sufficient to replenish the mineral salts consumed. Naturally, the supply of carbon dioxide to the tanks 42 must be assessed carefully according to the type of biodigester plant, to ensure the purity of the carbon dioxide introduced and to prevent contamination of the microorganism culture.

Naturally, without prejudice to the principle of the invention, the embodiments and the implementation details can vary greatly while remaining within the scope of the present invention .

Claims

1. Plant for producing microorganisms in aquaculture, comprising at least one container for a liquid culture medium suitable for growing microorganisms, the at least one container being placed inside at least one greenhouse equipped with a system for controlling the internal environmental conditions.

2. Microorganism production plant according to the preceding claim, wherein the at least one greenhouse comprises a piece of equipment for directly heating the air inside the greenhouse, which equipment is controllable by the system for controlling the internal environmental conditions.

3. Microorganism production plant according to any one of claims 1 to 2, wherein the at least one greenhouse is associated with a plant producing biogas by means of anaerobic digestion, which produces electrical power that is supplied to the equipment fitted in the greenhouse and produces thermal energy for heating the greenhouse.

4. Microorganism production plant according to claims 2 and 3, wherein the heating equipment is a heat exchanger supplied with the thermal energy produced by the biogas-production plant that is associated with the at least one greenhouse.

5. Microorganism production plant according to either claim 3 or claim 4, wherein the biogas-production plant produces carbon dioxide suitable for being used in the liquid culture medium to provide the nutrients required by the microorganisms .

6. Microorganism production plant according to any one of claims 3 to 5, wherein the biogas-production plant is supplied with biomass of vegetable origin.

7. Microorganism production plant according to any one of claims 3 to 6, wherein the biogas-production plant is positioned close to the at least one greenhouse.

8. Microorganism production plant according to any one of the preceding claims, wherein the at least one greenhouse comprises shading members that are controllable by the system for controlling the internal environmental conditions.

9. Microorganism production plant according to any one of the preceding claims, wherein the at least one greenhouse comprises lamps for artificial lighting that are controllable by the system for controlling the internal environmental conditions .

10. Microorganism production plant according to any one of the preceding claims, wherein the at least one greenhouse comprises air conditioning equipment that is controllable by the system for controlling the internal environmental conditions .

11. Microorganism production plant according to any one of the preceding claims, wherein the microorganisms produced are cyanobacteria and/or microalgae.

12. Microorganism production plant according to claim 11, wherein the microorganisms produced comprise Arthrospira platensis or Arthrospira maxima (spirulina) .

13. Microorganism production plant according to either claim 11 or claim 12, wherein the at least one container for the liquid culture medium is a tank having a capacity of over 1000 L.

14. Microorganism production plant according to claim 13, wherein the tank is ring-shaped and comprises means for generating a flow of liquid circulating in the ring-shaped tank .

15. Method for producing microorganisms in a plant according to any one of the preceding claims, comprising the steps of: providing an inoculum of at least one microorganism; providing a liquid culture medium suitable for growing said at least one microorganism; inoculating the microorganism into the liquid culture medium inside the at least one container of the at least one greenhouse; waiting for a predetermined period of acclimatisation and growth of the microorganism, while controlling the environmental conditions within the at least one greenhouse; taking a sample quantity of microorganism from the at least one container; restoring at least part of the liquid culture medium in the at least one container.