AU7906098A

AU7906098A - Method for producing biomass by photosynthesis

Info

Publication number: AU7906098A
Application number: AU79060/98A
Authority: AU
Inventors: Jurgen Broneske; Katrin Dobel; Horst Franke; Hans-Ulrich Oehlmann; Otto Pulz; Rainer Uphoff
Original assignee: Bioprodukte Prof Steinberg GmbH
Current assignee: BIOPRODUKTE PROF STEINBERG GmbH
Priority date: 1997-04-10
Filing date: 1998-04-09
Publication date: 1998-10-30
Anticipated expiration: 2018-04-09
Also published as: EP0874043A1; AU741734B2; ATE292672T1; DE59812723D1; DE19814253A1; EP1040182A1; WO1998045409A1; JP3600250B2; DK1040182T3; DE19814253C2; JP2001527394A; EP1040182B1; IS5195A

Abstract

Production of biomass by photosynthesis comprises inoculating a nutrient solution with microorganisms to obtain a culture suspension, conditioning the culture suspension with a CO2-containing gas until the suspension is saturated and free of bubbles, adjusting the pH to 5-7, adding an organic carbon source during the logarithmic to late stationary growth phase and passing the suspension through an illuminated reactor under uniform flow conditions.

Description

Method for producing biomass by photosynthesis Description The invention concerns a method for producing biomass by photosynthesis. The invention concerns in particular a method for the cultivation of microalgae and makes an effective production of biomass feasible by fixing carbon dioxide from C0 2 -containing waste gases. Microalgae can be cultivated easily, and techniques to use high photosynthetic productivity for this purpose are known from numerous patents and publications (Biotechnology and Bioengineering, Vol.35, 809, 1990, H.Gutermann). Phytoplankton is often harvested to obtain valuable materials or pharmaceutics like polysaccharides, polyunsaturated fatty acids, dyes, vitamins etc. (Algae and Human Affairs, Cambridge University Press, Cambridge, 1988, K.G.Sprenger et al.). However, biomass from algae are also used as protein-rich feed materials. Among the methods used to produce a mass product, first of all the often described open systems are to be named. As such raceway ponds, round basins and possibly natural lagoons with a depth of water of 10-30 cm, as a rule, are to be understood. In these systems the algae present in the suspensions are agitated with the aid of agitating devices, e.g. bucket wheels, propellers or Archimedes' screws, to prevent the settlement of the microorganisms. At the same time by virtue of intensive mixing all algae reach the upper water layers which are transilluminated by more intensive light (Applied Biochemistry and Biotechnology, Vol.51/52, 681, 1995, H.Matsumoto et al.). The productivity of microalgae in open systems depends to a 5 great extent from the light intensity as well as from the flow velocity, which is closely connected with the technique of agitation. Typical biomass outputs in open systems are 8-12 g/m2/day.

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A further increase of the biomass output can be achieved by methods, wherein closed reactors are used. In that case the cultivation parameters, like temperature, nutrient supply, CO 2 supply and light intensity can be regulated and controlled more accurately than in the open systems. As a rule, closed systems are plate or tube reactors, which are illuminated directly by solar light or artificial light or those where the light is bundled by light guidance systems, mirror and lenses (Advances in Biochemical Engineering/Biotechnology, Vol.46, 63, 1992, I.Karube et al.). Depending on the algae species as well as on the method of cultivation, the biomass yields in the systems mentioned are between 15-25 g/m 2 /day. Since the carbon content of the biomass is approx. 60 % by weight, it will be clear that during the photosynthesis CO 2 will be the largest contributor to the material decomposition and therefore one has to look for cost-effective CO 2 sources. The CO2 content in the atmosphere is too small to assure the required productivities. In contrast to this, flue gas and other industrial waste gases have the potential to supply even large algae farms with adequate quantities of CO 2 . A screening for algae species, which tolerate high CO 2 concentrations and trace gases like SO., NO, and CO, contained in the waste gas, is known from various publications (Plant Physiol.82, 610, 1986, Y.Marcus et al.; Plant Physiol.91, 514, 1989, G.D.Price; Plant Physiol.94, 760, 1990, T.Ogewa). Investigations regarding the growth behaviour when supplying microalgae with waste gases have shown that the growth of diverse marine algae, e.g. Tetraselmis suecicca, Nannochloropsis/Phaeodactylum, is not limited in their growth by using waste gases and the cultures remain stable for one year (Applied Biochemistry and Biotechnology, Vol.51/52, 681, 1995, H.Matsumoto et al.). These results, however, cannot be generalised and to a great extent depend from the species used and the cultivation conditions. Furthermore, the harvest of the biomass has to be reconciled with the respective microorganisms, since the relevant method depends to a great extent from the harvest concentration, the 2 size of the organism as well as the physiological properties. From the biotechnology as well as waste water technology a plurality of methods is known for harvesting cells and separating solids. Examples of this are plate centrifuge, lamination separator and filtration using flocculants ("Micro algal Technology", Cambridge University Press, 1988, M.A.Borowitzka, L.J.Borowitzka (eds)). The same, as for the harvest, is valid for the disintegration and extraction of microalgae. The methods to be used have to be harmonised to suit the cell and the purpose of application of the biomass. The object of this present invention is to produce a method for producing biomass by photosynthesis, in particular for the cultivation of microalgae, which makes an effective production of biomass feasible by fixing carbon dioxide from C0 2 containing waste gases while avoiding the disadvantages of the known methods. 'Such known methods should be improved with this method with regard to their energy balance, production costs and optimum use of the available luminous energy to that extent that they should be suitable for industrial use. According to the invention this objective is achieved by the features of claim 1. Advantageous refinements of the method are contained in claims 2 to 23. Furthermore, the invention comprises the use of particularly preferred algae species. According to this present invention the production of biomass is carried out by photosynthesis, whereby by injecting a nutrient solution containing microorganisms a culture suspension is produced and the culture suspension is conditioned with a CO2-containing gas by saturating the components contained in the gas until it is free of bubbles. At the same time the culture suspension is adjusted to a pH value of >5.0 and <7.0, due to which the microorganisms will be subjected to a conditioning stress (pH stress), consequently the fat proportion of the biomass altogether as well as the 3 proportion of special fatty acids like, for example, the proportion of multi-unsaturated fatty acids, can increase surprisingly. The quantitative composition of the cell materials can be modified in a targeted manner by a particular feature of the invention, whereby a fat content of more than 20 % is achieved in the biomass. A further supply of organic carbon sources takes place during the logarithmic growth phase of the microorganisms, well into the stationary one. According to a preferred feature of the invention as further sources of carbon simple and/or multiple sugar, in fact particularly glucose, is added to the culture suspension in a concentration of 0.3-10 g/L of culture suspension. The potential of the algae cells to assimilate carbon by various metabolisms with the aid of various biochemical processes to form cell-specific, energy-rich substance is fully utilised by virtue of this mixotrophic nourishing of the microorganisms. In addition, the processes of photorespiration, whereby 1 to 10 % of the newly assimilated carbon is lost, can be minimised. Subsequently, while producing a uniform flow pattern, the suspension is conveyed to a reactor, wherein under illumination and adjusting an optimum flow velocity of the solution a depletion of the Co2 contained in the culture suspension takes place by forming energy-rich carbon compounds and further products of metabolism. This present invention provi es a method which, when compared with the methods known so fat, is characterised by a reduced risk of contamination, reproduceable cultivation conditions, great flexibility with regard to environmental influences, reduced floor place and reproduceable qualities of the biomass. According to a particular feature of the invention in the case of metabolism products one deals with an 02-containing gas, which is separated by phase separation after having passed 4 through the reactor and can be conveyed to a further use. This oxygen production makes the method according to the invention particularly efficient. According to a preferred feature of the invention phototrophic organisms and/or cell cultures, in particular microalgae are used as microorganisms. It has been found that the above described advantages of the method can be particularly well reached with the microalgae species Chlorella vulgaris and/or Scenedesmus spp./Microcystis (mixed culture). According to a further feature of the invention with this present method algae biomasses and their metabolism products are produced, whereby as nutrient solution water and materials which are required for the decomposition of energy-rich carbon compounds, are used. In this conjunction one deals with nitrogen compounds, phosphates, potassium compounds, calcium compounds, magnesium compounds and tracer elements. According to another feature of the invention the nutrient solution also receives organic substances, e.g. carbohydrates as sugar and/or acetates and/or buffering substances like tris and/or borax and/or lime. According to a particular feature of the invention the nutrient solution may contain nutrient-containing waste water, including sewage slum and/or synthetic solutions. By virtue of this an extremely effective use of such waste waters is feasible. An equally preferred feature of the invention states that as c02-containing gas a waste gas, in fact particularly a waste gas from combustion processes, lime burning processes and/or metallurgical processes is used . Accordingly, such waste gases no longer have to be released into the environment, but can be conveyed to a meaningful application. Moreover, the algae species obtain sufficient quantities of C02 to ensure the required productivity. I JJ 5 According to a further feature of the invention the conditioning of the culture suspension with the CC.-containing gas is carried out by saturating the components contained in the gas in a gas scrubber. On this occasion the components of the gas dissolve in the culture suspension until saturation and the components of the culture suspension, the concentration of which exceeds the gas solubility in the culture suspension, are removed. The suspension leaves the gas scrubber free of bubbles. With the gas scrubber a regulated CO 2 input is feasible while the C02 losses are low. A preferred feature of the invention states that the C0 2 containing waste gas originating from combustion processes is tempered in a waste gas cooler and the resulting condensate with the CC 2 -containing waste gas is conveyed into a reactor via a gas scrubber. Thus the condensate becomes a component of the culture suspension and is conveyed to a meaningful application. According to a particularly preferred feature of the invention the uniform flow pattern of the conditioned culture suspension is produced by virtue of that it is conveyed into a reactor which comprises a photosynthetically and hydrodynamically active thin-layer system. According to this feature the thin-layer system comprises at least one module, while the module comprises a plurality of thin-layer vessels superposed parallel at a distance from each other, the entry and exit sides of which are connected with a module forward run and return run, respectively, by means of connecting pieces and connectors. At the same time the individual connectors have a cross-sectional area which makes a uniform flow pattern feasible through every glass tube. At the same time the cross-sectional area of the connector increases in the direction of flow of the module forward run, whereas the cross-sectional area of the connector of the module return run decreases in the direction of flow. Both the module forward run and return run consist of a tube with connecting pieces for the 6 connector, while the cross-sectional area of the module forward run decreases in the direction of the flow and the cross sectional area of the module return run increases in the direction of the flow. By virtue of the connectors with the reduced or increased, respectively, cross-section and the module forward run and module return run with the reduced or increased, respectively, cross-sections, a uniform flow pattern is made feasible through all thin-layer vessels. According to a further feature of the invention the uniform flow pattern of the conditioned culture suspension is produced by that it is introduced into a reactor, which comprises at least one single-piece extruded transparent plate module, which has a plurality of superposed and parallel continuous channels, the entry and exit sides of which open into a liquid collector and/or liquid distributor, while the liquid distributor has a reducing cross-section in the direction of flow and the liquid collector an increasing cross-section. Due to this according to the invention an optimum, uniform flow velocity of the solution can be adjusted in the reactor over all channels. By surprisingly simple means it will be achieved that at low flow velocities turbulent conditions are achieved in the culture medium which are necessary for an optimum utilisation of the luminous energy. Moreover, the resistance to flow can be considerably reduced in comparison with the known solutions. Finally, the required pump capacity will be essentially reduced by this, consequently improving the energy balance in a particularly advantageous manner. In addition, the joining of several plate modules makes the construction of a reactor of any size for industrial use feasible from simple standard components. In the following the invention is explained in detail based on an embodiment. They show in: 7 Fig.1 - an exemplary embodiment of the reactor and the schematic progress of the method, Fig.2 - a further version of a reactor with thin-layer systems. From the combustion process of a lime-burning kiln a waste gas 27a having a C02 concentration of approx. 25 % is conveyed to a waste gas cooler (not illustrated). In the waste gas cooler the C0 2 -containing waste gas is tempered to a temperature of approx. 350 C. During the cooling process a condensate is formed. The C0 2 -containing waste gas and the condensate are conveyed via a gas scrubber 5 to a reactor, while in the gas scrubber 5 a culture suspension 4a is conditioned with the CC.-containing waste gas 27a and the condensate (not illustrated) is conditioned by saturating the components contained in the waste gas until achieving a bubble-free condition. The culture suspension 4a comprises a nutrient solution and a microalgae species Chlorella vulgaris. The nutrient solution comprises water 9, inorganic nutrient materials and tris(hydroxymethyl)-aminomethane. At the same time the water is provided as waste water from the lime-burning kiln. Inorganic nutritive materials are dissolved in low concentration in the waste water. At the same time the nutrient solution (NL) contains the following inorganic nutritive materials. 8 Component Quantity (g/L)

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Macro components ''

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Urea 0.3

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Nitrate of ammonium 0.4

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Potassium dihydrogen phosphate 0.34

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Magnesium sulphate 0.5

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Iron sulphate 0.5

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Micro components I per litre of stock solution (0.01 mL/L of NL)

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Zinc sulphate 74.0

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Borax 5.7

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Cobalt sulphate 23.8

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Copper sulphate 23.6

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Manganese sulphate 410

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Micro components II per litre of stock solution (0.01 mL/L of NL)

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Ammonium molybdate 9.2 Tris(hydroxymethyl)-aminomethane is an organic substance which is added as acid buffer to the nutrient solution in a concentration of approx. 1.0 g/L. The conditioned culture suspension circulates in the reactor, which according to Fig.1 comprises a plate module installation 1 made up from single extruded transparent plate modules la... ln, an equalising container 6, a system pump 8, a measuring section 7, a forward run 2, a return run 3 and the 5 gas scrubber 5 and is illuminated with the aid of natural sunshine 12. The conditioned culture suspension travels from the equalising container 6 through the system pump 8 via the forward run 2 into the plate module la. 9 The plate module la has a plurality of superposed and parallel continuous channels the entry and exit sides of which open into a liquid collector 21 and liquid distributor 15, resp e ctively. At the same time the liquid distributor 15 has a'reducing cross-sectional area 19 in the direction of the flow and the liquid collector 21 has an increasing cross-sectional area 22 in the direction of the flow. By virtue of this a uniform flow pattern of the conditioned culture suspension is achieved in the plate module. The same effect, of course, is achieved also in a reactor according to Fig.2. This reactor comprises a photosynthetically and hydrodynamically active thin-layer system of at last one module A with a plurality of tubular thin-layer vessels B superposed parallel at a distance from each other, the entry and exit sides of which have a cross-sectional area via the connecting pieces C, D and connectors E for the superimposed thin-layer vessels, which increases in the direction of flow of the module forward run F and decreases in the direction of flow of the module return run G. 'At the same time the module forward run and module return run, F and G, each comprise a pipe with connecting pieces H and I for the connectors E, while the cross-sect onal area of the module forward run F decreases in the direct on of flow and the cross-sectional area of the module return flow G increases in the direction of flow. The growth of the biomass, the depletion of the dissolved CO 2 and the enrichment by oxygen 28 is carried out in the plate module. After the plate module the culture suspension 4a returns to the equalising container 6 via the return run 3 and the gas scrubber 5. The CO2-containing waste gas 27a is dissolved in the culture suspension 4a with the aid of the gas scrubber 5, and the 10

Claims

1. A method for producing biomass by photosynthesis, whereby by injecting a nutrient solution containing micrborganisms a culture suspension is produced, the culture suspension is saturated with a C0 2 -containing gas, adjusted to a pH value of >5.0 and <7.0 and conveyed to a reactor, wherein under illumination of the solution a depletion of the CO 2 contained in the culture suspension takes place by forming energy-rich carbon compounds and further products of metabolism, characterised in that the culture suspension saturated with the CO2-containing gas is conditioned until it is free of bubbles and by producing a uniform flow pattern and adjusting an optimum flow velocity it is conveyed to the reactor, that further organic carbon sources are conveyed to the culture suspension during the logarithmic growth phase, well into the stationary one.

2. A method according to claim 1, characterised in that as further organic sources of carbon simple and/or multiple sugar are added to the culture suspension.

3. A method according to claim 2, characterised in that as further organic source of carbon glucose is added to the culture suspension in a concentration of 0.3-10 g/L of culture suspension.

4. A method according to any one of the preceding claims, characterised in that the quantitative composition of the cell materials of the biomass is modified in a targeted 5 manner whereby a fat content of more than 20 % is achieved in the biomass. 13

5. A method according to any one of the preceding claims, characterised in that the metabolism product is an '2 containing gas, which is separated by phase separation after having passed through the reactor and is conveyed to a further use.

6. A method according to any one of the preceding claims, characterised in that the biomass produced in the reactor is separated from the culture suspension and is treated to become a water/biomass suspension rich in solids which can be still pumped.

7. A method according to claim 6, characterised in that the separation of the biomass is carried out by making use of the density differences in the settling vats and/or centrifuges and/or making use of the sizes of the particles in the filtering devices and the culture solution is returned to the\reactor system.

8. A method according to any one of the preceding claims, characterised in that algae biomass and its products of metabolism are produced and as nutrient solution water and materials which are necessary for the formation of energy rich carbon compounds are used.

9. A method according to claim 8, characterised in that the nutrition solution contains nitrogen compounds, phosphates, potassium compounds, calcium compounds, magnesium compounds and tracer elements.

10. A method according to claims 8 and 9, characterised in that the nutrient solution contains organic substances, e.g. carbohydrates like sugar and/or acetates and/or buffering substances like tris(hydroxymethyl)-aminomethane and/or borax and/or lime. 14

11. A method according to any one of claims 8 to 10, characterised in that as nutrient solution nutrient containing waste water, including sewage slum and/or synthetic solutions are used.

12. A method according to any one of the preceding claims, characterised in that phototrophic organisms and/or cell cultures are used as microorganisms.

13. A method according to claim 12, characterised in that microalgae are used as microorganisms.

14. A method according to claims 12 and 13, characterised in that the microalgae species Chlorella ssp. and/or Scenedesmus spp. and/or Microcystis ssp. are used as pure or mixed culture.

15. A method according to any one of the preceding claims, characterised in that a waste gas is used as CC2-containing gas.

16. A method according to any' one of the preceding claims, characterised in that a waste gas from combustion processes, lime-burning-processes and/or metallurgical processes is used as C0 2 -containing gas.

17. A method according to any one of the preceding claims, characterised in that the conditioning of the culture suspension is carried out with a CC2-containing gas in gas scrubbers, while the components of the gas dissolve in the suspension up to saturation and a bubble-free culture suspension is produced.

18. A method according to any one of the preceding claims, characterised in that the C02-containing waste gas is tempered in a waste gas cooler and the resulting condensate with the CC.-containing waste gas is conveyed into the reactor via a gas scrubber. 15

19. A method according to claim 18, characterised in thak the gas is conveyed in a counter-flow to the culture suspension via the gas scrubber. 4

20. A method according to any one of the preceding claims, characterised in that a uniform flow pattern of the conditioned culture suspension is produced, it is conveyed into a reactor which comprises a photosynthetically and hydrodynamically active thin-layer system comprising at least one module with a plurality of tubular thin-layer vessels superposed parallel at a distance from each other, the entry and exit sides of which are connected with a module forward run and a return run, respectively, via connecting pieces and connectors, while the individual connectors for the superposed thin-layer vessels have a cross-sectional area which increases in the direction of flow of the module forward run and decreases in the direction of flow of the module return run and the module forward run and module return run each comprise a pipe with connecting pieces for the connectors, while the cross sectional area of the module forward run decreases in the direction of flow and the cross-sectional area of the module return flow increases in the direction of flow.

21. A method according to any one of the preceding claims, characterised in that a uniform flow pattern of the conditioned culture suspension is produced, it is conveyed into a reactor which comprises at least one single-piece extruded transparent plate module, which has a plurality of superposed and parallel continuous channels, the entry and exit sides of which open into a liquid collector and/or liquid distributor, while the liquid distributor has a reducing cross-section in the direction of flow and the liquid collector an increasing cross-section, by means of which a uniform flow velocity of the solution is adjusted in the reactor over all channels. 16

22. A method according to any one of the preceding claims, characterised in that the illumination of the culture suspension situated in the reactor is carried out by natural direct and/or diffused solar radiation and/or _ artificial illumination.

23. A method according to any one of the preceding claims, characterised in that the biomass is further processed into an energy carrier.

24. A method according to any one of the preceding claims, characterised in that the biomass is further processed to produce chemical and/or pharmaceutical basic or active materials.

25. The use of the biomass produced in accordance with claims 1 to 24 as feedstock or feedstock additive. 17