FIELD OF THE INVENTION

The present invention relates to a method and an apparatus for harvest ing microalgae.

BACKGROUND OF THE INVENTION

Microalgae are unicellular microscopic algae found in freshwater and marine systems, living in both the water column and sediment. Microalgae have been indicated in numerous uses including production of pigments, antioxidants, vitamins, polymers and fatty acids. Microalgal cells or substances produced by mi croalgae are used in for example in food products, food supplements and animal feed. Fatty acids produced by microalgae are also used in producing biofuel such as biodiesel which is a promising alternative to liquid fossil fuels. An increasingly important application of microalgae is biogas production, where microalgal mass is subjected to anaerobic digestion to produce biofuel and/or energy in the form of heat and electricity. Among the benefits of culturing microalgae is that they can be cultivated in saline, relatively low-nutrient and low-cost liquids including seawater and wastewater, simultaneously effecting wastewater purification. There is a longstanding need to develop the value chain from microalgae propagated in wastewater into valuable products and energy.

A starting step of manufacturing algal products such as algal biofuel is harvesting microalgae from their growth medium to produce a concentrated mi croalgal mass. This concentrate can then be further treated to extract desired sub stances such as lipids. Techniques used in harvesting microalgae include sedimen tation, flotation, flocculation, filtration and centrifugation. Harvesting is a highly energy consuming step in treating microalgal cell mass, and often decreases prof itability of industrial scale algae refining.

The conventional techniques used in harvesting microalgae have sev eral drawbacks. Flocculation and sedimentation involve the use of flocculants that increase process costs, impede recirculation of culture medium and may need to be removed from the cell mass before further processing, thus adding extra process steps. Gravity-based sedimentation without flocculants is considered too ineffi cient for industrial scale applications. Centrifugation is highly cost- and energy-in tensive and the high gravitation and shear forces involved may cause damage to the microalgal cells. Filtration is an energy-intensive process that is also relatively slow and further slowed down by filtration membrane blocking, and the high shear forces involved in filtration may damage the microalgal cells. Flotation is a rela tively cost-intensive technique that may require the use of added flocculants.

There is a longstanding need for microalgal harvesting techniques that are simple to perform, energy-saving and cost-efficient also in industrial scale and avoid the use of additional chemicals such as flocculants.

BRIEF DESCRIPTION OF THE INVENTION

An object of the present invention to provide a new method and an ap paratus for harvesting microalgae to solve the above-mentioned drawbacks. The objects of the invention are achieved by a method and an apparatus which are char acterized by what is stated in the independent claims. The preferred embodiments of the invention are disclosed in the dependent claims.

In one aspect, the present invention provides a device unit for de-pres- surization of a liquid comprising microalgae to increase buoyancy of the microal gae. As a result of increased buoyancy, microalgal cells rise towards the surface of the liquid to provide concentrated microalgae for harvesting.

In another aspect, the present invention provides a method for harvest ing microalgae by providing de-pressurization to increase buoyancy of the micro algal cells.

In yet another aspect, the present invention provides a concentrated mi croalgae product produced by the method of the invention.

An advantage of the method and arrangement of the invention is that harvesting of microalgae is achieved in a rapid, simple and cost-efficient manner. A further advantage of the invention is that harvesting can be performed without us ing additional chemicals such as flocculants.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described in greater detail by means of preferred embodiments with reference to the accompanying drawings, in which

Figure 1 illustrates a first exemplary apparatus for harvesting microal gae;

Figure 2 illustrates a second exemplary apparatus for harvesting micro algae;

Figure 3 illustrates a third exemplary apparatus for harvesting microal gae;

Figure 4 illustrates a flow chart of a method for harvesting microalgae. DETAILED DESCRIPTION OF THE INVENTION

An object of the present invention is to provide an apparatus and a method for harvesting microalgae. The object of the invention is achieved by in creasing buoyancy of microalgal cells in order to cause the cells to float or to rise towards the surface of a liquid comprising microalgae.

As used herein, the term "buoyancy" refers to an upward force exerted by a fluid that opposes the weight of an immersed object. If the object is less dense than the liquid, the force can keep the object afloat i.e. the object becomes positively buoyant. Algae obtain buoyancy with help of several types of gas-filled structures. Macroalgae may have air-filled bladders called pneumatocysts, or gas bubbles may be entrapped in the central area of tubular hollow thallus or among filaments of macroalgae. In microalgae, buoyancy may be regulated by the algal cells. Regula tion may involve the production of intracellular gas-filled structures i.e. vacuoles a.k.a. gas vacuoles that are not delimited by membranes but are made up of assem blages of hollow cylinders with proteinaceous walls. The density of vacuoles is lower than that of water, and if sufficient gas-filled structures are present in a cell, it can become positively buoyant. The production of gas-filled structures is induced by lowlight conditions (e.g., in deep layers with insufficient light), causing cells to increase their buoyancy.

Some microalgae obtain buoyancy from liquids of lower specific gravity than seawater or freshwater in a way similar to a bathyscaphe. Liquid-filled floats have the advantage of being virtually incompressible; but because of their higher density they must comprise a much greater proportion of the organism’s overall volume than is necessary with gas-filled floats if they are to give equivalent lifts. For example, the large central vacuole of diatoms contains cell sap of reduced den sity, obtained by the selective accumulation of K ⁺ and Na ⁺ , which replace the heav ier divalent ions, conferring some buoyancy. Studies of the distribution of diatoms in the sea suggest that some species undergo diurnal changes of depth, usually ris ing nearer the surface during daylight and sinking lower in darkness, possibly due to slight alterations of their overall density affected by changes in specific gravity of the cell sap, or in some cases by formation or disappearance of gas vacuoles in the cytoplasm.

Without wishing to be limited to any certain theory, it is believed that applying de-pressurization causes an instant decrease in hydrostatic pressure sur rounding the microalgal cell, and the gas contained within the gas vacuoles of the cell expands and increases the volume of the microalgal cell. As a result, the density of the microalgal cell decreases in relation to the density of the liquid surrounding the microalgal cell, effecting more positive buoyancy to the microalgal cell. In addi tion, gas contained within a microalgal cell may expand to effect more positive buoyancy to the microalgal cell irrespective of the presence of gas vacuoles. This means that microalgal cells and species without gas vacuoles may be treated with the method and arrangement of the invention. Positive buoyancy causes the micro algal cells to rise towards the surface of the liquid column, leading to a formation of a concentrated mass of microalgal cells at the liquid surface and in its subsurface vicinity.

In an embodiment, the microalgae of the invention are microalgal spe cies having intracellular gas vacuoles.

The velocity or speed of rising may increase while the positively buoy ant microalgal cell rises towards the surface of the liquid. The cell may become more positively buoyant while approaching the surface due to reduction of air pressure and hydrostatic pressure which causes further expansion of intracellular gas. Furthermore, microalgal cells may form aggregates of cells, which may further increase in size i.e. number of individual cells while rising by joining into the aggre gate new cells that are possibly less buoyant and more static than the aggregate. These aggregates may have a more positive buoyancy than the sum of positive buoyancies of each individual microalgal cell. The reduction of hydrostatic pres sure affects the microalgal cell aggregates as well as the aggregates move higher towards the surface in the liquid column, possibly increasing the velocity or speed of rising.

The height of a water column that has a hydrostatic pressure of one at mosphere (1 atm, corresponds to 1,013 · 10 ⁵ Pa) at the bottom is approximately 10.3 meters. This means that if the water column is subjected to an external atmos pheric pressure (air pressure) of 1 atm, the pressure at the depth of 10,3 meters is 2 atm and decreases to about 1.1 atm at the depth of 1 m, where 1 atm of the pres sure is caused by atmospheric pressure and 0.1 atm by hydrostatic pressure. Cor respondingly, if the water column is subjected to external de-pressurization, the total pressure in the water column decreases. Typically, the majority of pressure a microalgal cell is subjected to in a shallow water column or water pool is caused by atmospheric pressure. In natural conditions microalgae are subjected to rapid changes in pressure for example in high surf conditions. Microalgae have evolved to endure these pressure changes without sustaining permanent damage - a qual ity which is utilized in the present invention. In the present invention, the de-pressurization in harvesting microalgae is to be applied in such a manner that maximal velocity of rising is achieved without damaging or rupturing the microalgal cells due to the underpressure. As de-pres- surization also may cause removal of gases such as carbon dioxide from the liquid as bubbles, de-pressurization is to be applied in such a manner that the cells rise rapidly and do not remain in the nutrient limited (e.g. carbon limited) conditions for extended periods of time. Nutrient limitation may lead to the microalgal cells utilizing for energy intracellular lipids and other (macro) molecules that are thus lost and cannot be retrieved for e.g. biofuel production.

Favourable conditions for further improving buoyancy of microalgal cells may also be achieved by controlling conditions including but not limited to nutritional status and light conditions of the microalgal cells before or during har vesting. Accumulation of macromolecules such as condensed carbohydrate in ac tively photosynthesizing microalgal cells may affect buoyancy in a negative manner by increasing intracellular ballast and turgor pressure. A high turgor pressure may induce collapse of gas vacuoles. Thus, long-term exposure to high-intensity light may decrease buoyancy. On the other hand, microalgal cells residing in lowlight conditions may produce gas vacuoles to increase buoyancy.

Nutritional conditions including but not limited to carbon, nitrogen, phosphate, and sulphate availability may affect buoyancy of microalgal cells. Lim ited carbon availability may increase microalgal cell buoyancy by e.g. preventing accumulation of condensed carbohydrates. Other conditions which reduce the car bon fixation rate are also likely to result in increased buoyancy, including but not limited to cell senescence and photo-oxidation of pigments.

While limited carbon availability may increase buoyancy, limited avail ability of other nutrients such as nitrogen, phosphate and sulphate may have the opposite effect. The presence of these nutrients reduces accumulation of con densed carbohydrate by facilitating production of other biopolymers such as pro teins. Nutrient availability conditions may also affect gas vacuole formation. Other conditions including but not limited to temperature and time of applying de-pres- surization or certain light or nutrient availability conditions may also affect buoy ancy.

As used herein, the term "positive buoyancy" refers to a microalgal cell that has a buoyancy that causes the cell to rise or float in a liquid or water column.

As used herein, the term "negative buoyancy" refers to a microalgal cell that has a buoyancy that causes the cell to sink in a liquid or water column. As used herein, the term "neutral buoyancy" refers to a microalgal cell that has a buoyancy that causes the cell to remain essentially at the same level in a liquid or water column.

Figure 1 illustrates a first embodiment of an apparatus 1 for harvesting microalgae. The apparatus 1 comprises an inlet 2 for liquid comprising non-con- centrated microalgae, a receptacle 3 for liquid comprising non-concentrated mi croalgae and an outlet 4 for liquid comprising concentrated microalgae at the up per section of the receptacle 3 for removing concentrated microalgae for further processing. The receptacle 3 can be made of corrosion resistant material such as stainless steel, polymer or glass, for instance.

In an embodiment, the receptacle 3 is made of material that is essen tially non-permeable to light. This ensures no extraneous light from the outside of the receptacle 3 can enter the receptacle 3 to interact or to exert an effect on the microalgae within the receptacle 3.

As used herein, the term "non-concentrated microalgae" refers to mi croalgae that has not been treated with the method of the invention or in the appa ratus 1 of the invention. The term "non-concentrated microalgae" thus does not exclude any pre-treatment performed before treating with the method of the in vention or in the apparatus 1 of the invention. The pre-treatment may or may not involve a step that includes, but is not limited to, concentrating, recovering, har vesting, thickening or dewatering of the microalgae.

As used herein, the term "concentrated microalgae" refers to microalgae that has been treated with the method of the invention or in the apparatus 1 of the invention. The term "concentrated microalgae" thus does not exclude any pre treatment performed before treating with the method of the invention or in the apparatus 1 of the invention. The pre-treatment may or may not involve a step that includes, but is not limited to, concentrating, recovering, harvesting, thickening or dewatering of the microalgae.

The apparatus 1 comprises at least one device unit for de-pressuriza- tion of the receptacle 3 to increase buoyancy of microalgal cells. As used herein, the term "de-pressurization" refers to reduction of pressure from the value wherein the liquid comprising microalgae resides outside the receptacle or outside the closed space containing the liquid comprising microalgae. De-pressurization is per formed within the receptacle 3 or closed space containing the liquid comprising microalgae. In the present invention, the effect of increasing buoyancy of the mi croalgal cells is not obtained by producing gas bubbles into the liquid comprising microalgal cells, which gas bubbles then attach to the microalgal cells to reduce density and to cause a more positive buoyancy. Therefore, to keep the harvesting expenses low, an aeration device or a flotation device and use of flocculants are to be avoided.

The device unit for de-pressurization can be for instance a pressure con trol valve 5 connected to a pump 6 arranged to adjust the pressure inside the re ceptacle 3. The pump 6 is arranged to pump air from the receptacle to lower the pressure inside the apparatus 1 compared to the ambient pressure outside the ap paratus 1. The pressure control valve 5 is arranged to prevent the pressure from returning to the receptacle 3 during the de-pressurization but let the pressure back in after the de-pressurization. The pump 6 can be for instance an air pump and connected to a controller which a user can actuate and set a preferred pressure within the receptacle 3. The pressure control valve 5 can be for instance a standard air pressure reducing valve which is normally open and allows 2-way air flow. When upstream pressure is lower than its setting, the valve closes and blocks air flow.

The inlet 2 is closable and covered by a lid, for instance, which is config ured to hermetically seal the apparatus 1 for keeping pressure inside the apparatus 1 unchangeable when the lid is closed. As shown in Fig. 1, the lid comprises at least one pressure control valve 5 which is connected to at least one pump 6 arranged to adjust the pressure inside the apparatus 1. However, the at least one pressure valve 5 can also be situated outside of the inlet 2 or on the side wall.

The outlet 4 in this embodiment can be a closable opening with a re movable plug or stopper or a pivotable piece or a slidable hatch, for instance, for transferring concentrated microalgae from the receptacle 3 to a separate container and is configured to hermetically seal the apparatus 1 during de-pressurization and after the de-pressurization pass on concentrated microalgae to other devices for further processing. The plug, stopper etc. can be made of corrosion resistant mate rial with a hermetic seal around it, for instance. The outlet 4 is provided at the up per section of the receptacle, preferably at the same height or slightly lower than the surface of the liquid comprising the concentrated microalgae.

As used herein, the term "upper section" refers to a section above the receptacle’s 3 midpoint and below the lid.

As used herein, the term "hermetic" refers to any type of sealing that makes the apparatus 1 airtight and impermeable to air, oxygen or other gases. Figure 2 illustrates a second embodiment of an apparatus 51 for har vesting microalgae. The embodiment is very similar to the one explained in con nection with Figure 1. Therefore, the embodiment of Figure 2 is mainly explained by pointing out the differences between these embodiments.

In this embodiment, a container 7 is provided next to the receptacle 3. A wall 9 extending from the bottom upwards to a height lower than the height of the container 7 separates the receptacle 3 from the container 7 and forms a first outlet 54. The first outlet 54 is provided above the top of the wall 9 and allows the surface layer of the liquid comprising concentrated microalgae to pass from the receptacle 3 to the container 7. The first outlet 54 in this context refers to an open top section of the wall 9. The wall 9 can be made of same material as the receptacle 3 and container 7.

A pivotable piece 10 on the top section of the wall 9 is arranged to facil itate adjustment of the height of the wall 9 and transfer of liquid comprising con centrated microalgae from the receptacle 3 to the container 7. The end of the pivot able piece 10 is directed upwards during the de-pressurization while keeping the liquid inside the receptacle 3 and angled towards the container 7 or the receptacle 3 during the harvesting of liquid comprising concentrated microalgae. The maxi mum angle a is preferable smaller than 180 degrees but larger than 45 degrees. For example, a hinge can be provided to enable pivoting. The pivotable piece 10 can comprise of a part of the horizontal length of the wall 9 or whole horizontal length of the wall 9 from one side of the receptacle 3 to another.

A device such as a scraper 20 can be used to speed up removing the sur face layer of the liquid comprising concentrated microalgae from the receptacle 3, but other instruments can also be used. The scraper 20 can be operated manually or automatically by a motor. It can be attached to a side wall at a height of the sur face layer in a way that it is retracted close the wall during the de-pressurization stage. The operation height may be adjustable. When the microalgae have floated to the surface, the scraper 20 is arranged to move horizontally with a telescopic arm or a scissor mechanism, for instance, towards the outlet 4 while pushing the liquid comprising concentrated microalgae through the outlet 4 and causing the pivotable piece 10 to angle towards the container 7 and transferring the liquid comprising concentrated microalgae from the receptacle 3 to the container 7. Al ternatively, the scraper may be an external device which can be used when the lid is opened. As used herein, the term "surface" or "surface layer" refers to the surface of the liquid and the subsurface zone in the vicinity of the surface. Specifically, the surface or the surface layer is the zone in the liquid column where microalgal cells concentrate after achieving positive buoyancy.

The liquid comprising concentrated microalgae can be collected from the container 7 from above or passed on via a second outlet 8 for further pro cessing. The second outlet 8 in this embodiment is similar as the outlet 4 described in Figure 1 and is provided at the bottom section of the container 7 so that the liquid comprising concentrated microalgae can be removed easily from the container 7 by gravity or a pump.

The inlet 2 can further comprise a valve 21 connected to at least one gas supply 22. The gas supply 22 is arranged to provide carbon dioxide or inert nitro gen gas via the valve 21 to a head space of the receptacle 3. Both gases may provide additional sustenance for the microalgae and accelerate its growth, and at the same time reduce the demand and adjustments of the microalgae through bioreactions such as reducing its volume or quantity of lipids. The gas supply 22 may provide the gas before the de-pressurization, during the de-pressurization or after the de pressurization or any combination of the above. However, the valve 21 can also be situated outside of the inlet 2 or on the side wall. The gas supply 22 may be con nected to a separate controller or the same controller connected to the pump 6.

The embodiment may further comprise at least one optical measure ment unit (not shown in the Figures). The optical measurement unit comprises at least one light source on one side wall and at least one optical sensor on the oppo site wall at the height of the surface layer, where the liquid comprising concen trated microalgae has gathered to. The light source may be a LED, UV or 1R source, for instance. The optical sensor is arranged to identify light intensity attenuation of the liquid. A light path length is fixed, and the microalgae has a specific absorption at the wavelength region. When the measured absorption exceeds a predetermined limit, which indicates the liquid comprising concentrated microalgae has reached a sufficient density level, the optical sensor may be arranged to send a signal to an user interface or a controller connected to the scraper motor to start moving the scraper 20 towards the outlet 4 at the operation height to move as much concen trated microalgae as possible.

Alternatively or additionally, the optical measurement unit may be pro vided at a height below the surface area to measure the clarity of the liquid below the liquid comprising concentrated microalgae layer. When the light intensity attenuation of the liquid reaches below a predetermined limit, which indicates the liquid below the liquid comprising concentrated microalgae layer has reached a certain transparency level, the optical sensor may be arranged to send a signal to the user interface or the controller connected to the scraper motor to start moving the scraper 20 towards the outlet 4 at the operation height to move as much con centrated microalgae as possible.

Alternatively or additionally, the optical measurement unit may be pro vided at a height below the surface area to measure the thickness of the liquid com prising concentrated microalgae layer. When the thickness of the liquid comprising concentrated microalgae layer reaches above a predetermined limit, the optical sensor may be arranged to send a signal to the user interface or the controller con nected to the scraper motor to start moving the scraper 20 towards the outlet 4 at the operation height to move as much concentrated microalgae as possible.

As used herein, the term "attenuation" refers to a gradual loss of light intensity along a fixed light path length of a liquid.

Alternatively, the optical measurement unit may be replaced with any other device unit for measuring the density level of the liquid.

Alternatively, a sample of the liquid comprising concentrated microal gae layer may be collected manually and measured outside the apparatus in a sep arate device such as hydrometer or pycnometer to determine if the liquid compris ing concentrated microalgae has exceeded a predetermined limit to indicate the liquid comprising concentrated microalgae has reached a sufficient density level and ready to be transferred from the receptacle 3 to the container 7.

Figure 3 illustrates a third embodiment of an apparatus 61 for harvest ing microalgae. The embodiment is very similar to the one explained in connection with Figures 1 and 2. Therefore, the embodiment of Figure 3 is mainly explained by pointing out the differences between these embodiments.

In the embodiment illustrated in Figure 3, at the bottom of the appa ratus 61, both a sound emitter 13 and a vibration emitter 14 are provided. These additional means may be utilized also in the embodiments of Figures 1 and 2. The sound emitter 13 is configured to emit soundwaves in the liquid comprising micro algae at a predetermined frequency in order to improve buoyancy of the microal gae. The vibration emitter 14 is configured to emit vibration in the liquid compris ing microalgae at a predetermined frequency in order to improve buoyancy of the microalgae. The sound emitter 13 and the vibration emitter 14 can also be config ured as one device unit. The sound emitter 13 and vibration emitter 14 may also be situated on the side wall or top of the receptacle 3. The predetermined frequen cies are such that the forces of the sound waves and/or the vibration in the liquid act to improve buoyancy of the microalgal cells comprised in the liquid.

In a further embodiment, the apparatus 61 comprises a sound emitter 13 and not a vibration emitter 14. The sound waves produced by the sound emitter 13 act as a force that is unidirectional to the upward force exerted by a fluid that opposes the weight of an immersed object. The combined effect of the two forces causes the immersed objects i.e. the microalgal cells to become more positively buoyant than in the case when only one force is applied.

In a yet further embodiment, the apparatus 61 comprises a vibration emitter 14 and not a sound emitter 13. The vibration produced by the vibration emitter 14 acts as a force that is unidirectional to the upward force exerted by a fluid that opposes the weight of an immersed object. The combined effect of the two forces causes the immersed objects i.e. the microalgal cells to become more positively buoyant than in the case when only one force is applied.

The apparatus can further comprise at least one light source 12 to fur ther improve buoyancy of the microalgae by adjusting wavelength and luminous flux of the light emitted from the at least one light source 12. The light source 12 can be on top of the receptacle 3, for instance, and arranged to be directed towards the microalgae in the surface layer of the liquid within the receptacle 3. The light source 12 can also be located on the bottom or on the side of the receptacle 3. Prop erties such as wavelength, intensity, position and direction of the light are arranged in such a manner that they provide positive buoyancy to the microalgal cells and slow down the decrease in intracellular energy levels of the microalgal cells to maintain positive buoyancy.

In an embodiment, the apparatus 61 comprises a vibration emitter 14 and a light source 12. In a further embodiment, the apparatus 61 comprises a sound emitter 13 and a light source 12. In a yet further embodiment, the apparatus com prises a vibration emitter 14, a sound emitter 13 and a light source 12.

As used herein, the term "a" or "an" may refer to the singular form or to the plural form of the word following the term.

As used herein, the term "and" may refer to the term "and" or the term

"and/or".

As used herein, the term "or" may refer to the term "or" or the term

"and/or". In the context of this application, the terms "microalgae", "microalgae cells" and "microalgal cells" and the like may be used interchangeably.

Instead of the pivotable piece 10, on top section of the wall 9 comprises a slidable hatch 11 to facilitate adjustment of the height of the wall 9 and transfer of liquid comprising concentrated microalgae from the receptacle 3 to the con tainer 7. The hatch 11 is configured to stay up during de-pressurization and slide down for harvesting liquid comprising concentrated microalgae. The hatch 11 can be operated on corrosion resistant rollers or along grooves on each side of the re ceptacle 3 where horizontal ends connect the receptacle 3, for instance, and can be lifted and lowered manually or automatically, for instance depending on the thick ness of the liquid comprising concentrated microalgae.

An advantage of the present invention is that the apparatus 61 of the present invention does not necessarily comprise mechanical means for agitating or mixing the liquid comprising microalgae. As used herein, the term "mechanical means for agitating or mixing" refers to a means such as a blade, rotor or impeller that induces turbulent, swirling or rotating motion in the liquid, thus possibly pre venting or disturbing the upward-rising motion or momentum of the positively buoyant microalgal cells.

A further advantage of the present invention is that the apparatus 61 of the present invention does not necessarily comprise a means for producing gas bubbles into the liquid such as an aeration device or a flotation device. In the pre sent invention, the effect of increasing buoyancy of the microalgal cells is not ob tained by producing gas bubbles into the liquid comprising microalgal cells, which gas bubbles then attach to the microalgal cells to reduce density and to cause a more positive buoyancy. Instead, buoyancy is increased through reducing density of microalgal cells by expansion of gas contained in the gas vacuoles by applying de-pressurization. When applied, de-pressurization may cause formation of gas bubbles from expanding gases contained in the liquid, but no extraneous gas is added to the liquid in the intention of forming bubbles.

In an embodiment, the apparatus does not comprise an aeration device or a flotation device.

Figure 4 illustrates a flow chart of a method for harvesting microalgae. The method comprises the steps of providing a liquid comprising non-concen- trated microalgae into a closed space 101, performing de-pressurization in the closed space to increase buoyancy of the microalgae 102, and collecting liquid com prising concentrated microalgae from the surface of the liquid 103. In an embodiment, the method further comprises the step of providing sound to the de-pressurized liquid to increase buoyancy of the microalgae. In an other embodiment, the method further comprises the step of providing vibration to the de-pressurized liquid to increase buoyancy of the microalgae. In another em bodiment, the method comprises the step of providing sound and vibration to the de-pressurized liquid to increase buoyancy of the microalgae. The sound emitter 13 and the vibration emitter 14 can be configured as one device unit.

In an embodiment, the method comprises the step of providing light to the de-pressurized liquid to increase buoyancy of the microalgae. In another em bodiment, the method comprises the step of providing sound and light to the de pressurized liquid to increase buoyancy of the microalgae. In another embodiment, the method comprises the step of providing vibration and light to the de-pressur- ized liquid to increase buoyancy of the microalgae. In another embodiment, the method comprises the step of providing sound and vibration and light to the de pressurized liquid to increase buoyancy of the microalgae.

A further object of the present invention is to provide a concentrated microalgae product produced by the method of the invention. The concentrated microalgae may be used as a starter or inoculum for a new culture of microalgae in a liquid culture medium such as wastewater. The concentrated microalgae may also be used as feedstock for biogas production or manufacturing or extraction of algal products.

In an embodiment, the method does not comprise a step of providing aeration by adding extraneous gas to the liquid or a step of flotation.

In both the apparatus and method, a depressurization i.e. underpres sure ranging from -0.2 bar to -0.8 bar may be used. Alternatively, a depressuriza tion ranging from -0.2 bar to -0.6 bar may be used. Alternatively, a depressurization ranging from -0.4 bar to -0.6 bar may be used. The depressurization time may be 10 min or longer. The depressurization time may be 15 min or longer, for example 15 min - several hours. The depressurization time may range from 15 min to 24 h or 15 min to 10 h or 15 min to 5 h or 10 min to 5 h or 15 min to 2 h or 10 min to 2 h. The depressurization time may be 10 min - 60 min or 15 min - 30 min. Example

Two algae species were tested: Chlorella vulgaris (N1VA-CHL 19) and Arthrospira platensis (N1VA-CYA 428). The species were purchased from The Nor wegian Culture Collection of Algae, NORCCA. Both species were first pre-cultivated in batch mode in cultivation ves sels (ca. 40 mL) at room temperature under a mixture of natural light and artificial light as used in the laboratory facilities. After the initial stage, both algae species were transferred into two 1 L glass bottles filled with 500 mL of growth medium. Spirulina medium was used for cultivating the Arthrospira and MWC medium for cultivating the Chlorella. More growth medium was added to the bottles on two occasions to increase the volume of the algae suspension. Bottles were kept closed and illuminated from the side under artificial light at photosynthetic photon flux density of ca. 50-55 gmol nr ² s ¹ . Cultures were illuminated for 12h:12h daymight in the beginning and 16h:8h later on. The algae were cultivated in a fume hood in the laboratory at room temperature. There was no control of pH or dissolved oxy gen, and no addition of carbon dioxide to the suspensions. Algae suspensions were shaken by hand twice every day and opened for gas exchange. After 8 days into the cultivation, the Chlorella culture was aerated continuously with the help of an aquarium pump and a pipette. Chlorella was pre-cultivated under the light scheme for 10 days, and Arthrospira for 14 days.

The apparatus for performing harvesting consisted of a glass cylinder of 50 cm height and 5 cm diameter attached to a base plate. The cylinder was clos- able with a lid, and the lid firmly attached to the glass cylinder with the help of a spiral and a metal bar. The cylinder could then be de-pressurized with the help of a hand-operated vacuum pump. De-pressurizations of -0.2 bar, -0.4 bar, -0.6 bar and -0.8 bar were tested. In the beginning of the test, the algae suspensions was poured to the experimental device. The device was filled with ca. 800 mL of algae suspension and the same algae suspension was used in the subsequent tests start ing with the control (no de-pressurization), de-pressurization at -0.2 bar, -0.4 bar, -0.6 bar and -0.8 bar, respectively. Algae concentration at near-surface was quanti fied gravimetrically. This was done by taking aliquots of algae suspension ca. 1-1.5 cm below the suspension surface using a pipette, filtering the suspension through pre-dried and pre-weighed filter papers (pore size 1.2 gm) and drying the filters in an oven at 105°C for 2 hours. Filters were then weighed again and the algae dry biomass was calculated as the difference between the two weights. Of Chlorella, 2 x 10 mL were sampled in each step, filtered and dried. In the Arthrospira tests, 1 x 10 mL was sampled in each step, filtered and dried. Each time when aliquots of algae suspension were removed from the test device, an approximate equal volume of algae suspension was poured to the cylinder, and the suspension in the test de vice mixed as well as possible by stirring with a glass rod in preparation for the next test performed with the same algae solution.

Results are shown in Table 1. The results varied based on algae species and the conditions used. With Arthrospira platensis, the best increase in algae cell density (as expressed in g/1 of dry algal biomass) was seen with 30 minutes of de pressurization at -0.2 bar or a shorter depressurization of 15 min at -0.4 bar or -0.6 bar. With Chlorella vulgaris, -0.2 bar was not sufficient to increase the algae cell density during either a 15 min or 30 min depressurization, but at -0.4 bar and -0.6 bar good results were seen with 30 min depressurization, and even with 15 min depressurization at -0.4 bar. The lowest pressure used, -0.8 bar, was not in this case effective in increasing cell density with the algal species tested.

Table 1. Results of algae harvesting using different underpressures and har vesting times

It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The inven tion and its embodiments are not limited to the examples described above but may vary within the scope of the claims.