Method for producing a micro gas bubble immobilised microalgae culture in a water or wastewater treatment process

Abstract

The invention relates to a method for producing a micro gas bubble immobilised microalgae culture for the removal of nitrogen and phosphorus in water and wastewater treatment. Said microalgae use nitrogen and phosphorus as nutrients while simultaneously forming biomass that can be used as a raw material for energy and other products. The method that is the subject of the invention enables the microalgal biomass to remain stable at the water surface by being immobilised with micro gas bubbles. In this way, the microalgae are able to receive the light required for photosynthesis and can be easily removed after the treatment process. This innovative approach aims to increase the efficiency of treatment processes while reducing energy consumption and operating costs.

Description

[0001] DESCRIPTION

[0002] METHOD FOR PRODUCING AN IMMOBILISED MICROALGAE CULTURE CONTAINING MICRO GAS BUBBLES IN A WATER OR WASTEWATER TREATMENT PROCESS

[0003] Technical Field

[0004] The invention relates to a method for producing immobilised algae cultures supported with micro gas bubbles for the removal of nitrogen and phosphorus pollution in water and wastewater treatment processes. By means of the method that is the subject of the invention, the produced microalgae culture (biomass) is enabled to remain in a floating form on the water surface, thereby allowing it to receive the light required for photosynthesis and to effectively remove nutrients present in the water. Compared to conventional methods, the production method that is the subject of the invention reduces energy consumption and operational costs, while enabling the biomass formed after treatment to be harvested and used as a raw material for energy or other products. The invention aims to reduce the environmental impacts of wastewaters and to provide a more sustainable treatment process.

[0005] State of the Art

[0006] Water pollution occurs as a result of the physical, chemical or biological contamination of water resources, leading to the deterioration of their natural properties. Among the damages caused to the environment are the destruction of ecosystems, the loss of aquatic life, the reduction in the quality of water used in agricultural and industrial activities, and the emergence of situations that threaten human health. Structures causing water pollution include industrial facilities, agricultural areas, domestic wastewater systems, mining operations and power plants. Wastewaters are defined as waters that have been contaminated as a result of various human activities and that must be treated before being discharged into the natural environment. These waters occur in domestic, industrial, agricultural or urban areas. For example, wastewater discharged from households, chemicals released from factories, and waters formed asa result of the washing of pesticides used in agriculture are considered within this scope.

[0007] Water treatment methods are divided into three main categories: physical, chemical and biological. Physical methods include processes such as screens, sedimentation tanks and filtration to separate large solid particles present in the water. In chemical methods, harmful substances in the water are attempted to be removed through processes such as pH adjustment, coagulation, flocculation, chlorination and ozonation. Biological methods involve the decomposition of organic pollutants with the aid of microorganisms, and active sludge systems or biological reactors are generally used. However, each of these methods has its own shortcomings. While physical methods can only separate large particles, chemical methods may be costly and may cause the release of additional chemicals into the environment. Biological methods, on the other hand, may be insufficient in effectively treating heavy metals or toxic chemicals.

[0008] The use of algae in water treatment methods has been attracting increasing attention, particularly in biological treatment processes. Algae have the capacity to clean water by absorbing pollutants present in the water, especially nutrients such as nitrate and phosphate, through photosynthesis. In addition, some algae species can also retain heavy metals. However, disadvantages of these methods include the continuous growth and control requirements of algae, management difficulties in large-scale applications, and the inability to completely remove pollution. Furthermore, there is a risk that organic substances released upon the death of algae may re-pollute the water. Therefore, algae-based treatment methods can yield more effective results when used together with other techniques.

[0009] A study conducted by Shelknanloymilan et al. in the state of the art examines how nitrogen and phosphate can be removed from wastewaters contaminated with synthetic and organic substances by means of the microalga Chlorella vulgaris [1], Said study aims to examine the relationship between improving the biological treatment capacity of microalgae and the removal of ammonium and phosphorus ions in wastewaters, and the growth support of microalgae. Said study demonstrates, based on observations carried out in photobioreactors for 30 days, that C. vulgaris is effective in nitrogen and phosphate removal in both synthetic and organic wastewaters. Inparticular, nitrogen and phosphate decreased more rapidly in synthetic wastewater, whereas this process occurred more slowly in wastewater contaminated with organic substances. In addition, factors such as the lower performance of algae in wastewaters contaminated with organic substances compared to synthetic wastewater, the potential negative effects of other elements on this process, and the consumption of algae by other organisms in such environments are present. Furthermore, the study was conducted only for a period of 30 days, and did not provide further data on long-term effects and efficiency. In addition, it is stated that the microalgae used are effective only under certain conditions and that their efficiency may vary with respect to different wastewater types.

[0010] The patent application numbered CN105174476A in the state of the art relates to a granulation system comprising a combination of activated sludge and microalgae for water treatment. Said system is designed to effectively remove pollutants such as nitrogen and phosphorus originating from both domestic and industrial wastewater sources. In addition, some information is presented regarding the potential of microalgae in biofuel production and the use of this system in renewable energy generation. However, said document contains certain deficiencies. For example, no detailed information is provided regarding the long-term operational durability of said system and its routine maintenance requirements. In addition, the document does not include any analysis regarding the quality standards of the by-products obtained after wastewater treatment or their economic value. Data regarding how system performance changes under real-world conditions outside the laboratory are also lacking. All these factors render the reliability of the system in terms of practical applicability and sustainability questionable.

[0011] The limitations and inadequacies of the solutions in the current state of the art, such as the need for additional devices or structures to enable microalgae to be positioned in a floating manner in water and the fact that these structures require maintenance difficulties and complex preparation processes, the mixing of suspended microalgae into water systems, and the need for secondary treatment methods to ensure their removal after treatment, have necessitated the development of an improvement in this field.Brief Description and Aims of the Invention

[0012] The invention describes a method for producing an immobilised algae culture containing micro gas bubbles for the treatment of nitrogen and phosphorus from water and wastewater. Said microalgae use nitrogen and phosphorus as nutrients while simultaneously producing biomass that can be utilised for energy and other industrial products. The immobilisation of the algal biomass and its retention at the surface by means of micro gas bubbles facilitate light uptake for photosynthesis while enabling the rapid collection of the biomass after the treatment process. This innovative method provides an environmentally friendly solution that increases treatment performance while reducing energy consumption and costs.

[0013] The main aim of the invention is to ensure the effective removal of nitrogen and phosphorus pollution in water and wastewaters. By means of the method that is the subject of the invention, nitrogen and phosphorus are naturally removed from the water due to the microalgae culture produced using these substances as nutrients. Considering the energy and cost burdens of the chemical or biological processes used in conventional treatment methods, the method that is the subject of the invention provides a more environmentally friendly and economical alternative. Both nitrogen and phosphorus removal can be achieved simultaneously in a single treatment unit. In particular, the removal of nitrogen in both ammonium and nitrate forms increases the effectiveness of this process and provides a wider field of application. In addition, the reuse potential of the treated water is also increased. By means of the use of immobilised algae cultures containing micro gas bubbles produced by the method that is the subject of the invention, both energy consumption and operational costs in water treatment are significantly reduced. The oxygen produced by the photosynthetic activities of the microalgae culture helps to meet the oxygen requirement in biological processes, thereby reducing energy consumption. In addition, retaining the microalgae culture in a floating form minimises the need for physical mixing and aeration equipment. Thus, a more economical and sustainable treatment process is provided. Another aim of the invention is to enable the microalgae culture (biomass) formed after the water and wastewater treatment process to be used as a valuable resource. The microalgae biomass obtained by means of the production method that is the subject of the invention can be utilised as a raw material in the production of biofuels, fertilisers,animal feed and other biotechnological products. In particular, its high organic content and nitrogen-phosphorus richness render this biomass an attractive resource for different industries. Thus, it becomes possible for the treatment process not only to provide an environmental solution but also to create economic added value. This provides an innovative approach to wastewater treatment processes.

[0014] Another aim of the invention is to minimise the environmental impacts of the water and wastewater treatment process and to ensure a low carbon footprint. The use of atmospheric carbon dioxide by the microalgae culture obtained by means of the method that is the subject of the invention during photosynthesis reduces greenhouse gas emissions arising in biological treatment processes. In addition, the floating form of said culture increases the use of natural light, thereby reducing the need for external energy input. The invention not only combats water pollution but also makes a significant contribution to combating climate change. Thus, a sustainable and environmentally friendly technology is provided.

[0015] Another aim of the invention is to provide a more effective and practical water treatment process. Retaining the microalgae culture produced by means of the method that is the subject of the invention in an immobilised and floating form enables it to be easily collected and reused after treatment. This eliminates the problems associated with waste sludge disposal frequently encountered in conventional treatment methods. In addition, the use of said microalgae culture in a fixed structure prevents its dispersion within the water and enables the treatment process to be managed in a more controlled manner. In this way, operational efficiency is increased, particularly in large-scale water treatment plants.

[0016] Description of the Drawings

[0017] Figure 1: Set forming micro gas bubble immobilised algae beads

[0018] Figure 2: Experimental set and operation used for forming micro gas bubble immobilised algae beads

[0019] Figure 3: Use of micro gas bubble immobilised algae biomass in conventional water / wastewater treatment plantsDescription of Reference Numbers in the Drawings

[0020] 1. Suspended algal biomass transfer line

[0021] 2. Pressure-dissolved gas gel transfer line

[0022] 3. Mixing zone

[0023] 4. Outlet pipe

[0024] 5. Micro gas bubble gel microalgae droplet

[0025] 6. Immobilised micro gas bubble microalgae beads

[0026] 7. Crosslinking solution

[0027] 8. Compressor producing pressurised gas

[0028] 9. Pressurised gas transfer line

[0029] 10. Gel tank

[0030] 11. Liqu id / gel transfer pumps

[0031] 12. Mixer

[0032] 13. Filtered airvent

[0033] 14. Suspended algal biomass tank

[0034] 15. Immobilised micro gas bubble algal beads transfer line

[0035] 16. Direction of movement of algal beads

[0036] 17. Separator

[0037] 18. Collection chamber

[0038] Detailed Description of the Invention

[0039] The invention relates to a method for producing immobilised microalgae cultures supported with micro gas bubbles for the removal of nitrogen and phosphorus from water and wastewaters. Said microalgae culture consumes nitrogen and phosphorus as nutrients while simultaneously producing biomass, thereby forming a valuable resource for energy and other products. Said method enables the microalgae cultureto remain on the surface in a stable structure by means of micro gas bubbles, thereby allowing the reception of light required for photosynthesis and enabling easy separation after treatment.

[0040] A method for producing a microalgae culture for use in the simultaneous removal of nitrogen and phosphorus from water or wastewater comprises the process steps of: i. preparing a gel-form polymer-microalgae mixture by mixing a polymeric gelform solution and suspended microalgae,

[0041] ii. mixing the polymer-microalgae mixture with pressurised gas or a gas mixture and subsequently converting it into droplet form to obtain a micro gas bubble gel-form microalgae culture droplet,

[0042] iii. dripping the microalgae culture droplets obtained in process step (ii) into a crosslinking solution, and

[0043] iv. obtaining an immobilised micro gas bubble microalgae bead by mixing the microalgae culture droplet with the crosslinking solution and polymerising it. In one embodiment of the method that is the subject of the invention, said polymeric gel-form solution comprises mixtures containing at least one of a polymer derived from monomeric structures containing sodium alginate, agar, agarose, calcium alginate, carrageenan, acrylamide, acrylonitrile, urethane, chitosan, polyvinyl alcohol, polyacrylamide, sodium carboxymethyl cellulose or hydroxyethyl methacrylate (HEMA).

[0044] In one embodiment of the method that is the subject of the invention, said crosslinking solution comprises mixtures containing at least one of calcium lactate, a crosslinking chemical solution containing +2 charged calcium cations, calcium chloride, magnesium chloride, glutaraldehyde, tannic acid, genipin, silica gel, activated carbon, polyurethane foam or polyethylene glycol, in order to enable the crosslinking of the polymers within the polymeric gel-form solution. Said structures have support and carrier properties and enable the formation of crosslinks. In another embodiment of the invention, the polymeric gel-form solution in which gas is dissolved under pressure is conveyed to the junction point via a transfer line. Here, when atmospheric pressure is reached, micro gas bubbles begin to form within it and, in droplet form, it drips and falls into the solution containing a crosslinking agent in the Erlenmeyer flask below. Crosslinking occurs here.In another embodiment of the invention, said polymeric gel-form solution contains sodium alginate.

[0045] In another embodiment of the method that is the subject of the invention, for preparing said gel-form polymer-microalgae mixture, suspended microalgae are mixed with a 1-5% by volume sodium alginate solution at a volume ratio of suspended microalgae:sodium alginate solution of 1 :5— 5:1.

[0046] In another embodiment of the method that is the subject of the invention, said microalgae is Chlorella vulgaris.

[0047] In another embodiment of the method that is the subject of the invention, said gas mixture contains 0.5-35% by volume carbon dioxide, 50-78% nitrogen and 2-21% oxygen. Said gas mixture may also contain other gases, since atmospheric air may also be used directly as the gas mixture. However, as the proportion of carbon dioxide in the gas mixture increases, the proportions of oxygen and nitrogen gas may also change. In particular, the use of flue gas discharged from the stack of an industry may also be possible in this process. In this case, the flue gas composition of that sector may also vary.

[0048] In another embodiment of the method that is the subject of the invention, said crosslinking solution is a crosslinking chemical solution containing +2 charged calcium cations.

[0049] In another embodiment of the method that is the subject of the invention, said crosslinking solution is a solution containing at least one of calcium lactate or a solution thereof, calcium chloride, magnesium chloride, glutaraldehyde, tannic acid, genipin, silica gel, activated carbon, polyurethane foam or polyethylene glycol as a crosslinking chemical agent. These structures are added to said solution as support and carrier materials.

[0050] In another embodiment of the method that is the subject of the invention, said crosslinking solution is a solution of calcium lactate at a concentration of 0.5-10% by weight per volume.

[0051] In another embodiment of the method that is the subject of the invention, the gas / polymeric gel ratio in said immobilised gas bubble microalgae bead is in the range of 1 / 50-1 / 1 by volume.The microalgae culture produced by the production method that is the subject of the invention comprises at least one immobilised gas bubble microalgae bead containing microalgae in polymeric gel form and micro gas bubbles.

[0052] A method for producing a microalgae culture for the simultaneous removal of nitrogen and phosphorus from water or wastewater consists of four basic process steps. First, a gel-form polymer-microalgae mixture is prepared by mixing a polymeric gel-form solution with suspended microalgae. Then, this polymer-microalgae mixture is mixed with pressurised gas or a gas mixture and converted into droplet form, thereby obtaining a micro gas bubble gel-form microalgae culture droplet. In the third step, the obtained microalgae culture droplets are dripped into a crosslinking solution. Finally, the microalgae culture droplet is mixed with the crosslinking solution and polymerised, and by this process an immobilised micro gas bubble microalgae bead is obtained. These steps ensure the effective production of the microalgae culture and render it suitable for use in water treatment. In order to obtain the immobilised microalgae culture, first, for the preparation of the gel polymer form, mixtures containing at least one or at least two of agar, agarose, sodium alginate, calcium alginate, a polymer derived from monomeric structures containing sodium alginate, carrageenan, acrylamide, acrylonitrile, urethane or chitosan, polyvinyl alcohol, polyacrylamide, sodium carboxymethyl cellulose or hydroxyethyl methacrylate (HEMA), and mixtures containing at least one or at least two of calcium lactate, calcium chloride, magnesium chloride, glutaraldehyde, tannic acid, genipin, silica gel, activated carbon, polyurethane foam or polyethylene glycol in order to enable the crosslinking of the polymers within the polymeric gel-form solution, are used. These chemical compounds are preferred due to properties such as enabling rapid production, forming a compound that is insoluble in water after immobilization, having a durable structure, being non-toxic, and having transparency that allows the transmission of the light required by the algae. In one embodiment of the invention, sodium alginate is used from among these materials. For the preparation of the microalgae culture, a suspended microalgae culture is mixed with a 1-5% sodium alginate solution at specific volume ratios. As the microalgae culture, a pure microalgae culture may be used, or multiple algae species may be used together; in one embodiment of the invention, Chlorella vulgaris is used, and other suitable algae species may also be preferred.In the method that is the subject of the invention, a batch reactor and a Chlorella vulgaris algae culture are used in the production of micro gas bubble immobilised algae beads. The invention is based on experimental studies carried out at different pH values, under light conditions of 8, 12 and 24 hours, with different air flow rates and at standard room temperature. The removal rates achieved at the initial nitrogen and phosphorus concentrations are extremely high; removal rates of 95% for ammonium nitrogen, 80% for nitrate and 99% for phosphate are obtained. The immobilization method used in the invention enables the microalgae culture (biomass) to be immobilised with various polymer-structured compounds and gelling chemicals. The immobilised microalgae biomass, in this way, maintains its viability and has the capacity to proliferate within the immobilised structure. At the same time, this structure prevents the algal biomass from mixing into the wastewater and thus enables the algal biomass to be easily harvested after treatment. The micro gas bubble immobilised algal biomass enables the simultaneous removal of both nitrogen and phosphorus in a single treatment unit. Said nitrogen and phosphorus removal may be applied to wastewaters as well as to waters that are not formed as waste as a result of a process. This is because nitrogen and phosphorus structures may also be present in waters that are not in waste form, and in such cases both nitrogen and phosphorus are removed from the water by means of the invention. In particular, in nitrogen removal, the microalgae culture can easily draw nitrogen species in both ammonium and nitrate forms from the water / wastewater as nutrients and can rapidly remove both of these nitrogen species. In addition, the micro gas bubble immobilised algae beads forming a layer on the surface can use the relatively high CO2 concentration in the wastewater after the biological treatment process for photosynthetic needs, thereby helping to reduce greenhouse gas emissions that arise intensively in biological treatment processes. Furthermore, since the immobilised microalgae beads in floating form are present in floating form on the surface in conventional circular cross-section treatment units, they can be easily harvested by skimming equipment. Since the immobilised microalgae culture is in the form of individual micro gas bubble immobilised beads, when the stability of the bead deteriorates during the treatment period, it loses the gas bubble and its density increases relative to the wastewater, causing it to sink to the bottom. However, even if the stability of the micro gas bubble immobilised algal biomass deteriorates, since not all of the micro gas bubbles are lost, its density remains lower than that of water and it can maintain its floating form. In addition, optionally, theimmobilised algal biomass may also be used together with bacterial microorganisms used in conventional treatment. In this case, while the oxygen required by the bacteria can be partially supplied by the photosynthesis of the algae, the CO2 produced by the bacteria during the treatment process can be used for photosynthesis by the immobilised algal biomass.

[0053] The invention relates to obtaining an immobilised algal culture containing micro gas bubbles that can be used for the removal of nitrogen and phosphorus from water and wastewater by saturating it with pressurised air to form micro gas bubbles, and to how benefits can be provided in this process. The micro gas bubble-containing structure of this immobilised algal culture enables the algae to come into contact with water or wastewater while allowing the algal biomass to be easily separated after treatment. In this way, there is no need for an additional process to remove the algae from the water after treatment. Similar to the dissolved air flotation (DAF) method commonly used in environmental engineering, by means of micro gas bubbles obtained by a pressurised gas dissolution method, the algal biomass becomes lower in density than water and remains suspended on the water surface. This enables the algae to receive the light required from the surface even in wastewaters with high turbidity. In addition, by means of the immobilised algal culture formed with micro gas bubbles, the algal biomass can form a structure that is more stable and capable of remaining on the surface for a longer time.

[0054] The production method of the microalgae culture can be carried out by means of a system comprising the following elements:

[0055] • a suspended algal biomass transfer line (1 ) that enables suspended microalgae coming from a suspended algal biomass tank (14) to be conveyed to a mixing zone (3),

[0056] • a pressure-dissolved gas gel transfer line (2) that enables gas or a gas mixture coming from a gel tank (10) to be conveyed to the mixing zone (3),

[0057] • a mixing zone (3) located at the junction point of the suspended algal biomass transfer line (1) and the pressure-dissolved gas gel transfer line (2), in which a gel-form polymer-microalgae mixture is mixed with gas or a gas mixture, • an outlet pipe (4) located downstream of the mixing zone (3), in which the mixture formed in the mixing zone (3) is converted into droplet form,• a crosslinking solution (7) into which droplets formed in the outlet pipe (4) drip, • a mixer (12) for mixing the crosslinking solution (7) into which the droplets formed in the outlet pipe (4) drip,

[0058] • a compressor (8) producing pressurised gas and a pressurised gas transfer line (9) that conveys the gas produced in the compressor (8) to the gel tank (10), • liquid / gel transfer pumps (11) located between the suspended algal biomass transfer line (1), the pressure-dissolved gas gel transfer line (2), and the mixing zone (3), and

[0059] • a filtered air vent (13) supplying air to the suspended algal biomass tank (14). An embodiment of the production method of the microalgae culture for use in the simultaneous removal of nitrogen and phosphorus from water or wastewater comprises the following process steps:

[0060] i. preparing a gel-form polymer-microalgae mixture by mixing a polymeric gelform solution and suspended microalgae,

[0061] ii. conveying the gel-form polymer-microalgae mixture from the suspended algal biomass transfer line (1) to the mixing zone (3),

[0062] iii. converting the gel-form polymer-microalgae mixture mixed in the mixing zone (3) with gas or a gas mixture coming from the pressure-dissolved gas gel transfer line (2) into droplet form in the outlet pipe (4), and obtaining a gel-form micro gas bubble gel microalgae droplet (5), and

[0063] iv. obtaining immobilised micro gas bubble microalgae beads (6) by mixing the micro gas bubble gel microalgae droplet (5) with a crosslinking solution (7) and polymerising it.

[0064] The invention explains a method for producing a micro gas bubble immobilised microalgae culture. The production method of the microalgae culture can be carried out by a system formed by the combination of various components. Suspended algal biomass is conveyed to the mixing zone (3) via a transfer line (1 ) coming from the tank (14). Likewise, gas or a gas mixture coming from the gel tank (10) reaches the mixing zone (3) via the pressure-dissolved gas gel transfer line (2). These two transfer lines merge in the mixing zone (3) and enable the gel-form polymer-microalgae mixture to be mixed with gas. Downstream of the mixing zone (3), this mixture is converted into droplet form by means of the outlet pipe (4). Droplets formed in the outlet pipe (4) aredripped into a crosslinking solution (7), and a mixer (12) is used to mix this solution. The compressor (8) producing pressurised gas conveys gas to the gel tank (10) and transfers this gas via the pressurised gas transfer line (9). In addition, the liquid / gel transfer pumps (11 ) located between the suspended algal biomass transfer line (1 ) and the pressure-dissolved gas gel transfer line (2) ensure the flow of these components. A filtered air vent (13) is used to supply air to the suspended algal biomass tank (14). The production of the microalgae culture is carried out by the combination of these elements.

[0065] Suspended microalgae biomass is conveyed from the tank (14) to the mixing zone (3) by means of the suspended algal biomass transfer line (1) and the liquid / gel transfer pumps (11). Here, gas reaching a pressure of 1-10 bar is pressurised by the compressor (8) producing pressurised gas. As the gas, atmospheric air may be used directly, or a gas mixture containing carbon dioxide or, optionally, a mixture containing nitrogen gas may also be used. In this gas mixture, the carbon dioxide content has a value in the range of 0.5-10% v / v. The pressurised gas enters the gel tank (10) and undergoes a dissolution process. The gas dissolution time lasts between 1 and 60 minutes. The flow rates of the suspended microalgae biomass and the gas-dissolved gel mixture vary depending on the pipe diameter and the gas-to-gel ratio. After the combination of the microalgae gel liquid mixed with pressurised gas, when atmospheric pressure is reached, the mass of gas dissolved in the liquid forms micro gas bubbles. The sizes of these micro gas bubbles may vary depending on factors such as the type and amount of algal biomass, the applied pressure, and the density and viscosity of the suspension. In the invention, silicone hoses (elements numbered 1 and 2) and Y- or T-type pipe connection fittings are used as the mixing zone (3). All materials used in the system employed in the production method are biocompatible and are capable of being sterilised for the production of pure algal cultures. A filtered air vent (13) is present in the suspended microalgae tank in order to balance the negative pressure formed while the suspended microalgae biomass is fed into the system. Both liquids are transferred to the mixing zone (3) at desired ratios by using the liquid / gel transfer pumps (11 ). In the mixing zone (3), the suspended algal biomass and the pressure-dissolved gel mixture are homogeneously mixed. The diameter of the outlet pipe (4) of this zone is in the range of 1-10 mm. The dissolved gases here become a supersaturated solution and form micro gas bubbles; these micro bubblesare converted into a micro gas bubble gel microalgae droplet (5). The formed micro gas bubble gel microalgae droplet (5) transforms into a spherical form and drips into the crosslinking solution (7) located below. Within the crosslinking solution (7), homogeneous formation of the micro gas bubble algal biomass is ensured, while the mixer (12) is operated at 100-500 rpm to ensure homogeneous distribution of polymerising chemicals in the solution. In one embodiment of the invention, a calcium chloride solution (0.5-20% m / v) is used, and similar chemical solutions such as calcium lactate may also be used. As a result, the micro gas bubble algal biomass polymerised in the crosslinking solution (7) is obtained as immobilised micro gas bubble microalgae beads (6). These micro gas bubble algal beads, due to the micro gas pockets they contain, have densities lower than that of water or wastewater and thus remain in a floating form. This feature enables the algal biomass to receive the light required for photosynthesis from the surface. The production method of the micro gas bubble immobilised microalgae culture can be used for nitrogen and phosphorus removal in conventional water / wastewater treatment plants. In Figure 3, an example of this application is shown in a secondary settling tank used after an activated sludge process in a typical wastewater treatment plant. The produced immobilised micro gas bubble microalgae beads (6) are conveyed to the suspended algal biomass tank (14) via an immobilised micro gas bubble microalgae beads transfer line (15) and combine with wastewater. At the centre of the tank (14), the microalgae beads move outward with radial flow (16). The hydraulic retention time in the tank ensures that an appropriate amount of immobilised micro gas bubble microalgae beads (6) accumulate on the surface in accordance with the removal rates of the algae. In continuously operating plants, newly added algal beads initiate nitrogen and phosphorus removal. The algal population increases at the outer section of the settling tank and significantly removes nitrogen and phosphorus from the water / wastewater. Aged algal beads are harvested by means of a collection chamber (18) placed in separators (17) located at the outer part of the tank. Thus, the algal biomass is removed from the water / wastewater. The harvested immobilised micro gas bubble microalgae beads (6), due to their high organic content and nitrogen-phosphorus richness, can be used as raw material in various sectors. This treatment technique can be easily adapted to many different treatment units. For example, it can be used on the upper surface of an activated sludge unit and enables carbon dioxide (CO2) generated in biological treatment together with activated sludge to be retained on the surface. In addition, byusing this system in membrane bioreactors, prevention of membrane fouling can be achieved.REFERENCES

[0066] [1] Shelknanloymilan L, Atici T, Obal O. “Removal of nitrogen and phosphate by using Choleralla vulgaris on synthetic and organic materials waste water”. Biological Diversity and Conservation, 5 / 2 (2012) 89-94.

Claims

CLAIMS1. A method for producing a microalgae culture for use in the simultaneous removal of nitrogen and phosphorus from water or wastewater, comprising the process steps of;i. preparing a gel-form polymer-microalgae mixture by mixing a polymeric gelform solution and suspended microalgae,ii. mixing the polymer-microalgae mixture with pressurised gas or a gas mixture and subsequently converting it into droplet form to obtain a gel-form microalgae culture droplet with micro gas bubbles,iii. dripping the microalgae culture droplets obtained in the previous step into a cross-linking solution, andiv. obtaining an immobilised microalgae bead with micro gas bubbles by mixing the microalgae culture droplet with the cross-linking solution and polymerising it.

2. A method for producing a microalgae culture according to claim 1, comprising the process steps of;i. preparing a gel-form polymer-microalgae mixture by mixing a polymeric gelform solution and suspended microalgae,ii. transferring the gel-form polymer-microalgae mixture from the suspended algal biomass transfer line (1) to the mixing region (3),iii. converting the gel-form polymer-microalgae mixture, which is mixed in the mixing region (3) with gas ora gas mixture coming from the pressurised gas- dissolved gel transfer line (2), into droplet form in the outlet pipe (4) to obtain a gel-form micro gas bubble gel microalgae droplet (5), andiv. obtaining immobilised microalgae beads with micro gas bubbles (6) by mixing the micro gas bubble gel microalgae droplet (5) with a cross-linking solution (7) and polymerising it.

3. A method for producing a microalgae culture according to claim 1 or 2, wherein said polymeric gel-form solution comprises mixtures containing at least one of a polymer derived from monomeric structures containing sodium alginate, agar, agarose, sodium alginate, calcium alginate, carrageenan, acrylamide, acrylonitrile, urethane,chitosan, polyvinyl alcohol, polyacrylamide, sodium carboxymethyl cellulose, or hydroxyethyl methacrylate (HEMA).

4. A method for producing a microalgae culture according to claim 1 or 2, wherein said cross-linking solution comprises mixtures containing at least one of calcium lactate, a cross-linking chemical solution containing +2 charged calcium cations, calcium chloride, magnesium chloride, glutaraldehyde, tannic acid, genipin, silica gel, activated carbon, polyurethane foam, or polyethylene glycol, in order to enable cross-linking of the polymers in the polymeric gel-form solution.

5. A method for producing a microalgae culture according to claim 1 or 2, wherein said polymeric gel-form solution comprises sodium alginate.

6. A method for producing a microalgae culture according to claim 1 or 2, wherein, for preparing the gel-form polymer-microalgae mixture, suspended microalgae are mixed with a sodium alginate solution at a concentration of 1-5% by volume, at a suspended microalgae:sodium alginate solution ratio of 1 :5- 5:1 by volume.

7. A method for producing a microalgae culture according to claim 1 or 2, wherein said microalgae are Chlorella vulgaris.

8. A method for producing a microalgae culture according to claim 1 or 2, wherein said gas mixture contains 0.5-35% by volume carbon dioxide, 50-78% nitrogen, and 2- 21% oxygen.

9. A method for producing a microalgae culture according to claim 8, wherein said gas mixture contains 0.5-10% by volume carbon dioxide.

10. A method for producing a microalgae culture according to claim 1 or 2, wherein said cross-linking solution is a cross-linking chemical solution containing +2 charged calcium cations.

11. A method for producing a microalgae culture according to claim 1 or 2, wherein said cross-linking solution contains, as a cross-linking chemical agent, at least one ofcalcium lactate or a solution thereof, calcium chloride, magnesium chloride, glutaraldehyde, tannic acid, genipin, silica gel, activated carbon, polyurethane foam, or polyethylene glycol.

12. A method for producing a microalgae culture according to claim 1 or 2, wherein said cross-linking solution is a solution of calcium lactate at a concentration of 0.5-10% by weight per volume.

13. A method for producing a microalgae culture according to claim 1 or 2, wherein the gas / polymeric gel ratio in said immobilised gas-bubble-containing microalgae bead is in the range of 1 / 50-1 / 1 by volume.

14. A microalgae culture produced by a method according to claim 1 or 2.15.A microalgae culture according to claim 14, comprising at least one immobilised gas-bubble-containing microalgae bead containing microalgae in a polymeric gel form and micro gas bubbles.

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