AN APPARATUS FOR MASS CULTIVATION OF MICRO ALGAE AND A METHOD FOR CULTIVATING THE SAME

FIELD OF INVENTION

The present invention relates to an apparatus or a device for mass-cultivation of micro-organisms, especially microalgae. More particularly, the present invention relates to an apparatus and a method for cultivating microalgae in different cultivation or nutrition modes, namely, phototrophically, heterotrophically or mixotrophically, which integrate the system of microalgae cultivation with waste treatment by using organic compounds from the wastewater and/or carbon dioxide as carbon sources and for energy generation; whereby the products of microalgae cultivation can be used in the manufacture of human food or nutritional supplements, animal feed as well as for the generation of renewable bio-fuels.

BACKGROUND OF THE INVENTION

In terms of biomass, microalgae form the largest group of primary producers. They play an important role in the conversion of solar energy to biomass. They are also known as microphyte, phytoplankton or planktonic algae. Microalgae are unicellular species which exist individually, or in chains (filaments) or in groups (colonies). They represent an immense range of genetic diversity. The sizes of microalgae can range from a few micrometers (μm) to a few hundreds of micrometers.

Microalgae are naturally found in freshwater and marine systems. However, they are presently cultivated abundantly for a variety of purposes and also to produce a wide range of commercially interesting byproducts. Microalgae function as oxygen providers among wastewater treatment plants for the removal of biochemical oxygen

demand (BOD) and as the primary producers in integrated aquaculture systems. As primary producers in an aquaculture system, microalgae convert waste materials, such as farm manure introduced into the aquaculture pond ecosystem as fertilizers into useable foods for aquaculture crops, such as fishes and shrimps.

In addition, microalgae are suitable to be made into human food or nutritional supplements as well as animal feed due to its high nutritional values, including vitamins and polyunsaturated fatty acids. The process of microalgae cultivation and its subsequent removal from the water bodies can also be used to control eutrophication, which is the consequence of nutrient input by anthropogenic activities on water bodies.

Enormous effort has been put into researching and solving the worldwide issues of waste disposal, the lack of food production and renewable energy sources. Mass cultivation of microalgae appears to offer solutions for these issues as the cultivation of microalgae can be incorporated into wastewater treatment plants and facilitates the capture of carbon dioxide at large point emitters, such as power plants, cement plants, steel plants, ethanol plants, petrochemical plants and others. Besides, the cultured microalgae can serve as feed for aquacultural crops in situ, or harvested and processed into human or animal foods as well as into renewable bio-fuels. Large quantities of organic carbon can be derived from saccharification of cellulosic materials from agricultural biomass wastes, such as empty fruit bunches, bargasse, corn stover, wheat stalks, rice stalks and others, and used for cultivation of microalgae from which second generation bio-fuels can be produced without competition with food crops for land resources.

Apparatuses for mass cultivation of microalgae are known in the art. Presently, the most widely used apparatus is the high rate pond for photoautotrophic cultivation of algae. However, due to design features resulting in poor light energy utilisation and low culture density, productivity per unit area has stagnated. To achieve higher

productivities, photobioreactors and heterotrophic fermentors are promoted. Photobioreactors use carbon dioxide as the source of carbon, photoautotrophic light as the source of energy, and are designed to optimize light management and growth conditions, such as pH, dissolved oxygen and others and thus a high culture density and productivity per unit volume of culture medium can be achieved. A great amount of developmental work is presently undertaken on photobioreactors of many configurations. Heterotrophic fermentors use organic carbon compounds as the carbon as well as energy source thus allowing them to operate in darkness and achieve very high culture densities and growth rates. However, the process is aerobic and unless the incoming air is sterilized, the culture broth is susceptible to bacterial infection or invasion of weed algae.

There are a few patented technologies over the prior arts relating to devices or apparatuses for cultivating microalgae. However, there is a wide variation in the system designs and qualities as well as the methods for operating the devices or practising the cultivating process thereof.

Of interest in respect to an apparatus for culturing microalgae is U. S. Patent No. US6037170. This apparatus comprises a waterway to be filled with a suspension containing a culture solution and microalgae, also known as a culture broth, and adapted to be irradiated with sun beams while being held in contact with ambient air. It also has a sealed storage tank for storing suspension to avoid it from contacting with ambient air. This invention uses a phototrophic mode to culture the microalgae, and pressurized gas is fed into the storage tank.

Another invention relates to a culture apparatus for microalgae is disclosed in Japanese Patent No. JP 1067174. This apparatus also applies the phototrophic cultivation mode which is effective in promoting the photosynthesis of microalgae and enabling high-density proliferation of microalgae in a short time. However, a source of photoautotrophic light is required to be accommodated inside the culture

tank for preventing the undesirable influence of natural environment.

To obtain a mass production of microalgae, a method for culturing microalgae is also disclosed in Japanese Patent No. JP8173139. This method comprises a mass culture system which has a control means that can be used to adjust the activated state, thus a activation of proliferating function of microalgae is performed.

There are also patented technologies applying a mixotrophic mode for algae cultivation. Of interest in connection with a method for mixotrophic culture for producing a biomass is U. S. Patent No. US2003017558. The method is characterized by the production of biomass which is rich in omega six polyunsaturated fatty acid. However, the method disclosed is merely suitable for the cultivation of Spirulina.

Generally, most of the apparatuses and methods disclosed in the prior arts use phototrophic cultivation mode to culture the microalgae. There are also inventions that apply heterotrophic modes in which nutrients and a source of carbon are supplied to the culture of microalgae. However, none of the patented technologies relate to an invention for mixotrophic culture of microalgae which is capable of providing the highest productivity rate of microalgae. Moreover, in its various embodiments, the apparatus can permit selection of the predominant cultivation mode, which is phototrophic, heterotrophic or mixotrophic.

To achieve the highest productivity of the microalgae, the art of cultivating microalgae must simultaneously create desirable growing conditions, without causing adverse effects. For example, in high rate ponds, the use of shallow depths and low culture density to facilitate light capture, also leads to photoinhibition, oxygen poisoning, and overall low productivity per unit area occupied by the apparatus. To overcome the drawbacks of the prior arts, it is desirable for the present invention to develop an improved and innovative apparatus as well as a systematic operating method which permits cultivation of microalgae under selected nutrition modes in

order to facilitate the rapid mass cultivation of the microalgae, while retaining low costs of apparatus construction and operation.

SUMMARY OF INVENTION

The primary object of the present invention is to provide an improved apparatus for mass-cultivating microalgae which allows different types of cultivation modes depending on the available sources of carbon and desired composition of the cultured algae and to achieve a higher production over a period of time from the area occupied by the apparatus through high growth rates and high culture density of the microalgae culture.

Another object of the present invention is to provide a method for operating a system for cultivating microalgae phototrophically, heterotrophically or mixotrophically, in which these cultivation modes can be altered by regulating the operation of one or more inlets for the feed and one or more circulating means to circulate the culture, thus avoiding photoinhibition and improving respiratory gas exchange, and the- microalgae cultured derive its carbon from carbon dioxide as well as organic carbon compounds, and energy from photoautotrophic light and organic compounds.

Still another object of the present invention is to develop an apparatus which incorporates the system of microalgae cultivation with the microalgae harvesting process, whereby the separation of microalgae from its culture broth can be performed directly in the apparatus.

Still another object of the present invention is to develop a system for cultivating microalgae which is capable of promoting sewage treatment and thus conserving the environment by resulting in valuable byproducts of microalgae such as animal feed and bio-fuels, instead of sewage sludge which requires further processing steps to

render it suitable for use as fertilizer or landfill.

Yet another object of the present invention is to develop an apparatus for cultivating microalgae massively whereby the microalgae can be used in the manufacture of animal feed.

A further object of the present invention is to develop an apparatus for cultivating microalgae massively which aids the carbon dioxide capture from large point emitters such as power plants, cement plants, steel plants, ethanol plants, petrochemical plants and the like, whereby the microalgae can be used in the manufacture of bio-fuels and/or animal feed.

Still another object of the present invention is to develop an apparatus for cultivating microalgae massively using organic compounds derived from saccharification of cellulosic material from agricultural biomass wastes such as empty fruit bunches, sugar cane bargasse, corn stover, wheat stalks, rice stalks and others, whereby the microalgae can be used in the manufacture of bio-fuels and/or animal feed.

At least one of the preceding objects is met, in whole or in part, by the present invention, in which one of the embodiments of the present invention describes an apparatus for cultivating photosynthetic microorganisms comprising a culture tank (51) containing the microorganisms grown in a culture broth fed with carbon sources and nutrients via one or more inlets (60, 61, 62); and one or more circulating means (54, 55, 56) located in the tank (51) for projecting the broth vertically from the bottom of the tank (51) up to the surface of the tank (51) and circulating the broth downwards to allow momentary exposure of the microorganism to light; wherein the operation of the inlets (60, 61, 62) and the circulating means (54, 55, 56) is regulated according to a selected cultivation mode. Preferably, the microorganisms cultured are microalgae.

In a preferred embodiment of the present invention, the predominant cultivation mode

is phototrophic, heterotrophic or mixotrophic cultivation. Preferably, the circulating means (54, 55, 56) are airlift tubes with different lengths. The airlift tubes (56) can also be equipped with a deflecting means (65). This apparatus can further comprises a controlling means (64) connected to the inlets (60, 61, 62) for controlling feed rate and/or sources of the feed.

Another embodiment of the present invention is an apparatus for cultivating photosynthetic microorganisms, further comprising a filtering means (52) located at the culture tank (51) for separating the photosynthetic microorganism from the culture broth and outputting it via an outlet for microalgae (57); and a storing means (53) receiving filtered water from the filtering means (52) and outputting clean water via an outlet for water (63). Preferably, the filtering means (52) is sand, microscreens or a combination thereof.

Still another embodiment of the present invention is an apparatus for cultivating photosynthetic microorganisms further comprising a pH sensor (59) for detemiining acidity of the culture broth.

In another embodiment of the present invention, the apparatus for cultivating photosynthetic microorganisms further comprises a light sensor (58) for determining light intensity in the culture tank (51).

Preferably, the apparatus according to the previous embodiments further comprises a means for covering the culture tank, wherein the means for covering is a photoautotrophic light transmitting material.

A further embodiment of the present invention is a method for cultivating photosynthetic microorganisms comprising the steps of feeding carbon sources and nutrients according to a selected cultivation mode via one or more inlets (60, 61, 62) into a culture tank containing the microorganisms grown in a culture broth; projecting

the broth vertically from the bottom of the tank (51) up to the surface of the tank (51) and circulating the broth downwards by one or more circulating means (54, 55, 56) to allow momentary exposure of the microorganisms to light; wherein the operation of the circulating means (54, 55, 56) is regulated according to the selected cultivation mode.

In another further preferred embodiment of the present invention, the method for cultivating photosynthetic microorganisms further comprises a step of separating the microorganisms from the culture broth by a filtering means (52) and thus outputting them via an outlet for microalgae (57) and sending filtered water to a storing means (53).

Preferably, the microorganisms are microalgae, such as Chlorella spp., Arthrospira spp., Dunaliella spp, Haematococcus spp, or a combination of any two or more thereof.

Still another embodiment of the present invention is a method for cultivating photosynthetic microorganisms further comprising a step of determining acidity of the culture broth for feed rate regulation.

Yet another embodiment of the present invention is a method for cultivating photosynthetic microorganisms further comprising a step of determining light intensity in the culture tank (51) for cultivation mode alteration.

One skilled in the art will readily appreciate that the present invention is well adapted to carry out the objects and obtain the ends and advantages mentioned, as well as those inherent therein. The embodiments described herein are not intended as limitations on the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of facilitating an understanding of the invention, there is illustrated in the accompanying drawing the preferred embodiments from an inspection of which when considered in connection with the following description, the invention, its construction and operation and many of its advantages would be readily understood and appreciated.

Figure 1 is a cross-sectional side view of an apparatus for cultivating photosynthetic microorganisms, as described by one of the preferred embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to an apparatus or a device for mass-cultivation of micro-organisms, especially microalgae. More particularly, the present invention relates to an apparatus and a method for cultivating microalgae in different cultivation or nutrition modes, namely, phototrophically, heterotrophically or mixotrophically, which integrate the system of microalgae cultivation with waste treatment by using organic compounds from the wastewater and/or carbon dioxide as carbon sources and for energy generation; whereby the products of microalgae cultivation can be used in the manufacture of human food or nutritional supplements, animal feed as well as for the generation of renewable bio-fuels.

Hereinafter, the invention shall be described according to the preferred embodiments of the present invention and by referring to the accompanying description and drawings. However, it is to be understood that limiting the description to the preferred embodiments of the invention and to the drawings is merely to facilitate discussion of the present invention and it is envisioned that those skilled in the art may devise

various modifications without departing from the scope of the appended claim.

The present invention discloses an apparatus for cultivating photosynthetic microorganisms comprising a culture tank (51) containing the microorganisms grown in a culture broth fed with carbon sources and nutrients via one or more inlet (60, 6I ₅

62); and one or more circulating means (54, 55, 56) located in the tank (51) for projecting the broth vertically from the bottom of the tank (51) up to the surface of the tank (51) and circulating the broth downwards to allow momentary exposure of the microorganisms to light; wherein the operation of the inlet (60, 61, 62) and the circulating means (54, 55, 56) is regulated according to a selected cultivation mode.

According to the preferred embodiment of the present invention, the photosynthetic microorganisms are preferably microalgae. The present invention is characterized by the three types of cultivation modes which can be applied in the apparatus and the cultivating method disclosed, which is phototrophic cultivation, heterotrophic cultivation or mixotrophic cultivation. Phototrophic cultivation or photoautotrophic cultivation allows the microalgae culture to obtain energy from light (sun) and carries out photosynthesis using inorganic simple materials, such as carbon dioxide, to generate energy for their growth and development. On the contrary, heterotrophic cultivation supplies the microalgae culture with organic substrates as their carbon sources as well as for the generation of energy.

The most preferred or the ultimate cultivation mode embodied in the present invention is the mixotrophic cultivation, whereby it allows the microalgae to derive metabolic energy both from photosynthesis and from external energy sources, such as organic waste materials present in or supplied into the culture tank.

As illustrated in Figure 1, the apparatus of the present invention comprises a culture tank (51) as its larger component. The culture tank (51) may be of any dimension as determined by the available space. However, an upper limit of 20 meters wide by 50

meters long with a central dividing wall taking the form of the well known high rate pond is recommended for use in the cultivation of microalgae, wastewater treatment and capture of carbon dioxide as well as for ease of operation management. For small scale operations, a lower limit of 2 meters wide and 10 meters long might be appropriate for cultivation of high value microalgae. The depth should not exceed 2 meters for safety reasons. In its larger form, the tank is essentially a pond and its walls and sides can be lined with impermeable material, optionally. In its smaller form, the tank can be made of steel, glass-reinforced plastics, cement, brickwork and others. As described in the preferred embodiment, the culture tank (51) contains culture broth in which the microalgae culture is grown. This culture broth or culture medium is supplied with carbon sources and nutrients via one or more inlets (60, 61, 62). The inlets can be a series of pipes and manifolds.

The apparatus disclosed in the preferred embodiment of the present invention comprises one or more inlets (60, 61, 62) connected to a controlling means (64) which is capable of controlling the feed rate as well as the sources of feed supplied to the culture tank (51) by opening and closing the inlets. Therefore, the carbon dioxide, organic carbon compounds and other nutrients can be inputted either via a single inlet, wherein the feedstocks are channeled together into the tank (51); or by multiple inlets, wherein the different types of feedstocks are channeled separately into the tank (51).

Preferably, three sets of inlets are assembled in the apparatus as disclosed in the preferred embodiment of the present invention. The first set of inlets (60) is feed for carbon dioxide; the second set of inlets (61) is feed for organic carbon compounds, such as domestic sewage, animal manures, leachate from landfills, sludge from sewage treatment plants, wastewater from food processing and vegetable oil (olive oil, palm oil and the like) plants, blood from slaughterhouses, simple and complex sugars from the saccharification of plant biomass; and the third set of inlets (62) is feed for other nutrients and trace elements, such as phosphorus, nitrogen, silica, calcium, magnesium, sodium, potassium, iron, manganese, sulphur, zinc, copper and

cobalt in a balanced fashion to foster optimal growth. Where the incoming feed for organic compounds is such that it causes the growth of excessive bacterial matter in the tank (51), a portion or all of the feed may be passed through an anaerobic digester, prior to discharge into the tank (51). The carbon dioxide produced by anaerobic digestion is fed to the tank (51) for phototrophic cultivation of the algae. Therefore, the culture medium generally comprises water, organic materials, dissolved oxygen, dissolved carbon dioxide, nutrients, trace elements and others.

Accordingly, the carbon sources are carbon dioxide for photoautotrophic growth and organic carbon compounds for heterotrophic growth, and a combination of the two for mixotrophic growth. Other nutrients such as nitrogen, phosphorus, trace elements like silica, calcium, magnesium, sodium, potassium, iron, manganese, sulphur, zinc, copper and cobalt are added so that the limiting factors for growth shall only be the carbon supply and photoautotrophic light.

As described by the preferred embodiment of title present invention, the apparatus for cultivating microalgae is equipped with one or more circulating means (54, 55, 56). Preferably, these circulating means (54, 55, 56) are airlift tubes of different lengths employed for different assemblies and purposes. In the present invention, three different types of airlift tubes are used. All airlift tubes are assembled vertically to ease the circulation process.

As illustrated in Figure 1, the first set of airlift tubes or airlift tubes type 1 (54) preferably begins slightly above the surface of the filtering means or bottom of the tank where the filtering means is not present at where the airlift tube is located, and terminates slightly lower than the level of the culture broth. It is a vertical circulation tube which projects the culture broth together with the microalgae so as to create a momentary exposure of the microalgae to high intensity sunlight, thus avoiding photoinhibition. This circulation of the airlift tubes type 1 (54) can facilitate removal of any excess oxygen during daylight hours when photosynthesis takes place and

removal of excess carbon dioxide during the night time. When the surface is exposed to photoautotrophic light, the microalgae will travel down to the bottom through the photic and dark zones due to the vertical mixing, experiencing a fluctuating light regime, or light and dark cycles.

The second set of airlift tubes or airlift tubes type 2 (55) begins slightly below the filtering means and terminates slightly lower than the level of the culture broth. The circulation of airlift tubes type 2 (55) is vital for transporting the culture medium, which microalgae has been separated therefrom, to replenish the culture tank (51). The airlift tubes type 2 (55) circulates the filtered water medium and assists to ensure that no anoxic pockets are formed throughout the culture tank (51).

The third set of airlift tubes or airlift tubes type 3 (56) begins slightly above the surface of the filtering means and terminates below the surface of culture broth, close to the commencement of the dark zone. These airlift tubes type 3 are preferably fitted with a deflecting means (65) or deflector plates to reduce or eliminate the vertical velocity component and induce a horizontal velocity component to the upcoming culture broth so that the culture broth circulates within the dark zone. The reason for employing the airlift tubes type 3 (56) is to promote the heterotrophic cultivation of the microalgae in the dark. For instance, this cultivation mode is capable of decreasing the proportion of protein content of the microalgae Chlorella protothecoides and increasing its fat content up to 55% or up to four times more than the fat content when cultivated by the phototrophic mode.

In the apparatus disclosed in the present invention, only the upper surface of the culture broth is exposed to sunlight. The deeper parts of the culture broth get progressively darker and devoid of sufficient light energy for photosynthesis due to absorption by the microalgae, and mutual shading. The photic zone or light fraction is the zone where sufficient light is available to support photosynthetic activity. This light will be absorbed by the algal culture for photosynthesis. Depending on biomass

concentration and absorption by algae present, there will be insufficient light to support photosynthesis beyond a certain depth. These deeper parts of the pond can be considered the 'dark zone', or dark fraction. Therefore, different types of circulating means are required to optimize the cultivation mode, production rate and culture density of microalgae.

In accordance with the preferred embodiment of the present invention, the operation of the inlets (60, 61, 62) and circulating means (54, 55, 56) can be regulated to achieve the growth condition for different types of cultivation modes. During phototrophic or photoautotrophic growth of microalgae, both sets of airlift tubes type 1 (54) and airlift tubes type 2 (55) are operated and the culture broth is fed with carbon dioxide from the first set of inlets (60), with other nutrients and trace elements from the third set of inlets (62). Organic materials are not needed in this cultivation mode as the microalgae can derive energy from sunlight to carry out photosynthesis process. The nutrients and trace elements are added to remove growth limiting factors of the microalgae so as to achieve an optimum growth rate and culture density of the microalgae.

During the heterotrophic cultivation mode where photoautotrophic light is not needed, the airlift tubes type 1 (54) is not operated, while the second and third sets of airlift tubes (55, 56) are operated. In this manner, the bulk of the microalgae remain in the dark zone. The culture broth is fed with organic material, other nutrients and trace elements, but the carbon dioxide is withheld, therefore, only the second set and third set of inlets (61, 62) are opened during this cultivation mode.

For mixotrophic growth, both airlift tubes type 1 and type 2 (54, 55) are operated, and all three sets of inlets (60, 61, 62) are opened to supply the culture broth with organic materials, carbon dioxide, other nutrients and trace elements.

Another embodiment of the present invention is an apparatus for cultivating

microorganisms, preferably microalgae, further comprising a filtering means (52) located at the culture tank (51) for separating the microalgae from the culture broth and outputting it via an outlet for microalgae (57); and a storing means (53) receiving filtered water from the filtered means (52) and outputting clean water via an outlet for water (63).

In accordance with another preferred embodiment of the present invention, the filtering means (52) is a sand bed or a microscreen. A semi-permeable membrane can also be included for further filtering purposes. It is capable of separating the microalgae from the culture broth, and the microalgae is removed from the tank via the outlet for microalgae (57). Preferably, the outlet for microalgae (57) is equipped with a pumping means externally or internally to assist the removal of microalgae.

After removal of microalgae, the resulting filtered broth or filtered water will be stored in a storing means (53). According to the preferred embodiment of the present invention, the storing means can be assembled at the bottom of the filtering means

(52). The storing means (53) can also be assembled externally. It can be another tank made of steel, glass reinforced plastics, cement, brickwork, etc and comprises an outlet for water (63). The outlet for water (63) is a means for discharging filtered water or filtered broth from the storing means (53). The dissolved organic materials in the incoming feed waters introduced through inlet pipes (60, 61, 62) is substantially incorporated into the microalgae through its growth process and removed together with the microalgae and other matters from this filtered water or filtered broth. The filtered water can be sent for any suitable applications directly or to be further processed and sterilized for more hygienic purposes.

Still another embodiment of the present invention is an apparatus for cultivating microorganisms, preferably microalgae, further comprising a pH sensor (59) for determining acidity of the culture broth. One or more pH sensor can be assembled according to the dimension of the culture tank (51). The reading of acidity or

alkalinity of the culture broth helps to regulate the feed rate of carbon dioxide and/or organic carbon compounds. The pH of the entire culture broth can be modified by varying the supply of these carbon compounds, increasing the supply to decrease pH and vice versa.

In another embodiment of the present invention, the apparatus for cultivating microorganisms, preferably microalgae, further comprises a light sensor (58) for determining light intensity in the culture tank (51). Preferably, one or more light sensor (58) is assembled at predetermined depths of the culture medium to obtain readings of the light intensity. The readings are used to determine the time to initiate the removal of the cultivated microalgae, as well as to determine the time to switch the cultivation mode to heterotrophic condition.

Preferably, the apparatus according to the previous embodiments further comprises a means for covering the culture tank, wherein the means for covering is a photoautotrophic light transmitting material, for example polyethylene film. The means for covering is capable of preventing contamination by weed algae and reducing water and heat losses.

A further embodiment of the present invention is a method for cultivating photosynthetic microorganisms comprising the steps of feeding carbon sources and nutrients, according to a selected cultivation mode via one or more inlets (60, 61, 62) into a culture tank containing the microorganisms grown in a culture broth; projecting the broth vertically from the bottom of the tank (51) up to the surface of the tank (51) and circulating the broth downwards by one or more circulating means (54, 55, 56) to allow momentary exposure of the microorganisms to light; wherein the operation of the circulating means (54, 55, 56) is regulated according to the selected cultivation mode.

As set forth in the preceding description, selection of the most preferred growth mode,

which is the mixotrophic cultivation mode, is performed by inputting organic carbon compounds, such as domestic sewage, animal manures, leachate from landfills, sludge from sewage treatment plants, waste water from food processing and vegetable oil

(olive oil, palm oil) plants, blood from slaughterhouses, simple and complex sugars from the saccharification of plant biomass, in addition to the carbon dioxide as feedstocks for the microalgae. Where the incoming feed for organic compounds is such that it causes the growth of excessive bacterial matter in the tank (51), a portion or all of the feed may be passed through an anaerobic digester, prior to discharge into the tank (51). The carbon dioxide produced by anaerobic digestion is fed to the tank (51) for phototrophic cultivation of the algae. According to the preferred embodiment of the present invention, vertical circulation is preferably achieved by utilizing airlift driven draught tubes to optimize light and gases utilization by product disposal.

In another further preferred embodiment of the present invention, the method for cultivating photosynthetic microorganisms, preferably microalgae, in an apparatus configured with a plurality of cultivation mode, further comprises a step of separating the microalgae from the culture broth by a filtering means (52) and thus outputting the microalgae via an outlet for microalgae (57) and sending filtered water to a storing means (53).

The microalgae cultured can be of any species, depending on the purpose and intent, including fast growing mixotrophic species, such as the Chlorella spp. for treatment of waste waters, capture of carbon dioxide and production of renewable fuels, high value species such as the Arthrospira spp. (Spirulina) for health food where the high culture density keeps invasion of weed algae to acceptable levels. However, the present invention does not limit the use of the apparatus and method embodied herein in the cultivation of other types of algae.

Still another embodiment of the present invention is a method for cultivating microalgae in an apparatus further comprising a step of determining acidity of the

culture broth for feed rate regulation.

Yet another embodiment of the present invention is a method for cultivating microalgae in an apparatus further comprising a step of determining light intensity in the culture tank (51) for cultivation mode alteration.

As described in the foregoing description, the cultivation mode can be altered by the operation of the inlets (60, 61, 62) and the circulating means (54, 55, 56). The cultivation mode selection is based on the light intensity obtained by the light sensor (58), or selected as the preferred mode due to the desirability of the algal contents when cultivated under the the preferred mode. The reading of the pH sensor (59) serves as a reference for the feedstock conditions and plays a role in the determination of the opening or closing of the feed inlets (60, 61, 62).

When the cultivated algae is harvested, all the inlets (60, 61, 62) are closed and the filtered water is pumped out, leaving the algae isolated above the filtering means (52). The microalgae is removed by the outlet for microalgae (57), only leaving behind sufficient microalgae as seed for the subsequent cultivation process.

The operating modes disclosed by the present invention are innovative in various aspects. Above all, the circulating means (54, 55, 56) operated in isolation of each other or in any combination avoids photoinhibition. In addition, the operating modes take advantage of the high light efficiencies of the microalgae in low light intensity environments as most of the time spent in the photic zone will be spent in a low light intensity regime. Besides, there are also the avoidance of oxygen poisoning and improved gas exchange at all times as circulation brings the entire contents of the culture tank (51) to the surface on a regular basis and enables the removal of excess dissolved oxygen during day time and removal of excess carbon dioxide during night time. Further, the circulating means (54, 55, 56) also help in the self-flocculation of the culture making the microalgae easier to be harvested.

The apparatus and the method invented also enables a deeper culture zone, which is approximately up to 2m from the surface level of the culture broth; and a higher areal productivity, which can be approximately 1,000 gms M2-1 d-1. As the culture density is higher, it can be used to control invasion by weed algae and maintain a monoculture. The incorporation of a pH sensor can also be used to regulate the extent of vertical circulation so that air exchange is adequate avoiding CO2 build up and sufficient supply of oxygen. By using the heterotrophic growth mode, the residues of microalgae after extraction of the valuable products can be used as feedstock for the growing system. In this manner, almost all the carbon compounds can be extracted and turned into fuel, which is a significant contributor to overall energy efficiency. Since the culture cycle is only approximately 48 hours, it is practicable to convert the entire feedstock into usable products.

The apparatus disclosed in the present invention can be employed to improve productivity and environmental sustainability in many aspects, including aquaculture, wastewater treatment, capture of carbon dioxide, production of human and animal food, control of eutrophication as well as production of renewable bio-fuels. With the apparatus invented, an avenue is opened to produce low cost high protein feeds for animal husbandry. Should the combination of transparent cover and high culture densities be sufficient to maintain culture purity, thus food and nutritional supplements for human consumption can also be cultivated with a lower production cost.

The apparatus may be used as replacement for conventional activated sludge and pond systems presently deployed for waste water treatment. Organic waste bearing waters from various sources, for example, sewage, sewage sludge, septage, animal manure, POME, blood from slaughterhouses, and others are transferred to the apparatus, where the microalgae is grown, using up the dissolved organic materials, phosphates and nitrates. In this manner, tertiary standards of treatment and beyond can be achieved.

According to the preferred embodiment of the present invention, the algae thus grown should be used for renewable fuels due to the likely presence of pathogens. The oil fraction would be processed into fuels for compression ignition engines, the carbohydrate fraction fermented into ethanol or butanol, and the mash anaerpbically digested into methane. The residues, containing phosphorus and residual nitrogen compounds, can be landfilled as fertilizer.

Being freely scalable, the devise can be deployed at community level septic tanks.

At municipal solid waste landfill sites, food material may be separated from the incoming waste by washing to substantially remove organic materials prior to land filling, thus substantially avoiding the formation of leachate. The wash water is processed by the apparatus and recovered for re-use and the microalgae removed for processing for bio-fuels. Where leachate already exists, the apparatus can be used to treat the leachate prior to discharge into waterways.

As described in the foregoing description, the apparatus can be used to treat wastewater to tertiary standards prior to discharge into the waterways or subjected to further processing steps. This serves to stem inflow of new nutrients, and help with slowing down the environmental degradation. The nutrients accumulated in the waterways and found in the bottom layers of the apparatus can also be processed therein, where the mud is converted into the microalgae returning relatively clean and oxygenated water into the environment. The algae so produced should be processed into fuel in view of the likelihood of pathogens inhabiting the mud. In this way, the revenues generated can be used to fund the control of eutrophication and minimize its impact in the worst affected areas.

The present disclosure includes as contained in the appended claims, as well as that of the foregoing description. Although this invention has been described in its preferred

form with a degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example and that numerous changes in the details of construction and the combination and arrangements of parts may be resorted to without departing from the scope of the invention.

EXAMPLE

The operation of inlets (60, 61, 62) and circulating means (54, 55, 56) according to different types of cultivation modes selected is shown in Table 1.

Table 1