WO2013186626A1

WO2013186626A1 - Raceway pond system for increased biomass productivity

Info

Publication number: WO2013186626A1
Application number: PCT/IB2013/001224
Authority: WO
Inventors: Arvind Mallinath Lali; Reena Ajit PANDIT; Gunjan PRAKASH; Channamallikarjun Sidramayya MATHPATI; Swanand Prakash GANGAL; Chaitali Paresh VIRA; Juilee Ajit PALKAR; Smita Dattatraya PATIL; Sujata Prakash GAIKWAD
Original assignee: Individual
Current assignee: Individual
Priority date: 2012-06-12
Filing date: 2013-06-12
Publication date: 2013-12-19
Anticipated expiration: 2014-12-12

Description

RACEWAY POND SYSTEM FOR INCREASED BIOMASS PRODUCTIVITY TECHNICAL FIELD OF INVENTION

The present subject matter relates to a raceway pond system, and its use for obtaining high productivity of algal biomass per unit light exposed area. The subject matter further relates to the use of said system for obtaining higher production of algal biomass per unit land area; reduced cost of production of algal biomass and associated products; and lower loss of water through evaporation, as compared to the corresponding performance of the conventional raceway pond systems through lower energy consumption per unit volume, and hence per unit biomass and associated products produced.

BACKGROUND OF THE INVENTION

Microalgae are photosynthetic microorganisms that grow in aqueous suspensions and utilize sunlight & carbon dioxide (C0₂) for their growth. Besides being a potentially important source of biomass and associated products like lipids or fat for production of green biofuels, they are also a rich source of varied bio-molecules such as neutraceuticals and phytonutrients, for instance single cell proteins and beta carotenes, PUFAs like DHA and EPA, minerals, vitamins and other products. Presently, extensive work is being carried out all over the world to determine the effectiveness of utilization of these photosynthetic algae as a green energy source for production of biodiesel, ethanol and hydrogen gas production. Large scale cultivation of algae is also considered for waste water treatment as algal species can use up the contaminants from domestic and industrial waste water while providing biomass for fuel or other products. Algae are also considered a possible technology for carbon dioxide sequestration from industries like thermal power plants. Cultivation of a variety of photosynthetic algae, including cyanobacteria and diatoms, is therefore a vigorously active field of research and development, and efforts are being made for improving production of photosynthetic algal biomass per unit photobioreactor volume at a lower cost of nutrient and energy inputs. Various methods and technologies have been developed for growing photosynthetic organisms/algal biomass. These include reactors called photobioreactors, nutrient optimization and recycle, and controlling cell metabolism using modern tools of biology. Great deal of attention needs to be paid to bioreactor design as it contributes to both capital and operating expenditure for growth of algae. Popular choices have included relatively inexpensive and thus by far the most popular reactors like open air systems such as ponds, lakes, raceway pond systems or the more expensive systems such as closed photo-bioreactors of one or the other designs. While the process parameters for growth of algal biomass can be controlled more efficiently in closed photobioreactors, the cost of building and operating such systems is too high for production of algal biomass or products especially for biofuels and bioenergy. On the other hand, the open air systems i.e. widely used raceway ponds, while suffer from lesser control over growth conditions they are far more cost effective for large scale production. Among the various open air reactor systems used for growing algae, raceway ponds are the most widely employed. Reportedly, raceway pond systems for photosynthetic organisms and algae cultivation have been in use since the 1950s (Mata, T.M., Martins, A.A., Caetano, N.S., 2010; Microalgae for biodiesel production and other applications: a review. Renew. Sustain. Energy Rev. 14, 217-232). Considerable experience exists on the operation and engineering of raceway pond systems and related facilities. Currently, 440,000m (44 hectares, or 108.7acres) of raceway pond systems are in use globally (Spolaore, P., C. Joannis-Cassan, E. Duran, and A. lsamberi. 2006; Commercial application of microalgae. Journal of Bioscience and Bioengineering 101: 87-96).

The raceway pond systems, though designed and deployed in various sizes, have all come to employ a common design that consists of a one or more paddle wheels employed to achieve circulation of the liquid content with suspended algal biomass around the pond that is filled with water containing nutrients filled to a depth of 30 to 50 cm (Figure 1 a, 1 b). The use of a depth of 30 to 50 cm and use of the paddle wheel impellers (x) as shown in Figure la and lb results from the need to have large enough surface area for incident sunlight per unit volume of the reactor with as low power consumption with as few number of impellers as possible. This design creates two major limitations in deployment of algal raceway ponds for large scale growth of algae : a) the area required for large scale growth of biomass becomes very large (for example, the area required to grow algae enough to sequester carbon dioxide from a thermal power plant of 100MW, or for providing enough algae for 100 ton liquid fuel/day, runs into several hundreds of hectares); and (b) the gas to liquid mass transfer of carbon dioxide is poor in shallow ponds due to lower gas-liquid surface area and lower time available for gas absorption. Any attempt to increase algal productivity per unit land area will greatly reduce land area requirement, a feature especially important for locations and countries where land is not available or is better used for growing food products. Secondly, better capture of carbon dioxide will also result in higher productivity of algal biomass and associated products per unit reactor volume. In a conventional system pond depths are typically restricted to 30-50 cm in order to maintain sufficient light exposure of algae. Any further increase in raceway pond system depth is discouraged due to poor light penetration with depth leading to low productivities as well as drastic increase in energy consumption by the traditional paddle wheel impellers. Attempts have been reported that have tried to achieve better designs of photobioreactors for growing algae on large scale.

A recent publication reported a paddlewheel-free, airlift-driven raceway configuration for microalgal cultivation (Ketheesan B, Nirmalakhandan N, 2012; Feasibility of microalgal cultivation in a pilot-scale airlift-driven raceway reactor, Biores Technol, 108, 196-202). This design provides two diagonally opposite airlift sections of 72 cm height added to the conventional raceway pond design. These airlift sections are partitioned into a downcomer and riser columns with sparger fitted at the base of the riser and a hydraulic head on its top to drive the flow of raceway pond system. This design was proven to be energy efficient and gave better C02 utilization. Another innovation aimed at better production of algal biomass has been reported in an Algae Raceway Integrated Design (ARID), which is a patented as cost and energy effective algae cultivation system (US 2011/0023360). This design enables high production of algal biomass by controlling pond water temperature variations and optimizing sunlight exposure, thus allowing algal cultivation in extreme environmental conditions. The design deploys circulating the culture through multiple basins thus maintaining the temperature effectively.

Another improvised raceway pond system is the Integrated Algal Ponding System (I APS). This system has been mainly used for treatment of wastewater by combining the principles of aerobic and anaerobic biological processes. This system comprises different chambers devoted for anaerobic digestion, photosynthetic oxidation and algae settling. This has a simultaneous benefit of wastewater treatment and algal biomass production {Render D, Cowan K, 2012; Integrated Algal Ponding Systems, Technical report, Institute of Environmental Biotechnology, Rhodes University). A computational detailed study has reported that improved algal growths can be achieved through optimization of raceway pond system parameters such as temperature and incident radiation through covering raceway pond systems with greenhouses; nutrient availability; depth flow characteristics, channel dimensions and geometry {James and Boriah, 2010; Modeling Algae Growth in an Open-Channel Raceway, J Computational Biology 17: 895-901)

Raceway pond parameters such as depth of the pond, and biomass concentration play important role in attaining desired biomass productivity of a raceway pond. Conventionally in a typical raceway pond design, both algal biomass concentration and pond depth have to be maintained low in order to ensure effective light penetration and sufficient exposure of algal biomass to light. Traditionally, large scale raceway ponds have been operated at depths of 30-50 cm {Brennan, L, Owende, P., 2010; Biofuels from microalgae - A review of technologies for production, processing, and extractions of biofuels and co-products. Renew. Sustain. Energy Rev. 14, 217-232) while maintaining low algal biomass concentration of 0.6 g Γ¹ (Tredici, M.R., 2004; Mass production of microalgae: photobioreactors. In: Richmond, A. (Ed.), Handbook of Microalgal Culture Biotechnology and Applied Phycology. Blackwell Publishing, Iowa, pp. 179-214). It was reported that any further increase in depth of raceway pond system with paddlewheel hampered light penetration and algal exposure to light, therefore resulted in low biomass productivities.

As stated above, large scale algal cultivation especially for low cost applications like production of biofuels and carbon dioxide sequestration is today hampered by the total area required for laying out the algal raceway pond systems. This large area requirement can be decreased by increasing the depth of the fluid channels. Increased depth can also improve gas-to-liquid mass transfer for carbon dioxide bubbled into the system. However, use of traditional paddle impellers at higher depths results in much higher power consumption per kilogram of algal biomass produced. For example, doubling the depth of the pond from 30cm to 60cm can result in more than 6 fold rise in power consumption per kilogram biomass produced.

The limitations with current raceway pond system designs, that is, high energy consumption by the mixing devices such as paddlewheels, and prohibitive use of several hundreds of hectares of raceway ponds at any single location necessitates improvements in raceway pond designs and alternative improved mixing mechanisms. Methods and devices designed to achieve better and more energy efficient mixing can provide viable options for increasing the light and carbon dioxide absorption efficiency of the biomass since these two are the major nutrients for photosynthetically growing algal biomass. This can be achieved through a combination of effects of type and speed of the mixing device and flow patterns in the photobioreactor to result in optimally required dark and light cycles for the growing algae. The flow patterns and depth of a raceway pond system can affect the operating dark and light cycle through surface exposure, distribution of nutrients, and gas to liquid mass transfer of C0₂ and therefore it becomes mandatory to design a raceway pond system. SUMMARY OF THE INVENTION

This summary is provided to introduce concepts related to raceway pond systems and the concepts are further described below in the detailed description. This summary is neither intended to identify essential features of the claimed subject matter nor is it intended for use in determining or limiting the scope of the claimed subject matter.

In one embodiment, the present subject matter provides a raceway pond system for cultivation of photosynthetic cells or organisms having a depth greater than 50 cm. The raceway pond system according to the present subject matter comprises of one or more substantially horizontal axial flow impellers disposed at one or more lateral ends of the raceway pond system, a drive means coupled with the one or more axial flow impellers, and a plurality of air spargers located throughout the tank length.

BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS

Figure la shows a top view of a conventional raceway pond system for growing photosynthetic organisms using a paddle wheel. Figure lb shows a side view of the conventional raceway pond system with working depth of 30 cm.

Figure 2a shows a top view of a raceway pond system for growth of photosynthetic cells or organisms, according to an embodiment of the present subject matter.

Figure 2b shows a side view of the raceway pond system according to the embodiment shown in figure 2a.

Figure 3a shows a top view of a raceway pond system for growth of photosynthetic cells or organisms, according to a further embodiment of the present subject matter.

Figure 3b shows a side view of the raceway pond system according to the embodiment shown in figure 3 a. Figure 4a shows a top view of a raceway pond system for growth of photosynthetic cells or organisms, according to another embodiment of the present subject matter.

Figure 4b shows a side view of the raceway pond system according to the embodiment shown, in figure 4a. DETAILED DESCRIPTION OF THE INVENTION

The present subject matter relates to a raceway pond system having a working depth ranging from about 60 to 150 cm without compromising effective light availability for the algal biomass, and achieving better gas-to-liquid mass transfer as compared to the conventional 30 to 50 cm depth raceway ponds. This has been achieved in the present subject matter by a novel method of mixing algal culture (algae biomass and nutrient fluid mixture), which results in continuous mixing of the biomass and effective light utilization.

In one embodiment, the subject matter relates to the raceway pond system comprising one or more substantially horizontal axial flow impellers disposed at one or more lateral ends of the raceway pond system, a drive means coupled with the one or more axial flow impellers, and a plurality of air spargers disposed along a major portion of a tank length.

In one embodiment, the raceway pond system according to the present subject matter has substantially rounded edges. In another embodiment, the raceway pond system according to the present subject matter has rectangular edges.

In another embodiment, the raceway pond system according to the present subject matter has a depth greater than 50cm In another embodiment, the raceway pond system according to the present subject matter has a depth ranging from 50-90 cm. In another embodiment, the raceway pond system according to the present subject matter has a depth ranging from 50-100 cm.

In yet another embodiment, the raceway pond system according to the present subject matter has a depth ranging from 60-90 cm. In yet another embodiment, the raceway pond system according to the present subject matter has a depth of 90cm. another embodiment, the raceway pond system according to the present subject matter has a depth of 100cm.

In yet another embodiment, the raceway pond system according to the present subject matter has a depth ranging from 60- 120cm. In still another embodiment, the raceway pond system according to the present subject matter has a depth ranging from 60- 150cm.

In yet another embodiment, the raceway pond system according to the present subject matter has a depth of 120cm. In yet another embodiment, the raceway pond system according to the present subject matter has a depth of 150 cm.

In one embodiment, the working depth of the raceway pond system is formed in a range of about 50 to 120 cm. This range of working depth allows for an increased volume of the raceway pond system per unit land area used, and thereby also facilitates an increased volume of algae that can be cultivated therein. Furthermore, in one embodiment, the axial flow impellers agitate the solution within the raceway pond system, which results in effective circulation and mixing of the biomass suspension with effective light availability resulting in increased biomass productivity per unit area

The term "raceway pond system" refers to an open aquaculture system or shallow artificial pond used for the cultivation of algae. The term "impellers" refers to a rotor inside a tube or conduit used to increase the pressure and flow of a fluid. The term "working depth" refers to a depth of the raceway pond system effectively available for the growth of the photosynthetic cells or organisms to be cultivated therein.

Yet another embodiment of the present subject matter provides for effective light utilization and increased gas-to-liquid mass transfer by continuous mixing of the biomass by the horizontal axial impellers resulting in increased photosynthetic activity of the algal biomass.

In yet another embodiment of the present subject matter, the combination of higher depth and impeller design provides for better carbon dioxide absorption by the photosynthetic organisms. These two aspects result in decreased energy consumption per unit volume and per kilogram algal biomass produced, and also lower rates of water loss to evaporation per unit volume of the raceway pond system.

Algal volumetric productivities are directly linked to high surface area for light incidence and specific requirements of dark and light cycles in photobioreactors. Figure la shows a top view of a conventional raceway pond system 100 for growing photosynthetic organisms using a paddle wheel impeller 102.

As shown in figure la, an overall structure of the conventional raceway pond system 100 is provided with substantially rounded edges 104. The conventional raceway pond system 100 also includes a paddle impeller 102 that provides circulation and mixing of the biomass suspension. Also illustrated are air flow channels 106, and C0₂ channels 108, wherein air and carbon dioxide are respectively introduced into the raceway pond system . for the growth .of photosynthetic_organisms. Eigure^lb shows a side_view-of the- conventional raceway pond system 100 at a working depth 1 10 of about 30 cm.

The present subject matter particularly relates to a raceway pond system that provides growth of algal cells with a dynamic light environment; power efficient mixing and achieves higher surface productivities at ponds depths between 50 and 120cm. Figures 2a and 2b show a top view, and a side view respectively, of a raceway pond 200 for growth of photosynthetic cells or organisms, according to an embodiment of the present subject matter. The raceway pond system 200 for cultivation of photosynthetic cells or organisms has substantially rounded edges 202 and a working depth 204 ranging from 60 to 150 cm. Furthermore, the raceway pond system 200 also includes one or more substantially horizontal axial flow impellers 206 in the raceway pond system 200, which are disposed at one or more lateral ends of the raceway pond system 200 and a drive means 208 coupled with the one or more axial flow impellers 206. The drive means 208 can be any conventionally known drive motor, such as a servo motor, pneumatic motor, or an electric motor. Also illustrated are air flow channels 210, and C0₂ channels 212, wherein air and C0₂ are respectively introduced into the raceway pond system 200 for the growth of photosynthetic organisms.

In one embodiment, the one or more axial flow impellers 206 are side entering hydrofoil impellers that are employed in the raceway pond system 200 to generate circulatory mixing and flow patterns in the liquid suspension of the raceway pond system 200. The side entering hydrofoils deployed in the present subject matter consume lower than one-third of the power per unit volume as consumed by the paddlewheels that are used in conventional raceway pond systems. Thus, the present subject matter provides for a means of algal biomass production with substantially reduced energy consumption per unit volume. Further, hydrofoils support uniform shear stress distribution without having localized high shear zones. Figures 3a and 3b show a top view, and a side view respectively, of a raceway pond system 300 for growth of photosynthetic cells or organisms, according to another embodiment of the present subject matter. The components of the raceway pond system 300 shown in figures 3 a and 3 b are widely similar in type and function and therefore have not been labelled for purposes of clarity. However, the raceway pond system 300 has sharp edges 302. The sharp edges 302, in one example, can be substantially rectangular or perpendicular in nature. The raceway pond system according to the present subject matter uses a light dilution principle wherein every cell is exposed to intermittent but sufficient light and dark cycles ensuring optimal photosynthetic efficiencies. Short exposure to high intensity sunlight on surface, typically available in tropical and desert climates, provides bursts of energy for exciting photosynthetic pigments in the algal cells and subsequent dark phase in lower depths to allow efficient assimilation of incident energy for metabolic activities. These alternate light and dark exposures of the algal cells help improve overall biomass productivities per unit area. The raceway pond system according to the present subject matter a ids in manipulating surface renewal of algae through fluid mechanics based on flow patterns, heat and mass transfer in bulk as well as at gas-liquid and solid-liquid interfaces.

Figures 4a and 4b show a top view and a side view respectively, of a raceway pond system 400 for growth of photosynthetic cells or organisms, according to another embodiment of the present subject matter. The raceway pond system 400 illustrated herein includes features similar to the embodiments shown in figures 2a, 2b, 3a, and 3b, but differs in that the raceway pond system 400 has a flow disturbance means 402 disposed at a bottom surface 404 of the raceway pond system 400. In an example, the flow disturbance means 402 can be formed integrally with the bottom surface 404 of the raceway pond system 400, or formed separately therefrom and fixed to the bottom surface 404. In one example, the flow disturbance means 402 can be formed selected from a group comprising a knurl, ridge or a saw-tooth contour to facilitate liquid circulation.

In^{^}the^"present^{^}subject ^"m eO

single celled photosynthetic organism, cyanobacteria, microalgae, diatoms.

The raceway pond system according to the present subject matter supports all apparatus suitable for submerged aerobic processes whenever aeration or enriched C0₂ supply is used for cultivation of photosynthetic organisms. The raceway pond system described in the present subject matter is seen to hold considerable promise for large scale cultivation of algae with better photosynthetic efficiencies through better absorption of light and carbon dioxide; lower power consumption; and lower evaporative loss of water and higher productivities per unit land area.

EXAMPLES

The disclosure will now be illustrated with working examples, which is intended to illustrate the working of disclosure and not intended to take restrictively to imply any limitations on the scope of the present disclosure. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this disclosure belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice of the disclosed methods and compositions, the exemplary methods, devices and materials are described herein. Example 1

Improved raceway pond system for cultivating photosynthetic organisms

In this example reference is made to figure 2 which illustrates an improved raceway pond system for growing Spirullina platensis in nutrient rich Zarrouk's culture media under natural environmental conditions with average light intensity 1500

average temperature 30°C, and average pH 10.5.

The raceway pond system has two channels with the dimensions of 312cm X 62 cm (length X breadth). The raceway pond was operated at a working depth of 90 cm. The circulation of culture media in these channels was achieved by a marine propeller comprising of three blades of diameter 28cm. The said propeller was located horizontally at small breadth side of the pond, and rotated at a controlled speed of 125 rpm, such as to generate average linear liquid flow velocity of 25 cm/sec. The raceway bottom was provided C0₂ and air mix spargers at multiple locations running alongside the tank length with flow velocity of 10 mm/sec; wherein the C0₂ content in the air mixture was maintained at 2000 ppm.

The photosynthetic organism, S. platensis was inoculated into the culture medium in the above described raceway pond and cultivated for 10 days to achieve a cell concentration of 0.96 g/L. On 10^th day cell biomass was harvested and biomass productivity was calculated on a dry weight basis. The total biomass productivity under the described cultivation conditions was found to be 86.23g/ m /day. A total power of 4 Watt/g dry cell biomass was consumed in the improved raceway pond design to achieve the high productivity of 86.23g/ m /day. The same experiment was performed using an improved raceway system as illustrated in figure 3 at a working depth of 100 cm to obtain comparable results.

Example 2

Comparison of performance of improved raceway pond with conventional raceway pond This example has reference to figure 1 which illustrates conventional raceway pond system which has two channels with the dimensions of 312cm X 62 cm (length X breadth). The conventional raceway pond system was operated at a working depth of 30 cm. The mixing of culture media was achieved by paddle wheel with surface linear velocity of 25cm/s. The improved raceway pond system as illustrated in figure 2 and described in example 1 was operated with working depth of 90 cm for comparative performance studies with the conventional raceway pond. The circulation of culture media was achieved by a marine propeller rotating at controlled speed 125rpm so as to achieve surface linear velocity 25cm/s which was same as conventional raceway pond system. Both raceway pond systems were inoculated with Spirullina platensis in nutrient rich Zarrouk's culture media. Cultivation was continued under natural environmental conditions with average light intensity 1500 μΕ/ιη²/8 for 10 days. The same comparative study was performed using the improved raceway pond system as illustrated in figure 3 at working depth of 100cm to obtain comparable results. ^*

The performance of both raceway pond systems was monitored for biomass productivity per unit area, evaporative water loss per day, power consumption, and photosynthetic efficiencies. Comparative data for the two raceway pond systems is shown in Table 1.

Table 1 : Performance comparison of conventional raceway pond and improved raceway pond

Modified Conventional

Raceway pond raceway pond raceway pond

( 90cm depth) (30m depth)

Linear water circulating velocity

25.0 25.0

(cm/s)

Water loss /day (L/d) 15.0 40.0

Power consumption

4 6

(Watt/g dry biomass)

Chlorophyll : Phycocyanin ratio 56.06 . 95.94

Photosynthetic efficiency (%) 36.4 32.1

Biomass productivity (g/m²/day) 86.23 29.33

Example 3

Improved raceway pond for cultivating photosynthetic organisms with 60 cm working depth

The improved raceway pond system of example 1 was used for growing Spirullina platensis in nutrient rich Zarrouk's culture media under natural environmental conditions with average light intensity 1500 μΕ/ιη²/8. The raceway pond was operated with working depth of 60 cm. The circulation of liquid media was achieved by a marine propeller rotating at controlled speed 125rpm. The cultivation of photosynthetic organism was continued for 10 days under natural light conditions. On 10^th day cell biomass was harvested and biomass productivity was found to be 60.00 g/ m²/day.

Example 4 Improved raceway pond for cultivating photosynthetic organisms with 90 cm working depth

The improved raceway pond system as illustrated in figure 4 was used for growing Spirullina platens is in nutrient rich Zarrouk's culture media under natural environmental conditions with average light intensity 1500 μΕ/ηι²/8. The raceway pond was operated at a working depth of 90 cm. The circulation of liquid media was achieved by synergistic effect of bottom formations and a marine propeller rotating at controlled speed of 80 rpm. The cultivation of photosynthetic organism was continued for 10 days under natural light conditions. On 10^th day cell biomass was harvested and biomass productivity was found to be 84.23g/ m²/day. Power consumption was decreased to l Watt/g dry biomass. In this example reference is made to figures 4a and 4b, which illustrates an improved raceway pond system 400 described in example 1 with bottom formations to create liquid circulation patterns. The same experiment was performed at a working depth of 100 cm to obtain comparable results.

Example 5 Modified raceway pond with marine propeller rotating at controlled speed lOOrpm

The improved raceway pond system of example 1 was used for growing Spirullina platensis in nutrient rich Zarrouk's culture media under natural environmental conditions with average light intensity 1500 μΕ/ι ²/8 for 10 days. The raceway pond system was operated at a working depth of 90 cm. The circulation of liquid media was achieved by a marine propeller rotating at controlled speed lOOrpm. On 10^th day cell biomass is harvested and biomass productivity was found to be 76.23g/ m²/day. The same experiment was performed at a working depth of 100 cm to obtain comparable results.

Example 6

Modified raceway pond with marine propeller rotating for growing Chlorella regularis

The improved raceway pond system of example 1 was used for growing Chlorella regularis in nutrient rich BGl l culture media under natural environmental condition with average light intensity 1500 μΕ/m /s for 10 days. The raceway pond system was operated at a working depth of 90 cm. The circulation of liquid media was achieved by a marine propeller rotating at controlled speed 125rpm. On 15^th day cell biomass is harvested and biomass productivity was found to be 62g/ m²/day. The same experiment was performed at a working depth of 100 cm to obtain comparable results.

Claims

I/We claim

1. A raceway pond system (200, 300, 400) for cultivation of photosynthetic cells or organisms having a depth ranging from 60-150cm comprising one or more substantially horizontal axial flow impellers disposed at one or more lateral ends of the raceway pond system, a drive means coupled with the one or more axial flow impellers, air spargers located through the tank length.

2. The raceway pond system as claimed in claim 1 , wherein the rotational speed of said one or more nearly horizontal axial flow impellers is in the range of 50 to 200 rpm.

3. The raceway pond system as claimed in claim 1 , wherein the bottom of said raceway pond system comprises at least one flow disturbance means to facilitate liquid circulation.

4. The raceway pond system as claimed in claim 3, wherein said flow disturbance means is in the form selected from a group comprising of a knurl, ridge and a saw-tooth contour.

5. The raceway pond system as claimed in claim 1, wherein the water temperature is maintained in the range of 10 to 45 °C.

6. The raceway pond system as claimed in claim 1 , wherein the pH of the liquid suspension is maintained in the range of 6 to 12.

7. The raceway pond system as claimed in claim 1 , wherein the raceway pond system has either round or rectangular edges.

8. The raceway pond system as claimed in claim 1, wherein the system is supplied with carbon dioxide at a flow rate in the range of 0.1 to 20 mm/sec to maintain a concentration in the range of 150 ppm to 10, 000 ppm.

9. The raceway pond system as claimed in claim 1, wherein the system is supplied with air at a flow rate in the range of 0.1 to 20 mm/sec.

10. The raceway pond system as claimed in claim 1 , wherein said raceway pond system facilitates increased photosynthetic cell biomass productivities per unit area (g/m²/day) at acceptable power consumption per unit weight of photosynthetic cell biomass.

1 1. The raceway pond system as claimed in claim 10, wherein the increased photosynthetic cell biomass productivity per unit area is in the range of 50-200 g dry/m²/day.

12. The raceway pond system as claimed in claim 10, wherein the acceptable power consumption per unit weight of cell biomass is in the range of 1.0-6.0 W/g dry cell.

13. The raceway pond system as claimed in claim 1 , wherein the photosynthetic cells or organisms include any filamentous/ single celled photosynthetic organism, cyanobacteria, microalgae, diatoms, phytoplanktons.