CLIAMS:1. A photo bioreactor for biofixing CO2 comprising of :
- a tank,
- a carbon source connected to the tank,
- an illumination source operably connected to the tank,
- a rotating shaft,
- a plurality of treated discs arranged around the shaft, the said discs having a layer of micro algae suspension on the treated discs surface,
- a plurality of spaces arranged alternating with plurality of said discs arranged around the shaft and
- a trough having an inlet at the top side, an outlet at the bottom side and within which the rotating shaft is mounted.

2. The photo bioreactor as claimed in claim 1 includes a liquid seal disposed at the outlet thereby correspondingly maintaining the level of micro algae suspension in the trough.

3. The photo bioreactor as claimed in claim 1 wherein the trough holds the micro algae suspension.

4. The photo bioreactor as claimed in claim 1 wherein the length of trough is greater than the diameter of disc, depth of trough is slightly greater than the radius of the disc and width of trough is same as shaft length.

5. The photo bioreactor as claimed in claim 1 wherein the trough has an inlet at the top for micro algae suspension on one side and an outlet at the bottom on the other side.

6. The photo bioreactor as claimed in claim 1 wherein the said treated disc means perforation on said discs adapted to hold a liquid film, wherein the said surface providing a highest photo receiving surface area to the volume of microalgae suspension and thereby increasing photosynthetic productivities.

7. The photo bioreactor as claimed in claim 1 wherein the said treated disc means rough surface formed on the said discs adapted to hold a liquid film, wherein the said surface providing a highest photo receiving surface area to the volume of microalgae suspension and thereby increasing photosynthetic productivities.

8. The photo bioreactor as claimed in claim 1 wherein the size and number of perforations on each disc is based correspondingly to hold a liquid film on the surface, and for providing a large surface area to volume ratio and thereby increasing light harvesting efficiency.

9. The photo bioreactor as claimed in claim 1 wherein the extent of roughness of the surface on each disc is based correspondingly to hold a liquid film on the surface, and for providing a large surface area to volume ratio and thereby increasing light harvesting efficiency.

10. The photo bioreactor as claimed in claim 1 wherein the said plurality of discs are generally spaced equally around rotary shaft.

11. The photo bioreactor as claimed in claim 1 wherein each disc is made of transparent or translucent material.

12. The photo bioreactor as claimed in claim 1 wherein the said tank may be of metal or non-metal.

13. The photo bioreactor as claimed in claim 1 wherein the rotary shaft is adapted to rotate at a speed corresponding to the rate of absorption of photons by micro algae and rate of absorption of carbon-di-oxide by water, thereby efficiently releasing O2.

14. The photo bioreactor as claimed in claim 1, wherein the tank includes a transparent closure.
,TagSPECI:FIELD OF INVENTION

Biological sequestration is a technique which utilizes carbon dioxide and light to produce value added products through photosynthesis by plants and microalgae. Photosynthesis is the growth of microalgae due to the reaction between carbon dioxide and water on photonic microalgae. The growth of microalgae undergoes four phases such as lag, log, and stationary and death phases. The science of microalgae growth is well established. The technology of microalgae growth varies due to the mode of absorption of carbon dioxide, adsorption of photons, mixing and harvesting. Variation made in each of these parameters resulted in different type of photobioreactor (PBR). Open and closed systems are the main classification of photobioreactor. Open pond systems are economical on large scale, however the problems are sustainability of the system to climatic changes over the year and subsequent contamination of the species resulting in lowering biomass yield and increasing the harvesting cost. Closed photobioreactor do overcome the problems of open pond systems, but their initial investment is too high besides the oxygen accumulation during operation. Closed photobioreactors are economical when value added products are obtained from the biomass.

Further many types of photobioreactor have been developed under each classification. There is not much of engineering gone into the design of photobioreactor. Hence, the main disadvantages of these reactors are poor exposed surface area per unit volume and difficulty in scaling-up.

Providing sufficient number of photons, carbon dioxide and mixing without cell damage pose a lot of challenges in designing a cost effective photobioreactor to produce required quantity of microalgae.

BACKGROUND OF INVENTION

In early stages, microalgae were grown for providing healthy food, treating waste water, producing fine chemicals and for fixing carbon dioxide. Closed ecological life support system (CELSS) can be developed through microalgal biotechnology. The lack of efficient photobioreactor hampers the commercialization of microalgae production. A thorough understanding of hydrodynamic and mass transfer is required for the efficient design of photobioreactor.

Beijerinck (1890) achieved the first unialgal cultures with Chlorella vulgaris. Warburg (1919) used these cultures for studying plant physiology in early 1900s. Research on mass culture of microalgae was focused after 1948. Burlew (1953) summarized many of these early studies in his classic book. Since then the growth of microalgae attracted the attention of many researches in several countries. As a result the commercial scale cultivation of microalgae were increased several folds.

The establishment of several industries has thrown light into challenges faced on mass cultivation. One of the challenges is the development of cost effective large scale culture systems for the microalgae. Several attempts have been reported by researchers on the development of commercial photobioreactor. The volume of existing commercial microalgae culture systems ranges from 102 L to 1010 L. Large open ponds, circular ponds with rotating arm to mix the cultures, raceway ponds and large bags are the predominating culture systems.

However, these culture systems remain as technology. Several engineering inputs are required to achieve a cost effective and reliable photobioreactors. Numerous configurations are available in photobioreactors (Lee 1986, Tredici and Materassi 1992, Borowitza 1996, Pulz and Scheinbenbogen 1998). However they are classified mostly as open pond and closed systems. Orientation of systems, the mechanism for circulating the culture, the method used to provide light, the type of gas exchange system, the arrangement of the individual growth units and the material of construction employed further categorize these photobioreactors. A comprehensive review on the developmental state of photobioreactor technology was reported (Lee 1986, Borowitza 1996, Pulz and Scheinbenbogen 1998, Vasumathi et al 2012).

OPEN POND SYSTEMS

Open pond systems are less expensive. They utilize direct sunlight for photosynthesis. There are four types of open pond systems. 1. shallow big ponds 2. tanks 3.circular ponds and 4. raceway ponds. The choice of a particular system depends upon the intrinsic properties of microalgae, local climatic conditions, the land cost and water. Cultivation of microalgae in open ponds has been extensively studied in the past few years (Boussiba et al. 1988, Tredici and Materassi 1992, Hase et al. 2000). The pond area and volume must be optimized and the cell density in the culture must be maximized in order to have an economical process. Further the power required for mixing also plays a decisive role. The productivities have to be maximized to offset the high capital cost of the ponds and associated production systems such as harvesting units and dryers. Further the pond depth is a compromise between providing adequate light to the microalgal cells and maintaining an adequate depth for mixing to avoid large changes in ionic concentration due to evaporation. Becker (1994) reported that only 13 - 20% of supplied carbon dioxide was absorbed in raceway ponds. Higher utilization rate of 70% was observed in raceway ponds by admitting 0.1 - 0.2 vol% carbon dioxide (Doucha et al. 2006) but increases the water requirement. Flue gas emitted from the thermal power plant has 15% carbon dioxide. Diluting this to such a lower concentration will involve handling of large volume of flue gas and increases land space requirement. Setlik et al. (1970) developed the cascade open pond systems. Doucha and Livansky (1995) used this concept and operated an open air system having 1 cm depth sloping culture surface made of glass. This system would be very competitive with paddle wheel raceway systems if located in a sunnier, warmer climate and constructed of lighter and cheaper material.

The actual productivity achieved in open pond systems is always less than the theoretical one as it is very difficult to control the culture environment. Hence, this system is limited to only few species.

CLOSED SYSTEMS

Closed systems will be required in future for microalgal mass culture of special interest, since contamination needs to be prevented at any cost. Even though the concept of closed systems has been around for a long time, the high cost of operation precluded the commercialization. In closed system, microalgae can be grown either photoautotrophically or heterotrophically. Heterotrophic cultivation is not possible for all microalgae cultivation since the chemical composition of microalgae, often changes.

In the aquaculture industry for the production of wide range of microalgal species, closed photoautotrophic systems are used. The need for providing artificial light and mixing result in high investment and operating cost. Flat plate reactor and tubular reactor are the widely used photobioreactors. The main features of these reactors are the reduction in the light path, well mixing to ensure optimum light availability to cells, high utilization efficiency of light and easy control of the variables.

Tubular reactor is one of the most suitable types for outdoor mass cultures. Glass or plastic tubes are the materials used for construction of reactor. Horizontal/serpentine (Chaumont et al. 1988, Molina et al. 2001), vertical (Pirt et al. 1983), near horizontal (Tredici and Zittelli 1998), conical (Watanabe and Saiki 1997), inclined (Lee and Low 1991, Ugwu et al. 2002) photobioreactor designs are so far reported. Increased surface area is the great advantage of the system. But, oxygen accumulation inside the reactor leads to the poor productivity (Chaumont et al. 1988, Molina et al. 2001, Pirt et al. 1983, Tredici and Zittelli 1998, Watanabe and Saiki 1997, Lee and Low 1991, Ugwu et al. 2002, Torzillo et al. 1986, Richmond et al. 1993). Also, photo inhibition is very commonly reported in outdoor tubular photobioreactors (Vonshak and Torzillo 2004). Efficient light distribution to the cells can be achieved by improving the mixing system in the tubes (Ugwu et al. 2003, 2005, Zhao et al. 2011). Culture density maintained inside the closed system is 5 - 6 times higher than that of the raceway ponds (Stephens et al. 2010). For scaling up of tubular photobioreactors, if diameter is increased then the surface area to volume ratio decreases and results in lesser productivity due to mutual shading, insufficient light and poor mass transfer rates. Hence scaling up is achieved by only increasing the length by multiple number of tubes of the same diameter. Hence the high cost remains still as a challenge. High temperature, oxygen accumulation, photoinhibition, carbon dioxide sparging, land requirement, more power consumption and scale up are the other problems with the tubular reactor.

Flat plate photobioreactors have received much attention for cultivation of photosynthetic microorganisms due to their large illumination surface area and smaller light path. The smaller light path enhances the higher photosynthetic efficiencies. Hu et al. (1998) reported higher productivity of 80 g/L in flat plate reactors. Accumulation of dissolved oxygen concentrations in flat plate photobioreactors is relatively low compared to horizontal tubular photobioreactors. High density cultures are easily maintained and achieved due to smaller path length. Higher productivity is very much suitable for mass cultures of microalgae. This reduces harvesting cost. The smaller path length provides the sufficient time of exposure thus this design results in high productivity (Hu et al. 1998, Degen et al. 2001, Richmond et al. 2003). The advantages are due to the highest surface area to volume ratio. Table.2 gives the comparison of different large scale microalgae culture systems used.

Table.2 Comparison of the properties of different large scale microalgal culture systems (Borowitzka 1999)

Reactor type Mixing Light utilization efficiency Temperature control Gas transfer Hydrodynamics stress on microalgae Species control Sterility Scale-up References
Unstirred shallow ponds Very poor Poor None Poor Very low Difficult None Very difficult Borowitzka and Borowitzka 1989
Tanks Poor Very poor None Poor Very low Difficult None Very difficult Fox, 1983
Circular stirred ponds Fair Fair-good None Poor Low Difficult None Very difficult Tamiya, 1957; Stengal, 1970; Soeder, 1981
Paddle-wheel Raceway ponds Fair-good Fair-good None Poor Low Difficult None Very difficult Weissman and Goebel, 1987; Oswald, 1988
Stirred tank reactor (internal or external lighting) Largely uniform Fair-good Excellent Low-high High Easy Easy achievable Difficult Pohl et al, 1988
Air-Lift reactor Generally uniform Good Excellent High Low Easy Easily achievable Difficult Juttner, 1977
Bag culture Variable Fair-good Good (indoors) Low-high Low Easy Easily achievable Difficult Baynes et al, 1979
Flat-plate reactor Uniform Excellent Excellent High Low-high Easy Achievable Difficult Hu et al 1996, tredici and Zitelli 1997
Tubular reactor (Serpentine type) Uniform Excellent Excellent Low-high Low-high Easy Achievable Reasonable Richmond et al, 1993; torzillo 1997
Tubular Reactor (Biocoil type) Uniform Excellent Excellent Low-high Low-high Easy Achievable Easy Borowitzka 1996

However, scaling up is limited with the large requirement of land area. It is evident from the above observations that none of the presently used photobioreactor is suitable for large scale implementation paving way for further improvement or development studies.

SUMMARY OF INVENTION

Designing an efficient photobioreactor is still an art rather than science. Design and construction of any photobioreactor should depend on the type of strain, the target product, geographical location as well as the overall cost of production. Further the requirements of large scale outdoor photobioreactors are 1. large volume with less ground area 2. transparent surfaces 3. high illumination surfaces 4. high mass transfer rate and 5. maintaining lower temperature. Therefore the main macroparameters used for designing and operating photobioreactors are surface productivity defined as the production per unit of time per unit of radiative surface of the reactor and volume productivity defined as the biomass concentration in dry weight per unit time per liter. The following issues need to be addressed for effective designing of photobioreactor 1. effective and efficient provision of light 2. supply rate of carbon dioxide while minimizing losses 3. removal of photosynthetically generated oxygen that may inhibit the metabolism or otherwise damage the culture if allowed to accumulate 4. sensible scalability of the photobioreactor technology (Weissman et al. 1988).

Corresponding to the fixation rate of microalgae, matching quantities of photons and molecules of carbon dioxide should be made available for photosynthesis. Rate of supply of photons and carbon dioxide molecules to cell should not control the growth rate of microalgae. Then only a maximum growth rate can be achieved.

The yield of microalgae from any type of photobioreactor is related to areal productivity (g/m2/day) and volume productivity (g/L/day). The length of light path affects the relationship between these two productivities. The areal productivity and volume productivity are the main factors to be considered for designing a cost effective photobioreactor. To reduce the shading effect and to increase the rate of adsorption of photons, the light path length should be reduced. Even though the areal productivity relates surface to microalgae productivity and volume productivity to growth rate, no such parameters is available for carbon dioxide transfer. However
there should be enough carbon dioxide available in suspension to achieve the desired growth rate. Similar to light adsorption, carbon dioxide absorption also requires large area for mass transfer to increase the carbon dioxide concentration in suspension providing the required surface area simultaneously for both light and carbon dioxide will solve the problem.

Therefore spreading the suspension of known volume to a large surface area would result in higher rates of adsorption of light and absorption of carbon dioxide. If a known volume of suspension with a given microalgae concentration is spread into a large surface area as a thin film, the length of light path would be reduced considerably and all the cells would be exposed to light. If carbon dioxide-air mixture were passed over this surface, the absorption would be better. Such an arrangement would increase the areal and volume productivities. Since no intensive mixing of suspension is required, the cell damage could be prevented. Two basic parameters relating the photobioreactor efficiency and the cost effectiveness are (i) total illuminated surface area per ground area required to produce a required quantity of product and (2) the culture volume required to produce that quantity. The photobioreactor would be the more efficient and cost effective as these values are lower.

DESCRIPTION OF THE INVENTION

Biological methods include photosynthesis of plants and microalgae. Photosynthesis of microalgae appears to be attractive over that of plants. The micro- algae growth kinetics indicates that the rate increases and decreases with the concentration of microalgae and highest at an optimum concentration. The mechanism of photosynthesis of algae involves providing highest spreading area to volume ratio, stabilizing the CO2, maintaining optimum concentration corresponding to highest growth rate and maintaining nutrient concentration. Ensuring the above criteria into the design of photobioreactor will be helpful to achieve an economical photobioreactor system.

The photobioreactor in one embodiment has a tank which is connected to CO2 source and an illumination source. A rotating shaft is installed within the trough of the tank onto which discs can be mounted. These discs have a layer of microalgae suspension on its surface and are assembled around the shaft. The trough has also an inlet and outlet. This is illustrated in fig. 1.

In another embodiment, the outlet has a liquid seal. In another embodiment, the trough is so designed wherein the length of the trough is greater than the diameter of the disc, its depth is greater than radius of the disc and also its width is same as shaft length. The novelty is in the formation of perforations on the said discs or roughening the surface, each adapted to increase the surface of the disc to hold more liquid film. The discs may be transparent or translucent and tank may be of metal or non-metalic. The rotation rate of shaft is adjustable and the tank also includes a transparent closure.

All embodiments are for purpose of understanding and do not limit the scope of invention. All modifications and variations are obvious to skilled persons are within the scope of the invention.