A High Productivity Photobioreactor

Key Word:

micro alga    spirulina    haematococcus   astaxanthin

• the whole set consisted of three parts, i.e.
  • the coiled large sized glass tube system that functions as the solar radiation receiver and with a large volume of algal culture medium(1000l/unit) circulated within it;
  • the glass towers, by connected closely to the tubular section that acts dual roles:
    • for the extraction of the D.O. in the growth medium ,
    • regulation of the growth parameters for attaining an optimized condition and,
  • the pump system, which propels the medium circulating in the VGPR with least injuries to the cells.
• the whole tubes and towers of the set were made of B & Si enhanced glass material with which forms a rational surface-to-volume ratio of ca 40, and being characterized of full orientation light acceptance, that ensures an extended day light hours and high efficiency photosynthesis of the cells.
• the VGPR was of such a flexibly joined section and was fully closed that it can be easily sterilized, thus ensures a strict monoculture in the whole cultivation process while excluding any of the external contaminants.
• the VGPR was set perpendicularly to the ground covering a land area of 5 square meters, which was only about 8% of the area for the pond systems as compared on equal yields.
• owing to above mentioned features, the VGPR, from an economic point of view, showed a high performance of lower energy consumption, longer durability and higher flexibility and can be easily scaled up to a size of commercial production plant (i.e.hundreds tons in volume or more), that makes it universal usefulness either for super quality production of micro-algae or for the culturing of other economic photophilic micro-organisms.

A pilot sized VGPR

Spirulina maxima

Haematococcus pluvialis

      Spirulina and Haematocuccus spp. are of microscopic plants, although they are biologically adaptable to a wide range of warm environments, the establishment and proliferation of themselves with high productivity depending on selective nutrients, the suitable growth temperature (28- 35 °C ), and in need of optimizing the light intensity (20k - 35k lux) and pH range (7.0 - 9.5 - 10.5), etc. In the last tree decades, some open pond systems has been chosen for the mass culturing of these algae, however, these essential requirements were hardly met in the open pond systems. As a result, people with full efforts in exploitation of the algae were sometimes getting setbacks or harvesting a poor yield.

       Considering all those limitations and shortcomings of the pond systems, most bioscientists and biotron-engineers, had oriented their research works towards the development of an unconventional way for micro-algae culture, which should be fully closed and compact with high surface-to-volume ratio and all the growth factors be optimized. With these desired characteristics as the main goals, researches on tubular systems were the right orientation and some forms of designs as the bag-shaped reactors, and fermenter-like bio-reactors etc. had in certain aspects succeeded when used in the experimental cultures. However, few of these forms could be really applicable in the pilot production scale, not saying that to be practised in the commercial production. The reasons were said mostly to be that there existed serious obstacles of operational problems and growth limitations. Amongst them, were primarily the oxygen build-up in the growth medium and the overheating inside the tube walls by the intensive solar radiation when operating in summer seasons. This is because the cells of the photophilic algae are highly sensitive to the oxygen tension (pO2) and the overheating (> 38 °C) when cell concentration over 35 - 40 mg/l, the algal growth and biomass would be inhibited in this case, especially in the midday light hours. Besides, the problems of leaks and annealing when plastic tubes being used, and the poor circulation of the growth medium causes the algal staining on the inner walls of the tubes, etc., gave eventually an uneconomic results.

       In order to overcoming above mentioned shortcomings of those forms of bio-reactors, some biotron engineerers had been exploring and performing certain improved tubular systems with special attention being paid on the oxygen build-up in the growth medium. For which, some people tried a way of degassing oxygen every 30 - 40 minutes in the degassing stations of the reactors and set up a maximum length-to-flow velocity ratio in the system, while others expel the oxygen in the medium with air and/or N2 bubbling at the bottoms of their reactors. But as far as the author knew, these systems still had some distance from practical use in mass production of the algae. The basic problem for those reactors was due that there still existed the O2 build-up, mainly because the dissolved oxygen (D.O.) can be still high up to an inhibitory level of 20 mg /l in the peak hours of photosynthesis in the daytime. And secondly, it was difficult to scaling up to a commercial production level due to these types can hardly be extended to large size, or only up to a few feet high (otherwise, they should be placed horizontally and that would occupy a large land area), and lastly, in connection of these units into series appeared to be technically and economically inefficiency.

       In view of these limitations and drawbacks, the author, based on his twenty more years' experience in micro-algae culture and biomass production, had specifically designed and manufactured a totally different kind of photo-bioreactor. This novel system has basically overcome those shortcomings as overheating of the growth medium, the high tension of the dissolved oxygen(D.O.), and the algal staining on the inner walls of the tubes, etc. After months of field cultural operation, it proved to be best suitable for the commercial production of Spirulina and Haematococcus spp. in particular and other photophilic micro-organisms in general.

       When outlining the features of the vertical glass photobioreactor (VGPR), there are several major structural differences in comparison with all previous types of tubular reactors. First of all, the whole set constituted of three parts, that is:

(1) the dozens array of horizontal glass tubes, which were connected to a desired length and took a double coiled compact form. By this form of tubular section, the system functions not only as a solar radiation receiver of full orientations, but also allows filling enough volume of growth medium and that maintains a steady speedy flow (50 cm/s) circulating inside the tubes.

(2) the glass towers, which connecting to the top and bottom valves of the arrayed tubes, with one tower acting for the extraction of D.O. from its installed degassing device, and with the other tower CO2 being added, temperature regulated (cooling in summer, heating in winter), the pH maintained and the medium refreshed and,

(3) the pump system, with which, the medium being propelled circulating from the tube section to the towers and return back to the tubes. The deoxygenation was occurring simultaneously along with the medium circulation. The whole combination of the set took a space of 5 metres in both height and width, 0.8 metres in depth, that made it a single unit, and occupied a ground area of only five square metres.

       As for the functional performances, the VGPR, passed through a running test of four months, was proved superior in all aspects over that of pond systems, and especially in deoxygenation merited over previous tublar reactors. The advantages of the VGPR could be highlighted as follows:

• this novel VGRP makes an optimum balance of the biological requirements of the micro-organisms with the physical performances of the engineered system, i.e. the cultural temperature can be effectively controlled, the light conversion efficiency be maximized and the pH value be dynamically regulated, etc. Owing to these main parameters being substantially optimized, that the VGPR keeps a mode of sustaining and continuous operation, thus it ensures the algal biomass being produced in all seasons and in whatever geographical locations.
• due to the whole set (tubes and towers) is made of boron and silicon enhanced glass materials (which is available and cost reasonable in China) and has a structurally rational surface-to-volume ratio of ca 40, so that a full orientation of light acceptance being achieved in the daytime with respect to the sunshine, and in the night to the artificial light. These features favoured not only with an extended light duration and a high efficiency of photosynthesis of the algal cells, but also with a longer working life. Besides, with this novel biotron, the problem of over saturation of light intensity could be effectively controlled, therefore, the photoinhibition and the dark respiration can be checked down to the lowest level. As a result, an averaged daily biomass of 1,000g (dry wt.) that means a higher productivity of 35 g (dry wt.) / m2.d, can be produced from this unit .
• deoxygenation, especially the D.O. is most vital to the algal cell growth, either in the closed systems or in the open ponds. There were several methods for this problem, as stirring the medium in the pons, air bubbling or N2 filling in the reactors, etc. which were usually used by those system for the repelling of oxygen out from the growth medium. Certainly, they had some degassing effects on the free oxygen, but had little effect on the D.O. However, for the VGPR, a specific physical way was invented and introduced on it by the author, with which an inherent and efficient extracting of the dissolved oxygen (D.O.) was fully succeeded and in the same time, not least to endanger the well-being of the algal cells. With this innovative degassing device being installed on the VGPR, the obstacle of the oxygen build-up in the growth medium has been basically eradicated even in the peak hours of the daytime. This merit of the VGPR favours greatly to the algal cells with a high efficiency of cell photosynthesis and a high rate of algal biomass.
• due to each tubular section of the VGPR is flexibly joined that formes the whole set being fully closed and in an easily sterilised condition. In addition, a deionized medium water being used, which is produced by a separate equipment, and the growth medium in the VGPR being kept a fixed flow speed, thus, it consequently ensures a strict and axenic monoculture running in the system while any external contaminants being effectively barred. This is why a VGPR could be best suitable to produce certain defined biochemicals as polysaccharides, pigments, lipids, or unsaturated fatty acids, etc. Therefore, it is of particular significance by using the VGPR for the exploitation of new products as astaxanthin from Haematococcus or poly- unsaturated fatty acids from Spirulina spp. or for culturing other micro-organisms of high economic values.
• the harvest mode of the VGPR is taking filtering the whole culture out from the medium at one time through the bottom valve, and at the same time, recycling the filtered medium back into the system by refreshing it with some kinds nutrient supplements. Meanwhile, in the reacting towers of the VGPR, there can be always retaining part of the growth medium during each harvest operation, this advantage helps to realize a continuous cultivation of the algae with no necessity of reinoculating the growth medium. With this mode, the cultivation periods between last and next harvesting operations could be shortened and the productivity be significantly raised. And lastly, the VGPR is set perpendicularly to the ground, covering a land area of 5m2, i.e. only about 8% of the area for the pond system as compared on equal yields. Besides, as the whole set runs independently, and gives a high biomass productivity that facilitates several to hundreds dozens of VGPRs making more production lines, or scaling up to an industrial production level.

The Inventor Miao Jian Ren

Patent Number: ZL95219504.6

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