New tubular photobioreactor design based on the Fibonacci configuration

Abstract

By 2050, the world will need to meet the needs of nine billion people. This will undoubtedly contribute to further increasing the demand for natural resources, food and energy supplies, causing an increase in pollution and favouring climate change. To accomplish these needs must be based on a sustainable development approach, that is, satisfying present needs without compromising future generations since the current pattern of consumption would require more planet Earth, which is not possible. In this context, microalgae could provide an interesting range of products, since they are a potential source of biochemical and nutritional substances such as proteins, lipids, carbohydrates, antioxidants, and pigments, with high economic possibilities, with uses in the food industry for human consumption and as additives for animal feed, pharmaceuticals and nutraceuticals. Microalgae convert solar energy into carbon storage products, which could become a sustainable renewable energy alternative, as they have a high potential to produce large volumes of biomass that in turn, can be used for the production of different biofuels. In addition, they would mitigate the accumulation of Co2 (carbon dioxide) in the atmosphere since they need carbon dioxide for photosynthesis. Moreover, they are very efficient in wastewater treatment, allowing to reduce water consumption, as well as recover nutrients from wastewater, such as phosphorus and nitrates. For all these reasons, processes based on the application of microalgae are receiving increasing interest within the industrial sector worldwide. Therefore, the challenge is to generate technology to industrialize the production of microalgae, for which it is necessary to have efficient technologies, i.e. photobioreactors. Relevant advances have been performed in this area, but many tasks remain to be done. Therefore, the great pending task is to design efficient photobioreactors, being necessary to capture as much light as possible, either distributed or diluted on the surface of the photobioreactor. The present research focuses on this topic by proposing a new photobioreactor desing inspired by the Fibonacci geometry, which allows optimizing light availability and productivity, with the possibility of being scaled up to an industrial level. The results show that the proposed design increases the solar radiation intercepted, compared to that received on a horizontal surface, providing optimal conditions for the growth of microalgae, while maintaining a moderate temperature, adequate pH and low dissolved oxygen concentration, improving the productivity of microalgae cultivation. This PhD thesis summarized the work performed to develop and scale up the Fibonacci type photobioreactor for the production of microalgae. In the first work, the new design of the tubular photobioreactor inspired into the Fibonacci geometry is showed. This comes from observing the geometry of the photosynthetic organisms in their natural state, where the plant growth forms turn, spirals or coils; patterns classified as helical. A small 6 L unit was evaluated indoors and a larger 250 L unit was evaluated outdoors; both units maintained the same concept and were developed using as the criterion to maintain the ratio of diameters of the upper and lower ellipses. The new photobioreactor design was tested with the Arthrospira (Spirulina) platensis strain. The results show that it is possible to maintain the temperature, pH and dissolved oxygen in the optimal ranges recommended for the cultivation of the strain. That is, there was no overheating of the photobioreactor, and the dissolved oxygen concentration remained below the of 200% Saturation limit. The growth model applied in both reactors showed that maximum specific outdoor growth rates reached 0.8 1/day, and photosynthetic efficiency reached 5.4%. In the second article, the Fibonacci-type photobioreactor is scaled up to 1200 L and the microalgae strain Dunaliella salina is used. The reactor was scaled up maintaining the design parameters that allow greater interception of solar radiation around its surface. The productivity of the photobioreactor was evaluated through the production of the Dunaliella salina strain under extreme environmental conditions, such as those of the Atacama Desert. The results show that the fibonacci type photobioreactor allows maintaining the temperature, pH and dissolved oxygen concentration within the optimal ranges for Dunaliella salina cultivation. The improved exposure to solar radiation in this photobioreactor, such as the dilution of the photonic flux, and the low thermal conductivity of the photobioreactor tube (PVC), avoid the use of cooling systems in outdoor conditions to maintain the temperature. The proposed photobioreactor can intercept up to 60% more solar radiation than the horizontal surface. Reaching a biomass concentration of 0.96 g L−1 , three times higher than the result obtained in a receway for dunaliella cultivation at commercial level, under the same extreme environmental conditions, while the productivity reached 0.12 g L−1 day-1 (2.41 g m−2 day-1 ). The specific growth rate reached up to 0.17 day−1 . Finally, the Fibonacci photobioreactor was scaled up to 2500 L and cultured in it of Chlorella sp. The results show that this photobioreactor design maintains the culture conditions at its optimal pH, temperature, and dissolved oxygen levels. Despite being a closed reactor, the temperature was kept below 30ºC while the dissolved oxygen was maintained below 200 % Sat. under outdoor conditions at which the mean daily solar radiation ranged from 139 to 450 µE/m2 s. Under these conditions, a maximum specific growth rate of 0.31 day-1 , a biomass concentration of 1.9 g/L, and volumetric biomass productivity of 0.37 g/L day were achieved. Biotechnological development for the industrial production of microalgae requires new photobioreactors capable of maximizing productivity and biomass production under high light conditions.. The Fibonacci-type tubular photobioreactor presented here offers substantial advances compared to classic tubular photobioreactors, as it has proven to be flexible in its geometric design and can adapt its parameters to the photosynthetic needs of the microalgae to ensure maximum efficiency, thus making it an alternative to conventional tubular systems. Lastly, the strategy for determining the optimal design of Fibonacci-type photobioreactors is presented. As an example of the application of the proposed technology, this thesis is completed with the tests carried out on the use of microalgae biomass produced as a nutritional supplement in livestock feed, specifically in chicken feed. In these trials, diets enriched with Spirulina platensis and Haematococcus pluvialis were tested, to analyze the improvement of the eggs produced by hens supplemented with these microalgae in terms of egg quality and egg laying. The results showed a significant improvement in egg production and egg quality when microalgae are incorporated into the feed, thus confirming the interest in the development of new studies that support the development of this type of applications of microalgae biomass, including improvements in its large-scale production using the technology developed in this thesis.