Mathematical Model of a Bubble Column for the Increased Growth of Arthrospira platensis and the Formation of Phycocyanin

The objective of this research was to develop a mathematical model for batch photoautotrophic cultivation of Arthrospira platensis and to validate it against data obtained in experiments. All trials were carried at 30°C, under a light intensity of 60 or 120 µmol m-2s-1. The purpose of the model was to determine the optimal concentration of carbon dioxide, as well as to investigate the formation of phycocyanin. For the experimental conditions in this study, the optimal concentration carbon dioxide (0.8% CO2, v/v) was predicted using the model according to the initial bicarbonate level, the carbon uptake by the microalga, the pH, and the mass transfer process. The use of this optimal value in the gas inlet seems to be a suitable option for maintaining the optimal pH (9.5), thereby eliminating the need for a pH controller in the bioreactor system. According to the simulations, the mass fraction of the phycocyanin formation rate seems to depend on the internal light level. The percentage of adjustment obtained (R2) was ?75%. The velocity of phycocyanin formation was enhanced at intensities up to 120 µmol m-2s-1. However, the actual internal irradiance values were lower than the light compensation point (4.5 µmol m-2s-1), so phycocyanin formation ceased. The mathematical model may facilitate the examination of optimal carbon delivery, as well as the light input, in several A. platensis culture conditions aimed at phycocyanin production

Eliminación de fosfatos y nitratos de agua residual municipal mediante un inóculo optimizado de Chlorella sp. en un sistema de fotobiorreactores verticales con columna de burbujeo a escala piloto

The discharge of municipal wastewater into local water bodies is one of the main causes of eutrophication. As a tertiary treatment for this problem, a culture of Chlorella sp. (recognized for its ability to remove phosphates and nitrates in wastewater) was used in a vertical system of photobioreactors with a bubble column at a pilot scale. An experimental factorial design was carried out, with three experimental phases, was used municipal wastewater from the WWTP “El Pueblo”. Initially, the inoculum size was determined at which the highest removal of NO3- and PO43- is obtained, was resulted as optimal cell density 3.6x106 cel/mL. The second phase allowed quantifying the concentration of phosphates and nitrates removed from wastewater evaluated according with the growth of the strain used, the experiment yielded removals greater than 95% in nitrate concentrations and 65.5% for phosphates. In the third phase, the inoculum size determined in first phase, was taken to a photobioreactor system evaluating its operating parameters regarding the nutrient removal capacity of the strain. With the results obtained, it was concluded that the proposed system is a viable alternative for the treatment of wastewater with different nutrient loads.El vertimiento de aguas residuales municipales en los cuerpos de agua locales constituye una de las principales causas de la eutroficación. Como tratamiento terciario para esta problemática se utilizó un cultivo de Chlorella sp., (reconocido por su capacidad de remoción de fosfatos y nitratos en aguas residuales) en un sistema de fotobiorreactores verticales con columna de burbujeo a escala piloto. Se realizó un diseño experimental de tipo factorial, con tres fases experimentales, en el que se empleó agua residual municipal proveniente de la EDAR “el Pueblo”. Inicialmente se determinó el tamaño de inóculo al que se obtiene la mayor remoción de NO3- y PO43-, con lo que se logró una densidad celular optima 3.6x106 cel/mL. La segunda fase permitió cuantificar la concentración de fosfatos y nitratos removido de aguas residuales evaluada respecto al crecimiento de la cepa utilizada, el experimento arrojó remociones superiores al 95% en concentraciones de nitratos y del 65.5% para fosfatos. En la tercera fase se llevó el tamaño de inoculo determinado en la primera fase, a un sistema de fotobiorreactores evaluando sus parámetros de operación respecto a la capacidad de remoción de nutrientes de la cepa. Con los resultados obtenidos se concluyó que el sistema propuesto es una alternativa viable para el tratamiento de aguas residuales con diferentes cargas de nutrientes

Optimization of microalgae production in industrial open reactors

The doctoral thesis has be developed in the framework of the research project "Control and optimization of biomass production with microalgae as a source of renewable energy" (DPI2014-55932-C2-1-R), which is focused on the modeling and control of the combined process of microalgae production and wastewater treatment with industrial reactors. This research project is a continuation of a previous project entitled "Modelling, Control and Optimization of Photobioreactors", where significant results were obtained in the field of modelling and automatic control of microalgae production in closed tubular photobioreactors. The current project continues in the same line, with the application of modelling and control techniques for the optimal production of biomass, but now focused on open photobioreactors, which are the most used worldwide. This thesis aims to improve the knowledge regarding to open reactors characterization, light distribution and utilization by microalgae, mass transfer and oxygen accumulation, as well as the use of control strategies to improve this technology. Both, raceway and thin-layer reactors are considered in this thesis. The information obtained from this thesis is being applied into the European Project Horizon 2020 SABANA focused to the development of microalgae based biorefineries for the improvement of agriculture and aquaculture sectors. The experimental work has been developed at three different locations: (i) “Las Palmerillas” Experimental Station (Almería, Spain) where experiments related with the improvement of open raceway reactors was performed, (ii) “Algatech” Experimental Station (Třeboň, Czech Republic) where experiments related with the evaluation of thin-layer reactors were performed, and (iii) “IFAPA” Experimental Station (Almería, Spain) where experiments related with the modelling of thin-layer cascade reactors were carried out. The major contributions on this thesis can be summarized such as: 1. Characterization and improvement of open raceway reactors Previous works demonstrated that dissolved oxygen accumulation affects to photosynthesis activity. This thesis demonstrates that dissolved oxygen accumulation limits the biomass productivity in raceway reactors if the mass transfer capacity is not improved. Although oxygen is desorbed to the air in the channel and the paddlewheel, this is not enough to remove the oxygen produced by photosynthesis when high biomass productivity is achieved, thus being necessary to include a sump, which adequately designed and operated, contributes to avoid oversaturation of oxygen. Therefore, the mass transfer capacity in the sump must be optimized to compensate the oxygen production rate in the system. Moreover, the influence of gas flow on the mass transfer coefficient was also determined, obtaining a calibrated empirical model. Using this model, it is possible to properlyregulate the air flow in the sump and thus, the reactor operation can be optimized. Full information is available in (Barceló-Villalobos et al., 2018). To improve the productivity of microalgae reactors in order to optimize the light pattern at which the cells are exposed to into the reactors must be optimized. For that, the first step is to know the real light pattern taking place in raceway reactors. This thesis demonstrates that microalgae cells are adapted to local irradiance because of the unfavourable cell movement pattern in raceway reactors. It has also been demonstrated how the light regime at which the microalgae cells are exposed to in a raceway reactor is far from the optimal one required to optimize the performance of microalgae cultures through light integration. Photosynthesis rate measurements were performed along different seasons at different daytime by using different light/dark cycles. These assays confirmed that no light integration exists at 0.15 m water depth. Moreover, it has also been confirmed that the cells are adapted to the local irradiance inside the reactor. Full information is available in (Barceló-Villalobos et al., 2019a). Regarding control strategies, a selective control strategy proposed previously by Pawlowski et al., 2015, has been used to control pH and dissolved oxygen simultaneously. In this control, the pH value is prioritized over the dissolved oxygen value since it has a critical influence on the process performance. This thesis demonstrates the correct functionality of this selective control approach in a semi-industrial raceway (100 m2) operated in semi-continuous mode. Furthermore, the oxygen mass transfer model already developed (Klal sump model) in the present thesis, has been validated in a simulation stage to demonstrate that it is possible to adjust the mass transfer capacity of the system close to the optimal value by controlling gas injections. It is shown that it is possible to reduce gas inflow actuations and control oxygen accumulation in the system by using a feedback control strategy. Finally, it has also been demonstrated that when the dissolved oxygen reference goes down respect to the initial reference (250% Sat), the necessary gas flow is higher (full information is available in Barceló-Villalobos et al., 2019c; Barceló-Villalobos et al., 2019d) 2. Characterization and improvement of thin-layer reactors It has been demonstrated that although thin-layer reactors are currently more productive than raceway reactors, their productivity can also be improved if the operating conditions are optimized close to the optimal culture values. This is the first step in optimizing and scaling-up this type of reactor for industrial applications. This thesis demonstrates the influence of variations of culture parameters (irradiance, temperature, pH and dissolved oxygen) on the performance of a microalgae culture. Different assays were done to analyse the system parameters in terms of position inside the reactor and time of the daylight cycle. Results demonstrate that average irradiance and temperature to which the cells are exposed are mainly a function of time, whereas pH and dissolved oxygen concentrations also showed relevant gradients depending on their position inside the reactor. VIII Furthermore, it has also been demonstrated that the existence of culture parameters gradients reduces the performance of the cultures (using two different methodologies: chlorophyll-fluorescence and net photosynthesis rate methods). Moreover, the influence of culture conditions on Scenedesmus almeriensis cell performance was modelled. Full information is available on (Barceló-Villalobos et al., 2019b). The performance of pilot scale thin-layer reactors located in Algatech (Trebon) has been also evaluated. Temperature and dissolved oxygen production have been analysed and modelled at three different pilot-scale thin-layer cascade reactors (small, medium, and large). Different assays were developed to analyse: (i) the variation of culture conditions, (ii) oxygen mass balance and (iii) model the oxygen production. Temperature is a stable parameter along the channel and through the day. On the other hand, dissolved oxygen increases along the channel through the day as it is expected by photosynthesis process. The modelling of oxygen production has been done by using light integration is here reported. Temperature and dissolved oxygen measurements were done along the thin-layer cascade reactor along the day. It was demonstrated that it is more accurately to use the integrated average irradiance parameter than the average irradiance concept, to demonstrate the effective light use into the culture. Full information is available on (Barceló-Villalobos et al., in review)

Análise de viabilidade de biodiesel derivado de microalgas

Doutoramento em Engenharia MecânicaA obtenção de biocombustíveis a partir de microalgas pode revelar-se uma alternativa potencialmente atrativa comparativamente à produção de combustíveis fósseis a sua utilização possui vantagens significativas do ponto de vista de minimização da poluição ambiental. O objetivo principal desta tese consiste em investigar os procedimentos necessários à eliminação das lacunas entre os testes laboratoriais e a produção industrial em grande escala. Para tal, procedeu-se à investigação e análise das tecnologias necessárias à viabilidade comercial da produção de biodiesel a partir de microalgas. Algumas estirpes de microalgas foram cultivadas em condições distintas e os seus lípidos foram extraídos usando várias técnicas. De seguida produziu-se biodiesel através do processo de transesterificação. Testes à qualidade do biodiesel produzido foram conduzidos de acordo com os padrões Europeus de qualidade com base na normativa EN14214:2003. Adicionalmente, foi feito um estudo de impacto económico e ambiental abrangendo todo o processo de produção. Os resultados experimentais obtidos permitiram determinar a altura ótima da colheita da biomassa e revelaram o papel do fotoperíodo e do fornecimento de CO2 à cultura. A análise da qualidade da produção de biodiesel a partir de microalgas cultivadas nas regiões de Aveiro, Almeria e Vigo confirmou a sua aptidão usando a normativa Europeia. Os resultados indicaram ainda que a utilização de águas residuais como meio de crescimento conduziu à necessidade de menos nutrientes. A análise económica comparativa sugeriu que a valorização dos co-produtos pode representar uma condição necessária à viabilidade do processo. uma modernização do projeto de instalação (por exemplo, a valorização do coproduto, de modo a aumentar as receitas do sistema). A Análise de Ciclo de Vida indicou que a fase de extração teve o maior impacto nas emissões (é responsável por 94% de gases por efeito de estufa – GEE e 84% de energia utilizada e obtida a partir de combustíveis fósseis – FEC) enquanto a fase de cultivo permitiu reduzir as emissões totais (-4% de GEE e -2% de FEC). O estudo de sensibilidade sugeriu um decréscimo acentuado nas emissões (8.9 e 4.5 vezes de GEE e FEC, respetivamente) usando recuperação dos solventes usados nos processo de extração a 95% face ao valor de 50%. A conclusão deste trabalho realçou a potencialidade das microalgas como matéria-prima no que concerne à produção de biodiesel. Se os riscos de contaminação forem evitados, as grandes lagoas abertas desempenharão um papel fundamental para a viabilidade global da otimização do processo produtivo de biomassa em grande escala.Microalgae derived biofuel is becoming attractive as a renewable fuel, promoting potential advantages resulting from its capability to reduce environmental pollution. The main objective of this thesis is to investigate the requirements to bridge the gap between laboratory scale testing and its industrial production. The research was focused on the characterization and analysis of the technologies required to obtain commercially viable microalgae derived biodiesel. Several microalgae strains were cultivated in different conditions and their lipids were extracted using different techniques and chemically trans-esterified to biodiesel. Quality testing of the produced biodiesel was performed (in accordance to the European biodiesel quality standard EN14214:2003) An economic and environmental study was developed for the whole production pathway. This approach projects that maximum biomass is obtained at the end of the ‘exponential phase’ of microalgae growth and that high illumination increases the biomass concentration irrespective of the aeration rate. Quality analysis confirmed that the analyzed properties of biodiesel produced from microalgae’s grown in Aveiro, Almería and Vigo region comply with the European standard. The use of wastewater as a growth medium does not compromise biodiesel quality and implies the use of less nutrients. A comparative economic analysis showed that the facility design used in this work requires upgrading since co-product valorization can add revenue and improve feasibility. The life cycle assessment study highlights the high contribution to emissions from the extraction step (94% greenhouse gas emission -GHG and 84% fossil energy consumption - FEC) while the cultivation step adds positive value to the total emissions (-4% GHG and -2% FEC). A sensitivity study highlighted the relevant reduction in emission (8.9 times GHG emission and 4.5 times FEC) with 95% solvent recovery when compared with 50% solvent recovery. The work concludes that microalgae have the potential to become a good feedstock for biodiesel production and if contamination risks can be avoided then large open ponds will play a vital role in large quantity biomass production hence optimizing overall feasibilit