A novel data-driven model for prediction and adaptive control of pH in raceway reactor for microalgae cultivation.

This work proposes a new data-driven model to estimate and predict pH dynamics in freshwater raceway photobioreactors. The resulting model is based purely on data measured from the reactor and divides the pH dynamics into two different behaviors. One behavior is described by the variation of pH due to the photosynthesis phenomena made by microalgae; and the other comes from the effect of CO2 injections into the medium for control purposes. Moreover, it was observed that the model parameters vary throughout the day depending on the weather conditions and reactor status. Thus, a decision tree algorithm is also developed to capture the parameter variation based on measured variables of the system, such as solar radiation, medium temperature, and medium level. The proposed model has been validated for a data set of more than 100 days during 10 months in a semi-industrial raceway reactor, covering a wide range of weather and system scenarios. Additionally, the proposed model was used to design an adaptive control algorithm which was also experimentally tested and compared with a classical fixed parameter control approach.

An artificial intelligence approach for identification of microalgae cultures.

In this work, a model for the characterization of microalgae cultures based on artificial neural networks has been developed. The characterization of microalgae cultures is essential to guarantee the quality of the biomass, and the objective of this work is to achieve a simple and fast method to address this issue. Data acquisition was performed using FlowCam, a device capable of capturing images of the cells detected in a culture sample, which are used as inputs by the model. The model can distinguish between 6 different genera of microalgae, having been trained with several species of each genus. It was further complemented with a classification threshold to discard unwanted objects while improving the overall accuracy of the model. The model achieved an accuracy of up to 97.27% when classifying a culture. The results demonstrate the effectiveness of the Deep Learning models for the characterization of microalgae cultures, it being a useful tool for the monitoring of microalgae cultures in large-scale production facilities while providing accurate characterization over a wide range of genera.

Productivity analysis in tubular photobioreactors using a dynamic photosynthesis model coupled to computational fluid dynamics particle tracking.

Tubular photobioreactors (TPBRs) are closed devices used for the mass culture of microalgae. TPBRs are supposed to be well-mixed, but the influence of their specific fluid dynamics in photosynthesis efficiency has never been studied in detail. Here, we use Computational Fluid Dynamics (CFD) coupled to a dynamic photosynthesis model to analyze the efficiency of the photosynthetic response in the loop of TPBRs of different sizes (14, 24, 44, 64, and 84 mm) and circulation velocities (0.4 to 1 m s-1). The results show that only the smallest diameters cause enough radial mixing for a photosynthesis-enhancing light regime (integration factor Γ = 0.199 for D = 14 mm and v = 1 m s-1) while high circulation velocities in larger diameters (up to 1 m s-1) increase operating costs but do not enhance photosynthetic productivity. It is also shown the relevance of the characteristic frequency of the strain (ß), which is crucial for high productivity.

Analysis of productivity in raceway photobioreactor using computational fluid dynamics particle tracking coupled to a dynamic photosynthesis model.

Raceway photobioreactors (RWPs) are the most common and affordable device for the mass culture of microalgae but due to geometry and the requirement of low input power, its photosynthetic performance is low. The fluid dynamics of RWPs have been studied for information such as energy dissipation and shear rate, CFD has never been used to analyze photosynthesis efficiency by coupling dynamic photosynthesis models with microalgae trajectories. In this work, we investigate by CFD simulation the effect of circulation velocities between 0.2 and 0.8 m s-1in a 0.15 m-1 deep RWPs under standard outdoor conditions to shows that in all circumstances the RWP from the point o view of photosynthesis operates as a perfectly segregated device (no mixing) and that the average growth rate is the result of the integration of the local growth rates at different depths (integration factor Γ = 0).

Indirect regulation of temperature in raceway reactors by optimal management of culture depth.

Temperature and irradiance are the two most relevant factors determining the performance of microalgae cultures in open raceway reactors. Moreover, inadequate temperature strongly reduces the biomass productivity in these systems even if enough sunlight is available. Controlling the temperature in large open raceway reactors is considered unaffordable because of the large amount of energy required. This study presents an indirect method for temperature regulation in microalgae raceway reactors by optimizing the culture depth. First, the effect of the culture depth on the raceway temperature is analyzed for different seasons of the year. Afterward, a simulation study is presented where the proposed control approach is compared to the normal operation mode with constant volume in the reactor. This study is also extended to industrial scale. Relevant improvements on the temperature factor and biomass production are presented. The developed knowledge allows the improvement of the performance in open raceway reactors up to 12% without involving additional energy and costs, being a suitable solution for large industrial reactors that until now have no options for controlling the temperature.

Diurnal and nocturnal pH control in microalgae raceway reactors by combining classical and event-based control approaches.

The pH control in raceway reactors is crucial for an optimal performance of the system. Classical pH control is exclusively performed during the daytime period for cost saving reasons. This paper demonstrates that pH can be controlled 24 hours a day by using both a continuous-based and an event-based control approach, being able to improve the system's performance and reducing costs at the same time. Thus, experimental tests on a raceway reactor for several days are presented to show a comparison between traditional control algorithms during the daytime period versus an event-based control approach operating during both daytime and night-time periods. As a result, the combination of classical PI control for the daytime period and the event-based control for the night-time period is presented as a promising pH control architecture in raceway reactors.

Evaluation of native microalgae from Tunisia using the pulse-amplitude-modulation measurement of chlorophyll fluorescence and a performance study in semi-continuous mode for biofuel production.

BACKGROUND: Microalgae are attracting much attention as a promising feedstock for renewable energy production, while simultaneously providing environmental benefits. So far, comparison studies for microalgae selection for this purpose were mainly based on data obtained from batch cultures, where the lipid content and the growth rate were the main selection parameters. The present study evaluates the performance of native microalgae strains in semi-continuous mode, considering the suitability of the algal-derived fatty acid composition and the saponifiable lipid productivity as selection criteria for microalgal fuel production. Evaluation of the photosynthetic performance and the robustness of the selected strain under outdoor conditions was conducted to assess its capability to grow and tolerate harsh environmental growth conditions. RESULTS: In this study, five native microalgae strains from Tunisia (one freshwater and four marine strains) were isolated and evaluated as potential raw material to produce biofuel. Firstly, molecular identification of the strains was performed. Then, experiments in semi-continuous mode at different dilution rates were carried out. The local microalgae strains were characterized in terms of biomass and lipid productivity, in addition to protein content, and fatty acid profile, content and productivity. The marine strain Chlorella sp. showed, at 0.20 1/day dilution rate, lipid and biomass productivities of 35.10 mg/L day and 0.2 g/L day, respectively. Moreover, data from chlorophyll fluorescence measurements demonstrated the robustness of this strain as it tolerated extreme outdoor conditions including high (38 °C) and low (10 °C) temperature, and high irradiance (1600 µmol/m2 s). CONCLUSIONS: Selection of native microalgae allows identifying potential strains suitable for use in the production of biofuels. The selected strain Chlorella sp. demonstrated adequate performance to be scaled up to outdoor conditions. Although experiments were performed at laboratory conditions, the methodology used in this paper allows a robust evaluation of microalgae strains for potential market applications.

Selection of native Tunisian microalgae for simultaneous wastewater treatment and biofuel production.

This paper focuses on the selection of native microalgae strains suitable for wastewater treatment and biofuel production. Four Chlorophyceae strains were isolated from North-eastern Tunisia. Their performances were compared in continuous mode at a 0.3 1/day dilution rate. The biomass productivity and nutrient removal capacity of each microalgae strain were studied. The most efficient strain was identified as Scenedesmus sp. and experiments at different dilution rates from 0.2 to 0.8 1/day were carried out. Maximal biomass productivity of 0.9 g/L day was obtained at 0.6 1/day. The removal of chemical oxygen demand (COD), ammonium and phosphorus was in the range of 92-94%, 61-99% and 93-99%, respectively. Carbohydrates were the major biomass fraction followed by lipids and then proteins. The saponifiable fatty acid content was in the 4.9-13.2% dry biomass range, with more than 50% of total fatty acids being composed of saturated and monosaturated fatty acids.

Utilization of secondary-treated wastewater for the production of freshwater microalgae.

In this work, we studied the potential use of secondary-treated wastewater as nutrient source in the production of freshwater microalgae strains. Experiments were performed indoors in a semicontinuous mode, at 0.3 day(-1), simulating outdoor conditions. We demonstrated that all the tested strains can be produced by using only secondary-treated wastewater as the nutrient source. The utilization of secondary-treated wastewater imposes nutrient-limiting conditions, with maximal biomass productivity dropping to 0.5 g l(-1) day(-1) and modifies the biochemical composition of the biomass by increasing the amount of lipids and carbohydrates while reducing the biomass protein content. We measured fatty acid content and productivity of up to 25 %d.wt. and 110 mg l(-1) day(-1), respectively. We demonstrated that all the tested strains were capable of completely removing the nitrogen and phosphorus contained in the secondary-treated wastewater, and while the use of this effluent reduced the cells' photosynthetic efficiency, the nitrogen and phosphorus coefficient yield increased. Muriellopsis sp. and S. subpicatus were selected as the most promising strains for outdoor production using secondary-treated wastewater as the culture medium; this was not only because of their high productivity but also their photosynthetic efficiency, of up to 2.5 %, along with nutrient coefficient yields of up to 96 gbiomass gN (-1) and 166 gbiomass gP (-1). Coupling microalgae production processes to tertiary treatment in wastewater treatment plants make it possible to recover nutrients contained in the water and to produce valuable biomass, especially where nutrient removal is required prior to wastewater discharge.

Effective utilization of flue gases in raceway reactor with event-based pH control for microalgae culture.

This work addresses effective utilization of flue gases through the proper pH control in raceway reactors. The pH control problem has been addressed with an event-based control approach using a Generalized Predictive Controller (GPC) with actuator deadband. Applying this control strategy it is possible to reduce the control effort, and at the same time saving control resources. In the pH process case, the event-based controller with actuator deadband can be tuned to supply only necessary amount of CO2 to keep the pH close to its optimal value. On the other hand, the evaluated control algorithm significantly improves the pH control accuracy, what has a direct influence on biomass production. In order to test the performance of the event-based GPC controller, several experiments have been performed on a real raceway reactor. Additionally, several control performance indexes have been used to compare the analyzed technique with commonly used on/off controller.

Outdoor pilot-scale production of Nannochloropsis gaditana: influence of culture parameters and lipid production rates in tubular photobioreactors.

This work studied outdoor pilot scale production of Nannochloropsis gaditana in tubular photobioreactors. The growth and biomass composition of the strain were studied under different culture strategies: continuous-mode (varying nutrient supply and dilution rate) and two-stage cultures aiming lipid enhancement. Besides, parameters such as irradiance, specific nitrate input and dilution rate were used to obtain models predicting growth, lipid and fatty acids production rates. The range of optimum dilution rate was 0.31-0.351/day with maximum biomass, lipid and fatty acids productivities of 590, 110 and 66.8 mg/l day, respectively. Nitrate limitation led to an increase in lipid and fatty acids contents (from 20.5% to 38.0% and from 16.9% to 23.5%, respectively). Two-stage culture strategy provided similar fatty acids productivities (56.4 mg/l day) but the neutral lipids content was doubled.

Evaluation of carbon dioxide mass transfer in raceway reactors for microalgae culture using flue gases.

Mass transfer of CO2 from flue gas was quantified in a 100m(2) raceway. The carbonation sump was operated with and without a baffle at different liquid/gas ratios, with the latter having the greatest influence on CO2 recovery from the flue gas. A rate of mass transfer sufficient to meet the demands of an actively growing algal culture was best achieved by maintaining pH at ∼8. Full optimisation of the process required both pH control and selection of the best liquid/gas flow ratio. A carbon transfer rate of 10gCmin(-1) supporting an algal productivity of 17gm(-2)day(-1) was achieved with only 4% direct loss of CO2 in the sump. 66% of the carbon was incorporated into biomass, while 6% was lost by outgassing and the remainder as dissolved carbon in the liquid phase. Use of a sump baffle required additional power without significantly improving carbon mass transfer.

Oxygen transfer and evolution in microalgal culture in open raceways.

The mass transfer characteristics of all sections of a 100 m(2) raceway were evaluated. The efficiency of different diffusers was determined dynamically and the most effective was used for steady state system characterisation at water depth 0.2 m and velocity 0.22 m s(-1). Mass transfer coefficients at a gas flow rate of 6 m(3) h(-1) were 164.50, 63.66, 0.87 and 0.94 h(-1) for the paddlewheel, sump, straight and curved channel sections, with associated oxygen transfer rates of 106, 172, 27 and 39 g h(-1). Oxygen supersaturation during algal cultivation led to a reduction in biomass productivity, which was more severe with pure CO2 than flue gas. Simulations showed the energy required to increase mass transfer and reduce oxygen concentrations was more than compensated for by increased biomass and potential energy yields. Oxygen removal is likely to be a critical criterion, and maintenance of mass transfer by sparging may be necessary even when CO2 is not required.

Marine microalgae selection and culture conditions optimization for biodiesel production.

Continuous cultures of Nannochloropsis gaditana, Tetraselmis chuii, Tetraselmis suecica and Phaeodactylum tricornutum were carried out at different dilution rates and culture media in order to check their influence on biomass productivity. N. gaditana attained maximum biomass productivity of 0.49 g/lday at a dilution rate of 0.421/day. The influence of nitrate concentration on biomass productivity was tested by continuous cultures of N. gaditana. At 8.0 mM nitrate and dilution rates ranging between 0.30 and 0.401/day, maximum biomass productivities were achieved. To enhance lipid accumulation, a two-stage culture strategy consisting in a first stage of nitrate-replete conditions followed by a nitrate-depleted phase was performed. The accumulated productivity was 51 mgFATTY ACIDS/l day. Results showed an important change in the fatty acids profile and an increase in the neutral lipids content, representing 73.1% of total lipids. Additionally, the combination of nitrogen depletion and light stress was proved to contribute to lipid enhancement.

Use of secondary-treated wastewater for the production of Muriellopsis sp.

In this paper, the use of secondary-treated wastewater as the culture medium for the production of Muriellopsis sp. microalgal biomass is analyzed. Using this wastewater, a maximum biomass productivity of 0.5 g l(-1) day(-1) was measured, it being only 38 % lower than that achieved using the standard culture medium. Due to the low nitrogen content of secondary-treated wastewater, cultures produced in a medium containing a high percentage of it become nitrate-limited, thus the quantum yield reduces by up to 0.38 g E(-1)--this compares to 0.67 g E(-1) when using a standard culture medium. On the other hand, nitrate limitation enhances the accumulation of lipids and carbohydrates, with values measured at 33 and 66 % dry weight, respectively. It was also demonstrated that secondary-treated wastewater does not have any toxic effect on the growth of Muriellopsis sp. in spite of nitrogen being in the form of ammonium rather than in nitrate. Moreover, the secondary-treated wastewater was depurated when used to produce Muriellopsis sp., with the outlet biological oxygen demand and chemical oxygen demand being lower than at the inlet; the nitrate and phosphate concentrations were zero. Therefore, Muriellopsis sp. production using secondary-treated wastewater allows a reduction in the process cost by decreasing freshwater and fertilizer use, as well as by depurating the water, thus greatly enhancing process sustainability.

Dynamic model of microalgal production in tubular photobioreactors.

A dynamic model for microalgal culture is presented. The model takes into account the fluid-dynamic and mass transfer, in addition to biological phenomena, it being based on fundamental principles. The model has been calibrated and validated using data from a pilot-scale tubular photobioreactor but it can be extended to other designs. It can be used to determine, from experimental measurements, the values of characteristic parameters. The model also allows a simulation of the system's dynamic behaviour in response to solar radiation, making it a useful tool for design and operation optimization of photobioreactors. Moreover, the model permits the identification of local pH gradients, dissolved oxygen and dissolved carbon dioxide; that can damage microalgae growth. In addition, the developed model can map the different characteristic time scales of phenomena inside microalgae cultures within tubular photobioreactors, meaning it is a valuable tool in the development of advanced control strategies for microalgae cultures.

Evaluation of flocculants for the recovery of freshwater microalgae.

The use flocculants on the recovery of freshwater microalgae is studied. Flocculants tested include metal salts, chitosan, and polyelectrolytes used in wastewater treatment processes. Influence of flocculant, but also the doses and biomass concentrations affecting biomass recovery as well as the concentration factor has been evaluated. Results showed that the use of metal salts or chitosan was not efficient, whereas polyelectrolytes allow the efficient recovery of biomass, at doses of 2-25 mg per gram of microalgae biomass. The required doses depend on the particular polyelectrolyte and the freshwater strain present; but cationic polyelectrolytes are generally recommended. The use of polyelectrolytes does not adversely affect water reuse in the production process. The concentration factors obtained are higher than 35 in most cases. Such high concentration factors allow a reduction in the equipment size necessary for biomass dewatering, thus improving the viability of using these microorganisms in biofuel or wastewater processes.

Production cost of a real microalgae production plant and strategies to reduce it.

The cost analysis of a real facility for the production of high value microalgae biomass is presented. The facility is based on ten 3 m3 tubular photobioreactors operated in continuous mode for 2 years, data of Scenedesmus almeriensis productivity but also of nutrients and power consumption from this facility being used. The yield of the facility was close to maximum expected for the location of Almería, the annual production capacity being 3.8 t/year (90 t/ha·year) and the photosynthetic efficiency being 3.6%. The production cost was 69 €/kg. Economic analysis shows that labor and depreciation are the major factors contributing to this cost. Simplification of the technology and scale-up to a production capacity of 200 t/year allows to reduce the production cost up to 12.6 €/kg. Moreover, to reduce the microalgae production cost to approaches the energy or commodities markets it is necessary to reduce the photobioreactor cost (by simplifying its design or materials used), use waste water and flue gases, and reduce the power consumption and labor required for the production step. It can be concluded that although it has been reported that production of biofuels from microalgae is relatively close to being economically feasible, data here reported demonstrated that to achieve it by using the current production technologies, it is necessary to substantially reduce their costs and to operate them near their optimum values.

Biomass and lutein productivity of Scenedesmus almeriensis: influence of irradiance, dilution rate and temperature.

In this paper, the biomass and lutein productivity of the lutein-rich new strain Scenedesmus almeriensis is modelled versus irradiance and temperature. The results demonstrate that S. almeriensis is a mesophile microorganism with an optimal growth temperature of 35 degrees C, and capable of withstanding up to 48 degrees C, which caused culture death. This strain is also tolerant to high irradiances, showing no signs of photoinhibition even at the maximum irradiance essayed of 1625 microE m(-2) s(-1) accumulating up to 0.55% dry weight (d.wt.) of lutein. The optimal conditions that maximise the biomass productivity also favour the lutein productivity, lutein being a primary metabolite. Maximal biomass and lutein productivities of 0.87 g l(-1) day(-1) and 4.77 mg l(-1) day(-1), respectively, were measured. The analysis of light availability inside the cultures, quantified as average irradiance, demonstrates that the cultures were mainly photo-limited, although photosaturation also took place at high external irradiances. The effect of temperature was also investigated finding that the specific maximal growth rate is modified by the temperature according to the Arrhenius equation. The influence of both light availability and temperature was included in an overall growth model, which showed, as a result, capable of fitting the whole set of experimental data. An overall lutein accumulation rate model was also proposed and used in a regression analysis. Simulations performed using the proposed models show that under outdoor conditions a biomass productivity of 0.95 g l(-1) day(-1) can be expected, with a lutein productivity up to 5.31 mg l(-1) day(-1). These models may be useful to assist the design and operation optimisation of outdoor cultures of this strain.

Antioxidant activity of Haematococcus pluvialis cells grown in continuous culture as a function of their carotenoid and fatty acid content.

The influence of culture conditions on the quality of Haematococcus pluvialis biomass is assessed. Continuously grown cells have been characterised with respect to their astaxanthin, fatty acid content, and antioxidant activity and compared with those of non-growing haematocysts. Moderate limitation of nitrate availability (1.7 mM) under continuous growth conditions favoured the production of reddish palmelloid cells whose extracts possessed antioxidant activity equivalent to that of haematocyst extracts, despite the lower astaxanthin content (0.6%d.wt.), which is compensated by a higher fatty acid level (7.6%d.wt.). Green cells produced under nitrate saturation conditions (>4.7 mM) exhibit only 40% antioxidant activity than palmelloid. In addition, the major fatty acid present in palmelloid cells was oleic acid (40%f.a.), whereas, in both green cells and haematocysts, the main fatty acids were myristic, palmitic, and oleic acid (20-30%f.a. each). Biomass extracts were fractionated and analysed. The antioxidant capacity was a function of both the carotenoid and the fatty acid profiles, the antioxidant capacity of astaxanthin diesters fraction being 60% higher than astaxanthin monoesters fraction and twice than free astaxanthin. In such a way, the evaluation of the quality of H. pluvialis biomass must take into account both variables. When considering the production of H. pluvialis biomass for human consumption, special attention should be paid to the one-step continuous system approach for the generation of cells rich in both astaxanthin and fatty acids, as they have high antioxidant activity but without thick hard cell wall.