F.G. Acién

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.

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.

Recovery of Lutein from Microalgae Biomass: Development of a Process for Scenedesmus almeriensis Biomass

In this work an optimized method for the extraction of lutein from microalgae biomass is presented. It has been developed using dry biomass of the lutein-rich microalga Scenedesmus almeriensis. The method comprises three steps, cell disruption, alkaline treatment, and solvent extraction, and renders a carotenoid extract rich in lutein. The results demonstrate that cell disruption is necessary and that the best option among the treatments tested with regard to industrial applications is the use of a bead mill with alumina in a 1:1 w/w proportion as disintegrating agent for 5 min. With regard to the alkaline treatment, the optimal conditions were obtained using 4% w/v KOH with a biomass concentration of 100 g/L for 5 min. Longer alkaline treatments or the use of higher KOH concentrations reduced the yield of the process. Finally, extraction with hexane is optimized. Using a 1:1 ratio hexane to sample volume, a total of eight extraction steps are necessary to recover 99% of lutein contained in the processed biomass. However, the optimal number of extraction steps is six, 95% of the lutein being recovered. In summary, the developed method allows the efficient recovery of lutein from microalgae biomass, it being a scaleable and industrially applicable method.

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 systems 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.

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.

The oxygen evolution methodology affects photosynthetic rate measurements of microalgae in well‐defined light regimes

Designing photobioreactors correctly is a must for the success of microalgal mass production. Optimal photobioreactor design requires a precise knowledge of photosynthesis dynamics in fluctuating light conditions and hence a method for the measurement of photosynthetic rates in specific light regimes. However, it is not uncommon in literature that experimental protocols used to obtain oxygen generation rates are described ambiguously and the reported rates of photosynthesis vary widely depending on the methodology. Additionally, quite a number of methods overlook certain aspects that can affect the estimated rates significantly, and can therefore affect photobioreactor design. We have developed a method based on oxygen evolution measurements that accurately determines photosynthetic rates under well‐defined light regimes. Our experimental protocol takes into account most of the issues that can affect the rates of oxygen generation, such as depletion of nutrients during the measurements and precision of the measurements. We have focused on the basic applications in photobioreactor design and used a dynamic model of photosynthesis to analyze our results and compare them with available published data. The results suggest that our oxygen evolution method is consistent. Biotechnol. Bioeng. 2010;106: 228–237.

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.

Wastewater treatment using microalgae: how realistic a contribution might it be to significant urban wastewater treatment?

Microalgae have been proposed as an option for wastewater treatment since the 1960s, but still, this technology has not been expanded to an industrial scale. In this paper, the major factors limiting the performance of these systems are analysed. The composition of the wastewater is highly relevant, and especially the presence of pollutants such as heavy metals and emerging compounds. Biological and engineering aspects are also critical and have to be improved to at least approximate the performance of conventional systems, not just in terms of capacity and efficiency but also in terms of robustness. Finally, the harvesting of the biomass and its processing into valuable products pose a challenge; yet at the same time, an opportunity exists to increase economic profitability. Land requirement is a major bottleneck that can be ameliorated by improving the system’s photosynthetic efficiency. Land requirement has a significant impact on the economic balance, but the profits from the biomass produced can enhance these systems’ reliability, especially in small cities.

Economics of Microalgae Biomass Production

Abstract Microalgae have been proposed as the potential source for a wide range of products, ranging from fine chemicals and pharmaceuticals to foods and feeds and as a biofuel source. However, only a few are successful on an industrial scale due to the high cost associated with the production of microalgae biomass. This chapter presents an economic analysis of various processes based on microalgae: from high-value products like astaxanthin to low-value products like biofuels. The target is to identify major factors contributing to production costs as a tool for the improvement of existing processes but also to assist in the design of new, more reliable processes. The results of our analysis demonstrate that for the production of high-value compounds, high-energy-demanding processes can be carried out, including with low productivity, whereas for the production of low-value compounds it is mandatory to minimize power consumption and improve productivity. More precisely, biomass productivity is the major factor impacting the unit production cost, whatever the product; therefore, to optimize the productivity of microalgae production systems, it is mandatory to improve the economic viability of microalgae-based processes.

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.