“Experimental method to evaluate the optical properties of microalgae suspensions”

JOSUE MIGUEL HEINRICH; ALEJANDRO RAUL TROMBERT; FAUSTO ADRIAN BOTTA; IGNACIO NIIZAWA; HORACIO ANTONIO IRAZOQUI

Praga

Congreso; “19th International Congress of Chemical and Process Engineering CHISA 2010 / 7th European Congress of Chemical Engineering ECCE-7”; 2010

There exists a high diversity of photosynthetic microorganisms which are potentially valuable for use in the food, cosmetics, pharmaceutical, agricultural and aquiculture industries (Spolaore et al., 2006). Fully closed photobioreactors (PBR) provide opportunities for axenic culture of a greater variety of algae than is possible in open systems. Microalgae provide all the benefits of plants coupled with high productivities associated with large scale microbial production approaches. Microalgae have the same basic photosynthetic mechanism as higher plants. As they are of simple structure -unicellular, filamentous or colonial-, energy is directed into photosynthesis, growth, metabolite production and reproduction rather than maintaining differentiated structures such as leafs and stalks. Furthermore, given that microalgae are microscopic in size and grow in liquid culture, nutrients can be maintained at or near optimal conditions potentially providing the benefits of high levels of controlled, continuous productivity similar to microbial fermentation (Walker et al., 2005) However, microalgae are still not a well-studied group from a biotechnological point of view (Spolaore et al., 2006). Mass culturing of microalgal species have been variously explored in the treatment of wastewater and control of water pollution, for atmosphere regeneration in biospheres (ie, spacecraft), as renewable fuels for transportation (biodiesel), as a source of high value natural health products (nutriceuticals) and lately in the mitigation of greenhouse gases and the production of hydrogen as a fuel source. Biomass productivity in any culture system depends on the degree to which the culture conditions match the requirements of the selected strain. Because mineral nutrient limitation is easily avoided in microalgal mass culture, light availability inside the PBR and temperature are the main factors that determine productivity. Once the temperature is suitably controlled, light availability becomes the only limiting factor. In an optimal system where no other factors limit, light availability determines the rate of photosynthesis and productivity. However, excessive light can be harmful and is known to produce a photoinhibitory response. In a well-mixed microalgal mass culture system, light attenuation by biomass gives rise to a heterogeneous illumination profile inside the culture bulk for which mathematical evaluation is essential to estimate the average irradiance on which the growth of the microalgae depends. The evaluation of the radiation field inside a PBR constitutes a central step in the study of photoassisted, photochemical and photocatalytic reactions. In PBRs with microalgae, the complexity of this task lies in the simultaneous existence of radiation absorption (by microalgae’s pigments) and scattering (by microalgae and gas bubbles). A rigorous approach to obtain the spatial and directional distributions of radiation intensities is the application of the Radiative Transfer Equation (RTE) to the system under study (Özisik, 1973). The RTE or “photon balance” can be written in terms of the monochromatic radiation intensity : (1) where is the coordinate in the sense of propagation of the beam in a participate medium (absorbent and dispersant). This equation considers the phenomena of radiation absorption, “out-scattering” (loss of radiant energy by dispersion) and also“in-scattering” (income of radiant energy by dispersion). To solve the RTE, three optical properties of the biological suspensions are required: the volumetric absorption coefficient ( ), the volumetric scattering coefficient ( ), and the phase function for scattering p. In the present work, a novel experimental method to measure the optical properties of aqueous microalgae suspensions is proposed. The method involved diffuse reflectance and transmittance spectrophotometric measurements of the algae suspensions, measurements of the energy flux transmitted through the microalgal suspension in different directions (angular measurements), the evaluation of the radiation field in the sample compartment, and the application of a genetic algorithm to adjust the model predictions to the experimental data in order to obtain the former parameters. Diffuse reflectance and transmittance were carried out in a spectroradiometer Optronic OL 750, equipped with an integrating sphere OL 740-70. Angular measurements were carried out in an experimental device constructed ad hoc. This Vis radiation emission-transmission-measuring apparatus o Vis-RETM apparatus allow to measure the transmitted energy flux in different directions through the microalgae suspension. Two different microalgae were investigated: Scenedemus quadricauda (from Culture Collection of Algae and Protozoa, UK) and Chlorella sp (kindly provided by Dr. A.M. Gagneten, FHUC, UNL). The absorption and scattering coefficients were estimated as specific properties (per unit microalgae mass concentration). The optical properties were determined from measurements of very small volumes of uniform microalgae suspensions in order to relate these values with the corresponding concentration; then, knowing the spatial and temporal variation of the microalgae concentration in a real photobioreactor, the reported optical properties can be readily used to compute the photon absorption rate.