coALescence and biofiLm in thIn GAp phoTObioreactoR – ALLIGATOR

Experiments are carried out in a confined cell of great extension adapted to the study of a swarm of bubbles in interaction. They consist in filming with a fast camera the evolution of the population of bubble sizes when the coalescence is not blocked. Tests are carried out in water, in a solution of non-Newtonian fluid and then in viscous fluid. Specific post-processing of image series is developed in order to extract useful information for modeling coalescence mechanisms.
In the task devoted to the study and control of biofouling, the adhesion rates of two species of microalgae of industrial interest are measured online in order to test the bubbling conditions allowing adhesion to be delayed. With the optical coherence tomograph (OCT), tests aiming at evaluating the impact of hydrodynamic conditions on the dynamics of biofilm formation of model microalgae are carried out.
For the study at the scale of the reactor, first in cold flow and then in intensified culture, a 2 mm gap photobioreactor in PMMA was designed and built . The overall behavior of this reactor is characterized from the hydrodynamic and gas-liquid transfer point of view for several model fluids: water (low viscosity Newtonian fluid), aqueous solution of glycerol at 50% by mass (viscous Newtonian fluid) and aqueous solutions of xanthan gum at 0.5 and 1 gL-1 (non-Newtonian fluids of increasing viscosity). Thus the respective influences of confinement and rheological properties of the liquid phase (representing increasing concentrations of microalgae cultures) can be highlighted.

Post-treatments of the images of coalescence in bubble swarms have been carried out for water as the liquid phase, they allowed developments of coalescence phenomena modeling. These results led to the submission of an article to Journal of Fluid Mechanics.

Regarding biofouling, the results show that the adhesion rate is highly dependent on the algal concentration and the type of strain under study. In situ observations resulting from the coupling of confocal laser scanning microscopy (CLSM) and OCT show a strong spatial heterogeneity of the biofilm, certainly associated with the different local hydrodynamic conditions. Furthermore, the first tests demonstrate an improvement in the transmission of light within the reactor for the highest gas flow rates applied. This suggests the interest of the approach proposed in this project to use bubbling to minimize the development of biofilm on the walls of the photobioreactor.

The hydrodynamics and gas-liquid mass transfer results at the global scale show, interestingly, that at a given superficial gas velocity, the void ratio is higher in viscous or non-Newtonian fluid than in water and in a column with a small air gap than in a traditional column. The mixing time is on the other hand significantly increased by the viscosity and the confinement. Finally, contrary to what was expected, the overall gas-liquid transfer coefficient is lower in a column with a small air gap compared to conventional columns, and it also decreases with the degree of confinement. The increase in viscosity and the non-Newtonian character have little influence on the global gas-liquid transfer.

Post-processing of bubble swarm images obtained in non-Newotnian fluid still needs to be done. On the other hand, the observation of the original behavior of bubble swarms in non-Newtonian fluid, led us to carry out complementary experiments to characterize the dynamics of isolated bubbles. The associated video recordings will also need to be processed and analyzed.

Now that the biofilm imaging tools (acquisition and processing) have been implemented and validated, the study of the bubbling conditions allowing adhesion to be delayed will be tested. The characterization of the local shear rate will soon be carried out by the GEPEA in collaboration with the LGPM in order to validate the hypothesis that the bubbling influences the nature of the biofilm. In this context, other bubbling conditions (frequency, flow) will be studied later in order to propose an optimized cleaning strategy.

Local ombroscopy and PIV studies will complement the study at the reactor scale, and wall shear stress will be measured to better quantify its influence on the possible development of biofilm. Finally, cultures will be carried out under conditions allowing to limit the coalescence of the bubbles and to limit the bifouling to validate the whole of the approach.

Submitted : J. Ruiz-Rus, P. Ern, V. Roig, C. Martinez-Bazan, Inertial self-induced collision and coalescence in a swarm of high-Re confined bubbles, submitted to Journal of Fluid Mechanics, January 2022

FANESI, Andrea, LAVAYSSIÈRE, Marc, BRETON, Cyril, et al. Shear stress affects the architecture and cohesion of Chlorella vulgaris biofilms. Scientific Reports, 2021, vol. 11, no 1, p. 1-11.

RUIZ RUS, Javier, ERN, Patricia, ROIG, Véronique & MARTINEZ BAZAN, Carlos, Exploration of coalescence mechanisms in confined bubble swarms, Dispersed Two-Phase Flows – SHF, 12-14 October 2021

FANESI, Andrea, MARTIN Thierry, BRETON, Cyril, and LOPES Filipa, Bubbling for light: microalgae biofilm control in PBRs. Algaeurope 7-10 December 2021 (Oral presentation, Online).

Photobioreactors dedicated to the culture of microalgae or cyanobacteria are a promising technology for numerous applications such as production of high added-value products, of bioenergy or for CO2 capture.
However, technological advances are still required to reduce production costs and environmental impacts and to increase energy efficiency. In this context, intensification of performances via an increase of culture concentration for a given illuminated surface represents a promising way to achieve optimized and eco-efficient production. This is why project ALLIGATOR aims at developing a new concept of intensified photobioreactor (PBR). The proposed configuration consists in a thin-gap bubble column expected to enhance gas-liquid mass transfer and to favor mixing, while allowing high biomass concentration and ensuring light availability on the whole reactor thickness. The problems of coalescence and biofilm formation constitute a major challenge for the development of this technology and will be specifically addressed in the project.
The program involves complementary studies at the local and reactor scales. Fundamental and original scientific developments are expected: (i) scientific knowledge about the complex two-phase hydrodynamics and gas-liquid mass transfer in thin-gap bubble column, this innovative intensified gas-liquid reactor being of interest for other applications than microalgae culture, (ii) better physical understanding of bubble coalescence mechanisms in non-Newtonian media and of the influence of the rheology on the 2D dynamics of a bubble swarm, (iii) deep knowledge on microalgae biofilm dynamics with a focus on the influence of two-phase hydrodynamics, which constitutes a breakthrough for PBR development and will contribute to increase the scientific knowledge on photosynthetic biofilms which are poorly studied.
Technologically, several advances are also expected: (i) a new technology of 2D confined PBR will be tested and developed at the laboratory scale, with the aim of significant gain in terms of productivity, (ii) scale-up rules will be developed to predict key information such as bubble residence time, mixing performance, gas-liquid mass transfer, wall shear stress, break-up and coalescence rates, (iii) recommendations for the design of such reactors will be deduced, (iv) conditions to limit biofilm formation and coalescence will be provided.
By joining academic skills in Fluid Mechanics (IMFT), Biofilm/Biofouling (LGPM) and PBR Engineering (GEPEA), this collaborative project will explore the feasibility of a new confined PBR and will improve physical understanding and prediction of bubbly flows and microalgae biofouling.