Exploring optical properties and radiation transfer in a mixed culture of purple phototrophic bacteria grown in a flat-plate photobioreactor via combined experimental and modelling approach

The optimization of radiation transfer in a mixed culture of purple phototrophic bacteria (PPB) inside a photobioreactor (PBR) is crucial for maximizing growth efficiency and resource recovery. In this study, the optical behavior of PPB mixed cultures was characterized from 300 nm to 1100 nm at different incident light intensities of 10 W center dot m- 2 , 20 W center dot m-2, 40 W center dot m-2, and 60 W center dot m- 2 , and at different biomass concentrations. Experimental data were processed via empirical and Monte Carlo methods to determine the optical properties. Various models, namely the Beer-Lambert law, the two-flux approximation model, and computational fluid dynamics (CFD) simulations, were then applied to describe the radiation transfer inside the PBR. Among the most relevant results, the adaptation of absorption peaks as a function of light intensity was observed. Moreover, the scattering effect was found to be non-negligible, characterized by a strong non-isotropic nature. Although the scattering effect was significant, the results showed that the Beer-Lambert law can effectively describe the light attenuation profiles, resulting in irrelevant errors in calculating the light intensity at each layer of the PBR compared to the two-flux approximation model and CFD simulations, in turn characterized by higher computational costs. However, simpler models could lead to higher values for the local volumetric rate of photon absorption along different layers, especially at lower concentrations. Finally, the results suggest that flat-plate PBRs illuminated from both sides can be an effective solution for PPB systems compared to raceway ponds, mainly due to the enhanced two-sided short-path light penetration.