Effect of hydrodynamics on the formation and removal of microalgal biofilm in photobioreactors

Abstract

To prevent the formation of biofilm or to remove already formed biofilm on the transparent walls of the closed cultivation systems, it is important to ensure that there are sufficient wall shear stresses to disrupt the stability of the biofilm. A pilot-scale flat-panel photobioreactor and laboratory tubular system were used to determine the critical value of the wall shear stress. The formation of biofilm on transparent walls of the cultivation systems reduces light penetration into the cultivation medium, which can subsequently reduce the production of microalgae, since light radiation a key parameter influencing the growth of microalgae. To study the influence of hydrodynamic conditions on the prevention of biofilm formation, a numerical model of a flat-panel photobioreactor was validated based on the experimental data. Using the validated model, a critical value of wall shear stress that above which avoids the formation of biofilm was defined to be 0.2 Pa. To refine the values from the numerical model, and to investigate the influence of hydrodynamic conditions on the process of disruption of the formed and stabilized biofilm, a simple experimental tubular system with the one-dimensional flow was developed. For one-dimensional flow, it is easier to describe the hydrodynamic behaviour and more precisely define the parameters influencing biofilm formation. The experiments in this simplified tubular system were focused on removing already formed biofilm. The formed biofilm on the transparent wall was completely removed from the surface when the wall shear stress reached a value of 53 Pa. However, the stability of the biofilm was already disturbed at a value of 6 Pa. The resulting values of critical wall shear stress could be used to select the operating conditions of the cultivation systems or to adjust their geometry affecting the hydrodynamic conditions. According to the velocity distribution based on the measured values, the thickness of the biofilm was less than the thickness of the viscous sublayer. Thus, the prevention of formation, disruption, or even removal of biofilm can be ensured by shear forces in the viscous sublayer.