Separate effect of turbulent pulsation on internal mass transfer in porous biofilms

Abstract

Turbulence is considered to be the result of coupled time-averaged and pulsating velocities, making it difficult to distinguish the respective effects, and the quantitative effect of turbulent pulsation alone on mass transfer within biofilms has not been discussed in the literature. In this study, we constructed a special oscillating grid biofilm reactor combining Particle Image Velocimetry (PIV) measurements and Computational Fluid Dynamics (CFD) simulations to achieve nearly isotropic turbulence in a designed ambient without time-averaged velocity and shear stress. Subsequently, velocity and contaminant concentration distributions were obtained by solving a mass transfer model with a k-ε turbulence model, combined with measurements of biofilm structure parameters. The results showed that the increase in turbulent pulsation intensity led to a significant stratification of the percolation velocity gradient in biofilms, which enhanced convective mass transfer. The changes of biofilm density and porosity under turbulent pulsation were more strongly correlated with convective mass transfer. When the turbulent intensity (q) increased to 2.50 cm s−1, the removal rate reached the highest value of 96.93%, accelerating the migration of contaminant concentration and the diffusive mass transfer effect was obvious. In addition, the trend of biofilm thickness under turbulent pulsation was consistent with the change of contaminant concentration distribution, and the correlation between them was greater. In summary, at q of 2.50 cm s−1, there was a positive effect on both convection and diffusion mechanisms in biofilms, and the contaminant removal rate and biofilm thickness reached the maximum, which was the recommended turbulent pulsation conditions.