Effect of Expansion Ratio on Biofilm Performance in an Aerobic Fluidized Bed

Biofilm performance at various bed expansion ratios was investigated in an aerobic biofilm fluidized bed. As the bed expansion was increased from 20 to 35%, bioparticle collision resulted in the decrease of biofilm thickness. However, shear stress became the dominant factor that resulted in the gradual decrease of biofilm thickness at high expansion ratios ranging from 65% to 90%. In general, biofilm thickness at the current study varied only slightly, remaining at a low level of 25±±5μm within the range of bed expansion from 20 to 90%, which is attributed to the characteristics of the spherical activated carbon particles. Moreover, there was no obvious change on the microbial count at low expansion ratios, with the bacteria count decreasing only gradually after the expansion ratio exceeded 65%. Although COD removal rate decreased slightly at high bed expansion ratio of 90%, up to 74% COD removal rate was still maintained at a short hydraulic retention time (HRT) of 45 min, which is attributed to a better nutrient mass transfer rate across the thin biofilm in the bioreactor. The axial biofilm thickness profile reflects a balance between the decreasing shear stress, and depleting nutrient concentration along the bed height along the height of the bed.