Fig. 4

Changes of dissolved oxygen in the biofilm with biofilm thickness, COD = 400 mg/L. The changes of DO along the biofilm depth under three wall-shear stresses were showed. Under three conditions, the DO fell along the biofilm depth and eventually decreased to zero. At the wall-shear stressed of 0.8, 1.29 and 2.0 Pa, the biofilm thicknesses where the dissolved oxygen reduced to zero were 2050, 1850 and 1450 μm, respectively. It revealed that the reduction rate of dissolved oxygen was positively correlated with the wall-shear stress