3.2. Numerical analysis of flow field in optimized CB FSB3.2.1. Axial distribution of phase volume fraction The axial distribution of the solid (silica sand)-phase volume fraction in the cross-section of y = 0 in a laboratory-scale CB FSB was shown in Fig. 4(a). The solid-phase volume fraction was randomly distributed at 0–0.05 m in the axial direction, which can be attributed to the backflow of a small amount of silica sand in the CB FSB. However, at 0.05–0.85 m in the axial direction, the solid-phase volume fraction was constant (0.25). These results indicated that the distribution of water was more uniform in the optimized CBFSB. At a given superficial velocity of 2.71 cm/s, the bed expanded uniformly, demonstrating that it worked as expected and there was no sand-pile which can increase sand friction and decrease biofilm growth. Finally, sand-pile formation reduces the efficiency of aqua-culture wastewater treatment (Summerfelt, 2006). At 0.85–0.88 min the axial direction, the solid-phase volume fraction decreased rapidly and ultimately reduced to zero. These results indicated that the bed was separated into three parts in the axial direction: the dense zone below 0.85 m, the transition zone between 0.85 and0.88 m, and the supernatant liquid zone above 0.88 m, in which only liquid exists. The delamination between solid and liquid was observed as expected. Otherwise, the silica sand would be washed out of the CB FSB by the water. One of the objectives is to control bed expansion as necessary by studying its relationship with the rate of inflow. According to the above-described results, this objective is attainable.