2.1. Experimental set-up
A schematic diagram of the recirculating system used to treat synthetic aquaculture wastewater is presented in Fig. 1. Synthetic aquaculture wastewater was pumped from the tank to the CB FSB through the main pipeline, while the bypass was used to control the inflow of CB FSB. Water was injected tangentially into an annular space. This continuous water flowing through the annular space created strong water rotation. The water then entered the FSB vessel through a slot at the base of the bed and around the circumference of the vessel. An inverted cone, incorporated into the center of the floor of the vessel, was used to rotate the upward-flowing water, which improved the bed expansion.1At low superficial velocity, the quartz silica sand (sieve analysis shown in Fig. 2) remained stationary at the base of the vessel, and water flowed upward through pores without moving the sand. How-ever, at higher superficial velocity, especially when the drag forces exceeded the down gravity forces on the sand grains, the particles were lifted and fluidization occurred. The dissolved waste in the water was absorbed and transformed by microbes attached to the surface of sand. Effluent from the CB FSB flowed into the sedimentation tank and then recirculated to the tank.