Fixed vs. Moving Bed Systems: Media Retention and Headloss

Submerged fixed-film media is utilized within aeration basins to optimize biological treatment and increase capacity. Therefore, proper design and installation of the equipment used to keep the media in the aeration basin and prevent it from passing downstream is critical to system function.

Physical Restraint of Submerged Media
Moving bed systems typically use submerged stainless steel screens to retain the media within the aeration basin. These are designed to prevent the media from passing while allowing wastewater to exit. Most also include an air scouring system to clean the screens as needed.

Retention screens need to be designed to pass the maximum hydraulic flow the plant may experience. Higher flows may result in flooding or overflow of the aeration basin and loss of media. The air scouring systems also typically include tank level measurement and electronic controls to activate the scour system when flow through the screens decreases.

Media retention is simple with fixed systems, like Brentwood’s AccuFAS: media blocks are secured to the floor of the tank using a specially designed restraint system. The structured-sheet media remains fixed in place while the wastewater being treated is pumped past the media using the airlift action of the diffusers. (The outlet of the tank requires a weir to maintain the liquid level, as with activated sludge systems). With fixed bed systems, media loss is not possible.

Hydraulics
Special attention needs to be paid to the hydraulic grade line when retrofitting a submerged fixed-film system into an existing wastewater treatment facility. The hydraulic grade line will be lowered by the media retention screens in moving bed systems. Typical losses are 50 to 150 mm (2 to 6 inches) for each screen.

Structured-sheet media modules will not change the hydraulic profile of an existing aeration basin.