Fixed vs. Moving Bed Systems: Basin Geometry

Aeration basins can have length to width (L:W) ratios varying from 1:1 (square) to over 30:1 (plug flow). The L:W ratio of an aeration basin directly impacts both the physical installation and biological performance of a submerged fixed-film system.

Physical Installation
Moving bed biomass carriers are free to move about the aeration basin. The aeration system provides energy to keep the biomass carriers suspended and in circulation. Due to the movement and effects of flow through the tanks, the distribution of biomass carriers is typically not uniform. Rather, the carriers have a tendency to bunch up on the outlet end of the tank. To minimize this and keep the carriers well-distributed, many manufacturers have a maximum L:W ratio of 2:1 for an aeration basin. Tanks with greater L:W ratios typically require multiple retention screens, at significantly higher costs, and with increased headloss. (The impact of media type on headloss in the tank will be addressed in a future post).

Fixed, structured-sheet systems (like Brentwood’s AccuFAS system) have no limitation on the maximum length to width ratio. The only provisions that may be necessary are for square tanks or tank geometry situations with a very low length to width ratio (2:1 or less) so that flow entering the tanks can be evenly distributed along the full width of the tower. This can easily be accomplished at minimal cost using pipe fitting or a channel with multiple openings into the aeration basin.

Biological Treatment
Submerged fixed-film systems are typically used to enhance nitrification by providing a surface to support the growth of nitrifying bacteria.

In our experience, when the fixed AccuFAS system is installed in a long (over 6:1), plug flow configuration, it allows for optimal biomass growth on each tower. As flow passes down the basin, the first towers will primarily support a biomass consuming carbon (BOD), while the following towers support nitrification for the removal of ammonia.

Without multiple partitions or retaining screens in a moving bed system, this is not possible as the biomass carriers should be moving the full length of the basin.


accufas basin geometry
AccuFAS system installed in an aeration basin with four channels, each 47 feet long by 6 feet wide. Overall length to width ration: 33:1.

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