Fixed vs. Moving Bed Systems: Surface Area and Kinetic Rates

The specific surface area of moving bed media is reported to be roughly 150 ft²/ft³ (500 m²/m³). Brentwood’s fixed bed system, AccuFAS, has a specific surface area of 50-96 ft²/ft³ (165-315 m²/m³). These “nominal” numbers may be somewhat misleading in terms of treatment capacity. Let’s take a closer look:

First, the effective surface area of AccuFAS media remains constant during the operation because of the effective scouring along the entire media surface area. Biofilm in an AccuFAS system is typically very thin (<0.5 mm). On the other hand, biofilm control is less efficient in a moving bed system due to the scouring mechanism. As biofilm grows, effective surface area is significantly reduced. For a 10 mm cylinder MBBR media, a 1 mm thick biofilm can reduce the effective surface area by 33%.

Second, the aeration in an AccuFAS system promotes mixing of wastewater and air through airlift pumping. Enhanced substrate and dissolved oxygen (DO) diffusions to the thin biofilm result in higher kinetic rates. In a moving bed system, the contact between biofilm, DO, and substrate is less intimated due to the mixing mechanism, allowing kinetic rates to be lower.

Therefore, despite the lower “nominal” surface area, AccuFAS can achieve comparable or higher treatment capacity than a moving bed system with similar media volume.