Fixed vs. Moving Bed Systems: Aeration and Mixing Efficiency

The aeration design for a biofilm system, such as moving bed or AccuFAS fixed-film systems, is more complex than a standard activated sludge system. In a biofilm system, the aeration design must satisfy the oxygen demand for required treatment, provide mixing, and control biofilm thickness.

In an AccuFAS system, diffusers are installed beneath each media tower, forming an airlift pumping mechanism during operation. This design promotes continuous mixing of wastewater, air, and biofilm. The airflow from underneath the media tower also acts as an “air knife,” effectively maintaining a thin biofilm along the media surface for optimum treatment. Due to the high-efficiency mixing design of the AccuFAS system, the process air requirement for treating pollutants is typically higher than the mixing requirement. In other words, no additional energy is wasted for mixing. High-efficiency fine bubble diffusers are typically used in combination with AccuFAS, which further improves the energy efficiency of the system.

The airlift pumping design of the AccuFAS system.

In a moving bed system, aeration needs to be designed to mix the floating media (i.e., keep the media uniformly distributed), control biofilm thickness, and provide process air. A high mixing air is typically required for media mixing. In addition, the biofilm control in a moving bed system can be less effective than that of the AccuFAS system because of the scouring mechanism, resulting in a higher mixing air to maintain a thinner biofilm. Moving bed systems typically use medium-coarse diffusers, and therefore, consume more energy for the same level of treatment.