1.Biological Activity: Increasing the fill ratio increases the surface area available for biofilm growth, which can enhance the system's biological treatment capacity. This is because a larger surface area provides more space for microorganisms to colonize and degrade pollutants.

2.Oxygen Transfer: The fill ratio can impact the oxygen transfer efficiency within the reactor. A very high fill ratio might impede the water flow and reduce the oxygen transfer rate, as the dense packing of media could limit the movement of water and air bubbles through the reactor. Conversely, too low a fill ratio might not provide enough surface area for efficient biological treatment.

3.Shear Stress: Biofilm thickness and health are influenced by shear stress, which is affected by media movement and reactor hydrodynamics. An optimal fill ratio helps maintain a balance where the biofilm is neither too thick (which can cause detachment due to excessive weight or internal anoxic conditions) nor too thin (which could reduce treatment efficiency).

4.Hydraulic Retention Time (HRT) and Solids Retention Time (SRT): The fill ratio affects both HRT and SRT, which are critical for the treatment process. A higher fill ratio can potentially reduce the HRT by increasing the system's treatment capacity but also increase the SRT by allowing more biomass to be retained in the system. The balance between these factors is crucial for achieving desired treatment outcomes.

5.Shock Loading Capacity: A higher fill ratio can provide resilience against shock loadings of high organic or hydraulic loads, as the increased biomass can absorb and process larger amounts of pollutants without significant performance drops.

6.Clogging and Maintenance: Higher fill ratios increase the risk of clogging and may require more frequent maintenance. As the amount of biofilm and the potential for debris entrapment grow, ensuring proper aeration and movement to prevent dead zones becomes more challenging.