Two-phase flow modelling for oxygen renewal estimation in vertical flow filter: luxury or necessity?

Area of Science:

• Environmental Engineering
• Fluid Dynamics
• Wastewater Treatment

Background:

• Effluent transfer and degradation modeling in Vertical Flow Sand Filters (VFSF) and Vertical Flow Constructed Wetlands (VFCW) are of increasing interest.
• Current monophasic modeling software often neglects air phase influence, limiting accuracy in unsaturated zones.
• Previous hydrological studies highlighted limitations of ignoring air movement, but this is less explored in vertical flow filter modeling.

Purpose of the Study:

• To describe complex air and water flows in intermittently fed vertical flow filters.
• To investigate the impact of two-phase flow modeling on understanding filter performance.
• To analyze the influence of ponding on flow behavior and oxygen transfer.

Main Methods:

• Utilized a two-phase flow model incorporating both numerical simulations and experimental results.
• Analyzed intermittent feeding conditions in vertical flow filters.
• Compared numerical predictions with experimental data on oxygen consumption.

Main Results:

• Observed distinct behaviors depending on whether ponding occurred.
• Identified air entrapment as a key factor during ponding, reducing infiltration speed.
• Demonstrated that air entrapment leads to water retention at the sand-drainage layer interface.
• Studied the relationship between hydraulic load and oxygen income by convection.

Conclusions:

• Two-phase flow modeling provides a more comprehensive understanding of vertical flow filter dynamics than monophasic models.
• Air entrapment significantly affects effluent transfer and oxygen availability in VFSF and VFCW systems.
• Accurate modeling of air-water interactions is essential for optimizing wastewater treatment processes in these systems.