Investigation of anaerobic zone short-circuiting using computational fluid dynamics

Area of Science:

• Environmental Engineering
• Fluid Dynamics
• Wastewater Treatment

Background:

• Anaerobic digestion tanks require optimal flow patterns to prevent solid settlement and ensure efficient treatment.
• Inlet/outlet baffle tanks with impellers are used to enhance mixing and solids suspension.

Purpose of the Study:

• To investigate the flow characteristics within an anaerobic zone using computational fluid dynamics (CFD).
• To evaluate the effectiveness of impellers in preventing solid settlement and maintaining flow circulation.

Main Methods:

• Utilized computational fluid dynamics (CFD) to model turbulent flow using the standard k-epsilon model.
• Employed the multiple rotating reference frame technique for impeller rotation and the Lagrangean discrete phase model for tracking solid settlement.

Main Results:

• CFD simulations showed completely mixed flow behavior in the anaerobic zone, with some degree of short-circuiting.
• Tracer simulation results from the model demonstrated reasonable agreement with experimental measurements.
• Investigated various configurations, confirming that impellers are essential for maintaining circulation and preventing solids from settling.

Conclusions:

• The anaerobic zone exhibits a completely mixed flow pattern primarily due to flow circulation around the central baffle.
• Impellers play a critical role in sustaining flow circulation and preventing solid settlement, thereby enhancing the efficiency of anaerobic digestion.
• Careful consideration of tank design options is necessary to avoid configurations that exacerbate short-circuiting.