Membrane bioreactor sludge rheology at different solid retention times

Area of Science:

• Environmental Engineering
• Biotechnology
• Fluid Mechanics

Background:

• Rheological characterization is vital for efficient sludge management, influencing dewatering, stabilization, and handling operations.
• Sludge retention time (SRT) is a key operational parameter affecting biomass properties in biological wastewater treatment, especially in membrane bioreactors (MBRs).

Purpose of the Study:

• To compare the rheological behavior of biomass in an MBR operated under varying SRTs.
• To establish correlations between apparent viscosity, solid concentration, and shear rate for MBR biomass.
• To evaluate and compare common rheological models for sludge simulation.

Main Methods:

• A bench-scale MBR was operated for four years with controlled SRTs ranging from 20 days to complete sludge retention.
• Rheological properties were measured over time, correlating apparent viscosity with mixed liquor suspended solids (MLSS) at equilibrium.
• Three common rheological models (Bingham, Ostwald) were evaluated using mean root square error (MRSE).

Main Results:

• The Bingham model generally provided slightly better fitting results than the Ostwald model based on MRSE.
• A strong correlation was found between the parameters of the Ostwald model across all tested SRTs.
• The relationship between apparent viscosity, shear rate, and solid concentration was determined and proposed.

Conclusions:

• The Ostwald model is proposed as a more convenient tool for determining sludge apparent viscosity due to the strong correlation between its parameters.
• Understanding sludge rheology under different SRTs is crucial for optimizing MBR operations and sludge management strategies.