Clarifier Settling Model
In primary and secondary wastewater treatment, sludge must be separated from the water column efficiently for the treatment process to work. The settling behavior of sludge varies with local concentration gradients, the velocity of the surrounding fluid, and the gravitational settling characteristics of the sludge itself. Clarifier settling behavior and efficiency affects everything from tank residence time and effluent quality to aeration demand and overall plant capacity.
The Clarifier Settling Model in FLOW-3D HYDRO simulates the transport and settling of sludge in septic tanks, clarifiers, secondary settling tanks, and other wastewater treatment equipment. It accounts for the three-way interaction between gravitational settling, advection with the water flow, and diffusion, giving engineers a spatially resolved picture of how sludge distributes and settles through the treatment process.
Sludge concentration (kg/m3) in a two-chamber settling tank at t = 160 seconds, showing accumulation at the bottom of the primary chamber and early settlement in the secondary chamber.
How the Model Works
Sludge is treated as a scalar quantity transported within the water-sludge mixture. Its motion is governed by three components: advection with the bulk fluid, and gravitational settling, and diffusion. The settling velocity is a function of the local sludge mass concentration – as concentration increases, settling slows due to particle-particle interactions, until a maximum settled concentration is reached. At that threshold, settling stops entirely, avoiding over-concentration of settled sludge, although the sludge can still be advected by the fluid. Diffusion accounts for both the molecular diffusion of the suspended sludge in water and turbulent diffusion.
Two methods are available for defining the settling velocity as a function of concentration. The Vesilind equation uses two calibrated parameters – maximum settling velocity and a concentration decay parameter – to describe the settling rate across the full concentration range. Alternatively, the relationship can be defined directly from laboratory settling test data as a tabular function, giving engineers full flexibility when measured data is available.
Modeling Capabilities
The Clarifier Settling Model in FLOW-3D HYDRO computes:
- Spatial distribution of sludge mass concentration throughout the tank at every time step
- Settling velocity as a function of local sludge concentration, using either the Vesilind equation or a user-defined tabular function
- Advection of sludge with the bulk water-sludge mixture velocity
- Turbulent and molecular diffusion of sludge within the flow field
- As settled sludge reaches a concentration threshold, the thickness of the settled sludge layer increases. Settled sludge is still advected by the fluid
- Suspended sludge at inflow boundaries, is defined as a constant or time-varying concentration
- Total sludge mass as a history output for monitoring sludge accumulation over time
- Sludge mass concentration, settling velocity, and settling mass flux as spatial outputs for postprocessing
Applications
The Clarifier Settling Model is used in FLOW-3D HYDRO for:
- Predicting sludge accumulation zones in primary and secondary clarifiers to optimize tank geometry and baffle placement
- Evaluating the effect of inlet flow rate and sludge loading on settling performance and effluent quality
- Evaluating sludge flushing efficiency
- Assessing short-circuiting in settling tanks where flow bypasses the intended settling zone
- Designing and optimizing septic tanks, Imhoff tanks, and other decentralized wastewater treatment systems
- Investigating the resuspension of settled sludge during flow surges or peak loading events
- Supporting regulatory compliance assessments by predicting settled sludge volumes and effluent concentrations