Solving the World’s Toughest CFD Problems

# Sludge Settling Model

Accurately estimating the amount of sludge reaching the aeration tank is important for the efficient operation of a wastewater treatment plant. FLOW-3D’s sludge settling model can simulate the settling dynamics of sludge in the primary and secondary processes of a wastewater treatment plant, offering wastewater engineers the tools for better analysis and more efficient design.

Sludge wastewater dynamics are dependent on three factors:

1. The rate of change in the concentration of the sludge
2. The advection of the sludge based on gravity and the velocity of the sludge-water mixture
3. Diffusion of sludge in the wastewater

FLOW-3D‘s sludge settling model takes all three factors into account to accurately estimate the sludge settling in a wastewater treatment plant.

Intuitively, settling velocity of the sludge decreases as the concentration gradient of the sludge increases. A threshold is reached when the concentration of sludge is high enough that no more sludge can settle, and therefore the settling velocity becomes zero. This threshold can be based on the Vesilind equation [1], or can be manually defined by the user.

Simulation result of modeled sludge settling in a septic tank.

In this simulation, wastewater flows into the tank from the inlet on the left for eight seconds with a sludge concentration of 5 kg/m3. The threshold concentration is specified as 10 kg/m3 and can be found in the fully settled regions at the bottom of the tank. The Vesilind equation is used to estimate the settling velocity. Most of the sludge stays in the main chamber of the tank, with a small amount of it entering the secondary settlement chamber through the hole in the separation wall. The initially settled sludge is disturbed by the incoming sludge, but then resettles.

### Reference

[1] Vesilind, P.A., 1968, Design of prototype thickeners from batch settling tests, Water Sewage Works 115(7), pp. 302-307.