Flocculation Calculator

Velocity gradient equals square root of power over viscosity times volume

Solution

—
Share:

How It Works

Flocculation gently stirs chemically treated water so that tiny destabilized particles collide and stick together into larger clumps called flocs. The velocity gradient G quantifies the shear intensity -- too low and particles rarely collide, too high and fragile flocs break apart. Designers also calculate the power needed by paddle mixers using fluid density, drag coefficient, and blade velocity.

Typical G values for flocculation range from 20 to 80 s−1, with detention times of 20–40 minutes, giving G×t products of roughly 10,000–100,000.

Example Problem

A flocculation tank has 150 W of power input, a volume of 50 m³, and the water viscosity is 0.001 Pa·s. What is the velocity gradient?

  1. G = √(P / μV) = √(150 / (0.001 × 50))
  2. G = √(3,000,000) ≈ 1,732 s−1

This is well above typical flocculation ranges, suggesting the power input should be reduced or the tank enlarged.

Frequently Asked Questions

What is the velocity gradient in flocculation?

The velocity gradient G measures the intensity of shear mixing in a flocculation basin. It is calculated as the square root of power input divided by viscosity and volume, expressed in s−1. Values of 20–80 s−1 are typical for gentle flocculation.

What is the G times t value for flocculation?

G×t is a dimensionless product of velocity gradient and detention time that characterizes the total mixing energy applied. For water treatment flocculation, G×t typically ranges from 10,000 to 100,000 to ensure adequate particle collisions without floc breakup.

How does paddle speed affect flocculation?

Faster paddles increase the velocity gradient and collision rate, promoting rapid floc growth. However, excessive speed can shear apart delicate flocs. Paddle tip speeds are usually kept between 0.3 and 0.9 m/s in practice.

Related Calculators