What is the ideal GT value for flocculation?

The product of velocity gradient (G) and detention time (T), known as the GT value, should typically be between 10,000 and 100,000 for effective flocculation (Clause 5.2).

What is tapered flocculation?

It is a process of applying a decreasing energy level (velocity gradient) in a series of compartments to allow large, settleable flocs to form without being broken apart by excessive turbulence (Clause 5.3).

What is the typical detention time in a flocculator?

The detention time (T) generally ranges from 10 to 30 minutes, depending on the type of water, temperature, and coagulant used (Table 1).

What is the recommended velocity for paddle flocculators?

The peripheral velocity of the outermost paddles should be between 0.15 and 0.6 m/s (Clause 5.2.2).

IS 7208

: 1992

Guidelines for flocculator devices

CurrentSpecializedGuidelinesBIMWater Resources · Public Health Engineering

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This standard provides guidelines for the design and construction of flocculator devices used in water treatment plants. It covers different types of flocculators, such as mechanical and hydraulic, and specifies key design parameters like velocity gradient (G), detention time (T), and power requirements to ensure effective floc formation.

Guidelines for flocculator devices

Quick Reference Values

Recommended Velocity Gradient (G)10 - 75 s⁻¹

Recommended Detention Time (T)10 - 30 min

Recommended GT value (Camp's Number)10,000 - 100,000

Peripheral Velocity of Paddles0.15 - 0.6 m/s

Ratio of paddle area to tank cross-section15 - 20 %

Key Formulas

G = sqrt(P / (μ * V)) — Velocity Gradient Calculation

Practical Notes

Tapered flocculation, where the velocity gradient (G) is decreased in successive compartments, is highly recommended for better floc growth and to prevent floc shear.

Ensure sufficient flexibility in the design to vary the speed of mechanical paddles, allowing operators to adjust for changes in raw water quality and flow rate.

Proper baffling and inlet/outlet design are critical for achieving the theoretical detention time and preventing short-circuiting, a common operational problem.