Authoritative engineering reference for flocculator sizing: Camp & Stein G-value calculation, GT product selection, tapered staging, power input, retention time, and jar-test protocol.
The root-mean-square velocity gradient G (s−¹) is the cornerstone of flocculator design. It quantifies the intensity of fluid shear available to promote particle–particle collisions without breaking floc aggregates:
G = √(P / μ · V)
Where: P = net power dissipated in fluid (W) | μ = dynamic viscosity (Pa·s, ≈ 0.001 at 20°C) | V = tank liquid volume (m³)
Rearranged for power: P = G² · μ · V
The GT product (dimensionless) — G multiplied by the hydraulic retention time t (s) — characterises the cumulative mixing work delivered to the suspension. GT values in the range 10,000–150,000 are recommended for most coagulation applications. Jar-test results should confirm the optimum GT for each specific raw water and coagulant regime.
| Application | Target G (s−¹) | HRT (min) | GT (dimensionless) |
|---|---|---|---|
| Drinking water — surface (turbid) | 20–50 | 20–30 | 24,000–90,000 |
| Drinking water — low turbidity / coloured | 10–30 | 25–40 | 15,000–72,000 |
| DAF pre-flocculation | 15–40 | 10–20 | 9,000–48,000 |
| Lamella pre-treatment | 20–50 | 15–25 | 18,000–75,000 |
| Industrial wastewater (food / dairy) | 30–60 | 15–20 | 27,000–72,000 |
| Phosphorus precipitation | 30–60 | 10–20 | 18,000–72,000 |
| Heavy metal precipitation | 40–80 | 10–15 | 24,000–72,000 |
The power input for a paddle flocculator is calculated from the drag force on the blades:
P = CD · A · ρ · vrel³ / 2
Where: CD = 1.5 (flat blade) | A = total blade area (m²) | ρ = 1,000 kg/m³ | vrel = 0.75 × tip speed (m/s, accounting for the water dragged with the blade)
Given: Q = 2,000 m³/day, target G = 35 s−¹, HRT = 20 min, T = 15°C (μ = 0.00114 Pa·s)
Tank volume: V = (2,000/86,400) × 20 × 60 = 27.8 m³
Power required: P = G² × μ × V = 35² × 0.00114 × 27.8 = 38.8 W
GT product: GT = 35 × 20 × 60 = 42,000 (within the 10,000–150,000 range — suitable)
Motor selection: Allow 60% mechanical and drive efficiency → installed power = 38.8 / 0.6 = 65 W. Standard 0.37 kW gear-motor with VSD provides comfortable headroom.
Single-chamber flocculators operated at a constant G-value are a compromise. A tapered (staged) design applies a descending G-value sequence that promotes fast particle collision in the first chamber and protects growing flocs from shear in the final chamber.
| Chamber | G (s−¹) | HRT (min) | Purpose |
|---|---|---|---|
| Chamber 1 (rapid) | 50–80 | 5–8 | High collision frequency — grow micro-flocs from primary particles |
| Chamber 2 (intermediate) | 25–50 | 8–12 | Aggregate growth — floc particle approaches 50–200 μm |
| Chamber 3 (gentle) | 10–25 | 5–10 | Consolidation — densify and strengthen floc prior to separation |
Design principle: Tapered flocculation consistently produces 15–30% improvement in settled turbidity and 10–20% reduction in polymer dose compared to a constant-G single-chamber design of equivalent total HRT. The benefit is greatest when the upstream coagulant is alum or ferric, which produce large, fragile aluminium/iron hydroxide flocs.
Theoretical G-value calculations set the design envelope; jar tests (British Standard BS EN 14899 or AWWA Manual M37) confirm the optimum coagulant dose, pH, and G-value for the specific raw water. Our engineers conduct both laboratory and on-site jar tests as part of the feasibility and pilot-testing service.
Add coagulant to each jar. Mix at 300 rpm for 30 seconds to simulate flash-mix conditions. Adjust coagulant dose across jars (e.g., 2, 5, 10, 20, 40 mg/L as Al).
Reduce to 20–50 rpm (target G ≈ 20–50 s−¹) for 20 minutes. Observe floc formation rate and particle size. Repeat at different G-values to find optimum.
Allow 30 minutes quiescent settling. Withdraw supernatant at 5 cm depth. Measure turbidity (NTU), UV254, colour, and pH. Plot residual turbidity vs dose to find optimum.
Repeat the optimum-dose jar with varied slow-mix times (5, 10, 20, 30 min) to confirm the minimum GT for target effluent quality. This sets the design HRT for the full-scale flocculator.
Pilot testing: For critical projects — new WTPs, desalination pre-treatment, or industrial streams with variable quality — Reynolds & Bauhm operates a mobile pilot plant that reproduces the full coagulation–flocculation–separation train at 1–5 m³/h to validate design G-values under real operating conditions before capital expenditure is committed.
Slow-speed horizontal or vertical paddle wheels for gentle, sustained floc growth.
View PageAxial and radial turbine impellers for higher-intensity or multi-stage flocculation.
View PageChemical coagulant flash-mixing without moving parts using pipe-mounted static elements.
View PageCamp & Stein G-value calculations, GT product, tapered flocculation staging, and chamber sizing.
View PageDiagnose pin floc, carry-over, excessive breakup, and drive mechanical faults.
View PageDiscuss your specific requirements with our technical team and receive a tailored proposal for your project.
Contact UsOur process engineers will review your raw water quality, coagulant regime, and downstream separation method to recommend the optimum flocculator type, G-value profile, and chamber configuration.
Our expertise spans multiple industries with sector-specific water treatment solutions.