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Coagulation and flocculation

Enter the jar test dose and basin sizes to see how much coagulant to buy and feed, whether mixing is in the right range and what motor each mixer needs.

Pre-sizing and estimating tool: results do not replace design, calculations or reports signed by a licensed professional, nor verification against the applicable codes and standards.

Pre-sizing
Raw water
Coagulant

Strength must be on the same basis as the jar test dose. If the jar test used the product as delivered, enter 100%.

Rapid mix
Mechanical flocculation

Stages have equal volumes. G should taper from one stage to the next so flocs grow without breaking.

—Commercial coagulant use
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Velocity gradient at each step

Light bars: range recommended by the chosen reference. Points: design G. Log scale.

Tap the chart to see the values.

Rapid mix power versus temperature

Power into the water to hold the target G: cold water is more viscous and needs more power.

Tap the chart to see the values.

Mixing and flocculation steps

The report gathers this page's inputs, results, equations, assumptions and references, with fields for project, engineer and revision. In the print dialog, choose Save as PDF.

Method, assumptions and references

Equations used

Active product: m = Q · dose (kg/h with Q in m³/h and dose in mg/L ÷ 1000; lb/d = 8.34 · MGD · mg/L); commercial = active ÷ strength; volume = commercial ÷ density
G = √(P ÷ (μ · V)); P = G² · μ · V; motor = P ÷ η; t = V ÷ Q; Camp = Σ Gᵢ · tᵢ
Water viscosity: μ = 2.414 × 10⁻⁵ · 10^(247.8 ÷ (T + 133.15)) Pa·s (T in °C)
Alkalinity: residual = raw alkalinity − k · dose; k = 0.50 (Al₂(SO₄)₃·14H₂O), 0.92 (FeCl₃), 0.75 (Fe₂(SO₄)₃) mg CaCO₃/mg

Assumptions and limits

  • Power dissipated uniformly in the volume (complete mix per stage); flocculation stages of equal volume. Motor power includes the entered mixer, gearbox and motor efficiency.
  • Ranges: AWWA and MWH (rapid mix G 600 to 1,000 s⁻¹) and Ten States Standards (rapid mix up to 30 s; flocculation at least 30 min, paddle tip speed 0.5 to 3.0 ft/s); NBR 12216 (G 700 to 1,100 s⁻¹ within 5 s; flocculation G from 70 down to 10 s⁻¹, 30 to 40 min mechanical).
  • Stoichiometric alkalinity consumption; PACl consumes less (depends on basicity). The best dose depends on raw water and must be checked with new jar tests.

Standards and technical documents

  • ABNT NBR 12216:1992: Projeto de estação de tratamento de água para abastecimento público
  • Recommended Standards for Water Works (Ten States Standards), 4.2 Clarification
  • ASTM D2035: Standard Practice for Coagulation-Flocculation Jar Test of Water
  • AWWA. Water Quality and Treatment, 6th ed., McGraw-Hill, 2011
  • MWH. Water Treatment: Principles and Design, 3rd ed., Wiley, 2012
  • Camp, T. R. Flocculation and flocculation basins. Trans. ASCE 120, 1955
  • ABNT NBR 15784: Produtos químicos utilizados no tratamento de água para consumo humano — Efeitos à saúde

U.S. regulations

  • NSF/ANSI/CAN 60: Drinking Water Treatment Chemicals — Health Effects (required by most states)
  • EPA 40 CFR 141 and 143: turbidity rules and secondary standards for aluminum and iron
  • OSHA 29 CFR 1910.1200 (Hazard Communication) and 1910.132 (PPE)
  • DOT 49 CFR 171–180: transport of hazardous materials

Frequently asked questions

How do I convert a jar test dose to a feed rate?

Multiply flow by dose and divide by product strength: lb/d = 8.34 × MGD × mg/L. For 1,500 gpm (2.16 MGD) at 30 mg/L of alum, that is 540 lb/d of dry alum; as 48.5% liquid alum at 11.1 lb/gal, about 1,110 lb/d or 4.2 gal/h.

What G value should rapid mix and flocculation use?

Rapid mix typically runs at 600 to 1,000 s⁻¹ and Ten States Standards limit detention to 30 s. Flocculation tapers from about 70 to 10 s⁻¹ over at least 30 minutes, with Gt between 10,000 and 100,000.

How do I size a flocculator mixer?

Use P = G² · μ · V. For G = 60 s⁻¹ in 16,700 gal (63 m³) of 60 °F water (μ ≈ 1.13 cP), the power into water is about 0.26 kW (0.35 hp); with 70% efficiency, use a 0.5 hp motor.

Why does alkalinity matter?

Alum consumes about 0.5 mg/L of alkalinity as CaCO₃ per mg/L of dose, ferric chloride about 0.92. Soft water with low alkalinity loses pH and may need lime or soda ash.