Static Mixers for Coagulation & Flocculation: Flash Mixing Guide

KEY TAKEAWAYS

Flash mixing is the critical first step — coagulant (alum, ferric, PAC) must disperse in < 1 second or floc quality suffers permanently.
The mixing intensity target is the velocity gradient G = 300–1000 s⁻¹ for coagulation; flocculation runs gentler at G = 20–80 s⁻¹ .
CoV < 0.05 is the dispersion target — static mixers hit it inline where mechanical flash mixers often don't.
Poor flash mixing costs chemical overfeed (5-20%) and worse turbidity — both measurable plant KPIs .
Retrofit-friendly: replaces the mechanical flash mixer in the discharge line, ΔP 0.3–1.0 bar.

1. Coagulation: Why the First Second Wins

When coagulant hits raw water, it must disperse instantly and uniformly. Slow or uneven dispersion means:

Some water gets excess coagulant → wasted chemical
Some water gets too little → turbidity breakthrough
Flocs form at different times → inconsistent settling

The static mixer fix: inline flash mixing at the coagulant injection point, < 1 second, CoV < 0.05. This is the classic static mixer water-treatment duty.

2. G-Factor Targets

Stage	G (s⁻¹)	Mixer type
Flash mixing (coagulation)	300 – 1000	Static mixer, 6 elements
Flocculation	20 – 80	Tanks/pipes (low shear)

Design note: The static mixer handles the flash stage (high G); floc growth then happens in downstream tanks or pipes at low G. Don't try to flocculate inside the mixer — that requires long residence and low shear, the opposite of flash mixing.

3. Coagulant Types & Materials

Coagulant	Typical dose	Material
Alum (aluminum sulfate)	10–100 mg/L	PVC / PP
Ferric chloride	5–50 mg/L	PP (aggressive)
PAC (polyaluminum chloride)	5–50 mg/L	PVC / PP
Cationic polymer (primary)	1–10 mg/L	PP

Material rule: ferric and other acidic coagulants → PP; standard alum/PAC → PVC (NSF 61 for potable).

4. The Mixer Configuration

Element	Recommended
Element type	Blade (turbulent water flow)
Count	6 elements (CoV < 0.05)
Injection	Integrated injection port (coagulant enters mixing zone)
Position	Immediately downstream of chemical injection point
ΔP	0.3–1.0 bar

5. Retrofitting From Mechanical Flash Mixers

Many plants still use motor-driven flash mixers. The retrofit case:

Factor	Mechanical	Static mixer
Mixing time	10–60 s	< 1 s
Moving parts	Motor/impeller/seals	None
Maintenance	Schedule	Zero
Energy	Motor kW	Pump head only
Space	Tank + footprint	Inline

Retrofit path: install the static mixer in the discharge line, verify ΔP vs pump, decommission the mechanical mixer. Most plants see equal-or-better CoV at lower operating cost.

6. FAQ

Q: Can a static mixer replace my flash mixer? Yes — for inline continuous coagulant dosing, it's the standard modern replacement. Verify ΔP and mixing time.

Q: Does mixing intensity matter after the coagulant is in? Yes but differently — flocculation needs gentle, long mixing. The static mixer does flash; downstream does floc.

Q: What material for ferric chloride? PP — ferric chloride is aggressive to PVC at working concentrations. Confirm with the compatibility table.

Q: Do you supply mixers with coagulant injection ports? Yes — NPT/flanged injection ports are standard options.

Your Action Roadmap

Confirm your coagulant & dose — material follows chemistry.
Target G and CoV — 300-1000 s⁻¹, < 0.05.
Position the mixer — at the injection point.
Get a quote — flash mixer replacement or new line.

→ Send us your plant duty spec for a flash mixing recommendation — typically answered within 24 hours.