A perfectly calculated dose is wasted if it isn't dispersed into the flow fast enough to react uniformly. This page covers the physical delivery of reagent: the injection quill, rapid-mix energy (the velocity-gradient G-value), in-line static mixers, and sizing the contact volume so the reaction actually completes before the next process step or analyser.
Reaction starts on contact — if mixing is slow, you treat a slug, not the stream.
Many dosing reactions (coagulation, neutralisation, oxidation) are fast relative to mixing. If reagent enters as a concentrated stream and disperses slowly, it over-reacts locally and under-treats the bulk — you see high reagent consumption, poor floc and a wandering analyser even though the calculated dose is correct. Good injection plus rapid mixing makes the dose uniform; that is what turns the number on the pump into the result in the water.
Get the reagent into the centre of the flow, cleanly and maintainably.
A carrier-water ring around the quill dilutes concentrated acid/caustic on entry, cutting local corrosion and scaling and speeding dispersion.
Wall injection (reagent runs down the wall, scales, never mixes), injecting into a stopped line (no-flow interlock needed), and a quill so long it vibrates and snaps.
The velocity gradient that quantifies mixing intensity.
Mixing intensity is expressed as the mean velocity gradient G (s⁻¹). Rapid mix (dispersion) needs high G for a short time; flocculation needs low G for longer to grow floc without shearing it.
G = mean velocity gradient (s⁻¹)P = power dissipated (W); μ = dynamic viscosity (Pa·s); V = mixing volume (m³)| Duty | G (s⁻¹) | Time | Gt |
|---|---|---|---|
| Rapid mix (coagulant) | 300–1000 | 1–30 s | ~10⁴–10⁵ |
| pH neutralisation mix | 300–600 | seconds | — |
| Flocculation | 20–80 | 10–30 min | ~10⁴–10⁵ |
In-line static mixer, mechanical rapid-mix, or hydraulic jump.
Fixed elements in the pipe create high-G dispersion with no moving parts — compact, low-maintenance, ideal for dosing injection. Sized on pressure drop vs flow.
Static mixersA stirred chamber delivering controllable G, used where flow varies widely or a defined detention time is needed. More flexible, needs power and maintenance.
Flocculator equipmentWeirs, flumes and hydraulic jumps provide free mixing energy where head is available — no power, but G is flow-dependent and uncontrolled.
Give the reaction time to finish before you measure or move on.
After mixing, the stream needs enough residence time for the reaction to complete — floc to form, neutralisation to settle, oxidant to act for its CT credit. Size the contact volume from flow and required reaction time.
t = required reaction/contact time; for disinfection, sized to meet the CT targetHigh reagent use, patchy floc or a wandering analyser are often a mixing problem, not a dose problem. We assess injection geometry and mixing energy and specify the quill, static mixer or rapid-mix that makes your calculated dose actually work.
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