Dark Blue-Green Reactive Dye Wastewater Brought to a Clear Yellow Supernatant
Dark Blue-Green Reactive Dye Wastewater Brought to a Clear Yellow Supernatant
Reactive dyes are the hardest class to remove — strongly hydrophilic, they form a true solution that will not settle. This dark blue-green liquor at pH above 8 was treated with 5 kg/t decolorant , 5 kg/t PAC and 20 g/t anionic PAM to yield a clear yellow supernatant.
Left to right: the near-black raw reactive dye liquor, two jar-test beakers showing dark floc settling out, and the filtered final sample. Bench-scale record from our laboratory.
What makes this effluent difficult
Reactive dyes behave differently from every other class, and conventional treatment reflects that:
They form a true solution
Reactive dyes are strongly hydrophilic. Once dissolved they are not suspended matter, so no settling tank will remove them regardless of retention time.
Conventional coagulation barely works
Standard coagulation and sedimentation are almost ineffective on this class. The effluent can be clear of turbidity and still be deeply coloured.
High chroma hides the endpoint
At this colour depth, partial removal is invisible. Progress only becomes apparent once most of the chromophore load has gone.
Alkaline dyebath chemistry
Reactive dyeing runs alkaline, so the effluent arrives above pH 8 — outside the window where the decolorant and polymer work best.
Reactive dye colour cannot be settled out as it stands. It has to be converted into something separable first — that conversion is the whole point of the programme below.
The dosing programme, step by step
Six steps. The pH is set first, then the colour is converted from dissolved to particulate, then it is made large enough to settle.
| Step | Action | Dosage / Setpoint | Why It Matters |
|---|---|---|---|
| 1 | Acidify to pH 7–8 | acid, to pH 7 – 8 | Brings the water to neutral, creating suitable pH conditions and disrupting part of the stable dye molecular structure. |
| 2 | Dose decolorant | 5 kg/t | Core oxidation and preliminary decolorization. |
| 3 | Dose PAC | 5 kg/t | Strengthens coagulation and sedimentation, building the destabilised colour into a settleable floc. |
| 4 | Confirm pH above 7 | pH > 7 | Within this range the molecular chains of anionic PAM extend best and flocculation is strongest. Add caustic flake if the liquor has dropped below it. |
| 5 | Dose anionic PAM | 20 g/t | Bridges the flocs into large aggregates ready to separate. |
| 6 | Settle 2–3 min | — | Sludge–water separation, leaving a clear yellow supernatant. |
What happened at each stage
Raw sample
Dark blue-green liquor, so deeply coloured it reads as near-black in the bottle. Alkaline at pH above 8.
After acidification to pH 7–8
The liquor is brought to neutral and part of the dye structure is disrupted.
After decolorant
The chromophores break down and the colour begins to transfer out of solution.
After PAC
Dark flocs form throughout the liquor, carrying the colour with them.
After anionic PAM and settling
The dark floc mass collects at the base and a clear yellow supernatant separates above it.
Colour transferred into a dark floc mass, leaving clear yellow water
From a liquor that read as near-black, the programme produced a clear yellow supernatant with the colour concentrated in a settled floc bed — using 5 kg/t decolorant, 5 kg/t PAC and 20 g/t anionic PAM. For a reactive dye liquor at this chroma, converting a true solution into a separable solid is the substantive result.
Works where coagulation alone fails
Reactive dyes resist conventional coagulation. The decolorant is what makes them separable at all.
Large drop in visible chroma
From near-black to a clear yellow supernatant in a single treatment stage.
Well-defined floc
The dark floc mass settles as a distinct bed, separating sharply from the water phase.
Honest limit
Yellow remains. Residual dissolved colour needs a polishing stage — see the FAQ.
From jar test to full-scale dosing
| Treated Flow | Decolorant @ 5 kg/t | PAC @ 5 kg/t | Anionic PAM @ 20 g/t |
|---|---|---|---|
| 10 m³ / day | 50 kg / day | 50 kg / day | 200 g / day |
| 50 m³ / day | 250 kg / day | 250 kg / day | 1 kg / day |
| 100 m³ / day | 500 kg / day | 500 kg / day | 2 kg / day |
| 500 m³ / day | 2500 kg / day | 2500 kg / day | 10 kg / day |
Where this programme applies
The same logic transfers to other strongly coloured industrial effluents. The chemistry is the same; the dosage and the decolorant grade change with the water.
Questions we are asked most
Why are reactive dyes harder to treat than other dyes?
They are strongly hydrophilic and form a stable true solution rather than a suspension. Disperse or vat dyes are largely particulate and settle far more readily; reactive dye colour has to be chemically converted before any physical separation will touch it.
Why grade rather than a general decolorant?
Decolorant grades differ in charge density and mechanism, and reactive-dye chromophores respond to some and not others. was the grade that performed on this liquor — the right grade for your water is determined by jar test.
Why is the treated water yellow rather than colourless?
Coagulation removes what can be made particulate. Small, highly water-soluble residual molecules stay dissolved, and additional coagulant will not capture them. ReAching colourless requires a polishing step — activated carbon, catalytic ozonation, or membrane filtration.
Can I use less than 10 kg/t of combined reagent?
Only if your colour load is lower. This dosage matches this chroma. A lighter reactive dye rinse will need considerably less — test on your own water rather than adopting this figure.
Does this handle the salt in a reactive dyebath?
No. Reactive dyeing uses large quantities of electrolyte, and this programme addresses colour and suspended load, not dissolved salts. Salt removal requires membrane or evaporative treatment.
Can you test my reactive dye effluent?
Yes. Send a sample with your target discharge standard and we will run the same bench-scale procedure and report the grade and dosage.
Send us your wastewater — we will run the jar test
Tell us your industry, daily flow and target discharge standard. Our laboratory will run a bench-scale trial on your own sample and report the exact chemical and dosage.
Products referenced in this case
| Product | Role in This Case | Dosage |
|---|---|---|
| Water Decoloring Agent | Chromophore breakdown on reactive dyes | 5 kg/t |
| Poly Aluminium Chloride (PAC) | Coagulation and floc formation | 5 kg/t |
| Anionic Polyacrylamide (APAM) | Floc bridging and settling | 20 g/t |
| Caustic Soda Flake | pH correction above 7 before PAM | as required |