Stage one: after acidification to pH 3–4 and anionic PAM, the liquor separates into a clear pink supernatant above a settled floc bed.
01 — The Challenge
What makes this effluent difficult
This water carries two problems at once, and they have to be solved in the right order:
Chloride interferes with coagulation
A high chloride background compresses the electrical double layer and changes how coagulants behave. Dosages proven on low-salinity effluent do not transfer.
Alkaline start, acidic reaction window
The liquor arrives at pH 8–9 but the dye structures are most vulnerable in acid. The pH has to be driven down before anything else will work.
CODcr above 11,000 mg/L
This is a strong organic load, not a colour problem with a little COD attached. Reagents are consumed by both.
Single-stage treatment falls short
Attempting colour and load in one pass leaves reagent competing against suspended matter. Removing the solids first is what makes the decolorant effective.
The strategy that worked: strip the bulk load first under acid conditions, then decolorize a cleaner liquor.
02 — The Programme
The dosing programme, step by step
Eight steps in two distinct stages. Stage one removes the bulk load; stage two treats the recovered supernatant.
| Step |
Action |
Dosage / Setpoint |
Why It Matters |
| Stage 1 — Acid Flocculation and Bulk Solids Removal |
| 1 |
Acidify |
to pH 3 – 4 |
Destroys the stable structure of the dyes in the wastewater and makes them far easier to decolorize downstream. |
| 2 |
Dose anionic PAM |
5 g/t |
Preliminary flocculation and settling aid, bringing the destabilised material together. |
| 3 |
Settle 2–3 min |
— |
Solid–liquid separation. Removes coarse particles and reduces turbidity, yielding a clear pink supernatant. |
| Stage 2 — Decolorization of the Recovered Supernatant |
| 1 |
Recover the supernatant |
— |
The clarified liquor is filtered off and carried into stage two. The bulk load stays behind in the settled sludge. |
| 2 |
Dose the decoloring agent |
500 g/t |
Applied to the recovered supernatant, where it acts on colour rather than being consumed by suspended solids. |
| 3 |
Dose PAC |
5 kg/t |
Strengthens coagulation, neutralises colloidal charge, and removes further suspended solids and part of the COD. |
| 4 |
Correct pH with caustic flake |
to pH 4 – 5 |
Returns the liquor to weak acid, the condition the second polymer dose needs. |
| 5 |
Dose anionic PAM |
— |
Secondary flocculation, promoting floc growth ahead of the final settle. |
| 6 |
Settle 2–3 min |
— |
The flocs settle fully, leaving a clear, pale yellow supernatant. |
Why acidify to 3–4 and then come back to 4–5? Two different jobs. pH 3–4 is where the dye structures break down; pH 4–5 is where the anionic polymer bridges flocs efficiently. Trying to run both at one pH compromises each of them.
Why decolorize after the first settle? Because reagent dosed into a liquor full of suspended solids is partly wasted on them. Removing the bulk load first means the 500 g/t of decolorant acts on colour — which is why this stage needs grams per tonne rather than kilograms.
Stage two: after decolorant, PAC, pH correction and a second PAM dose, the final supernatant is clear and pale yellow.
03 — The Result
CODcr more than halved, with a clear pale yellow supernatant
This is the only case in the series with laboratory-measured COD data. The two-stage programme reduced CODcr from 11,300 mg/L to 5,200 mg/L — a 53.98% reduction — and produced a clear, pale yellow supernatant from an alkaline, high-chloride starting point.
After Treatment
5,200 mg/L
Laboratory-measured values for this sample.
Measured, not estimated
CODcr was determined in the laboratory before and after treatment. The 53.98% figure is a measurement.
Effective under high chloride
The two-stage route works in a salinity background where single-stage coagulation typically underperforms.
Efficient use of decolorant
Removing bulk solids first cuts the decolorant requirement to 500 g/t.
Realistic expectation
COD is halved, not eliminated. A biological or advanced-oxidation stage would normally follow.
04 — Scale-Up
From jar test to full-scale dosing
1 kg/t = 1,000 ppm · 1 g/t = 1 ppm · 1 tonne of wastewater = 1 m³
| Treated Flow |
Anionic PAM
@ 5 g/t |
Decoloring Agent
@ 500 g/t |
PAC
@ 5 kg/t |
| 10 m³ / day |
50 g / day |
5 kg / day |
50 kg / day |
| 50 m³ / day |
250 g / day |
25 kg / day |
250 kg / day |
| 100 m³ / day |
500 g / day |
50 kg / day |
500 kg / day |
| 500 m³ / day |
2.5 kg / day |
250 kg / day |
2500 kg / day |
Prepare PAM as a dilute solution Anionic PAM powder is normally made up to a 0.1–0.2 % working solution and allowed to dissolve fully before dosing. Dosing dry powder gives poor results and blocks lines.
Treat these as a starting point These quantities are arithmetic conversions of one jar test. Use them to size a trial and plan storage, not as a guaranteed design dosage.
05 — Applications
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.
Firecracker & pyrotechnic manufacturing
High-chloride dye effluent
Dye & pigment production
Saline industrial wastewater
Chemical process wastewater
High-COD coloured effluent
06 — FAQ
Questions we are asked most
Is a 53.98% COD reduction good?
For a physico-chemical stage on an 11,300 mg/L influent, yes. Coagulation removes the fraction that can be made particulate; the dissolved, biodegradable remainder is normally handled by a biological stage downstream. Halving the load ahead of that stage is exactly what this programme is for.
Why two separate PAM doses?
They serve different stages. The first (5 g/t) settles the bulk load under acid conditions; the second flocculates the material generated by the decolorant and PAC. One combined dose cannot do both.
Does the high chloride damage equipment?
Chloride is corrosive to standard carbon steel, and operating at pH 3–4 compounds it. Specify acid-resistant materials for the dosing and settling equipment on this kind of water.
Can I skip stage one and dose the decolorant directly?
You can, but the decolorant is then consumed by suspended solids as well as colour, so the dosage rises sharply and the result is usually worse. Sequencing is what makes this programme economical.
What is the sludge volume from the two stages?
Not quantified in this test. Two settling stages produce two sludge streams — account for both in your dewatering capacity.
Will this work on my high-salinity effluent?
The two-stage logic transfers, but the dosages will not. Chloride level, COD and dye chemistry all shift the requirement. Send a sample and we will run the same procedure.
Free Laboratory Service
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
The results described in this document were obtained from a bench-scale jar test on a single customer sample under laboratory conditions. Actual performance depends on the characteristics of each effluent. We recommend a jar test on your own wastewater before finalising a dosing programme.
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