APPLICATION
PAC for Textile and Dyeing Wastewater: Colour, COD and the Two-Step Train
Dyeing wastewater changes personality with every fabric and recipe: salt loads, dyebath pH and colour all move together. Coagulation with PAC is the front line for colour and part of the COD, and it works well - when it is matched to the dye classes actually running on the floor. This guide sets realistic expectations and shows where PAC fits in the train.
What coagulation removes - and what it does not
Insoluble and dispersed colour comes out well: disperse, vat and sulphur dyes, pigment padding residue and most finishing oils are caught by charge neutralisation and swept into floc. Soluble reactive dyes are the hard case - hydrolysed reactive dye stays dissolved and coagulation removes only part of it, which is why a PAC plant can run crystal-clear on one week's production and stay tinted the next.
COD removal by coagulation is real but partial, typically a third to a half of the raw load, because a large fraction of dye-house COD is soluble acids, salts and low-molecular organics that no coagulant can catch. Design the biological stage for the remainder instead of expecting chemistry to carry it.
Dosing in practice
Dyeing effluent sits at the high end of PAC dosing: jar tests on mixed effluent usually land in the several-hundreds of mg/L range, far above drinking-water doses, and the number moves with the production schedule. PAC at 28% industrial grade (GB/T 22627-2022) is the normal choice; the drinking-water 29% grade adds cost without benefit here.
Pair PAC with a small polymer dose: 1 - 3 mg/L of anionic or low-charge cationic PAM added after the coagulant turns fine flocs into fast-settling ones and cuts chemical consumption. Add PAC first, mix, then polymer - reversing the order wastes both. For colour that survives coagulation, a second stage at higher PAC dose on the clarified stream often beats one giant dose, because the first pass has already removed competing colloids.
Design notes that save operators pain
- Equalise hard. A buffer tank of several hours flattens the salt, pH and colour swings that make dosing chase a moving target.
- Watch pH: dye-bath alkalinity reaching the coagulation stage at pH above 9 will undercut PAC; trim with acid ahead of the dose point.
- Plan the sludge: dye flocs are voluminous and dewater poorly without polymer. Budget for a dewatering stage, not just a settling tank.
- Re-test monthly. When the fabric mix changes, yesterday's dose ladder quietly expires.
Frequently asked questions
Can PAC remove reactive dye colour completely?
Partially. Hydrolysed reactive dyes are soluble and coagulation catches only a fraction; expect meaningful colour reduction but not full decolorization on reactive-dye streams. Disperse and vat colours, by contrast, come out well. Bleaching or adsorption stages cover the residual tint.
Which PAM works with PAC on dyeing effluent?
Start with anionic PAM at medium charge for the typical slightly acidic to neutral clarified stream; low-charge cationic can win on effluent with more soluble organics. The jar test with both options on your actual effluent settles it in an hour.
Does the salt in dyeing wastewater hurt PAC?
Moderate salt does not stop coagulation - PAC tolerates the brackish loads common in dye houses better than alum - but very high salt slows floc growth and changes settling. Test on the real effluent, and equalise before dosing to avoid the worst swings.