The treatment train for dyehouse and finishing effluent: stream composition, reagents and order of dose, water reuse, and the five costliest mistakes.
Textile finishing is one of the most water-intensive industries in Uzbekistan. A tonne of finished fabric consumes 60 to 150 m³ of water, and almost all of it leaves the plant as coloured, alkaline, hot effluent. That creates three problems at once: meeting discharge limits, not overspending on reagents, and recovering as much water as the process allows.
What follows is the working logic for treating dyehouse and finishing effluent: what the stream is made of, which treatment train handles it, which reagents apply at what order of dose, and where plants most often lose money.
Why textile effluent is harder than ordinary industrial wastewater
- Colour. Reactive and disperse dyes produce intense colour at very low concentration, and biological treatment barely touches them — colour passes straight through the aeration tank.
- High COD with relatively low BOD. Much of the organic load resists biological oxidation: surfactants, sizing agents, levelling agents, thickeners.
- pH swings. Mercerising and alkaline scouring give pH 10–12, acid dyeing gives pH 4–5. The stream arrives in batches, and without equalisation the dosing system is working blind.
- Temperature. 50–70 °C leaving the shop floor. Hot effluent reduces biological efficiency and accelerates corrosion.
- Salinity. Reactive dyeing needs large doses of sodium chloride and sulfate, which complicates both biology and any downstream reverse osmosis.
What the finishing effluent is made of
| Process | Contribution | Typical problem |
|---|---|---|
| Desizing | Starch, PVA, enzymes | Sharp COD spike |
| Scouring and mercerising | Caustic, waxes, surfactants | pH 10–12, high alkalinity |
| Bleaching | Peroxide, stabilisers | Residual oxidiser |
| Dyeing | Dyes, salts, levelling agents | Colour, salinity |
| Washing and finishing | Resins, softeners | Foaming |
The treatment train, from equalisation to reuse
1. Equalisation
The first and most underrated unit. An 8–24 hour equalisation tank smooths out batch discharges of pH, temperature and colour. Without it, coagulant dose is calculated against an average that never actually occurs, and reagent overspend becomes permanent. Money saved on the tank comes back as chemical cost.
2. Neutralisation and mechanical treatment
pH correction into the coagulant's working window, plus fibre and coarse solids removal on screens and drum filters. Fibre that gets further down the train blocks pumps and membranes.
3. Coagulation and flocculation
The core of the train. The coagulant destabilises colloids and binds part of the dye; the flocculant grows the floc to a settleable size. For textile work the usual candidates are ferric chloride and polyaluminium chloride (PAC). Iron produces a denser, faster-settling floc and removes more colour; PAC tolerates a wider pH window and leaves less sludge.
4. Decolourisation
If colour still exceeds the limit after coagulation, add a colour remover stage. A cationic decolourant binds the anionic dye residues the coagulant missed — cheaper than pushing coagulant dose indefinitely.
5. Biological treatment
Removes the remaining biodegradable organics. It needs stable pH, temperature and nutrient ratio. If dye and high salinity are still present at the inlet, the sludge will be inhibited.
6. Polishing and reuse
Mechanical filtration and, where reuse is the goal, reverse osmosis with membranes protected by antiscalant. Returning wash water to the shop floor is the fastest payback item at plants where water is constrained.
Reagents and order of dose
| Reagent | Role | Indicative dose |
|---|---|---|
| Ferric chloride (III) | Coagulation, colour removal | 150–600 mg/l |
| PAC | Coagulation over a wide pH range | 100–400 mg/l |
| Anionic polyacrylamide | Flocculation after iron | 0.5–3 mg/l |
| Cationic polyacrylamide | Sludge dewatering | 2–8 kg/t dry solids |
| Colour remover | Residual colour | 50–300 mg/l |
| Caustic soda / acid | pH correction | as required |
Important: these ranges are for estimating order of cost, not a working recipe. The real dose comes from a jar test on your own effluent — two plants running similar fabric ranges can differ twofold in coagulant consumption, and the difference in cost per cubic metre is significant.
The regulatory side
In Uzbekistan the requirements differ depending on where the effluent goes: into the municipal sewer or into a water body. Discharge to sewer is governed by the agreement with the utility and allows softer figures; discharge to a water body requires compliance with MPC values and formal permitting. The practical conclusion is simple — fix the discharge point and obtain the list of monitored parameters with their limits before designing anything. Designing "to the standard" in the abstract is meaningless, because the parameter set differs by discharge point.
Five mistakes that cost the most
- A train without equalisation. Saving on the tank turns into permanent reagent overspend and unstable effluent quality.
- Selecting coagulant by price per kilogram. The number that matters is cost per cubic metre treated: a more expensive reagent at half the working dose usually wins.
- Expecting biology to remove colour. The aeration tank does not remove colour. That happens at the chemical stage.
- No plan for the sludge. The train starts up but dewatering and disposal were never worked out, and the yard is full within a month.
- Recalculating dose only after an incident. A change of fabric range changes the effluent. Dose should be reviewed when the dyeing programme changes, not when the numbers have already breached the limit.
Frequently asked questions
Can colour be removed by biological treatment alone?
Effectively no. Reactive dyes resist biological oxidation and pass through the aeration tank largely unchanged. Colour is removed by coagulation and a decolourant; biology handles the residual organics.
Ferric chloride or PAC for textile effluent?
Ferric chloride is usually stronger on colour and gives a denser sludge, but needs a narrower pH window and tints the water if overdosed. PAC forgives pH fluctuation and produces less sludge. In practice a jar test on the real effluent decides it — and a combination often wins.
How much water can realistically be recovered?
It depends where it goes back. Washing and auxiliary duties accept polished water easily. Returning water to dyeing requires salinity and hardness control, which means a membrane stage. A realistic industry benchmark is 30 to 60 % with a properly designed RO-based scheme.
Where do we start if there is no treatment plant yet?
With an effluent analysis per shop and a 24-hour discharge profile. Without those two, any train is designed by guesswork. Then a jar test to select the reagent, and a mass balance.
What we do on these sites
Aqua Global specifies reagent schemes for textile plants across Uzbekistan: effluent analysis, jar testing to select coagulant and flocculant, working dose and cost-per-cubic-metre calculation, and regular supply from Tashkent stock. More in textile industry solutions, or send us your effluent analysis through the contact form.
