Printing and Dyeing Wastewater: Where the Load Comes From and Why Biology Struggles
Every one of the four processing stages in a printing and dyeing mill discharges wastewater, and they are not remotely alike. Pretreatment, meaning singeing, desizing, boiling, bleaching and mercerizing, produces the desizing, boiling, bleaching and mercerizing streams. Dyeing produces dyeing wastewater. Printing produces printing and soaping wastewater. Finishing produces finishing wastewater. What arrives at the treatment plant is the mixture of all of them, or the combined flow with the bleaching stream excluded.
Stream by stream
Desizing waste is small in volume but concentrated, carrying sizing agents and their decomposition products, fibre debris, starch alkali and assorted auxiliaries at around pH 12. Where starch sizing is used, as on cotton, COD and BOD are both very high and the waste degrades readily. Where PVA sizing is used, as on polyester-cotton warp, COD is high but BOD is low and the stream resists biology; synthetic sizing generally leaves BOD below 5 mg/L. That one substitution changes the whole treatment strategy.
Boiling waste is large in volume and heavily loaded, carrying cellulose, pectic acid, wax, grease, alkali, surfactants and nitrogen compounds at high temperature and a brown colour; cotton-fibre scouring waste is strongly alkaline with COD and BOD running to several thousand mg/L. Bleaching waste is large in volume but comparatively light, with residual bleach, a little acetic and oxalic acid and sodium thiosulfate. Mercerizing waste sits at pH 12-13 and, although most plants recover NaOH by evaporation at 3%-5% concentration, what finally leaves after repeated reuse is still strongly alkaline, full of fibre debris and suspended solids, with very high BOD and COD.
Dyeing waste varies with the fibre and the dye class. It is normally alkaline, intensely coloured and low in suspended solids, with COD far above BOD and correspondingly poor biodegradability. Printing waste, including the soaping and washing water that follows it, runs high in BOD and COD. Finishing waste is small, mostly fibre debris, resin, formaldehyde, oil agents and sizing. Wool dyeing and finishing is in a class of its own: roughly 318 t of wastewater per 454 kg of clean wool, brown and colloidal, carrying 91-114 kg of BOD-based organic pollutants.
Alkali-peeling waste from polyester simulation-silk work is the worst of the lot. It consists mainly of polyester hydrolysis products, terephthalic acid and ethylene glycol, with terephthalic acid reaching 75%. pH generally runs above 12 and CODcr can reach 90,000 mg/L. The high-molecular fraction and part of the dye load will not biodegrade, which makes it a high-concentration refractory stream that belongs in a segregated line rather than in the composite.
How big the problem actually is
Discharge volumes are enormous. European statistics put the fabric-to-wastewater weight ratio at 1:150-1:200; in China it runs about 1:200-1:400. Textile-industry wastewater ranks sixth among all industrial discharges in China, and around 80% of that is printing and dyeing. The sector has been one of the country's largest industrial water users since the 1970s, and its competition with agricultural water supply is a standing conflict that regulators take seriously.
The general composite is unpleasant enough: pH 6-10, CODcr 400-1000 mg/L, BOD5 100-400 mg/L, suspended solids 100-200 mg/L and colour 100-400 times. That envelope is only a starting point. Bring alkali-peeling waste into the mix and CODcr climbs past 2000-3000 mg/L, BOD5 goes above 800 mg/L and pH reaches 11.5-12, with quality worsening as more alkali-peeling waste is added. Once the COD contributed by alkali peeling exceeds 20% of the total CODcr, biological treatment adapts poorly and the plant starts chasing its tail.
Composition also drifts. Unreacted dyes and pigments give the colour; unreacted auxiliaries, reaction products and fabric debris add the rest; and the mix shifts with market demand, season and supplier turnover rather than with anything the treatment operator controls. Some of what is in there is carcinogenic or teratogenic, and toxic heavy metals are present too.
Why treatment keeps disappointing
Printing and dyeing wastewater is among the harder industrial streams to treat, and mostly for economic rather than technical reasons. The biological-physical combinations in common use reach the basic discharge limit and no further. Colour drops somewhat, but the organic matter is only broken into smaller fragments whose behaviour nobody can predict, so there is no guarantee the discharge is harmless.
Adsorption is the workhorse physical method. Powdered or granular porous media such as activated carbon (PAC) or clay are mixed with the wastewater, or the wastewater is passed through a filter bed of such particles, so pollutants are held on the porous surface or filtered out. Overseas plants lean on activated-carbon adsorption, mostly as tertiary treatment. It is very effective on dissolved organics but cannot remove colloids or hydrophobic dyes, and it only handles water-soluble types: cationic, direct, acid and reactive. Reported performance reaches 93% adsorption, 92% BOD removal and 63% COD removal, with capacity up to 500 mg COD per gram of carbon.
The economics are the real constraint. Treatment needs land, capital and running cost, and production cost rises with it. Meeting the secondary discharge standard is estimated to cost about as much per cubic metre as city tap water, and reuse costs considerably more, which is why so many plants struggle to run what they have already built.