Eight Waste Streams Behind One Dye House Outlet, and How They Are Actually Treated

A dye house has one discharge point and eight different wastewaters behind it. Treating them as a single stream is the most common reason a well-built plant underperforms.

The Eight Streams

• Desizing wastewater - relatively small in volume but high in pollutant concentration. It carries sizing agents and their decomposition products, fibre debris, acids, starch alkalis and enzyme pollutants, with high turbidity and pH around 12. Where starch sizing is used both BOD and COD are high and biodegradability is good; with synthetic sizing COD is very high and BOD drops below 5 mg/L, so biodegradability is poor.
• Scouring wastewater - large volume, high concentration: cellulose, pectic acid, wax, grease, alkali, surfactants and nitrogenous compounds. Strongly alkaline, hot and brownish, with very high COD and BOD reaching several thousand mg/L. Scouring wastewater from chemical fibres pollutes comparatively lightly.
• Bleaching wastewater - large volume, relatively lightly polluted: residual bleaching agents, small amounts of acetic acid, oxalic acid and sodium thiosulfate.
• Mercerizing wastewater - NaOH at 3%-5%. Most mills recover it by evaporation and concentration, so it is rarely discharged; the final effluent after multiple reuse cycles stays strongly alkaline with high BOD, COD and SS.
• Dyeing wastewater - highly variable, sometimes carrying sodium sulfide, tartar emetic, aniline, copper sulfate and phenol. Alkaline, pH sometimes above 10 when sulfur or vat dyes are used, high colour but low SS, COD above BOD, and poor biodegradability.
• Printing wastewater - sizing agents, with high BOD and COD.
• Finishing-process wastewater - fibre debris, resins, formaldehyde, finishing oils and sizing agents, small in volume.
• Alkali-reduction wastewater - from polyester imitation silk: terephthalic acid and ethylene glycol, with terephthalic acid content as high as 75%. pH generally above 12, organic concentration very high, and CODCr from the alkali-reduction process can reach 90,000 mg/L. The macromolecular organics and some dyes resist biodegradation, making it a high-concentration refractory stream.

Nationally the daily discharge of printing and dyeing wastewater ranges from 3x10^6 to 4x10^6 m3. Large volume, high organic pollutant content, deep colour, strong alkalinity and highly variable quality - which is why it has a reputation as a difficult industrial wastewater.

Reduce Before You Treat

Treatment should start with reuse and comprehensive utilisation, combined with reforms to the production process itself, so consumption of water, alkali and auxiliaries falls and dyes and sizing agents are recovered. Dry printing suits synthetic fibres and fabrics containing more than 75% synthetic fibre, and eliminates printing wastewater at source. Replacing potassium dichromate with sodium nitrate or hydrogen peroxide as the oxidant in acid mordant dyeing removes chromium pollution from the effluent. Mercerizing alkali liquor is commonly reused as scouring liquor, and scouring waste alkali goes to the desizing stage - repeated reuse cuts total alkali discharge across the whole process. Sulfur-dye wastewater can be acidified first in a reaction vessel to release hydrogen sulfide, recovered after precipitation and filtration; vat and disperse dye particles can be recovered from wastewater by ultrafiltration.

Coagulation, Then Biology

The applicable discharge standard is the "Discharge Standard of Water Pollutants for Dyeing and Finishing of Textile Industry" (GB 4287-1992). It sorts pollutants into Class 1 - substances that accumulate in the environment or in animals and plants with long-term effects on human health - and Class 2, whose long-term effects are less severe, with maximum allowable discharge concentrations tabulated for each. Those limits include 0.05 (expressed as Hg), 0.5 (expressed as Cr6+), 1 mg/L (expressed as Cu), 5 mg/L (expressed as Zn) and 10 mg/L (expressed as F).

Coagulation adds reagents so most water-insoluble dye particles and colloidal organics coagulate into larger flocs, removed by natural sedimentation or flotation. Because the flocs adsorb strongly, some water-soluble organics go with them. After coagulation, more than 80% of suspended organic pollutants are removed and the colour removal rate can reach 50%-95%. Flocculant choice decides the outcome: aluminium sulfate, ferric sulfate and ferric chloride show obvious effects on water-insoluble dye wastewaters such as disperse, vat and sulfur dyes, with very high COD and colour removal.

The mainstream flowsheet in China combines physicochemical treatment with secondary specialised treatment - coagulation-sedimentation and coagulation-flotation at the front, and mostly activated sludge among the biochemical facilities already in operation, though SBR (sequencing batch reactor) application is gradually increasing. A typical train runs equalization tank, hydrolysis acidification, biological contact oxidation, intermediate sedimentation tank, coagulation reaction tank, dissolved-air flotation tank, then effluent. Treatment chemicals are polyacrylamide (PAM) and polyaluminum chloride (PAC), with calcium hypochlorite as the decolorizing agent. For PAM selection, anionic and nonionic grades are generally chosen for coagulation while cationic PAM is used for sludge dewatering, at relatively low charge density and molecular weight above 10 million. In the United States most printing and dyeing wastewater is treated by secondary treatment - physicochemical pretreatment plus biochemical - with a few enterprises running a tertiary system that adds activated-carbon adsorption; Japanese plants use similar flows and apply ozonation somewhat more often.

Why Good Plants Still Miss

Five failure modes account for most of it. Mills copy other mills' experience without analysing their own wastewater characteristics, quality and quantity. Design specifications for municipal wastewater get applied to printing and dyeing wastewater with only a few parameters changed - especially in the early period, when centralised treatment of large mills was handled by large design institutes that lacked deep understanding of this wastewater. New technologies, processes and chemicals go straight into projects without pilot testing; pilot scale is usually 3%-5% of project flow, meaning magnification of at most about 20x, and applying laboratory results directly to engineering rarely succeeds. Wastewaters with similar production processes may share a flowsheet but still need technical parameters tuned to actual water quality and quantity. And operation and management that fails to adjust as the wastewater changes leaves the plant unstable.

The load has moved too. Imitation silk and advances in dyeing and finishing technology pushed PVA sizing, viscose alkali-hydrolysis products (mainly phthalates) and new auxiliaries into the wastewater, lifting COD concentration from several hundred mg/L to 2,000-3,000 mg/L and dropping COD removal in original biological systems from 70% to around 50% or lower. Conventional chemical precipitation and flotation achieve only about 30% COD removal on such wastewater.

Reuse: AFF and MBFB

Lime as pH regulator and ferrous sulfate as coagulant leave the effluent high in iron, so it cannot be reused directly. After intensified aeration in the contact oxidation tank, ferrous iron converts to ferric and forms ferric hydroxide micro-flocs - the main reason for turbidity and colour in the treated effluent. Simply adding polyaluminum chloride and PAM combines those micro-flocs into larger flocs that high-efficiency filtration can catch, which is where the AFF asymmetric-fiber filter comes in. AFF integrates dosing, micro-flocculation, sedimentation and filtration: filtration rate more than ten times that of sand filtration, filtration precision 5 um, four times that of ordinary sand filtration, easy backwashing and convenient management.

AFF does not solve COD. Reclaimed water leaving it still carries a COD of around 100 mg/L, mainly soluble COD (SCOD), which directly affects reuse value; organics also shorten reverse-osmosis membrane life, so it must be reduced below 30 mg/L by an appropriate process. The membrane biological fluidized bed (MBFB) does that, using specially treated ceramic membranes to combine membrane separation with a high-load biological fluidized bed. Except for conductivity, MBFB effluent meets the industry standard for workshop reuse water in the textile printing and dyeing sector and can be used directly for water washing, soaping and rinsing, achieving a reuse rate of about 60%. It also serves as RO pretreatment - MBFB effluent enters the RO membrane directly without complex security filtration and ultrafiltration, cutting pretreatment cost and extending membrane service life.

The pressure to get there is structural. About 80% of textile wastewater comes from printing and dyeing. In 2008 the textile industry discharged 2.3 billion tons of wastewater, third among all industrial sectors and 10.60% of national industrial wastewater discharge, with CODCr emission of 314,000 tons, fourth and 7.76% of the national industrial total - and those figures cover only above-scale enterprises. SMEs account for 99.6% of the sector and non-public enterprises 95%, so the real numbers are larger. If 70% of fibre processing volume requires printing and dyeing, annual wastewater discharge is about 3 billion tons. The state asked for a 60% reuse rate during the Eleventh Five-Year Plan; the rate after treatment remains below 7%.