How to Treat Printing and Dyeing Wastewater? Common Process Methods
(1) Desizing wastewater. It contains various sizing agents and their decomposition products, fiber debris, acid/alkali and enzyme pollutants, etc. Wastewater using starch sizing has high BOD and COD, while that using synthetic sizing has higher COD and BOD below 5 mg/L;
(2) Boiling/scouring wastewater. Cotton-fiber wastewater is strongly alkaline, with high COD and BOD (up to several thousand mg/L), large volume, high pollution and brown color, while chemical-fiber wastewater is less polluted;
(3) Bleaching wastewater. Large volume, lighter pollution;
(4) Mercerizing wastewater. Alkaline, pH 12–13, containing much fiber debris and other suspended solids, with very high BOD and COD;
(5) Dyeing wastewater. Pollution varies with fiber type, dye kind and concentration, auxiliaries and scale; it mainly contains organic dyes and surfactants, is alkaline with high COD and BOD but few suspended solids;
(6) Printing wastewater. Mainly contains organic dyes and surfactants, with high COD and BOD;
(7) Finishing-stage wastewater. Mainly contains fiber debris, resin, formaldehyde, oil agents and sizing, with small volume. Wool dyeing/finishing plants have high-concentration wastewater: about 318 t of wastewater per 454 kg of clean wool, brown and colloidal, with 91–114 kg of BOD-based organic pollutants [1].
Printing/dyeing wastewater discharge is very large; European statistics show a fabric-to-wastewater weight ratio of 1:150–1:200, and in China about 1:200–1:400. China's textile-industry wastewater ranks sixth among all industrial wastewater discharges, of which 80% is printing/dyeing wastewater.
Printing/dyeing wastewater contains unreacted dyes and pigments (coatings) with heavy color, plus unreacted auxiliaries, reaction products and fabric debris. More seriously, it also contains carcinogenic and teratogenic organic compounds and toxic heavy metals.
The combination and nature of various components in the wastewater change irregularly with market shifts, season changes and supply turnover.
Printing/dyeing wastewater is one of the harder industrial wastewaters to treat. For technical and economic reasons, most biological–physical methods used can only meet basic discharge requirements. Although color decreases somewhat, organic matter is only broken into smaller substances whose properties are hard to control or master, giving no guarantee of no environmental harm.
(5) Heavy economic treatment burden
Current treatment methods need large land area, high investment and costly treatment, keeping production costs high. It is estimated that meeting the secondary discharge standard costs about the same as city tap-water price; meeting reuse requirements costs even more, so actual operation is quite difficult.
Printing/dyeing wastewater keeps conflicting with agricultural production and must be taken seriously [2].
All four printing/dyeing processing stages discharge wastewater: the pretreatment stage (singeing, desizing, boiling, bleaching, mercerizing, etc.) discharges desizing, boiling, bleaching and mercerizing wastewater; the dyeing stage discharges dyeing wastewater; the printing stage discharges printing and soaping wastewater; and the finishing stage discharges finishing wastewater. Printing/dyeing wastewater is the mixed wastewater of all the above, or the comprehensive wastewater excluding bleaching wastewater.
Printing/dyeing wastewater quality varies with fiber type and processing technology, with great differences in pollutant composition. Generally its pH is 6–10, CODcr 400–1000 mg/L, BOD5 100–400 mg/L, SS 100–200 mg/L and colority 100–400 times. But when the printing/dyeing process or fiber/process changes, quality changes greatly. For example, when it contains alkali-peeling wastewater from polyester simulation-silk printing/dyeing, CODcr rises above 2000–3000 mg/L, BOD5 above 800 mg/L and pH to 11.5–12, and quality worsens as more alkali-peeling wastewater is added; when the CODcr from added alkali-peeling wastewater exceeds 20% of total CODcr, biological treatment adapts poorly. The drainage of each stage is generally:
(1) Desizing wastewater: small volume but high pollutant concentration, containing various sizing agents, their decomposition products, fiber debris, starch alkali and various auxiliaries. Alkaline, pH about 12. Starch-based sizing (e.g., cotton) desizing wastewater has very high COD and BOD and good biodegradability; PVA-based sizing (e.g., polyester-cotton warp) desizing wastewater has high COD but low BOD and poor biodegradability.
(2) Boiling wastewater: large volume, high pollutant concentration, containing cellulose, pectic acid, wax, grease, alkali, surfactants, nitrogen compounds, etc.; strongly alkaline, high temperature, brown.
(3) Bleaching wastewater: large volume but lighter pollution, containing residual bleach, a little acetic acid, oxalic acid, sodium thiosulfate, etc.
(4) Mercerizing wastewater: high alkali, NaOH 3%–5%; most plants recover NaOH by evaporation concentration, so little is discharged, but the finally discharged wastewater after repeated reuse is still strongly alkaline with high BOD, COD and SS.
(8) Alkali-peeling wastewater: produced by the polyester simulation-silk alkali-peeling step, mainly containing polyester hydrolysis products terephthalic acid and ethylene glycol, with terephthalic acid up to 75%. It has high pH (generally >12) and high organic concentration; CODcr can reach 90,000 mg/L, and high-molecular organics and some dyes are hard to biodegrade, making it a high-concentration refractory organic wastewater [3].
(6) Printing wastewater: fairly large volume, including not only printing-process wastewater but also soaping and washing wastewater after printing, with high pollutant concentration containing sizing, dyes and auxiliaries, and high BOD and COD.
(7) Finishing wastewater: small volume, containing fiber debris, resin, oil agents, sizing, etc.
(5) Dyeing wastewater: fairly large volume, quality varying with the dye used, containing sizing, dyes, auxiliaries and surfactants; generally strongly alkaline, very high colority, COD much higher than BOD, poor biodegradability.
Printing/dyeing wastewater treatment methods include physical, chemical and biological methods.
The most used physical method is adsorption: powdered or granular porous materials such as activated carbon or clay are mixed with the wastewater, or the wastewater is passed through a filter bed of such particles, so pollutants are adsorbed on the porous surface or filtered out. Abroad, activated-carbon adsorption is mainly used (mostly for tertiary treatment). It is very effective for dissolved organics but cannot remove colloids or hydrophobic dyes, and adsorbs well only water-soluble dyes such as cationic, direct, acid and reactive dyes. Saito T et al. found adsorption, BOD and COD removal rates of 93%, 92% and 63% respectively, with capacity up to 500 mg COD/g carbon; pre-aeration speeds adsorption, but if BOD5 > 200 mg/L the method is uneconomic.
Many adsorbents are used; engineering must consider selectivity for dyes and choose by wastewater quality. Studies show that in printing/dyeing wastewater at pH=12, a siloxane polymer (methyl) adsorbent removes anionic dyes by 95%–100%.
Kaolin is an adsorbent; studies show that after treatment with long-chain organic cations, kaolin effectively adsorbs yellow direct dyes from wastewater. Domestically, activated diatomite and coal cinder are also used for traditional printing/dyeing wastewater at lower cost and good decolorization, but with the drawback of large slime production and difficult further treatment.
Mainly coagulation-sedimentation and coagulation-flotation are used, with coagulants mostly aluminum or iron salts; polyaluminum chloride (PAC) has better bridging-adsorption performance, while ferrous sulfate is cheapest. In recent years abroad polymeric coagulants are increasingly used and tend to replace inorganic ones, but in China they are rare due to cost. Reportedly, weakly anionic polymeric coagulants have the widest use and work better with aluminum sulfate. Coagulation's main advantages are simple flow, easy operation, low equipment investment and small footprint, and high decolorization for hydrophobic dyes; drawbacks are higher running cost, large and hard-to-dewater slime, and poor performance on hydrophilic dyes.
Ozone oxidation is widely used abroad; Zima S.V. et al. derived a mathematical model for ozone decolorization of printing/dyeing wastewater, showing 80% decolorization of light-brown dye wastewater at 0.886 g O3/g dye; they also found continuous operation needs more ozone than intermittent, and installing baffles in the reactor cuts ozone use by 16.7%. Thus ozone decolorization is best designed as an intermittent reactor, possibly with baffles. Ozone oxidation decolorizes most dyes well but poorly for water-insoluble sulfide, vat and coating dyes. Domestic and foreign experience shows good decolorization but high power use, making large-scale application difficult.
Photoxidation decolorizes printing/dyeing wastewater with high efficiency, but equipment investment and power use still need to be lowered;
Electrolysis treats printing/dyeing wastewater containing acid dyes well, with 50%–70% decolorization, but poorly for deeply colored, high-CODcr wastewater. Electrochemical studies show the CODcr removal order among dyes under electrolysis is: sulfide and vat dyes > acid and reactive dyes > neutral and direct dyes > cationic dyes. This method is being promoted.
Since the 1970s, domestic printing/dyeing wastewater treatment has been mainly biological, over 80%, mostly aerobic. Currently, surface-accelerated aeration and contact oxidation dominate China's biological treatment; blast-aeration activated sludge, jet-aeration activated sludge and biodiscs are also used, while biological fluidized beds are still experimental. But biological color removal is not high, generally around 50%, so physical or chemical treatment is needed as supplement when effluent colority is strictly required.
Aerobic biological treatment clearly removes BOD, generally about 80%, but color and COD removal are low. Especially with widespread PVA and other chemical sizings, surfactants, solvents and fabric alkali-peeling, COD reaches 2,000–3,000 mg/L and BOD/COD falls from 0.4–0.5 to below 0.2, making sole aerobic treatment harder and effluent hard to meet standards. Also, aerobic treatment's high running cost and the sludge handling/disposal problem have long been unsolved in wastewater treatment — reported sludge costs are 50%–70% of total plant cost abroad and about 40% in China. For these reasons, anaerobic biological treatment of printing/dyeing wastewater has begun to receive attention.
The printing/dyeing industry is a big water consumer; its wastewater discharge and total pollutants rank second and fourth among national industrial sectors, making it a key polluting industry in China. Printing/dyeing wastewater, with large discharge and high treatment difficulty, has long been a focus and difficulty of treatment-process research. Meanwhile, with China's rapid economic growth, water scarcity has become a limiting factor for further development of the industry. To achieve sustainable development, resource-reuse of printing/dyeing wastewater is key.
Production enterprises focused on garment dyeing, washing and ironing discharge large volumes of wastewater containing some organics and colority, requiring advanced treatment before reuse. The state required the whole industry's sewage reuse rate to reach 60% during the 11th Five-Year Plan, but the post-treatment reuse rate is still below 7%. As China is severely short of water, limited resources dictate that the printing/dyeing industry must follow a circular-economy path; thus vigorously developing reclaimed-water reuse is a wise long-term choice.
