Water Quality Characteristics of Printing and Dyeing Wastewater and Its Treatment Solutions
The principal methods used to treat printing and dyeing wastewater are physicochemical, biochemical and chemical processes, along with combinations of several of them. Pretreatment within the treatment train mainly serves to improve water quality, remove suspended solids and directly settleable impurities, balance water quality and flow, and lower wastewater temperature, thereby improving overall treatment performance and ensuring the stability of the whole system. Pretreatment therefore holds an extremely important place in printing and dyeing wastewater treatment. [1]
The printing and dyeing industry is a major industrial wastewater discharger; incomplete statistics put national printing and dyeing wastewater discharge at 3×10⁶–4×10⁶ m³ per day. This wastewater is high in volume, high in organic pollutant content, deeply coloured, strongly alkaline and highly variable in quality, making it one of the harder industrial wastewaters to treat.
① Desizing wastewater: low in volume but high in pollutant concentration, [1] containing mainly sizing agents and their decomposition products, fibre debris, acids, starch, alkali and enzymes, with high turbidity. The wastewater is alkaline, with pH around 12. With starch sizing agents, BOD and COD are both high and biodegradability is good; with synthetic sizing agents, COD is very high while BOD is below 5 mg/L and biodegradability is poor.
② Scouring wastewater: high in volume and high in pollutant concentration, containing mainly cellulose, pectic acid, waxes, oils and fats, alkali, surfactants and nitrogen compounds. It is strongly alkaline and hot, brown in colour, with COD and BOD reaching several thousand milligrams per litre. Scouring wastewater from synthetic fibres is less polluted.
③ Bleaching wastewater: high in volume but relatively lightly polluted, containing mainly residual bleaching agents along with small amounts of acetic acid, oxalic acid and sodium thiosulfate.
④ Mercerising wastewater: high in alkali, with NaOH content of 3%–5%. Most printing and dyeing mills recover NaOH by evaporation and concentration, so mercerising wastewater is rarely discharged; the wastewater finally released after repeated reuse remains strongly alkaline, with relatively high BOD, COD and SS.
⑤ Dyeing wastewater: highly variable in quality, sometimes containing toxic substances associated with particular dyes (sodium sulfide, tartar emetic, aniline, copper sulfate, phenols and so on). It is alkaline, with pH sometimes above 10 (when sulfur or vat dyes are used), and contains organic dyes and surfactants. Colour is very intense while SS is low, COD is higher than BOD, and biodegradability is poor.
⑥ Printing wastewater: contains sizing agents, with high BOD and COD.
⑦ Finishing wastewater: contains mainly fibre debris, resins, formaldehyde, oils and sizing agents; volume is small.
⑧ Alkali-reduction wastewater: generated by the alkali-reduction step in polyester imitation-silk processing, containing mainly polyester hydrolysates such as terephthalic acid and ethylene glycol, with terephthalic acid content as high as 75%. Alkali-reduction wastewater is not only high in pH (generally >12) but also high in organic concentration: CODCr in the wastewater from the alkali-reduction step can reach 90,000 mg/L, and the macromolecular organics and some dyes are very hard to biodegrade. It is a high-concentration, refractory organic wastewater.
The discharge standard for printing and dyeing wastewater is the Discharge Standards of Water Pollutants for the Textile Dyeing and Finishing Industry (GB 4287-1992).
Category I: harmful substances that can accumulate in the environment or in animals and plants and have long-term effects on human health; maximum permissible discharge concentrations are listed in Table 5-4.
Category II: harmful substances whose long-term effects are smaller than those of Category I; maximum permissible discharge concentrations are listed in Table 5-5.
Maximum permissible discharge concentration (mg/L)
0.05 (as Hg)
0.5 (as Cr6+)
Suspended solids (hydraulic ash sluicing, coal washing water, etc.)
Biochemical oxygen demand (5 days at 20°C
Chemical oxygen demand (potassium dichromate method)
Cyanide (as CN-)
1 mg/L (as Cu)
5 mg/L (as Zn)
10 mg/L (as F)
Given the nature of the textile printing and dyeing industry, wastewater treatment should make the fullest possible use of reuse and comprehensive utilisation measures, tie in with reform of the production process itself, minimise consumption of water, alkali and other dyeing auxiliaries, and recover dyes and sizing agents from the wastewater. For instance, using dry printing processes for synthetic fibres and fabrics containing more than 75% synthetic fibre can eliminate printing wastewater from the production process; and when acid mordant dyes are used, replacing potassium dichromate with sodium nitrate or hydrogen peroxide as the oxidant can eliminate chromium pollution in the wastewater. Many printing and dyeing plants routinely send the alkali liquor discharged from mercerising to the scouring step for use as scouring liquor, and the spent alkali from scouring to the desizing step; repeated reuse of this kind can greatly reduce the total alkali discharged across the whole process. For wastewater containing sulfur dyes, acid can first be added in a reaction vessel to release the hydrogen sulfide, after which the water is settled, filtered and reused. For wastewater containing vat and disperse dyes, ultrafiltration can be used to recover the water-insoluble dye particles for reuse. Production innovations of this kind can effectively cut pollutant discharge from the textile printing and dyeing industry while saving raw materials and improving the economic returns of the enterprise.
The main treatment targets in cotton textile industry wastewater are alkalinity, organic matter that is not readily biodegradable or degrades extremely slowly, dye colourants and toxic substances. In the United States most printing and dyeing wastewater receives secondary treatment — a physicochemical pretreatment stage combined with biochemical treatment — while a few enterprises use a tertiary system that adds activated carbon adsorption after the biochemical stage. Japanese textile printing and dyeing enterprises use processes similar to those in the United States, though ozonation is somewhat more common. In China, printing and dyeing wastewater is likewise treated mainly by combining physicochemical treatment with a second specialised chemical stage; coagulation-sedimentation and coagulation-flotation dominate the physicochemical side, while most biochemical facilities already in operation use the activated sludge process, with SBR applications increasing steadily. The sections below focus on coagulation pretreatment and the subsequent biochemical treatment.
Coagulation involves dosing chemicals into the wastewater so that most of the water-insoluble dye particles and colloidal organics in the printing and dyeing wastewater agglomerate into larger particles, which are then removed by natural sedimentation, flotation or similar means. Because the flocs formed during coagulation have strong adsorption capacity, a portion of the water-soluble organics is also removed by adsorption. Coagulation removes more than 80% of the suspended organic pollutants from printing and dyeing wastewater, with colour removal reaching 50–95%.
The key to coagulation treatment of printing and dyeing wastewater is choosing the right flocculant. The flocculants conventionally used are aluminium sulfate, ferric sulfate and ferric chloride, which work well on wastewater containing water-insoluble dyes — disperse dyes, vat dyes and sulfur dyes, for example — where COD and colour removal rates are both very high.
The treatment train runs: process wastewater — equalisation tank — hydrolytic acidification — biological contact oxidation — intermediate settling tank —
— coagulation reaction tank — flotation tank — effluent.
The treatment chemicals used in this process are polyacrylamide and polyaluminium chloride, with calcium hypochlorite as the decolourising agent. The heart of the process is aerobic biological contact oxidation, whose main function is to degrade organics. On the polyacrylamide side, coagulation treatment generally calls for anionic and non-ionic polyacrylamide, while sludge dewatering calls for cationic polyacrylamide; a cationic grade with relatively low charge density and a molecular weight above 10 million generally performs best.
One of the main obstacles to the development of the textile industry is environmental protection and energy conservation (low carbon), and the principal environmental issue is wastewater — around 80% of textile wastewater comes from printing and dyeing. Statistics show that in 2008 the textile industry discharged 2.3 billion tonnes of wastewater, third among all industrial sectors and 10.60% of national industrial wastewater discharge. Chemical oxygen demand (CODCr) discharged in textile industry wastewater came to 314,000 tonnes, fourth among all industrial sectors and 7.76% of national industrial wastewater CODCr. These figures cover only enterprises above a designated size, so the real numbers may be considerably higher. The printing and dyeing industry is in fact a competitive sector dominated by small and medium-sized enterprises — SMEs account for 99.6% and non-public enterprises 95% — and data from large numbers of small firms are not captured. If 70% of fibre processing volume requires printing and dyeing, annual wastewater discharge is on the order of 3 billion tonnes.
Analysing the Problems in Printing and Dyeing Plant Wastewater Treatment
There are many successful examples of printing and dyeing plant wastewater treatment, but plenty of disappointing ones too, broadly for the following reasons. (1) The plant fails to analyse the characteristics of its own wastewater (quality and quantity) and simply copies another plant's experience, which rarely turns out well. (2) Design codes for municipal sewage treatment are applied to printing and dyeing wastewater with only a few parameters changed, causing heavy losses. This was particularly common in the early years, when centralised wastewater treatment at large printing and dyeing plants was handled by large design institutes that did not understand the nature of the wastewater well enough, resulting in substantial losses. (3) New technologies, processes and chemicals are taken straight into full-scale projects without pilot testing, which causes many failures. New technology should generally pass bench and pilot trials first; pilot scale is typically 3%–5% of project flow, meaning a scale-up of at most about 20 times. Taking laboratory results directly into a project rarely succeeds. Projects should use the most mature and reliable technology available. (4) Wastewater from similar production processes can use similar treatment processes, but technical parameters still need appropriate adjustment for water quality and quantity in order to maintain treatment performance. (5) Poor operating technique and management, with no suitable adjustment as the wastewater changes, is another cause of unstable operation.
The rise of imitation silk and advances in printing and dyeing finishing technology have brought large quantities of poorly biodegradable organics into printing and dyeing wastewater — PVA sizing agents, rayon alkali hydrolysates (mainly phthalate compounds) and new auxiliaries. COD concentrations have risen from a few hundred mg/L to 2,000–3,000 mg/L, pushing COD removal by existing biological treatment systems down from 70% to around 50% or even lower. Conventional biological treatment processes are under serious pressure; conventional chemical precipitation and flotation achieve COD removal of only about 30% on this kind of wastewater. Developing economical and effective printing and dyeing wastewater treatment technology has therefore become an increasingly prominent topic in the environmental sector.
The printing and dyeing industry is a heavy water user, ranking second in wastewater discharge volume and fourth in total pollutant load among all national industrial sectors — one of China's key polluting industries. Its large discharge volume and treatment difficulty have long made printing and dyeing wastewater both a focus and a challenge for treatment process research. At the same time, as China's economy has grown rapidly, water scarcity has become a constraint on the industry's further development. Achieving sustainable development in printing and dyeing therefore hinges on recycling and reusing the wastewater as a resource.
Production enterprises centred on garment dyeing, washing, and finishing and pressing discharge large volumes of wastewater containing a certain amount of organic matter and colour, and this wastewater requires advanced treatment before it can be reused. The state required the industry to reach a 60% wastewater reuse rate during the 11th Five-Year Plan, yet the post-treatment reuse rate has not even reached 7%. Since China is a country with severely limited water resources, those limits alone dictate that the printing and dyeing industry must follow a circular economy path, which makes vigorous development of reclaimed water reuse a far-sighted choice.
1. Comply with the relevant environmental regulations and ensure every effluent indicator meets national and local water quality standards;
2. Choose a reasonably mature treatment process so the system runs simply, reliably and safely, is easy to operate, and keeps operating costs and capital expenditure as low as possible;
3. Choose a short process flow that is technically feasible, resistant to shock loads and stable in treatment performance;
Reclaimed water that has passed through AFF filtration still has a COD of around 100 mg/L, mostly soluble COD (SCOD), which directly undermines its reuse value; organics also shorten reverse osmosis membrane life considerably, so a suitable treatment process must bring COD below 30 mg/L.
5. The construction site and land use should take full account of the user's existing conditions, use the land at the location specified by the plant, and allow for sensible pipe network layout;
6. The water treatment station should produce no secondary pollution, so as to limit its impact on the surrounding living environment.
In this printing and dyeing wastewater reuse process, lime serves as the pH adjuster and ferrous sulfate as the coagulant, so effluent iron content is relatively high and the water cannot be reused directly. This project, however, uses a physicochemical plus biochemical front end, and the intensified aeration in the contact oxidation tank in particular converts all ferrous iron in the water to ferric iron, forming ferric hydroxide microflocs in the effluent — the main reason the treatment station's effluent is turbid and coloured.
Adding a certain amount of polyaluminium chloride and PAM to the effluent binds these ferric hydroxide microflocs into larger flocs, and high-efficiency filtration then removes the iron from the water. This project therefore uses an AFF asymmetric fibre filter, a high-efficiency filtration unit that integrates dosing, microflocculation, sedimentation and filtration. It offers fast filtration (more than 10 times the rate of sand filtration), high filtration precision (5 μm, four times that of ordinary sand filters), easy backwashing and convenient management. In this project, AFF mainly serves to remove iron and suspended solids from the reclaimed water.
4. Convenient to operate and manage, and easy to maintain;
A membrane biological fluidised bed (MBFB) process is therefore adopted, combining a specially treated ceramic membrane separation system with a high-load biological fluidised bed to obtain stable treated water quality. The process has already been promoted and applied in sewage and wastewater treatment in the United States, Japan, the United Kingdom, Germany, South Africa, Australia and other countries and regions.
Apart from conductivity, MBFB-treated effluent meets the industry standard for workshop reuse water in the textile printing and dyeing sector and can be used directly in washing, soaping and rinsing workshops, giving a reuse rate of roughly 60%. MBFB can also serve as the pretreatment stage ahead of reverse osmosis, feeding the RO membranes directly for desalination without the need for complex cartridge filtration and ultrafiltration stages.
Adopting an advanced reclaimed water reuse process further reduces iron and COD concentrations on top of compliant wastewater discharge. On one hand the water can be reused directly in washing, soaping, front-end rinsing and other stages with modest water quality requirements; on the other, the treated reclaimed water can be desalinated directly by reverse osmosis or ion exchange, dispensing with the multi-stage cartridge filtration and ultrafiltration steps of a conventional RO train, cutting pretreatment costs and extending RO membrane life.
