Characteristics and Treatment Solutions of Printing and Dyeing Wastewater
The main methods for treating printing and dyeing wastewater include physicochemical, biochemical, and chemical processes, as well as combined process approaches. Pretreatment in wastewater treatment primarily aims to improve water quality, remove suspended solids and readily settleable impurities, regulate water quality and quantity, and lower the wastewater temperature, thereby enhancing the overall treatment performance and ensuring the stability of the entire treatment system. Pretreatment therefore holds an extremely important position in printing and dyeing wastewater treatment.
The printing and dyeing industry is a major source of industrial wastewater discharge. According to incomplete statistics, the daily discharge of printing and dyeing wastewater nationwide ranges from 3x10^6 to 4x10^6 m3. Such wastewater is characterized by large volume, high organic pollutant content, deep color, strong alkalinity, and highly variable quality, making it a difficult-to-treat industrial wastewater.
1 Desizing wastewater: relatively small in volume but high in pollutant concentration. It mainly contains sizing agents and their decomposition products, fiber debris, acids, starch alkalis, and enzyme pollutants, with high turbidity. The wastewater is alkaline, with a pH around 12. When starch sizing is used, both BOD and COD are high and biodegradability is good; when synthetic sizing is used, COD is very high, BOD is below 5 mg/L, and biodegradability is poor.
2 Scouring wastewater: large in volume and high in pollutant concentration. It mainly contains cellulose, pectic acid, wax, grease, alkali, surfactants, and nitrogenous compounds. The wastewater is strongly alkaline, hot, and brownish, with very high COD and BOD reaching several thousand mg/L. Scouring wastewater from chemical fibers causes relatively lighter pollution.
3 Bleaching wastewater: large in volume but relatively lightly polluted. It mainly contains residual bleaching agents, small amounts of acetic acid, oxalic acid, sodium thiosulfate, etc.
4 Mercerizing wastewater: high in alkali content, with NaOH at 3%-5%. Most printing and dyeing mills recover NaOH by evaporation and concentration, so mercerizing wastewater is rarely discharged; the final effluent after multiple reuse cycles remains strongly alkaline, with high BOD, COD, and SS.
5 Dyeing wastewater: highly variable in quality and sometimes containing toxic substances from various dyes (sodium sulfide, tartar emetic, aniline, copper sulfate, phenol, etc.). It is alkaline, with pH sometimes above 10 (when sulfur or vat dyes are used), and contains organic dyes and surfactants. It has high color but low SS; COD exceeds BOD, and biodegradability is poor.
6 Printing wastewater: contains sizing agents, with high BOD and COD.
7 Finishing-process wastewater: mainly contains fiber debris, resins, formaldehyde, finishing oils, and sizing agents, with small volume.
8 Alkali-reduction wastewater: generated from the alkali-reduction process for polyester imitation silk, mainly containing polyester hydrolysis products such as terephthalic acid and ethylene glycol, with terephthalic acid content as high as 75%. This wastewater not only has a high pH (generally >12) but also high organic concentration; the CODCr in the effluent from the alkali-reduction process can reach up to 90,000 mg/L. The macromolecular organics and some dyes are difficult to biodegrade, making this a high-concentration, refractory organic wastewater.
The discharge standard for printing and dyeing wastewater is the 'Discharge Standard of Water Pollutants for Dyeing and Finishing of Textile Industry' (GB 4287-1992).
Class 1: harmful substances that can accumulate in the environment or in animals and plants and have long-term effects on human health; their maximum allowable discharge concentrations are listed in Table 5-4.
Class 2: harmful substances whose long-term effects are less severe than those of Class 1; their maximum allowable discharge concentrations are listed in Table 5-5.
Maximum allowable discharge concentration (mg/L)
0.05 (expressed as Hg)
0.5 (expressed as Cr6+)
Suspended solids (e.g., hydraulic ash discharge, coal-washing water)
Biochemical oxygen demand (5 days, 20 C)
Chemical oxygen demand (dichromate method)
Cyanide (expressed as CN-)
1 mg/L (expressed as Cu)
5 mg/L (expressed as Zn)
10 mg/L (expressed as F)
In light of the characteristics of the textile printing and dyeing industry, the treatment of printing and dyeing wastewater should, as far as possible, adopt reuse and comprehensive utilization measures, combined with reforms of the printing and dyeing production process, so as to minimize the consumption of water, alkali, and other auxiliaries and to recover dyes and sizing agents from the wastewater. For example, dry printing can be adopted for synthetic fibers and fabrics containing more than 75% synthetic fiber, which eliminates printing wastewater during production. In the use of acid mordant dyes, replacing potassium dichromate with sodium nitrate or hydrogen peroxide as the oxidant can eliminate chromium pollution in the wastewater. Many printing and dyeing enterprises commonly reuse the alkaline liquor from the mercerizing process as the scouring liquor in the scouring stage, and the waste alkali liquor from scouring is reused in the desizing stage; repeated reuse can greatly reduce the total alkali discharged in the whole process. For wastewater containing sulfur dyes, acid can first be added in a reaction vessel to release hydrogen sulfide, which is then recovered after precipitation and filtration. For wastewater containing vat and disperse dyes, ultrafiltration can be used to recover water-insoluble dye particles. Through such technological innovations, pollutant emissions from the textile printing and dyeing industry can be effectively reduced, while also saving raw materials for producers and increasing their economic benefits.
The main treatment targets of cotton textile industry wastewater are alkalinity, organic matter that is difficult or extremely slow to biodegrade, dye pigments, and toxic substances. In the United States, most printing and dyeing wastewater is treated by secondary treatment-a process route combining physicochemical pretreatment with biochemical treatment-and a few enterprises use a tertiary treatment system that adds activated-carbon adsorption after biochemical treatment. Japanese textile printing and dyeing enterprises adopt processes similar to those in the United States, but apply ozonation somewhat more often. In China, printing and dyeing wastewater treatment also mainly combines physicochemical treatment with secondary specialized treatment; physicochemical treatment is dominated by coagulation-sedimentation and coagulation-flotation, while among the biochemical facilities already in operation, most adopt the activated sludge process, and the application of SBR (sequencing batch reactor) is gradually increasing. Below we mainly introduce the coagulation pretreatment process and the subsequent biochemical treatment process.
The coagulation method involves adding chemical reagents to the wastewater so that most of the water-insoluble dye particles and colloidal organics coagulate into larger flocs, which are then removed by natural sedimentation, flotation, etc. Because the flocs formed during coagulation have strong adsorption capacity, some water-soluble organics can also be removed by adsorption. After coagulation treatment, more than 80% of the suspended organic pollutants in printing and dyeing wastewater are removed, while the color removal rate can reach 50%-95%.
The key to the coagulation treatment of printing and dyeing wastewater lies in selecting a suitable flocculant. Conventional flocculants suitable for printing and dyeing wastewater treatment mainly include aluminum sulfate, ferric sulfate, and ferric chloride; these flocculants show obvious effects on some water-insoluble dye wastewaters-such as disperse, vat, and sulfur dyes-with very high removal rates of COD and color.
The treatment process flow: equalization tank -> hydrolysis acidification -> biological contact oxidation -> intermediate sedimentation tank ->
-> coagulation reaction tank -> dissolved-air flotation tank -> effluent.
The wastewater treatment chemicals used in this process are polyacrylamide (PAM) and polyaluminum chloride (PAC), with calcium hypochlorite as the decolorizing agent. The core of the process is aerobic biological contact oxidation, whose main function is to degrade organics. For PAM selection in coagulation, anionic PAM and nonionic PAM are generally chosen, while cationic PAM is selected for sludge dewatering. For cationic types, a relatively low charge density is selected, and a molecular weight above 10 million generally gives better results.
The printing and dyeing industry is a large water consumer; its wastewater discharge and total pollutant load rank second and fourth respectively among national industrial sectors, making it one of China's key polluting industries. Printing and dyeing wastewater has long been a focus and difficulty in wastewater treatment research because of its large discharge and high treatment difficulty. Meanwhile, with the rapid development of China's economy, water scarcity has become a limiting factor constraining the further development of the printing and dyeing industry. To achieve the sustainable development of the industry, the resource-based reuse of printing and dyeing wastewater has become the key to this goal.
Analysis of Problems in Printing and Dyeing Mill Wastewater Treatment
There are many successful cases of printing and dyeing mill wastewater treatment, but also quite a few with poor results. The reasons are roughly as follows: (1) The mill did not analyze its own wastewater characteristics (quality and quantity) and simply copied other mills' experience, often yielding unsatisfactory results. (2) Design specifications for municipal wastewater treatment were applied to printing and dyeing wastewater with only a few parameters changed, causing significant losses. Especially in the early period, centralized treatment of large mills was handled by large design institutes that lacked deep understanding of printing and dyeing wastewater, leading to major losses. (3) New technologies, processes, and chemicals were applied directly to projects without pilot testing, causing many failures. New technologies should generally undergo lab and pilot tests before engineering application, with the pilot scale usually 3%-5% of the project flow (i.e., magnification of at most about 20x). Applying lab results directly to engineering rarely succeeds. Projects should adopt the most mature and reliable technologies. (4) Wastewaters with similar production processes may use similar treatment processes, but technical parameters must still be adjusted according to water quality and quantity to ensure treatment performance. (5) Improper operation and management techniques-failing to make timely adjustments to changing wastewater-also cause unstable operation.
The rise of imitation silk and advances in dyeing and finishing technologies have caused large amounts of hard-to-biodegrade organics-such as PVA sizing, viscose alkali-hydrolysis products (mainly phthalates), and new auxiliaries-to enter printing and dyeing wastewater. Their COD concentration has risen from several hundred mg/L to 2,000-3,000 mg/L, causing the COD removal rate of original biological systems to drop from 70% to around 50% or even lower. Traditional biological processes face serious challenges; conventional chemical precipitation and flotation achieve only about 30% COD removal for such wastewater. Therefore, developing economical and effective printing and dyeing wastewater treatment technologies has increasingly become a focus of the environmental protection industry.
One of the main constraints on the development of the textile industry is environmental protection and energy conservation (low carbon). The main environmental issue is wastewater, and about 80% of textile wastewater comes from the printing and dyeing sector. Statistics show that in 2008 the textile industry discharged 2.3 billion tons of wastewater, ranking third among all industrial sectors and accounting for 10.60% of national industrial wastewater discharge. The textile industry's CODCr emission was 314,000 tons, ranking fourth and accounting for 7.76% of the national industrial CODCr. These figures cover only above-scale enterprises; the actual figures are likely much larger. In reality, the printing and dyeing industry is a competitive sector dominated by small and medium enterprises-SMEs account for 99.6% and non-public enterprises for 95%-and many small enterprises are not included in the statistics. If 70% of fiber processing volume requires printing and dyeing, the annual wastewater discharge is about 3 billion tons.
Production enterprises focused on garment dyeing, washing, and ironing discharge large volumes of wastewater containing certain organics and color during production, which must undergo advanced treatment before reuse. The state required the industry's wastewater reuse rate to reach 60% during the Eleventh Five-Year Plan, yet the reuse rate after treatment remains below 7%. Moreover, as China is a country severely short of water resources, limited water supplies determine that the printing and dyeing industry must follow a circular-economy path. Therefore, vigorously promoting reclaimed-water reuse is a far-sighted and wise choice.
1. Comply with relevant environmental regulations to ensure that all effluent indicators meet the requirements of national and local water-quality standards;
2. Select a relatively mature treatment process with simple, reliable, safe, and easy operation, minimizing operating costs and capital investment;
3. Select a process with a short flow, feasibility, shock-load resistance, and stable treatment performance;
4. Easy operation, management, and maintenance;
5. The construction site and land use should fully consider the user's existing conditions, designate the site per the client's requirements, and allow for rational pipe-network layout;
6. The water treatment station should cause no secondary pollution, minimizing impact on the surrounding living environment.
In the printing and dyeing wastewater reuse process, lime is used as the pH regulator and ferrous sulfate as the coagulant, so the effluent has a high iron content and cannot be used directly for reuse. However, this project adopts a physicochemical + biochemical process as the front-end treatment; especially after intensified aeration in the contact oxidation tank, the ferrous iron in the water is converted into ferric iron, forming ferric hydroxide micro-flocs in the effluent, which is also the main reason for the turbidity and color of the treatment-station effluent.
Simply adding a certain amount of polyaluminum chloride and PAM to the effluent combines the ferric hydroxide micro-flocs into larger flocs that can be removed by high-efficiency filtration. This project therefore adopts the AFF asymmetric-fiber filter, an integrated high-efficiency filtration device combining dosing, micro-flocculation, sedimentation, and filtration. Its features include fast filtration rate (more than 10x that of sand filtration), high filtration precision (5 um, 4x that of ordinary sand filtration), easy backwashing, and convenient management. In this project, AFF is mainly used to remove iron and suspended solids from the reclaimed water.
After AFF filtration, the reclaimed water still has a COD of around 100 mg/L, mainly soluble COD (SCOD), which directly affects reuse value; meanwhile, organics greatly affect the service life of reverse osmosis membranes. An appropriate process must be applied to reduce it below 30 mg/L.
Therefore, the membrane biological fluidized bed (MBFB) process is adopted, using specially treated ceramic membranes to combine membrane separation with a high-load biological fluidized bed, achieving stable treated-water quality. This process has been promoted and applied in the sewage and wastewater treatment fields of countries and regions including the United States, Japan, the United Kingdom, Germany, South Africa, and Australia.
Except for conductivity, the effluent from the MBFB process meets the industry standard for workshop reuse water in the textile printing and dyeing sector, and can be used directly for production processes such as water washing, soaping, and rinsing, achieving a reuse rate of about 60%. At the same time, the MBFB process can serve as the pretreatment stage for reverse osmosis; the MBFB effluent can enter the RO membrane directly for desalination without the need for complex security filtration and ultrafiltration stages.
By adopting an advanced reclaimed-water reuse process on top of the original compliant discharge, the iron and COD concentrations in the water are further reduced. On one hand, it can be used directly as reuse water for stages with lower water-quality requirements such as water washing, soaping, and front-end rinsing; on the other hand, the treated reclaimed water can be directly desalinated by reverse osmosis or ion exchange, eliminating the multi-stage security filtration and ultrafiltration in the RO process, reducing pretreatment costs and extending the RO membrane service life.
