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Table of contents（Click to go to where you want to see）

5.1. Physical treatment methods ( Our company’s main products and cases)

5.2. Chemical method
5.3. Biological treatment
5.4. New treatment technolog
5.5 Hydrolysis and acidification treatment process
5.6 Photocatalytic oxidation technology
5.7 Ozone oxidation technology
5.8 Ectrochemical oxidation method

6. Research on classification of textile printing and dyeing wastewater treatment (under development)

1. How are textile printing and dyeing wastewater generated?

Printing and dyeing wastewater is generated during textile printing and dyeing production. It is one of the primary sources of pollution in the textile industry. Printing and dyeing wastewater contains a large amount of refractory organic matter, inorganic salts, and dyes, and the concentration of these pollutants is high, and the color is prominent.

With the improvement of global economic development and people's quality of life, higher requirements have been put forward for the quality of textile printing and dyeing products. Therefore, the current textile printing and dyeing wastewater treatment technology and processes have broad development prospects.

Most pollutants in printing and dyeing wastewater are organic matter and vary with the fiber type and processing technology.

Under normal circumstances, the pH value of printing and dyeing wastewater is 6-10, COD (chemical oxygen demand) is 400-1000 mg/L, BOD (biological oxygen demand) is 100-400 mg/L, and SS (suspended solids) is 100-200 mg/L, chroma is 100-400 times.

Types of textile printing and dyeing wastewater

Desizing wastewater

Desizing is using chemicals to remove the sizing on the fabric (hydrolyzed or enzymatically decomposed into water-soluble decomposition products). At the same time, it also removes some impurities in the fiber itself. Desizing wastewater is organic wastewater, which is light yellow and contains pulp decomposition products, fiber scraps, enzymes, etc. The wastewater is alkaline with a pH value of about 12; the COD and BOD content accounts for about 45% of the printing and dyeing wastewater. When PVA or CMC chemical slurry is used, the BOD of the wastewater decreases, but the COD is high, and the wastewater is more challenging to treat. PVA slurry is one of the main reasons for the poor treatment effect of printing and dyeing wastewater.
scouring wastewater

Scouring uses aqueous caustic soda and surfactant to scour cotton fabrics under high temperature (120°C) and alkaline (PH = 10-13) conditions to remove the grease, wax, and fruit contained in the fiber. Glue and other impurities are removed to ensure the processing quality of bleaching, dyeing, and finishing. The scouring wastewater has a large volume, high water temperature, dark brown color, and alkaline solid content (alkali concentration is about 0.3%). Scouring wastewater contains cellulose, fruit acid, wax, grease, alkali, surfactant, nitrogen-containing compounds, and other substances. Its BOD and COD values are high (up to several thousand milligrams per liter), and the concentration of pollutants is high.
Bleaching wastewater

The bleaching process generally uses oxidants such as sodium hypochlorite, hydrogen peroxide, and sodium chlorite to remove impurities on the surface and interior of the fiber. Bleaching wastewater is characterized by large water volume, light pollution, low BOD and COD, and is relatively clean wastewater that can be directly discharged or recycled after treatment.
Mercerized wastewater

Mercerizing is the solution treatment of fabrics in a concentrated solution of sodium hydroxide to improve the tensile strength of the fibers.
Degree, increase the fiber's surface gloss, reduce the fabric's potential shrinkage, and improve the affinity for dyes. Mercerizing wastewater is highly alkaline (containing about 3%-5% NaOH). Most printing and dyeing factories recover NaOH through evaporation and concentration, so mercerizing wastewater is generally rarely discharged. After repeated use, the final discharged wastewater is still highly alkaline, with higher BOD, COD, and SS values.
Dyeing wastewater

The primary pollutants in dyeing wastewater are dyes and auxiliaries. Since different fiber raw materials and products require the use of other dyes, auxiliaries, and dyeing methods, as well as the different coloring rates, dye liquors, and concentrations of various dyes, the quality of dyeing wastewater changes considerably. Dyeing wastewater is generally highly alkaline and has a large amount of water. The water quality contains slurry, dyes, auxiliaries, surfactants, etc. The color of the wastewater can be as high as thousands of times, and the COD is much higher than BOD. The COD is generally 300-700. mg/L, BOD/COD is usually less than 0.2, and biodegradability is poor.
Printing wastewater

Printing wastewater mainly comes from flushing wastewater from mixing toners, printing rollers, and printing screens, as well as soaping and washing wastewater during post-printing treatment. Since the amount of slurry in printing paste is several to dozens of times greater than the amount of dye, printing wastewater also contains a large amount of slurry in addition to pigments and auxiliaries, and both BOD5 and CODcr are high. The printing wastewater is large, and the concentration of pollutants is high. Potassium dichromate is used when the printing roller is chromium-plated, and chromium trioxide is produced when the roller is stripped of chromium. These chromium-containing wastewater poisons must be treated separately.
Treating wastewater

The treated wastewater is small and contains fiber scraps, resin, oil, slurry, surfactant, formaldehyde, etc. The amount of treated wastewater is minimal and has little impact on the quality and quantity of mixed wastewater in the entire process.
Alkali reduction wastewater

Produced by the alkali reduction process of polyester imitation silk, it mainly contains polyester hydrolyzate terephthalic acid, ethylene glycol, etc., of which the terephthalic acid content is as high as 75%. Alkali reduction wastewater has a high pH value (generally >12) and a high concentration of organic matter. The COD can be as high as 90,000 mg/L. High molecular organic matter and some dyes are difficult to biodegrade. This kind of wastewater is a high-concentration refractory organic wastewater.

What are the characteristics of textile printing and dyeing wastewater?

Large amount of water

High concentration

Most wastewater is alkaline, has high COD, and is dark in color.

Large fluctuations in water quality

The production process and dyeing materials used in printing and dyeing factories vary with the type of textiles and management level, and for each factory, its products are constantly changing. Therefore, the concentration of pollutant components in wastewater changes and fluctuates frequently.

Mainly organic pollution

Except for acids and alkalis, most pollutants in wastewater are natural or synthetic organic matter.

Difficult to process

Changes in dye varieties and the extensive use of chemical slurries make wastewater contain organic matter that is difficult to biodegrade and has poor biodegradability. Therefore, printing and dyeing wastewater is one of the more difficult industrial wastewater to treat.

Some wastewater contains toxic and harmful substances

For example, printing and engraving wastewater contains hexavalent chromium; some dyes (such as aniline dyes) are highly toxic.

The ratio of BOD5 to CODcrD is generally below 0.4, and the direct biochemical performance is not very good.

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What are the hazards of textile printing and dyeing wastewater?

Printing and dyeing wastewater consists of auxiliaries, dyes, slurries, etc., discharged from the dyeing and finishing process. What causes the color of printing and dyeing wastewater is the discharged dye. During printing and dyeing, about 10%-20% of the dye is discharged into the wastewater. The dye in the wastewater can absorb light, reduce the transparency of the water body, and affect aquatic organisms and microorganisms. It is conducive to the self-purification of water bodies while causing visual pollution and causing significant damage to the environment.

Moreover, as the variety of designs and colors increases, dyeing and finishing processes are constantly updated, some of which lead to increased pollution. For example, in the alkali reduction process widely used in recent years, a large amount of terephthalic acid in the fiber is dissolved, resulting in a significant increase in COD content. The COD in the wastewater can reach 20,000-80,000mg/l;

In the same principle, the COD in wastewater from the island silk process is as high as 20,000-100,000 mg/l. The adoption of these new processes makes the treatment of printing and dyeing wastewater more difficult.

What are the primary treatment methods for textile printing and dyeing wastewater at this stage?

Physical treatment methods(Our company’s main products and cases)

The physical method is a method of removing pollutants through physical action. It has the characteristics of simple operation and no secondary pollution. It is widely used in printing and dyeing wastewater treatment.

Adsorption method

The adsorption method uses substances with adsorption capacity to separate pollutants in wastewater, such as activated carbon, zeolite, and carbon fiber. During the adsorption process, the force between pollutants and adsorbents is the van der Waals force, and the adsorbent can combine with pollutants through van der Waals forces to purify wastewater.

Flocculation method

The flocculation method is when the flocculant reacts with water to form macromolecular polymers and fine suspended solid particles in the water. After flocculation, the impurities in the wastewater are adsorbed on the macromolecular polymer, thereby purifying the wastewater.

Ilter method

This method uses media to separate suspended particles or colloidal particles, thereby removing suspended particles or colloidal particles from water. Filtration methods can be divided into two processes: filtration and air flotation. The filtration method can effectively remove impurities and suspended solids in the water, but the operating cost is high because the filter material or filter needs to be replaced.
The air flotation method combines gases dissolved in water with pollutants to purify wastewater. During the flotation process, pollutants will be adsorbed on the bubbles to form scum, which then floats to the water surface. Floats can also be reused.

Application case(Preprocessing-physical method)

Case Studies-Nonwoven Spunlace Wastewater Treatment

The Hangzhou Nbond Nonwovens Co., Ltd project uses high-efficiency dissolved air flotation produced by our company for pretreatment to reduce the content of organic matter and suspended matter in the landfill leachate. After air flotation pretreatment, the suspended matter concentration removal rate reaches over 90%. The grease removal rate reaches over 95%, significantly reducing the burden of subsequent biochemical treatment.

Case Studies-Egypt printing and dyeing wastewater renovation project

This case is the upgrading and transforming of the Egyptian printing and dyeing factory. Our factory's high-efficiency dissolved air flotation tank has a good treatment effect, high removal efficiency, and low sludge production. It can remove the residual colloids, insoluble organic matter, and most of the suspended solids in the effluent of the secondary sedimentation tank and has achieved good results.

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Chemical method

The chemical method is a method of decomposing and removing pollutants in wastewater through chemical reactions. Commonly used chemical methods include coagulation precipitation, coagulation flotation, iron-carbon micro-electrolysis, adsorption, etc.

Coagulation sedimentation method

This method separates and removes pollutants from wastewater. The principle is to use flocculants or chemicals to react with contaminants in wastewater to separate pollutants from water to achieve the purpose of purification. Common flocculants include alum, poly ferric sulfate, polyaluminum chloride, etc. The coagulation and sedimentation method usually requires the addition of a coagulant. The dosage depends on the quality of the wastewater, usually 5 to 15 mg/L. The more significant the dosage, the better the wastewater treatment effect. The coagulation sedimentation method mainly treats printing and dyeing wastewater with high chroma. Generally, more than 70% of the chroma can be removed, and the COD content in the wastewater can also reach less than 100 mg/L. The coagulation and precipitation method is divided into poly aluminum chloride ferric salt coagulant, polymeric ferric sulfate ferric salt coagulant, and poly aluminum ferric sulfate coagulant.
Coagulation flotation

The coagulation air flotation method uses the gas-liquid two-phase separation phenomenon at the gas-liquid two-phase interface formed by air, air, and water for purification. Its principle is to separate and separate colloidal particles or suspended solids dispersed in water through air flotation. Remove. Standard air flotation methods include air flotation and air sedimentation. Air flotation mainly treats printing and dyeing wastewater containing suspended solids and macromolecular organic matter. It can effectively remove suspended solids in printing and dyeing wastewater, and at the same time, it can also remove part of the color of the wastewater. The air sedimentation method discharges the flocculated activated sludge suspension through gravity under stirring conditions. Because this method has the advantages of high efficiency and flexibility, it is widely used.
Iron-carbon microelectrolysis method

Iron-carbon micro-electrolysis is a method that uses the electrochemical reaction of iron and carbon to remove pollutants. This method has the advantages of a simple process, easy operation, and low cost, and is currently widely used in printing and dyeing wastewater treatment.
Adsorption method

The adsorption method is a method that uses activated carbon, resin, etc., to adsorb pollutants in printing and dyeing wastewater. The adsorption method can effectively remove some organic contaminants in wastewater and most of the wastewater color, but its cost is high.

Biological treatment

Biological methods use the metabolic effects of microorganisms to convert pollutants in wastewater into stable and harmless substances, thereby achieving the purpose of purifying wastewater.

This method has the advantages of low cost, high treatment efficiency, and no secondary pollution. It is currently widely used in printing and dyeing wastewater treatment.

Currently, the commonly used biological treatment technologies mainly include activated sludge and biofilm methods. The activated sludge method is one of the earliest technologies to treat printing and dyeing wastewater. It refers to inoculating a certain number of microorganisms into a closed, liquid-filled system. The microorganisms decompose and transform organic pollutants in the wastewater to achieve the purpose of purifying water quality.

The biofilm method is a new biological treatment technology that mixes anaerobic and aerobic microorganisms to form a biofilm layer. It intercepts impurities such as suspended solids and organic matter, thereby purifying wastewater. This technology has the advantages of having a good processing effect and low cost.

In addition, biological treatment methods for printing and dyeing wastewater also include aerobic treatment and anaerobic treatment. Aerobic treatment refers to the complete oxidation of organic matter in sewage containing pollutants into harmless substances;

Anaerobic treatment refers to the chemical reaction of microorganisms to decompose organic matter and oxygen in wastewater to generate carbon dioxide and water.

Since printing and dyeing wastewater contains many organic pollutants, a certain amount of activated sludge must be added to remove them. Aerobic biological treatment technology is still in its infancy, and its application scope is not broad enough.

New treatment technology

The new treatment process refers to using membrane separation technology to treat sewage. It has the advantages of high efficiency, energy saving, simple operation, etc., and has a wide range of applications. In printing and dyeing wastewater treatment, membrane separation technology is widely used and has achieved good results. Membrane separation technologies include microfiltration, ultrafiltration, nanofiltration, and reverse osmosis.

During the microfiltration process, suspended solids and colloidal substances in the sewage are intercepted, which improves the removal rate of suspended solids in the water; during the ultrafiltration process, the suspended solids in the sewage are trapped on the membrane surface, thereby improving the filter membrane surface biomass; during the nanofiltration process, the colloidal substances in the sewage are intercepted, thereby increasing the biomass on the filter membrane surface.

Therefore, methods such as microfiltration and ultrafiltration can effectively remove suspended solids, colloidal substances, and chemical oxygen demand (COD) in printing and dyeing wastewater. The reverse osmosis process means water is filtered through a reverse osmosis membrane to trap pollutants.

As a new process, reverse osmosis has a high removal rate. At present, research on reverse osmosis technology is relatively extensive. Still, there are no complete research results on the application of reverse osmosis membrane filtration technology in printing and dyeing wastewater treatment, so it is of great significance to conduct in-depth research on it.

Hydrolysis and acidification treatment process

In this system, hydrolysis and acidification technology are used to anaerobically treat sewage, which significantly improves the biodegradability of sewage and provides a suitable environment for later biological treatment.

In this process, the contact oxidation process provides a suitable environment for the treated materials. In this process, the contact oxidation process is applied to the aerobic process. This method uses fixed biological filler as a matrix, immerses the matrix containing bacteria in water, and provides oxygen to the bacteria through ventilation. Biological contact oxidation technology is an efficient, stable and efficient treatment method, and its advantage lies in its good biological activity.

The treatment effect of each process on pollution factors

Photocatalytic oxidation technology

Photocatalytic oxidation technology uses semiconductor photocatalysts to generate electron-hole pairs under ultraviolet and visible light irradiating, thereby degrading refractory organic pollutants. Photocatalytic oxidation technology has the advantages of simple operation and high treatment efficiency and is widely used in printing and dyeing wastewater treatment.

Studies have shown that using TiO₂ as a catalyst can degrade CODCr, chroma, and other pollutants in wastewater under ultraviolet and visible light irradiation. Under the TiO2 photocatalyst, the removal rate of CODCr in printing and dyeing wastewater by activated sludge is as high as 90%.

Printing and dyeing wastewater is treated through photocatalytic oxidation technology. After activated carbon adsorption-photocatalytic oxidation treatment, the COD removal rate in the wastewater reaches more than 90%, and the chroma removal rate reaches more than 95%.

Compared with other advanced oxidation technologies, photocatalytic oxidation technology has the advantages of low energy consumption, simple operation, and no secondary pollution. Therefore, it has good application prospects in printing and dyeing wastewater treatment. However, this technology still has problems, such as low catalyst activity, difficulty in catalyst recovery, and high wastewater treatment costs.

Ozone oxidation technology

Ozone (O³) is a strong oxidant. Its oxidizing properties are similar to the structure of organic pollutants. It can oxidize and decompose organic matter in a very short time. It can also react with organic contaminants in wastewater to form new and efficient Degradation products.
Ozone has a strong oxidizing ability and can oxidize and decompose all organic pollutants in wastewater. Therefore, it has been widely used in printing and dyeing wastewater treatment. Ozone's oxidation of organic matter in water mainly includes direct and indirect oxidation. Direct oxidation refers to the direct reaction of ozone with organic matter in water; indirect oxidation refers to using active oxygen radicals (·OH) and superoxide radicals (O^-2) generated by ozone decomposition in water to oxidize organic matter.
Currently, the ozone oxidation method mainly removes chroma, COD, etc., in wastewater. It is used chiefly to treat printing and dyeing wastewater under acidic conditions. Compared with other advanced oxidation technologies, ozone oxidation technology has the advantages of fast reaction speed, continuous operation, and strong oxidation performance.
However, since O³ is a strong oxidant, it will hurt microorganisms in the water body, especially since it has a specific toxic effect on microbial cells. In addition, O³ has a particular selectivity for organic matter, and only a tiny amount of organic matter can be decomposed into CO₂ and H2O. Therefore, the ozone oxidation method has problems such as low treatment efficiency and high cost in practical applications.

Ectrochemical oxidation method

Electrochemical oxidation technology uses electrodes as reactors for oxidation reactions to oxidize and remove pollutants in water in the form of electric current.
The electrocatalytic oxidation method is currently the most widely researched electrochemical advanced oxidation technology. This technology uses active free radicals generated by electric current to oxidize pollutants and convert pollutants into low-toxic or non-toxic substances.
When electrochemical methods treat printing and dyeing wastewater in an electrolysis device with a platinum carbon electrode as the anode, the COD removal rate reaches more than 80% after catalytic oxidation. As the current intensity and electrode spacing increase, the COD removal rate increases; as the electrolysis time increases, the COD removal rate first increases and then decreases when the current intensity is 0.1 A/cm2, and the electrode spacing is 1.0 cm. The best processing effect is when

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