The textile printing and dyeing wastewater industry is an industry with high water consumption, high organic content in wastewater, and high pollution.

Textile printing and dyeing wastewater mainly comes from processes such as pulp (weaving) wastewater, refining wastewater, bleaching wastewater, silk dyeing wastewater, dyeing wastewater, printing wastewater, and finishing wastewater. The wastewater has a large volume of water, contains a large amount of alkali, has a high pH value, and also contains a large amount of suspended fiber particles, dyes, additives, slurries, finishing agents, etc. Therefore, the color is high, the organic matter content is high, and there are heavy metal toxins from copper, lead, zinc, chromium, mercury, cyanide ions. If discharged directly without treatment, it will cause serious pollution to the water and environment. The treatment of textile printing and dyeing wastewater should first focus on prevention, actively transform production processes and equipment, and reduce waste and waste generation; Reduce pollutant emissions and improve water reuse rates through reverse flow and reuse of water; Reuse dyeing raw materials, reduce production costs, and alleviate environmental pollution, achieving multiple benefits at once. The final wastewater is treated and discharged.

Due to the characteristics of large wastewater volume, complex water quality, and large changes in water quality and quantity, the treatment of textile printing and dyeing wastewater is relatively complex. Its treatment is generally divided into three stages: first, second, and third.

The primary treatment often uses grids, pre settling tanks, or initial settling tanks to separate suspended solids or lumps from wastewater using simple physical, mechanical, or chemical methods, or to neutralize the acidity or alkalinity of the wastewater.

Secondary treatment is mostly biochemical treatment, which can effectively remove gelatinous dissolved organic pollutants and improve water quality. When the biodegradability of wastewater is good, biochemical methods can be chosen; When the biodegradability of wastewater is poor, chemical methods such as coagulation sedimentation or pressurized air flotation can be chosen.

Third level treatment often uses physical or chemical methods for deep treatment, standard discharge or reuse.

2、 Common treatment methods for textile printing and dyeing wastewater

1. Physical Law

(1) Fence method: used to remove floating and suspended particles such as yarn heads and cloth blocks from wastewater. There are mainly grilles, grids, screens, etc.

(2) Regulating tank: Due to the large variation in water quality and quantity of textile printing and dyeing wastewater, a regulating tank must be installed. Generally, when the wastewater volume is 5000t/d, the retention time of the regulating tank is 4h; When the wastewater volume is 2000t/d, the residence time of the regulating tank is 5-6 hours; When the wastewater volume is less than 1000t/d, the retention time in the regulating tank is 7-8 hours.

(3) Sedimentation tank: The suspended particles in printing and dyeing wastewater are small, so it is not suitable to directly carry out sedimentation treatment without other (such as chemical) pretreatment. Sedimentation tanks are divided into horizontal flow, vertical flow, and radial flow, with the former being the most commonly used.

(4) Filtration method: The filtration method used in printing and dyeing wastewater is mostly fast filter, which means that water passes through the filter at a speed of 6-12m/h under the action of gravity to complete the filtration process.

2. Chemical treatment method

(1) Neutralization method: In printing and dyeing wastewater, this method can only adjust the pH of the wastewater and cannot remove pollutants from the wastewater. When using biological treatment, the pH should be controlled between 6-9 before entering the biological treatment equipment.

(2) Coagulation method: Chemical agents are used to combine a large number of dye, detergent and other particles in wastewater into large particles for removal. coagulants used in the treatment of printing and dyeing wastewater include basic aluminum chloride, polyacrylamide, aluminum sulfate, alum, ferric chloride, etc.

(3) Air flotation method: Printing and dyeing wastewater contains a large amount of organic colloidal particles in the form of emulsions of various oils and fats. These impurities are coagulated to form floc particles that are small, lightweight, and have poor sedimentation performance. Air flotation method can be used to separate them; At present, in the treatment of printing and dyeing wastewater, the air flotation method has a trend of replacing the sedimentation method and is a major treatment method for printing and dyeing wastewater. The air flotation treatment in printing and dyeing wastewater mainly adopts pressurized dissolved air flotation.

(4) Electrolysis method: This method has good decolorization effect, with a decolorization rate of over 90% for direct dyes, media dyes, sulfide dyes, dispersed dyes and other printing and dyeing wastewater, and over 70% for acidic dye wastewater. Disadvantages of this method: high power consumption and electrode material consumption, requiring DC power supply, suitable for small-scale wastewater treatment.

(5) Adsorption method: Adsorption method is very effective in removing COD and BOB color from printing and dyeing wastewater. Due to the large investment in activated carbon adsorption, it is generally not given priority consideration. In recent years, activated porous materials such as peat, diatomaceous earth, and kaolin have been used instead of activated carbon for adsorption. Activated adsorption materials with developed filter pores are recommended for printing and dyeing wastewater.

(6) The oxidation decolorization efficiency is low, only 40-50%, while the coagulation decolorization efficiency is relatively high, reaching between 50-90%. However, after treatment with these methods, the effluent still has a deep color and needs further decolorization treatment. Currently, the main methods used for dewatering printing and dyeing wastewater are photooxidation, ozone oxidation, and chlorine oxidation. Due to price and other reasons, the most commonly used method is chlorine oxidation, which uses liquid chlorine, bleaching powder, and sodium hypochlorite as oxidants. This method is less suitable due to high treatment costs and high operating conditions.

3. Biochemical method

(1) Biological pond method: There are four types of biological ponds: aerobic, facultative, anaerobic, and aerated. Their characteristics are shown in the table:

name

Aerobic biological sugar

Facultative biogenic sugar

Anaerobic biological sugar

Aerated biological sugar

Water depth (m)

0.2～0.4

1～2.5

2.5～4

2.5～4.5

Stay time (d)

2~6

7~30

30~50

2~10

BOD load (g/m2 · d)

10~20

2~10

2~100

BOD removal rate (%)

80~95

35~75

50~70

50~80

photosynthetic reaction

have

have

Algae concentration (g/t)

>100

10~50

The main one suitable for the treatment of printing and dyeing wastewater is the facultative biological pond.

(2) Anaerobic fermentation method: If aerobic biochemical treatment or additional coagulation treatment is used alone for textile printing and dyeing wastewater, the power consumption is high, and many wastewater substrates are difficult to decompose and decolorize. Practice has proven that anaerobic technology is effective in treating this type of wastewater. Anaerobic fermentation processes can be divided into conventional anaerobic fermentation, high-efficiency anaerobic fermentation, anaerobic contact method, anaerobic filtration method, upflow anaerobic sludge bed (UASB), improved anaerobic fermentation device (UASB+AF), anaerobic reflux process, anaerobic fluidized bed or expanded bed process, downflow anaerobic filtration (fixed membrane) reactor and other processes.

(3) Biofilm method: It can be divided into biological filter, biological turntable, and biological contact oxidation method. Among them, the latter two methods are widely used in the treatment of printing and dyeing wastewater in China. The biological turntable method is suitable for the treatment of small amounts of printing and dyeing wastewater, while the biological contact oxidation method is often used for the treatment of printing and dyeing wastewater. The tower type biological filter in the biological filter is also increasingly applied to printing and dyeing wastewater.

(4) Activated sludge method: The oxidation ditch method is also applied to the treatment of printing and dyeing wastewater due to its advantages of convenient construction and low daily operating costs. The quality of textile printing and dyeing wastewater varies greatly due to differences in its products and production processes. According to the degree of difference in water quality, textile printing and dyeing industry wastewater can be divided into three major categories with obvious characteristics.

（1） It's hair washing wastewater

（2） It is chemical fiber industry wastewater;

（3） It is wastewater from textile printing and dyeing, as well as wastewater from water wool spinning.

The treatment plans for industrial wastewater such as hemp and silk, wool washing wastewater, chemical fiber industrial wastewater, and cotton textile printing and dyeing wastewater are all described in detail below. However, the treatment methods for cotton textile printing and dyeing wastewater can also be used due to their similar water quality and pollution level as cotton textile printing and dyeing wastewater. For other textile dyeing wastewater, it is only necessary to add appropriate treatment units based on the characteristics of wool textile dyeing wastewater and hemp and silk wastewater. For example, using electrolysis to treat chromium in wool dyeing and finishing wastewater, so that the wastewater meets the discharge standards after treatment.