Analysis of the Generation and Characteristics of Printing and Dyeing Wastewater

The pollutants in printing and dyeing wastewater mainly come from the fabric fibers themselves and the dyeing materials used in the processing. During the pre-treatment process of printing and dyeing production, desizing wastewater, boiling wastewater, bleaching wastewater, and silk polishing wastewater are discharged. Dyeing and printing processes discharge dyeing wastewater, soap washing wastewater, and printing wastewater, while finishing processes discharge finishing wastewater.

Characteristics and hazards of printing and dyeing wastewater:

Characteristics:

1. Large water volume

2. High concentration

3. Large fluctuations in water quality

4. Mainly organic pollution

5. Difficult to handle

6. Some wastewater contains toxic and harmful substances

harm:

Printing and dyeing wastewater contains a large amount of organic pollutants, which will consume dissolved oxygen when discharged into water bodies, disrupt the ecological balance of water, and endanger the survival of fish and other aquatic organisms. Organic matter that sinks to the bottom of the water can decompose anaerobically and produce harmful gases such as hydrogen sulfide, deteriorating the environment. The color of printing and dyeing wastewater is dark, which seriously affects the appearance of hand-held water bodies. The main factor causing discoloration of water bodies is dyes. In textile printing and dyeing processing, 10% -20% of dyes are discharged as waste. The chromaticity of printing and dyeing wastewater is particularly severe, and it is difficult to remove it using general biochemical methods. Colored water bodies can also affect the transmission of sunlight, which is not conducive to the growth of aquatic organisms. Most of the printing and dyeing wastewater is alkaline, and when it enters farmland, it can cause soil salinization. The sulfate in the dyeing wastewater can be converted into sulfide under the reducing conditions of the soil, producing hydrogen sulfide.

Wastewater generated from pre-treatment

1. Resizing wastewater: Resizing is the process of using chemical agents to remove the pulp from the fabric and also remove some impurities from the fibers themselves. The desizing wastewater is alkaline organic wastewater, containing decomposition products of oily slurry, fiber debris, enzymes, etc. Its COD and BOD5 are high. The amount of desizing wastewater is small, but the pollution is heavy, making it the main source of organic pollutants in pre-treatment wastewater.

2. Boiling wastewater: Boiling is the process of using an aqueous solution of caustic soda and surfactants to boil cotton fabrics under high temperature (120 ℃) and alkaline (PH10-13) conditions, removing impurities such as oil, wax, and pectin from the fibers to ensure the quality of bleaching and dyeing processes. Boiling and refining wastewater is strongly alkaline, with an alkali concentration of about 0.3%, dark brown in color, and high BOD5 and COD values.

3. Bleach wastewater: Bleach is the process of using oxidants such as sodium hypochlorite, hydrogen peroxide, and sodium chlorite to remove colored impurities from the surface and interior of fibers. The characteristics of bleaching wastewater are large water volume, light pollution level, low BOD5 and COD, and it belongs to relatively clean wastewater.

4. Mercerization wastewater: Mercerization is the process of treating fabrics in concentrated sodium hydroxide solution to increase the tensile strength of fibers, enhance the surface gloss of fibers, reduce the potential shrinkage rate of fabrics, and improve their affinity for dyes. Silk wastewater is generally recovered after evaporation and concentration, and the small amount of silk wastewater discharged from the end is highly alkaline.

Dyeing and printing wastewater

1. Dyeing wastewater

The main pollutants in dyeing wastewater are dyes and auxiliaries. Due to the use of different dyes, auxiliaries, and dyeing methods for different fiber raw materials and products, as well as varying dye uptake rates and dye concentrations, the quality of dyeing wastewater can vary greatly. The color of dyeing wastewater is generally dark and has poor biodegradability. Its COD is generally 300-700mg/L, BOD5/COD is generally less than 0.2, and the chromaticity can reach several thousand times.

2. Printing wastewater

The printing wastewater mainly comes from the washing wastewater of color matching slurry, printing drum, printing screen, as well as soap washing and water washing wastewater during post printing treatment. Due to the fact that the amount of paste in printing ink is several to tens of times higher than the amount of dye, in addition to dyes and additives, printing wastewater also contains a large amount of paste, high BOD5 and COD. Due to the use of potassium dichromate during printing drum plating, chromium trioxide is produced during drum stripping. These chromium containing carving wastewater should be treated separately.

3. Wastewater treatment

Wastewater treatment contains resin, formaldehyde, surfactants, etc. The amount of wastewater sorted is relatively small, and the impact on the water quality and quantity of mixed wastewater in the entire plant is also relatively small.

Handling method:

Printing and dyeing wastewater is mainly composed of organic pollution, with complex components. The main targets of treatment are BOD, organic matter that is not easily biodegradable or has a slow biodegradation rate, alkalinity, dye pigments, and small amounts of toxic substances. Although the biodegradability of printing and dyeing wastewater is generally poor, except for a few special types of printing and dyeing wastewater (such as pure chemical fiber fabric dyeing), it still belongs to biodegradable organic wastewater. The main treatment method is biological treatment, supplemented by necessary pretreatment and physical and chemical deep treatment.

technological process: