Printing and dyeing wastewater treatment process

Printing and dyeing wastewater treatment, the commonly used physical and chemical treatment process is mainly coagulation and precipitation method and coagulation flotation method. In addition, electrolysis, bio-activated carbon method and chemical oxidation method is also sometimes used in printing and dyeing wastewater treatment:

1. Coagulation

Coagulation is the most widely used method of printing and dyeing wastewater treatment, coagulation and precipitation and coagulation and flotation of two kinds of methods. Commonly used coagulants are alkaline aluminum chloride, polymeric ferric sulfate and so on. Coagulation method has a good effect on the removal of COD and color.

Coagulation method set in the biological treatment before the coagulant dosage is large, the sludge volume is large, easy to make the treatment cost increase, and increase the difficulty of sludge treatment and final treatment. The COD removal rate of coagulation is generally 30% to 60%, and the BOD5 removal rate is generally 20% to 50%.

As a deep treatment of wastewater, coagulation is set after the biological treatment structure, with the advantage of flexible operation. When the influent concentration is low, biochemical operation is good, can not add coagulant, in order to save costs; when the biological contact oxidation method, you can consider not to set up the second sedimentation tank, so that the biological treatment structure of the effluent directly into the coagulation treatment facilities. In printing and dyeing wastewater treatment, most of the coagulation method is set after biological treatment. Its COD removal rate is generally 15% to 40%.

When the original wastewater pollutant concentration is low, only coagulation can already meet the discharge standards, can consider only setting coagulation treatment facilities.

2. Chemical oxidation

One of the characteristics of textile printing and dyeing wastewater is with a darker color. Mainly caused by residual dyes in the wastewater. In addition, some suspensions, slurries and auxiliaries can also produce color. Wastewater decolorization is to remove the above mentioned colorful organic matter in wastewater. After the dyeing wastewater is treated by biological method or coagulation method, with the removal of BOD and some suspended matter, the color degree is also reduced to some extent. In general, the decolorization rate of biological method is low, only 40% to 50%. The decolorization rate of the coagulation method is slightly higher, but it varies greatly depending on the dye species and coagulant, and the decolorization rate is between 50% and 90%. Therefore, after treatment with the above methods, the effluent still has a darker color, which is detrimental to both discharge and reuse. For this reason, further decolorization treatment must be carried out. Commonly used decolorization treatment methods include oxidation and adsorption method. Oxidative decolorization method of chlorine oxidation, ozone oxidation and photo-oxidation method of three.

Chemical oxidation is generally used as a deep treatment facility, set in the last stage of the process. The main purpose is to remove the chromaticity, but also to reduce some of the COD. after chemical oxidation, the chromaticity can be reduced to less than 50 times, COD removal rate is low, generally only 5% to 15%.

3. Electrolysis

With the help of the role of the applied current to produce chemical reactions, the conversion of electrical energy into chemical energy process called electrolysis. The use of electrolysis of chemical reactions, so that the harmful impurities in the wastewater into the removal of the method known as wastewater electrolysis treatment, referred to as electrolysis.

Electrolysis method used to deal with cyanide, chromium-containing plating wastewater, only in recent years began to be used in the treatment of textile printing and dyeing wastewater management, but there is still a lack of mature experience. Research shows that the decolorization effect of electrolysis is remarkable, for some reactive dyes, direct dyes, mordant dyes, sulfur dyes and disperse dyes printing and dyeing wastewater, the decolorization rate can reach more than 90%, and the decolorization rate of acid dyes wastewater is more than 70%. Electrolysis method for the treatment of small amount of printing and dyeing wastewater, with simple equipment, convenient management and better results. Fixed-bed electrolysis method has also been applied in engineering and achieved better results. Its disadvantage is that it consumes more electricity and electrodes, and is not suitable for use when the water volume is large. Electrolysis is generally used as a depth treatment, set after biological treatment. Its COD removal rate is 20% to 50%, and the chromaticity can be reduced to less than 50 times.

When the concentration of the original wastewater is low, only electrolysis can meet the discharge standards, can consider only set up electrolysis treatment facilities. When treated only with electrolysis, the COD removal rate is 40% to 75%.

4. Activated carbon adsorption

Activated carbon adsorption technology in the domestic pharmaceutical, chemical and food industries such as refining and decolorization has been many years of history. 70's began to be used for industrial wastewater treatment. Production practice shows that activated carbon has excellent adsorption of trace organic pollutants in water, which has good adsorption effect on industrial wastewater of textile printing and dyeing, dyestuffs and chemicals, food processing and organic chemicals. In general, it has unique removal ability for organic substances expressed in wastewater by BOD, COD and other comprehensive indicators, such as synthetic dyes, surface agents, phenols, benzene, organochlorine, pesticides and petrochemicals. Therefore, activated carbon adsorption has gradually become one of the main methods of industrial wastewater secondary or tertiary treatment.

Adsorption is the process of one substance attaching to the surface of another substance. Adsorption is an interfacial phenomenon, which is related to the change of surface tension and surface energy. There are two driving forces that cause adsorption, one is the repulsive force of solvent water on hydrophobic substances, and the other is the affinity attraction of solids to solutes. Most of the adsorption in wastewater treatment is the result of the combined effect of these two forces. The specific surface area and pore structure of activated carbon directly affects its adsorption capacity, and when selecting activated carbon, it should be determined by test according to the water quality of wastewater. For printing and dyeing wastewater, it is preferable to choose the charcoal with developed transition pores. In addition, ash also has an effect, the smaller the ash, the better the adsorption performance; adsorbent molecular size and carbon pore diameter is closer to the more easily adsorbed; adsorbent concentration of activated carbon adsorption also has an effect. In a certain concentration range, the adsorption amount is increased with the increase of adsorbent concentration. In addition, the water temperature and pH value also have an effect. The adsorption amount decreases with the increase of water temperature and increases with the decrease of pH value. Therefore, low water temperature and low pH are favorable to the adsorption of activated carbon.