Dyeing Wastewater Treatment Methods
The dyeing industry is a typical high water consumption industry that consumes a large amount of softened water for processes every year. Its wastewater contains dyestuffs, slurries, dyeing auxiliaries, fiber impurities, oil agents, acid, alkali and inorganic salts. It is characterized by an unstable quality of water, high organic content, high chromaticity (mainly colored dyestuff), high alkalinity and containing many low-biodegradable substances, etc., and recognized as one of the main wastewaters that are most difficult to be treated. Its direct discharge brings great harm to the living environment and causes waste of water resources.
The adsorption method is the most applied physical treatment method. It mixes powder or particles of porous substances such as activated carbon and clay with wastewater, or lets wastewater pass through a filter bed composed of its granular materials so that pollutants in wastewater are adsorbed on the surface of porous substances or removed by filtration. The adsorption rate, BOD removal rate, and COD removal rate of activated carbon reach 93%, 92%, and 63% respectively.
There are mainly the coagulation sedimentation method and the coagulation–air flotation method. The coagulant used is mostly based on aluminum or iron salts, among which the bridging and adsorption performance of polymeric aluminum chloride (PAC) is better. The main advantages of the coagulation method include simple process flow, easy operation and management, less equipment investment and space required, high efficiency of decolorization of hydrophobic dyes. Disadvantages are higher operating costs, more sludge and dewatering difficulties, poor treatment of hydrophilic dyes.
The ozone oxidation method can obtain a good decolorization effect for most dyestuffs, but poor decolorization effect for water-insoluble dyestuffs such as sulfur dyes, vat dyes and paint. From the current operating experience and results, the method requires high equipment investment and consumes more electricity, which is difficult for large-scale promotion in uses and needs to be further reduced.
The electrolytic method has a good effect on the treatment of dyeing wastewater containing acid dyes, with a decolorization rate of 50% to 70%, but it is less effective in treating wastewater with deep color and high CODCr.
The biological treatment method of dyeing wastewater is mainly by surface aeration and biological contact oxidation. In addition, activated sludge, rotating biological contactors, etc. are also applied, and the biological fluidized bed is still in the experimental application stage. Since the biological removal rate of chromaticity is not high, generally around 50%, so when the effluent chromaticity requirement is high, it needs to be supplemented with physical or chemical treatment.
Activated Carbon in Dyeing Wastewater Treatment
The excellent adsorption property of activated carbon makes it suitable for use in dyeing wastewater treatment. Experiment results show that for methyl orange, crystal violet, direct black 19 and reactive blue 231 solutions with an initial concentration of 30 mg/L, the removal rates of all four dyes are 97%-99% at pH 7, with a powdered activated carbon dosage of 6 g/L and an adsorption time of 20 min.
Among acidic fuchsin, basic fuchsin and reactive black B-133 dye wastewater, acid fuchsin is the easiest to be decolorized by activated carbon, followed by basic fuchsin, and reactive black B-133 wastewater is the most difficult to be decolorized. With an initial concentration of 250 mg/L, the removal rates are all above 97% when a coconut shell activated carbon dosage is 0.8%, 1.0% and 2.0%, and the adsorption time is 35h, 6h and 17h, respectively. The decolorization rate of dye wastewater increases with the increase of temperature, and the pH value does not have much effect on the decolorization effect of dye wastewater. The amount of activated carbon is the main factor affecting the decolorization rate of dye wastewater.
Considering that the separated activated carbon still has partial adsorption capacity, these activated carbons can be used to treat the dye wastewater to reach a low intermediate concentration, and then new activated carbons can be used to make the dye wastewater at the intermediate concentration meet the discharge standard in order to reduce the cost.