Wastewater decolorization refers to the process of removing chromogenic pollutants from wastewater using physical, chemical, or biological methods to meet discharge standards or reuse requirements. The color in industrial wastewater mainly comes from industries such as dyeing, printing and dyeing, chemical, papermaking, and food. These pollutants not only affect the appearance of water bodies but may also contain toxic or teratogenic and carcinogenic substances, posing a threat to the ecological environment and human health. The following describes decolorization methods, technical characteristics, and application status:

I. Main Methods and Principles of Wastewater Decolorization

Physical Methods

Adsorption method: Utilizing the porous structure of adsorbents such as activated carbon, diatomaceous earth, and resins to adsorb pigment molecules in water. For example, activated carbon can achieve a decolorization rate of over 90% for water-soluble dyes, but the effect is poor for hydrophobic dyes, and regeneration or replacement is required after adsorption saturation, resulting in high costs.
Membrane separation method: Utilizing membrane technologies such as reverse osmosis (RO) and nanofiltration (NF), the selective permeability of the membrane is used to retain pigment particles. This method has high decolorization efficiency (up to 95% or more), stable effluent water quality, but the membrane is easily polluted, and the operating cost is high, suitable for the deep treatment of high-value-added wastewater.
Coagulation-sedimentation method: Adding coagulants (such as polyaluminum chloride, polyacrylamide) to coagulate and precipitate pigment particles through charge neutralization and bridging adsorption. This method is simple to operate and has low cost, and the removal effect on colloidal and suspended pigments is good, but attention should be paid to chemical residues and sludge treatment problems.

Chemical Methods

Oxidation method: Utilizing oxidants such as ozone, hydrogen peroxide, and sodium hypochlorite, or photocatalysis (such as TiO₂), electrochemical oxidation, etc., to destroy the conjugated structure of pigment molecules, causing them to fade. For example, the ozone oxidation method can achieve a decolorization rate of 80% to 90% in the treatment of dyeing wastewater, but the energy consumption is high, and by-products may be produced.
Reduction method: Adding reducing agents (such as sodium sulfide, sodium sulfite) to reduce colored pollutants into colorless substances. This method is suitable for wastewater containing reducible pigments such as azo dyes, but it may produce toxic intermediate products and should be used cautiously.

Biological Methods

Activated sludge method/biofilm method: Utilizing the metabolic action of microorganisms to degrade pigment molecules, suitable for wastewater with good biodegradability (such as food processing wastewater). For example, anaerobic biological treatment can achieve a decolorization rate of over 70% for certain dyes, but the treatment cycle is long, and the effect on refractory pigments is limited.
Microbial agent method: Adding high-efficiency decolorizing strains (such as white rot fungi, Bacillus), decomposing pigments through enzymatic hydrolysis. This method is environmentally friendly and has no secondary pollution, but the strains are sensitive to environmental conditions (pH, temperature), and operating parameters need to be optimized.

II. Technical Characteristics and Application Status

Limitations of single methods: Physical adsorption and chemical oxidation have significant decolorization effects on high-concentration wastewater, but the cost is high; biological methods are environmentally friendly, but the efficiency is low, and they often need to be combined with other methods. For example, the "coagulation-sedimentation + ozone oxidation" process can achieve a decolorization rate of over 95% for dyeing wastewater, while reducing COD; "biological treatment + membrane separation" can achieve wastewater reuse.

Development of new technologies: New technologies such as nano-material adsorption (such as magnetic nanoparticles), advanced oxidation technologies (such as Fenton oxidation, persulfate oxidation), and electrochemical-biological coupling systems are gradually being promoted due to their high efficiency and low energy consumption. For example, the adsorption capacity of graphene-based adsorption materials for dyes is 3 to 5 times that of traditional activated carbon, and they can be reused.

Industry application characteristics: The dyeing industry mostly uses a combination process of "pretreatment (coagulation) + biological treatment + deep oxidation"; papermaking wastewater often uses "flotation + anaerobic-aerobic biological treatment + membrane filtration" for treatment; chemical wastewater has complex components and needs to be treated in combination with advanced oxidation and adsorption technologies.

III. Future Development Directions

Develop low-cost, high-adsorption capacity new materials (such as modified biomass, MOFs);

Promote "intelligent + green" processes, such as photocatalytic-biological coupling systems, using solar energy to drive decolorization;

Strengthen wastewater source control, promote clean production, and reduce the generation of chromogenic pollutants.