Wastewater decolorization refers to the process of removing color 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, chemical, papermaking, and food. These pollutants not only affect the appearance of water bodies but may also contain toxic, teratogenic, or carcinogenic substances, posing threats to the ecological environment and human health. The following explains the decolorization methods, technical characteristics, and current applications:

1. Main Methods and Principles of Wastewater Decolorization

Physical Methods

Adsorption: Using 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 is less effective for hydrophobic dyes. After adsorption saturation, regeneration or replacement is required, which is costly.
Membrane Separation: Using membrane technologies such as reverse osmosis (RO) and nanofiltration (NF) to selectively retain pigment particles through membrane permeability. This method has a high decolorization efficiency (up to over 95%) and stable effluent quality, but membranes are prone to fouling, have high operating costs, and are suitable for advanced treatment of high-value wastewater.
Coagulation and Sedimentation: Adding coagulants (such as polyaluminum chloride, polyacrylamide) to aggregate pigment particles through charge neutralization and bridging adsorption. This method is simple, low-cost, and effective for removing colloidal and suspended pigments but requires attention to chemical residues and sludge treatment.

Chemical Methods

Oxidation: Using oxidants such as ozone, hydrogen peroxide, sodium hypochlorite, or technologies like photocatalysis (e.g., TiO₂) and electrochemical oxidation to break the conjugated structure of pigment molecules, causing fading. For example, ozone oxidation can achieve an 80%-90% decolorization rate for printing and dyeing wastewater but has high energy consumption and may produce by-products.
Reduction: 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 like azo dyes but may produce toxic intermediates and should be used cautiously.

Biological Methods

Activated Sludge/Biofilm: Using microbial metabolism to degrade pigment molecules, suitable for wastewater with good biodegradability (e.g., food processing wastewater). For example, anaerobic biological treatment can achieve over 70% decolorization for certain dyes but has a long treatment cycle and limited effect on hard-to-degrade pigments.
Microbial Preparations: Adding efficient decolorizing strains (such as white rot fungi, Bacillus) that secrete enzymes to decompose pigments. This method is environmentally friendly with no secondary pollution but strains are sensitive to environmental conditions (pH, temperature) and require optimized operation parameters.

2. Technical Characteristics and Current Applications

Limitations of Single Methods: Physical adsorption and chemical oxidation are effective for high-concentration wastewater but costly; biological methods are environmentally friendly but less efficient and often combined with other methods. For example, the "coagulation + ozone oxidation" process can achieve over 95% decolorization for printing and dyeing wastewater while reducing COD; "biological treatment + membrane separation" can enable wastewater reuse.

Development of New Technologies: New processes such as nanomaterial adsorption (e.g., magnetic nanoparticles), advanced oxidation technologies (e.g., Fenton oxidation, persulfate oxidation), and electrochemical-biological coupling systems are gradually promoted due to their high efficiency and low consumption. For example, graphene-based adsorbents have 3 to 5 times the adsorption capacity of traditional activated carbon and are reusable.

Industry Application Characteristics: The printing and dyeing industry often uses a combination of "pretreatment (coagulation) + biological treatment + advanced oxidation"; papermaking wastewater is commonly treated with "air flotation + anaerobic-aerobic biological treatment + membrane filtration"; chemical wastewater is complex and requires combined advanced oxidation and adsorption technologies.

3. 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 driven by solar energy for decolorization;

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