Dyeing and printing wastewater can only be treated through a combination of processes. While developing these processes, it’s also crucial to achieve recycling and reuse, striving for zero wastewater discharge.
(1) Conditioning: For wastewater with significant variations in water quality and quantity, the conditioning tank should have a longer retention time. Generally, when subsequent treatment units involve hydrolysis acidification or anaerobic treatment, aeration should not be used for mixing during conditioning.
(2) Coagulation: When the wastewater contains a large amount of hydrophobic dyes, the coagulation process should be placed before biological treatment to remove insoluble dye substances and reduce the load on subsequent biological treatment.
(3) Neutralization: When the raw water pH is high, H2SO4 or HCl is usually used for neutralization. To save on reagent usage, this can be done after the conditioning process.
(4) Sedimentation (Air Flotation): Due to the large amount of sludge, sedimentation should be prioritized for separation of physicochemical reactions (inclined tube sedimentation is prone to clogging and is not recommended). Generally, radial flow sedimentation tanks are suitable for large volumes of water, while vertical flow sedimentation tanks are suitable for small volumes. When land is available, horizontal flow sedimentation tanks can also be used when sludge is suctioned. Large dosages result in large sludge volumes, and radial flow tanks may cause density currents. Novel peripheral inlet and outlet sedimentation tanks can overcome this drawback.
(5) Filtration: When effluent requires clarification or reuse, sand filtration or two-layer coal sand filtration should be used.
(6) Electrolysis: Titanium-plated ruthenium inert electrode electrolysis is effective in decolorizing acid dyeing and printing wastewater. It also shows high removal rates for sulfide dyes, vat dyes, acid dyes, and reactive dyes when removing COD. Metal anode electrolysis is less commonly used due to the large amount of sludge.
(7) Anaerobic hydrolysis: Dyeing and printing wastewater has high organic matter content and COD, and a low B/C ratio. Hydrolysis acidification should be considered, and the packing material should be increased. A hydraulic mixer should be installed at the bottom of the biofilm-coated tank to ensure extensive contact between suspended activated sludge and organic matter in the water. For larger tanks, a series system should be used to avoid short circuits.
(8) Aerobic biodegradation: For dyeing and printing wastewater with large volume and high concentration, activated sludge processes such as oxidation ditches, intermittent activated sludge processes (SBR), and circulating activated sludge processes (CSTR) are preferred. For wastewater with small volume and low concentration, biological contact oxidation can be considered, but the packing material should ensure density and volume ratio, and a multi-stage series method is preferable.
(9) Decolorization: When using Cl2, the decolorization oxidation time should be at least 1 hour. Cl2 decolorization also has the function of adjusting the pH value. For small-scale applications, ClO2, NaClO bleaching powder [Ca(ClO)2], ultraviolet light, etc., can be used. The decolorization reaction tank can use a rotating baffle or folding plate; mechanical stirring or compressed air reaction is not recommended.
(10) Activated carbon adsorption: Activated carbon has good adsorption performance for wastewater containing water-soluble dyes such as cationic dyes, direct dyes, acid dyes, and reactive dyes (but is less effective for wastewater containing insoluble dyes such as sulfur dyes and vat dyes). Biological activated carbon (BAC) method is a biological technology for activated carbon adsorption. It utilizes the exoenzymes secreted by added microorganisms to penetrate the microporous structure of the carbon, causing the adsorbed organic matter to continuously decompose into CO2, H2O, or synthesize new cells, eventually seeping out of the carbon structure and being removed.
(11) Diatomaceous earth adsorption: Diatomaceous earth has both coagulation and adsorption effects in dyeing and printing wastewater, resulting in good decolorization. Generally, activated diatomaceous earth has varying decolorization effects on hydrophilic dyes, but is more effective on hydrophobic dyes. When there are many surfactants and leveling agents in the wastewater, the effect will significantly decrease.
(12) Oxidation: Ozone oxidation is fast and effective for hydrophilic dyes such as direct dyes, acid dyes, basic dyes, and reactive dyes; however, it is less effective for hydrophobic dyes such as vat dyes, naftate dyes, oxidized dyes, sulfide dyes, and disperse dyes, requiring a larger amount of ozone. Ozone decolorization does not produce carcinogenic, teratogenic, or mutagenic substances, ensuring the safety of wastewater effluent.
(13) Membrane separation technology ① Ultrafiltration: Due to the precise fine pores of ultrafiltration membranes, large molecules and other particles in water can be retained. It operates at low pressure and requires simple equipment, making it suitable for dye recovery or deep treatment of effluent. ② Nanofiltration: This is a new technology that uses nanofiltration membranes to retain pollutants. The separation pressure is generally 0.5~2.0 MPa. It treats water-soluble (hydrophilic) dye wastewater and can recover useful dyes. Nanofiltration membranes are used to recover Direct Black, Reactive Brilliant Red, Acid Orange II, and Acid Red dye wastewater.
