Color removal is one of the most challenging aspects of printing and dyeing wastewater treatment. High color intensity, color reversion, and fluctuations between clear and yellow effluent are common problems encountered during plant operation.
When effluent color exceeds the required limit, operators may be tempted to simply increase PAC and PAM dosages. However, excessive chemical addition can cause chemical costs to rise sharply, increase sludge production, interfere with sedimentation, and sometimes even result in unstable decolorization or yellowish effluent.
The key is to identify the source and type of color before selecting the appropriate treatment method.
1. Two Main Types of Color
Suspended Color
Suspended color is associated with dye particles, dye flocs, sizing agents, and suspended solids. It can generally be removed relatively effectively through coagulation and sedimentation and is therefore easier to treat.
Dissolved Color
Dissolved color is caused by dyes that remain dissolved in the wastewater, including reactive dyes, acid dyes, and azo dyes. This type of color is much more difficult to remove through conventional coagulation alone.
When dissolved color is the main problem, simply increasing PAC or PAM dosage may provide limited improvement. Biological treatment, chemical oxidation, or a combination of treatment processes may be required.
2. Common Decolorization Chemicals
PAC
Polyaluminum chloride (PAC) is widely used for coagulation and the removal of suspended solids and certain types of dye.
It is commonly applied to suspended color and many disperse dyes and generally provides moderate sludge generation and stable operation.
A wastewater pH of approximately 7.0–8.0 may be used as a starting range. Decolorization efficiency can decrease significantly when the pH is below 6.5.
Excessive PAC dosage should be avoided. Overdosing can cause turbidity to reappear and may contribute to yellowing, potentially increasing apparent color rather than reducing it.
Ferrous Sulfate
Ferrous sulfate can be effective for certain reactive and azo dyes and may provide better decolorization performance than PAC under suitable conditions.
Its decolorization effect is associated with reduction reactions and the breakdown of dye chromophores.
An alkaline pH of approximately 8.5–10.0 may be required, with alkali added for pH adjustment.
However, excessive dosing can cause reddish effluent, while poor sedimentation may result in yellow-tinted treated water. Ferrous sulfate also generates significant quantities of iron-containing sludge, increasing the overall sludge volume.
PAM
Polyacrylamide (PAM) primarily acts as a coagulation or flocculation aid rather than a decolorizing agent. It helps suspended particles and dye-containing flocs aggregate and settle but does not normally destroy dissolved dye molecules by itself.
Anionic PAM is commonly used to assist coagulation and sedimentation in printing and dyeing wastewater.
Cationic PAM is mainly used for sludge conditioning and dewatering after biological treatment and is rarely used for initial decolorization coagulation.
The optimum PAM type and dosage should be determined through jar testing.
Sodium Hypochlorite
Sodium hypochlorite can oxidize and destroy chromophores and may be used for emergency or final-stage decolorization.
Excessive dosing at the biochemical effluent stage should be strictly avoided. Residual chlorine flowing back into a biological treatment system can inhibit activated sludge, potentially causing sludge toxicity and deterioration of biological performance.
Bench-scale testing should be conducted before full-scale application. Sufficient aeration or other appropriate conditions should also be provided to minimize residual chlorine.
Fenton Process
The Fenton process, using hydrogen peroxide and ferrous sulfate, is highly effective for treating difficult dissolved color and can be used as an advanced decolorization process.
A strongly acidic environment, typically pH 2.5–3.5, is required during the reaction. After treatment, the pH should be adjusted to approximately 7–8 to precipitate iron sludge.
The main disadvantages are high chemical consumption, significant sludge generation, and more complicated operation. Therefore, Fenton treatment is generally more suitable for terminal advanced treatment than for long-term, high-volume continuous treatment.
3. Process Control
Hydrolysis-Acidification
The hydrolysis-acidification stage can help break down complex dye molecules before downstream biological and physicochemical treatment.
An ORP of approximately -100 to -300 mV can be used as a reference for maintaining reducing conditions.
For textile wastewater, an HRT of approximately 8–12 hours may be used as a starting range. Insufficient retention time can result in incomplete breakdown of dye molecules.
The effluent may sometimes appear darker than the influent after hydrolysis-acidification. This can be a normal result of hydrolysis-induced changes in dye compounds, which may subsequently be addressed through aerobic and coagulation treatment.
Aerobic Biological Treatment
A DO concentration of approximately 2–3 mg/L can be used as a reference for aerobic treatment. Insufficient oxygen may cause the accumulation of dye intermediates, resulting in yellowish or dark effluent.
MLSS of approximately 3,500–4,500 mg/L may be suitable depending on the process. Sludge aging or deflocculation can release intracellular or adsorbed color compounds and contribute to increased effluent color.
Long-term low-load operation should also be avoided because it can contribute to sludge deterioration, poor floc formation, and color carry-over.
Coagulation and Sedimentation
A typical dosing sequence is:
Wastewater → Alkali addition and pH adjustment → Decolorizing agent → Rapid mixing → PAM addition → Slow mixing → Sedimentation
Rapid mixing at approximately 150–200 rpm for 1–2 minutes can help distribute chemicals effectively, followed by slow mixing at approximately 50–80 rpm for 3–5 minutes to promote floc formation.
For textile wastewater, a sedimentation tank surface loading rate of approximately 0.6–0.8 m³/(m²·h) may be used as a reference. Excessive loading can cause short-circuiting, poor settling, and the carry-over of colored flocs.
4. Color Reversion After Decolorization
Color reversion occurs when treated water initially appears clear but becomes visibly colored again after standing.
Common causes include:
Residual dissolved color: Coagulation and sedimentation mainly remove suspended and colloidal dye particles, while dissolved chromophores may remain in the water.
Incomplete chemical reaction: Insufficient ferrous sulfate dosage or unsuitable reaction conditions can result in incomplete treatment.
Insufficient oxidation: Inadequate oxidation time with sodium hypochlorite may leave chromogenic intermediates.
Unstable biological treatment: Sludge aging, deflocculation, insufficient oxygen, or poor biological conditions can contribute to color carry-over.
Rather than simply increasing chemical dosage, operators should first investigate the source of the color.
Upstream hydrolysis-acidification and biological treatment should be optimized to reduce the dye and organic load reaching the final treatment stage. When persistent dissolved color remains, Fenton-based advanced oxidation may be considered instead of continuously increasing coagulant dosage.
A simple 30-minute static beaker test can also be used. After coagulation and sedimentation, allow the treated sample to stand undisturbed and observe whether the color becomes stronger. Testing different chemical dosages can help identify the optimum operating condition and potential for color reversion.
FAQ
What type of color is most difficult to remove?
Dissolved color is generally more difficult to remove because dissolved dye molecules are not readily captured by conventional coagulation and sedimentation.
Can increasing PAC dosage solve high color?
Not necessarily. PAC can be effective for suspended and colloidal color, but it may have limited effectiveness against dissolved dyes. Excessive PAC can also increase sludge production and cause turbidity or yellowing.
Does PAM remove color?
PAM mainly acts as a coagulation or flocculation aid. It improves floc formation but does not normally destroy dissolved dye molecules.
Why does treated water become yellow again?
Possible causes include residual dissolved chromophores, incomplete chemical reactions, insufficient oxidation, and unstable biological sludge.