Why Most Industrial Wastewater Color Cannot Be Removed by Biological Treatment

In many industrial wastewater treatment plants, operators often encounter a common problem:
COD is reduced successfully, but color remains visible in the effluent.

This phenomenon is especially common in textile, dyeing, printing, and pigment wastewater. To understand why this happens—and how color can be effectively removed—it is essential to look at the chemical nature of color in wastewater, rather than biological degradation alone.

What Makes Industrial Wastewater Colored?

Color in industrial wastewater is mainly caused by dissolved dye molecules, not suspended solids. These dyes contain chromophoric groups, such as:

• Azo bonds (–N=N–)

• Aromatic rings

• Conjugated double-bond structures

These structures absorb visible light and are designed to be chemically stable, resisting light, heat, and biological breakdown.

Most importantly, the majority of industrial dyes carry a negative charge in water, making them highly soluble and difficult to remove.

Why Biological Treatment Cannot Remove Color Effectively

Biological treatment systems are designed to remove biodegradable organic matter, not chemically stable dyes.

The main limitations include:

1. Dyes Are Not Easily Biodegradable

Many dye molecules are engineered to resist microbial attack, allowing them to survive biological reactors almost unchanged.

2. Color Is Caused by Dissolved Molecules

Unlike suspended solids, dissolved dyes do not settle or float naturally, even after biological oxidation.

3. Negative Charge Prevents Aggregation

Negatively charged dye molecules repel each other, remaining dispersed in water and maintaining visible color.

As a result, even after effective biological treatment, color often passes through the system untreated.

Why Most Dyes in Wastewater Are Negatively Charged

In industrial applications, dyes are intentionally designed to bond with fibers. To achieve this, many dyes are manufactured as:

• Reactive dyes

• Acid dyes

• Direct dyes

These dye types typically dissociate in water and form anionic species, which enhances fiber attraction but also increases wastewater stability.

This negative charge is the fundamental reason why conventional settling, filtration, and biological methods fail to remove color.

How Chemical Decolorization Solves the Problem

To remove color effectively, the electrical stability of dye molecules must be destroyed.

This is achieved through chemical decolorization using cationic polymers.

Step 1: Charge Neutralization

Cationic decolorization agents introduce positively charged functional groups into the wastewater, which attract and neutralize negatively charged dyes.

Step 2: Destabilization of Dye Molecules

Once neutralized, dye molecules lose water solubility and structural stability.

Step 3: Formation of Insoluble Aggregates

The neutralized dye–polymer complexes form insoluble particles that can be removed through sedimentation, flotation, or filtration.

This process targets the root cause of color, rather than treating color as a secondary symptom.

Why Charge Density Is Critical in Decolorization

The effectiveness of a decolorization agent depends primarily on its cationic charge density, not its molecular size.

• Higher charge density provides stronger neutralization

• Faster reaction kinetics

• Lower chemical dosage

This is why low to medium molecular weight cationic polymers with high charge density are widely used for industrial wastewater color removal.

Typical Applications of Chemical Decolorization

Chemical decolorization is commonly applied in:

• Textile dyeing wastewater

• Dyestuff manufacturing effluent

• Printing and pigment wastewater

• Color polishing after biological treatment

In many systems, decolorization agents are used together with inorganic coagulants and flocculants to optimize overall treatment efficiency.

Conclusion

Color in industrial wastewater is a chemical stability problem, not a biological one.
As long as dye molecules remain electrically stable and dissolved, color will persist.

Effective color removal requires targeted chemical neutralization, making cationic decolorization agents an essential tool for industries facing strict discharge or reuse standards.