Overview
Colour in wastewater arises from dissolved organic chromophores, which are molecules with conjugated double-bond systems that absorb visible light. Sources include synthetic dyes (from textile and paper manufacturing), natural humic and tannin compounds from biological degradation of plant matter, and petroleum-derived aromatic compounds (naphthalenes, polycyclic aromatics, asphaltene fragments) from refinery and petrochemical operations. Even at very low concentrations below 1 mg/L, these compounds can impart deep colour to water that is objectionable visually and that may indicate persistent organic contamination.
Colour removal is challenging because the responsible molecules are often highly soluble, resistant to biological treatment, and not effectively removed by conventional coagulation alone. Depending on the colour source, effective treatment requires a tailored approach: high-charge cationic coagulants for anionic dye removal, activated carbon adsorption for dissolved aromatics, or oxidation to break chromophore bonds. A combined approach is frequently necessary for complex industrial effluents.
Colour vs. turbidity: These are distinct parameters. Turbidity is caused by suspended or colloidal particles; colour is caused by dissolved compounds. A wastewater can be low-turbidity but highly coloured (true colour). For example, a clear but deeply brown refinery wastewater containing dissolved aromatic compounds. Conventional coagulation removes turbidity but may leave dissolved colour almost unaffected. Decolourant selection must target the true colour fraction, not just the turbidity.
Decolourant Types
Applications
| Application | Colour Source | Recommended Treatment |
|---|---|---|
| Textile wastewater (reactive dye effluent) | Reactive dyes (anionic, highly soluble) | Polyamine decolourant + PAC or ferric coagulant; AOP polishing for non-reactive fractions |
| Palm oil mill effluent (POME) | Melanoidins, tannins, carotene | Ferric coagulant + flocculant; GAC for final colour polish after biological treatment |
| Refinery and petrochemical effluent | Aromatic compounds, asphaltene fragments, crude oil derivatives | Enhanced coagulation (ferric) + GAC polishing; AOP where PAH levels require oxidative treatment |
| Paper & pulp mill effluent | Lignin, tannins, black liquor organics | Ferric chloride coagulation + GAC; ozone effective for lignin colour removal at paper mills |
| Municipal wastewater, secondary effluent colour | Humic and fulvic acids from biological treatment | Enhanced ferric coagulation; PAC dosing for humic removal before discharge |
| LNG condensate treatment | Aromatic hydrocarbons in stripping condensate water | Air stripping for dissolved BTEX; GAC polishing for residual colour and dissolved organics |
Petrochemical & LNG Applications
Colour in refinery, petrochemical, and LNG wastewater has a different character from textile or paper mill colour. It arises primarily from dissolved aromatic hydrocarbons, asphaltene fragments, and polycyclic aromatic hydrocarbons (PAHs) that are present in crude oil and natural gas condensates and that partition into wastewater streams during processing.
In refineries, the effluent treatment plant receives process drains from across the facility (crude unit desalter brine, tank farm drainage, heat exchanger cleaning water, and utility system blowdown) which may contain petroleum-derived colour compounds. After API separation and DAF treatment remove bulk oil and grease, a significant dissolved colour load often remains. Enhanced coagulation using ferric salts, followed by GAC polishing, is the standard treatment sequence for colour reduction to PROPER-compliant levels. PAH removal is a particular concern because Indonesian PROPER regulations include specific limits for total and individual PAH species.
At LNG terminals, process condensate water produced during gas dehydration and inlet separation contains BTEX compounds (benzene, toluene, ethylbenzene, xylenes) and other aromatic hydrocarbons that impart colour and that are toxic at low concentrations. Air or steam stripping removes most of the volatile aromatics; GAC polishing captures the residual dissolved fraction and achieves the low concentrations required before discharge to coastal waters where stringent marine environment standards apply.