Colour Removal in Wastewater Treatment

Colour removal is not a single technology but a category of treatment techniques specifically targeting dye and chromophore removal from industrial effluent, applied where biological treatment alone is insufficient. The principal approaches are coagulation-flocculation, where alum, ferric, or polymeric coagulants destabilise dye-bearing colloidal particles so they can be settled or floated out; activated carbon adsorption, typically via granular activated carbon (GAC) columns, which is effective across many dye classes; ozonation, which oxidatively cleaves the chromophore double bonds responsible for colour and is particularly effective on azo dyes; and advanced oxidation processes such as Fenton oxidation, UV/ozone combinations, and electrocoagulation for difficult or mixed dye streams.

Colour compliance is regulated independently of BOD and COD in many Indian states. Pollution control boards such as TNPCB commonly impose colour limits using Pt-Co (platinum-cobalt) or APHA colour unit scales, or simply require effluent to be visually colourless before discharge — regardless of whether organic load parameters are already within limits. This creates a distinct compliance risk for textile dyeing clusters and other coloured-effluent industries: a plant can be fully compliant on COD and BOD while still receiving notices for visible colour in its discharge.

Azo dyes are the most common reactive dye class used in cotton and textile dyeing, and they are particularly persistent and resistant to biological degradation. Their characteristic colour comes from azo (-N=N-) chromophore bonds that conventional activated sludge or anaerobic biological treatment cannot reliably break down. This means biological treatment, while essential for BOD/COD reduction, frequently leaves visible residual colour in the treated effluent even after full organic load compliance is achieved — making a dedicated tertiary colour removal stage necessary rather than optional.

The standard combined approach is to let biological treatment handle BOD/COD reduction as the secondary stage, followed by a tertiary colour removal step — ozone, activated carbon, or advanced oxidation — sized specifically for the residual colour load. The industries most often requiring a dedicated colour removal stage are textile dyeing, where azo dyes are the dominant colour source; paper and pulp, where lignin-derived colour persists after pulping; and distillery, where melanoidin colour forms during caramelisation reactions in spent wash. Each of these colour types responds differently to oxidative and adsorptive treatment, so the right combination of technologies depends on the specific dye or pigment class present.