Determination of Colority in Textile Dyeing and Printing Wastewater: Standards, Methods and Applicat

Colority (chromaticity) is one of the most critical pollution indicators for textile dyeing and printing wastewater, directly reflecting the concentration of residual dyes, pigments, and dyeing auxiliaries in the effluent. Accurate determination of colority—typically performed using the dilution multiple method or platinum–cobalt colorimetry in accordance with standards such as GB 11903, ISO 7887, and ASTM D1209—is essential for verifying compliance with discharge regulations and for optimizing decolorization treatment processes. This article provides a technical overview of the principles, sampling requirements, testing items, analytical methods, and practical applications of colority determination in dyeing and printing wastewater.

Technical Overview

Colority is a quantitative measure of the degree of coloration in water caused by dissolved and colloidal chromophoric substances. In textile dyeing and printing wastewater, the dominant contributors are unfixed reactive dyes, disperse dyes, azo dyes, indigo, sulfur dyes, and hydrolyzed dye by-products. Because many of these chromophores are poorly biodegradable and remain visible at very low concentrations, colority serves as a fast, sensitive indicator of both treatment performance and environmental impact.

Raw dyeing and printing effluent frequently exhibits colority values ranging from several hundred to more than one thousand dilution multiples, while regulatory discharge limits are far stricter. Under the Chinese national discharge standard for the dyeing and finishing industry, GB 4287-2012, the colority limit for direct discharge is typically expressed as 80 dilution multiples, with more stringent values applied in environmentally sensitive regions. Related integrated and municipal discharge standards such as GB 8978-1996 and GB 18918-2002 specify colority limits generally in the range of 30 to 50 dilution multiples, and water reuse specifications commonly require values of 30 or lower.

An important technical distinction in colority measurement is the difference between true color and apparent color. True color is the color of the water sample after suspended solids have been removed by settling, centrifugation, or filtration, whereas apparent color includes the light-scattering effect of turbidity. Regulatory colority determinations are based on true color, which is why sample pretreatment to eliminate suspended matter is an integral part of every recognized method.

Testing Samples

Raw and Untreated Dyeing Effluent

Grab samples collected from the equalization tank or collection sump represent the combined wastewater from desizing, scouring, dyeing, rinsing, and printing operations. These samples characteristically show high and strongly variable colority, so representative sampling across production shifts is recommended.

Intermediate Treatment Stage Samples

Samples taken after each unit process—such as coagulation and flotation, biological treatment, chemical oxidation, or activated carbon adsorption—allow the decolorization contribution of each stage to be quantified individually and are valuable for treatment process diagnosis.

Final Discharge and Reuse Water Samples

Effluent collected at the outfall is tested against statutory discharge limits, while reclaimed water destined for reuse in washing or dyeing rinse stages is evaluated against internal or national reuse criteria, which impose tighter colority requirements to prevent product quality defects.

Sample Collection and Preservation

• Collect samples in clean glass or polyethylene containers with a minimum volume of 500 mL.
• Remove floating oil, grease, and large debris at the time of sampling.
• Analyze samples as soon as possible; when storage is unavoidable, preserve them at approximately 4 °C in the dark and complete the determination within 48 hours.
• Do not add chemical preservatives, as they may alter dye speciation and hue.

Testing Items

The core testing item is the colority of the wastewater sample, which may be reported in different units depending on the method applied:

• Colority by the dilution multiple method, expressed in dilution multiples (times), per GB 11903.
• Color in platinum–cobalt (Pt-Co, Hazen) color units or degrees, per ASTM D1209 and ISO 6271, where one unit corresponds to the color produced by 1 mg/L of platinum and 0.5 mg/L of cobalt.
• Spectral absorption coefficients determined at 436 nm, 525 nm, and 620 nm, or ADMI tristimulus color values, per ISO 7887.
• Supporting parameters that influence color interpretation, including pH, suspended solids, and turbidity.
• Calculated decolorization efficiency, defined as the percentage reduction in colority across a treatment stage or the entire plant.

Testing Methods

Dilution Multiple Method (GB 11903)

The dilution multiple method is the reference procedure for strongly colored dyeing and printing wastewater and forms the basis of statutory colority limits expressed in dilution multiples. In this method, suspended solids are first removed by settling or centrifugation so that true color is measured. The clarified sample is then progressively diluted with colorless pure water in matched colorimetric cylinders. Starting from a low dilution ratio and increasing stepwise, the observer views the diluted sample vertically against a white background under diffuse daylight and compares it with a blank of pure water. The reported result is the smallest dilution factor at which no perceptible color difference remains between the diluted sample and the blank.

For example, if the sample must be diluted 80-fold before the color becomes undetectable, the colority is reported as 80 dilution multiples. The method is simple, robust, and applicable to any hue, but it depends on standardized lighting conditions and consistent observer judgment, so duplicate determinations and confirmation of the transition point are recommended good practices.

Platinum–Cobalt Visual Colorimetry (ASTM D1209 / ISO 6271)

In this classical colorimetric approach, a series of platinum–cobalt standard color solutions is prepared from potassium chloroplatinate and cobalt chloride in hydrochloric acid, covering a scale of 0 to 500 Hazen units. The sample is matched visually against the standards in Nessler-type tubes. The method is precise and well suited to clear, pale yellowish waters such as polished reuse water, but it is less appropriate for raw dyeing wastewater whose hue differs markedly from the yellow-brown Pt-Co scale or whose color exceeds the standard range. High-colority samples may first be diluted and the result multiplied back by the dilution factor.

Spectrophotometric and Instrumental Methods (ISO 7887)

ISO 7887 describes instrumental alternatives that eliminate subjective visual judgment. These include measurement of the true color absorbance at the three complementary wavelengths of 436 nm, 525 nm, and 620 nm, with results expressed as spectral absorption coefficients, as well as tristimulus-based ADMI color determination, which provides a single-number color value independent of hue. Portable colorimeters and online colority analyzers operating on these optical principles are increasingly used for continuous monitoring of treatment plant performance, after initial correlation with the standard visual method has been established.

Quality Control Considerations

• Use matched colorimetric glassware and calibrated volumetric equipment for all dilutions.
• Perform determinations under consistent, diffuse illumination against a white background.
• Verify complete removal of turbidity before measurement, as suspended solids cause systematic overestimation.
• Analyze samples in duplicate and record the sample pH alongside the colority result, since pH shifts can change the apparent hue of many dye classes.

Applications

Colority determination supports a wide range of regulatory, operational, and technical objectives across the textile dyeing and finishing sector:

• Statutory discharge compliance testing against GB 4287-2012, GB 8978-1996, GB 18918-2002, and applicable local discharge permit limits.
• Performance evaluation and optimization of decolorization processes, including coagulant and flocculant dosing, adsorption, Fenton and ozone oxidation, and membrane treatment.
• Calculation of overall and stage-wise decolorization efficiency for environmental performance reporting.
• Quality verification of reclaimed water reused in dyeing, rinsing, and washing operations to prevent fabric staining and shade variation.
• Environmental impact assessments, cleaner production audits, and pollution discharge licensing documentation.
• Research and development of advanced treatment technologies for recalcitrant synthetic dyes.

FAQ

Question: What is the difference between true color and apparent color?

True color is measured after suspended solids have been removed, while apparent color includes the contribution of turbidity and suspended matter. Regulatory colority values are based on true color, which is why settling or centrifugation of the sample is required before determination.

Question: Which method should be used for strongly colored dyeing wastewater?

The dilution multiple method described in GB 11903 is the preferred approach, because it applies to any hue and any concentration level. The platinum–cobalt scale is mainly used for lightly colored, clear effluents such as polished reuse water.

Question: In what units is colority reported?

Results are reported either as dilution multiples (times), which is the basis of the discharge limits in GB 4287-2012, or in platinum–cobalt color units (Hazen degrees) when colorimetric comparison against Pt-Co standards is used.

Question: What interferences should be controlled during testing?

Suspended solids and turbidity are the principal interferences and must be removed by settling, centrifugation, or filtration. Floating oils should be eliminated, and the sample pH should be recorded because it can influence the visible hue of many dyes.

Question: Can accredited third-party laboratories perform this determination?

Yes. Accredited third-party laboratories routinely conduct colority testing of dyeing and printing wastewater according to GB 11903, ISO 7887, and ASTM D1209, and issue test reports that can be used for regulatory submission, customer verification, and internal performance benchmarking.