Colority is one of the most critical and visually apparent pollution indicators in wastewater generated by the textile printing and dyeing industry. Accurate colority analysis, performed in accordance with recognized standards such as GB 11903-89, GB 4287-2012, and ISO 7887, is essential for regulatory compliance, treatment process optimization, and water reuse evaluation. This article provides a comprehensive overview of the sampling procedures, testing items, analytical methods, and practical applications of colority analysis for textile dyeing and printing industrial wastewater.
Technical Overview
Colority, also referred to as chroma or color degree, is a quantitative measure of the coloration of water caused by dissolved and colloidal substances. In the textile printing and dyeing industry, wastewater coloration originates primarily from residual dyes that are not fixed onto fibers during dyeing and printing operations, as well as from hydrolyzed dye by-products, dyeing auxiliaries, tannins, and sizing agents. Depending on the dyes used, including reactive, disperse, azo, vat, sulfur, and anthraquinone classes, untreated wastewater can exhibit intense hues of red, blue, yellow, green, or black, with colority values frequently reaching several hundred to several thousand dilution multiples.
Colority is regulated for several reasons. Highly colored effluent degrades the aesthetic quality of receiving water bodies, reduces sunlight penetration and photosynthetic activity, and may indicate the presence of synthetic dyes and their degradation intermediates, some of which are persistent or potentially harmful. Because modern dyes are deliberately engineered to resist light, washing, and oxidation, color removal is one of the most technically challenging aspects of dyeing wastewater treatment, which makes reliable colority measurement indispensable.
In China, the discharge standard GB 4287-2012 (Discharge Standard of Water Pollutants for Dyeing and Finishing of Textile Industry) sets a maximum colority limit of 80 dilution multiples for discharged wastewater, with stricter special emission limits, such as 30 dilution multiples, applicable in environmentally sensitive regions. The determination of colority itself is governed by GB 11903-89 (Water Quality - Determination of Colority), while ISO 7887 provides internationally recognized procedures for the examination and determination of colour in water. Accredited third-party laboratories routinely apply these standards to support compliance verification and process improvement.
Testing Samples
Untreated Production Wastewater
Raw wastewater samples are collected from the combined effluent of dyeing, printing, desizing, scouring, bleaching, mercerizing, and washing operations. Because textile production is typically batch-based, pollutant concentrations fluctuate significantly over time, so time-proportional or flow-proportional composite samples across complete production cycles are recommended to obtain representative colority values. Grab samples may be taken for rapid internal screening or to capture peak-loading episodes.
Treated Effluent and Final Outfall Samples
Final effluent samples are collected after the complete treatment train, typically including screening, equalization, coagulation-flotation or sedimentation, biological treatment, and where applicable, advanced decolorization stages such as chemical oxidation, adsorption, or membrane filtration. These samples are the basis for demonstrating compliance with the applicable discharge limits and are usually sampled at the statutory outfall monitoring point.
Intermediate Process Stream Samples
Samples taken before and after individual treatment units, such as the coagulation tank, biological reactor, ozone contactor, Fenton reactor, or activated carbon column, allow the color removal performance of each stage to be quantified separately. This is essential for diagnosing treatment bottlenecks, calculating unit-wise decolorization efficiency, and optimizing chemical dosages.
Receiving Water and Water Reuse Samples
Colority testing is also applied to receiving water bodies upstream and downstream of the discharge point for environmental impact assessment, and to treated water intended for internal recycling in the mill. For reuse applications, such as rinsing or washing water, low colority is critical to prevent staining of subsequent fabric batches and maintain product quality.
Testing Items
Colority analysis of textile printing and dyeing wastewater encompasses several related test items, each suited to different purposes and reporting conventions:
- Colority expressed as dilution multiples (the primary regulatory parameter under GB 4287-2012 and GB 11903-89)
- Chroma in platinum-cobalt units (PCU, Hazen degrees) by visual comparison or spectrophotometric calibration against Pt-Co standards
- Spectral absorbance at the ISO 7887 reference wavelengths of 436 nm (yellow), 525 nm (red), and 620 nm (blue), reported as spectral absorption coefficients
- Full-wavelength absorbance scanning (typically 400-700 nm) to identify the maximum absorption wavelength characteristic of the dyes present
- ADMI tristimulus color value, which converts multi-wavelength transmittance data into a single color number applicable to hues that differ from the yellow Pt-Co scale
- Decolorization rate (color removal efficiency) calculated from absorbance values measured before and after treatment
- Light transmittance of the sample at specified wavelengths, used for effluent clarity characterization
- Supporting parameters such as pH, suspended solids, and turbidity, which influence the interpretation of colority results
Testing Methods
Sample Pretreatment
Colority results are strongly affected by suspended matter, so samples are first homogenized and, where the "true color" is required, clarified by centrifugation or filtration through a 0.45 micrometer membrane. Unfiltered samples yield "apparent color," which includes the light-scattering contribution of suspended solids; the measurement basis must always be stated with the result. Samples should be collected in clean glass containers, protected from light, stored at 2-5 degrees Celsius, and analyzed as soon as possible, preferably within 24 hours, because dye degradation, microbial activity, and pH drift can alter coloration during storage.
Dilution Multiple Method (GB 11903-89)
The dilution multiple method is the standard regulatory procedure for colored industrial wastewater in China and is particularly suited to the complex, intense hues of dyeing effluent. The sample is progressively diluted with colorless dilution water in a geometric dilution series until trained observers, viewing against a uniform white background under standardized illumination, can just no longer perceive any difference between the diluted sample and the blank dilution water. The cumulative dilution factor at this endpoint is reported as the colority, expressed in "times" or dilution multiples, for example 80 times. The method covers a wide range from a few multiples to several thousand and is robust for red, blue, green, and black wastewaters that cannot be matched against Pt-Co standards.
Platinum-Cobalt Comparison Method
The platinum-cobalt method determines chroma by visual comparison of the sample against a series of calibrated Pt-Co color standards, in which one color unit corresponds to the color produced by 1 mg of platinum per liter in the form of chloroplatinate with cobalt chloride in hydrochloric acid solution. The scale, also described in ISO 6271 and ASTM D1209 for clear liquids, is well established and instrument-compatible. However, because the Pt-Co scale is defined for yellow hues, it is mainly applicable to lightly colored, treated effluents and is generally unsuitable for the deep, non-yellow colors typical of raw dyeing wastewater.
Spectrophotometric Method (ISO 7887)
ISO 7887 specifies the determination of true color by measuring the absorbance of the filtered sample at 436 nm, 525 nm, and 620 nm in a spectrophotometer, using matched cuvettes and a distilled water reference, with results reported as spectral absorption coefficients. Full spectral scanning between 400 and 700 nm further reveals the maximum absorption wavelength of the specific dye mixture, which is widely used in research and process control. Single-wavelength absorbance at the maximum absorption wavelength provides a convenient basis for calculating decolorization efficiency, defined as (A0 minus At) divided by A0, multiplied by 100 percent, where A0 and At are the absorbances before and after treatment.
ADMI Tristimulus Method
The ADMI (American Dye Manufacturers Institute) tristimulus method overcomes the hue limitation of the Pt-Co scale. Transmittance of the sample is measured at multiple wavelengths across the visible spectrum, and the data are transformed through tristimulus weighting functions into a single ADMI color value expressed on a scale equivalent to Pt-Co units. This allows differently hued wastewaters, such as blue, red, or green dye effluents, to be compared and ranked on one consistent numerical scale, which is valuable when evaluating alternative decolorization technologies.
Quality Assurance and Quality Control
Reliable colority data depend on rigorous quality control. Key measures include the use of freshly prepared and verified Pt-Co calibration standards, consistent observer training and standardized viewing conditions for visual methods, replicate determinations with agreement criteria, dilution water blanks, calibrated spectrophotometers with wavelength and photometric accuracy checks, and documented sample pretreatment. Testing performed by accredited third-party laboratories operating under ISO/IEC 17025 quality management systems provides the traceability and defensibility required for regulatory reporting and dispute resolution.
Applications
Colority analysis serves a wide range of practical purposes across the textile printing and dyeing sector:
- Regulatory compliance monitoring against the colority limits specified in GB 4287-2012 and applicable local discharge permits
- Design and optimization of decolorization processes, including coagulation with polyaluminum chloride or ferrous salts, activated carbon adsorption, ozonation, Fenton and other advanced oxidation processes, and membrane separation
- Performance evaluation and acceptance testing of wastewater treatment plants and their individual treatment units
- Chemical dosing control and real-time process adjustment based on absorbance monitoring
- Quality assurance of recycled water for internal reuse in dyeing and washing operations
- Environmental impact assessment of effluent discharge on receiving water bodies
- Research and development, including the evaluation of new dye formulations, biodegradable dyes, and dye-degrading microorganisms
FAQ
What is the difference between colority in dilution multiples and chroma in Pt-Co units?
Dilution multiples express how many times a sample must be diluted before its color becomes visually imperceptible, and this is the reporting basis used for regulatory compliance of dyeing wastewater under GB 4287-2012. Pt-Co units (PCU or Hazen degrees) express color by comparison with platinum-cobalt standards and are best suited to light yellowish waters. The two values are not directly interchangeable, so the method and units must always accompany the result.
Why must suspended solids be removed before measuring colority?
Suspended particles scatter and absorb light, which artificially inflates the perceived color and distorts spectrophotometric readings. Filtration or centrifugation separates the "true color" contributed by dissolved and colloidal dyes from the "apparent color" that includes turbidity effects, ensuring that results reflect actual dye concentration and are reproducible between laboratories.
How long can wastewater samples be stored before colority testing?
Colority should be determined as soon as possible after collection. If immediate analysis is not feasible, samples should be stored in full, tightly sealed glass bottles at 2-5 degrees Celsius in the dark and analyzed preferably within 24 hours, because dye hydrolysis, oxidation by residual chemicals, and microbial activity can change the coloration during storage.
Which method is most appropriate for dark, highly colored dyeing wastewater?
The dilution multiple method of GB 11903-89 is the preferred regulatory method for intensely colored industrial effluents with complex hues, since it does not depend on matching the sample to a yellow reference scale. Spectrophotometric absorbance measurement and the ADMI method are valuable supplements for quantifying color removal during treatment optimization and research.
How is decolorization efficiency calculated?
Decolorization efficiency is calculated from the absorbance at the maximum absorption wavelength of the dye mixture, using the formula (A0 minus At) divided by A0 multiplied by 100 percent, where A0 is the initial absorbance and At is the absorbance after treatment. For compliance purposes, however, the colority of the final effluent must still be verified by the dilution multiple method.
Does low colority guarantee that the wastewater is fully treated?
No. Some decolorization processes, particularly certain oxidative treatments, can break chromophore groups and eliminate visible color while leaving soluble organic intermediates in the water. Colority must therefore always be interpreted together with complementary parameters such as COD, BOD, ammonia nitrogen, total nitrogen, and aniline residues to comprehensively assess treatment performance and regulatory compliance.