Technical Bulletin No. 0803: An Update of Procedures for the Measurement of Color in Pulp Mill Wastewaters

NCASI Method 253 was originally developed in 1971 to provide a technique for color measurement in pulping wastewaters and their receiving waters. Because the processes of pulp and paper production have changed considerably since that time, a study was undertaken with the objective of updating the method. The color of pulp mill effluents is highly pH-dependent, and the pH adjustment procedure included in Method 253 sometimes resulted in less than optimum stability of pH during the analytical procedure. The use of a buffer to stabilize pH and reduce the erroneous increase in absorbance was investigated and successfully implemented. Experiments were conducted to investigate approaches for removing turbidity from pulp mill wastewater samples, since turbidity can interfere with the assessment of color by scattering light and introducing a high bias. Centrifugation was investigated as an alternative to filtration for the removal of turbidity. The percent decrease in turbidity was consistently higher for aliquots treated using filtration vs. centrifugation. In addition, the resultant color values were consistently higher for samples that were centrifuged. The current technique, use of a 0.8 µm membrane filter, proved to be an effective method of turbidity removal in pulp mill wastewaters. Pre-filtration of the samples using a 1 µm glass fiber filter was confirmed as an acceptable method for reducing filter plugging of the 0.8 µm membrane filters. Comparisons of three different types of membrane filters demonstrated that similar results could be obtained with any of the three filter types. The selection of a wavelength of 465 m was confirmed to be valid in low color effluents for the spectrophotometric measurement of absorbance. Finally, the method was assessed at both the single- and the inter-laboratory level for ruggedness, precision, and accuracy.