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Ozone residual is used to calculate CT-value and disinfection credits for Giardiacyst, virus, and Cryptosporidium oocyst. Accurate ozone residual data willgenerate correct disinfection reports, lower ozone disinfection byproductformation, reduce plant operating cost, and provide consistency and accuracy ofreported bench-scale and pilot-scale research results. Undervalued ozone residualwill increase ozone dose, which will increase plant operating costs and mightincrease disinfection byproduct formation.Ozone residuals are determined using indigo colorimetric procedures either byStandard Methods 4500-Ozone or Hach Accuvac ampules. The method presumesthat high-purity indigo trisulfonate is used. If not, then reported residual is undervalued.This paper describes how utilities might consistently measure and reportthe true ozone residual and determine when measured residual data are inaccurate.The Standard Methods proportionality constant, f, (0.42 L mg-1 cm-1) that is usedto calculate ozone residual is based on an indigo molar absorbance, e, of~20,000 M-1 cm-1. Gordon and co-workers reported recently that indigotrisulfonate molar absorbance is variable (Gordon et al., 2000a, 2000b). Datafrom several water utilities have shown that molar absorbance might easily be16,000 M-1 cm-1 and could be as low as 6,000 M-1 cm-1. At these low molarabsorbance values, the reported ozone residual is approximately 10 percent to35 percent below the true residual, respectively. Standard Methods currentlyallows e = 12,800 M-1 cm-1, which undervalues residual by about 20 percent. Includes 4 references, table, figures.

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Edition: Vol. - No. Published: 06/16/2002 Number of Pages: 14File Size: 1 file , 360 KB