AWWA ACE56322

Ozone residual is used to calculate CT-value and disinfection credits for Giardia cyst, virus, and Cryptosporidium oocyst. Accurate ozone residual data will generate correct disinfection reports, lower ozone disinfection byproduct formation, reduce plant operating cost, and provide consistency and accuracy of reported bench-scale and pilot-scale research results. Undervalued ozone residual will increase ozone dose, which will increase plant operating costs and might increase disinfection byproduct formation. Ozone residuals are determined using indigo colorimetric procedures either by Standard Methods 4500-Ozone or Hach Accuvac ampules. The method presumes that high-purity indigo trisulfonate is used. If not, then reported residual is undervalued. This paper describes how utilities might consistently measure and report the 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 used to 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 indigo trisulfonate molar absorbance is variable (Gordon et al., 2000a, 2000b). Data from several water utilities have shown that molar absorbance might easily be 16,000 M-1 cm-1 and could be as low as 6,000 M-1 cm-1. At these low molar absorbance values, the reported ozone residual is approximately 10 percent to 35 percent below the true residual, respectively. Standard Methods currently allows e = 12,800 M-1 cm-1, which undervalues residual by about 20 percent. Includes 4 references, table, figures.