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.
| Edition : | Vol. - No. |
| File Size : | 1
file
, 360 KB |
| Note : | This product is unavailable in Ukraine, Russia, Belarus |
| Number of Pages : | 14 |
| Published : | 06/16/2002 |
