Numerous studies have been conducted to compare the performance of various TOC methodologies on a wide range of raw source water and finished drinking water samples. The methodologies studied are those that are approved for TOC monitoring under the USEPA’s Disinfectants and Disinfection Byproducts Rule (D/DBPR).
The results of these studies demonstrate that UVPersulfate oxidation combined with membrane conductometric detection produces results comparable to combustion oxidation combined with NDIR detection. The studies discussed and summarized in this application note include:
- USEPA study referenced in USEPA Method 415.3 for TOC and SUVA
- City of Fort Collins study
- Central Kagaku Corporation study
1. USEPA study referenced in USEPA Method 415.3 for TOC and SUVA
The USEPA method for TOC measurement was developed for use with D/DBPR compliance monitoring. A copy of the method can be reviewed at http://www. epa.gov/nerlcwww/documents/Method%20415_3_ Rev1_2_Final.pdf
Section 17 of this method lists the results of testing accomplished on the various TOC measuring methodologies covered by the method, including UV-Persulfate / Membrane Conductometric and Combustion / NDIR. Testing was performed on realworld samples collected from sites around the United States. Table 1 summarizes the TOC recoveries for three instruments compared in the study.
2. City of Fort Collins
The City of Fort Collins, Colorado conducted a comparison study between three instruments manufactured by Sievers1 (UV-Persulfate / Membrane Conductometric), Shimadzu (Combustion / NDIR) and OI (Combustion / NDIR). This study differs from the EPA study in that this study evaluated seven known compounds considered to be common to drinking water sources and that would present a significant analytical challenge to the instruments. The same concentration standard solutions were prepared on several different days, and then run on each instrument.
In general, the results show good comparison between the different TOC methodologies. For Humic Acid, one of the Combustion/ NDIR methods yielded significantly lower than expected recoveries. Other studies have suggested that this effect—not limited to Humic Acid—is a result of significant loss of Dissolved Organic Carbon (DOC) during the sparging step, which is conducted to remove background inorganic carbon.2
3. Central Kagaku Corporation Study
In preparation of TOC regulations in Japan, Central Kagaku Corporation (distributor at the time for Hach Company and Sievers Instruments) conducted a study comparing the UV-Persulfate method to the Combustion/NDIR method on real-world raw water samples.
Three pretreatment techniques were utilized on split samples:
- Filtration with a 0.45-micron filter to measure DOC
- Filtration through a 100-micron filter followed by filtration through a 50-micron filter
- Homogenization, per the Japanese regulation which specified homogenization for samples containing particulates
The study evaluated performance of a Sievers TOC Analyzer and a Shimadzu TOC analyzer. The results are summarized in Figures 3–5. Turbidity and suspended solids are graphed with the TOC data.