Total Organic Carbon (TOC) Methodology
The organic carbon in water and wastewater is composed of a variety of organic compounds in various oxidation states. Some of these carbon compounds can be oxidized further by biological or chemical processes, and the biochemical oxygen demand (BOD) and chemical oxygen demand (COD) may be used to characterise these fractions.
The presence of organic carbon that does not respond to either the BOD or COD test makes them unsuitable for the measurement of total organic carbon. Total organic carbon (TOC) is a more convenient and direct expression of total organic content than either BOD or COD, but does not provide the same kind of information. If a repeatable empirical relationship is established between TOC and BOD or COD, then TOC can be used to estimate the accompanying BOD or COD. This relationship must be established independently for each set of matrix conditions, such as various points in a treatment process. Unlike BOD or COD, TOC is independent of the oxidation state of the organic matter and does not measure other organically bound elements, such as nitrogen and hydrogen, and inorganics that can contribute to the oxygen demand measured by BOD and COD. TOC measurement does not replace BOD and COD testing.
1. TOC Analyser Configuration
Conductivity and Non dispersive infrared (NDIR) are the two common detection methods used in modern TOC analysers.
2. Conductivity based TOC analysers
Direct conductivity provides an all encompassing approach of measuring CO2. This detection method uses no carrier gas, is good at the parts per billion (ppb) ranges, but has a very limited analytical range.
Membrane conductivity relies upon the filtering of the CO2 prior to measuring it with a conductivity cell. Both methods analyse sample conductivity before and after oxidization, attributing this differential measurement to the TOC of the sample.
Conductivity analysis assumes that only CO2 is present within the solution. As long as this holds true, then the TOC calculation by this differential measurement is valid.
3. Non Dispersive Infrared (NDIR) TOC Analysers
The principal advantage of using NDIR is that it directly and specifically measures the CO2 generated by oxidation of the organic carbon in the oxidation reactor, rather than relying on a measurement of a secondary, corrected effect, such as used in conductivity measurements.
Merits
i. The low temperature techniques have the advantage of allowing a large volume of sample to be analysed thereby improving the low limit of detection.
ii. Also the blank value is very low as long as the reagents are pure, which makes the analysis more accurate
Limitations
i. Usually the particulates are more difficult to oxidise by nature or organics escape exposure to the reagents by being within the interstitial spaces of the particles.
ii. High molecular weight compounds such as proteins may be slow to oxidise with the low temperature techniques.
4. Combustion Technique
The combustion technique uses heat at 680oC or higher temperature in a stream of air, oxygen or nitrogen and usually in presence of a catalyst. Dissolved organics and particulate organics are expected to oxidise fully to carbon dioxide under these conditions. The catalysts vary from cupric oxide, cobalt oxide or platinum on an alumina support.
Analysis range
The range of TOC measurement varies with oxidation method and detection technique. A combustion/TOD method may measure upto 100% carbon in a sample, whereas the NDIR and conductivity detectors vary in range from as low as 0.5ppb to 25,000 ppm.
