• Two ways to measure TOC levels in water: UV persulphate NDIR versus UV correlation


Two ways to measure TOC levels in water: UV persulphate NDIR versus UV correlation

Jan 28 2021

Plant engineers responsible for water and wastewater treatment in municipal and industrial plants will find that Electro-Chemical Devices (ECD) now offers them two different highly accurate and reliable solutions to measuring total organic carbon (TOC) levels to protect water quality.

Organic compounds are found in almost all types of water – from natural and treated drinking water to process water, cooling water, and water used in pharmaceuticals and food production. Too much organic contamination in the water interferes with many municipal and industrial processes. For example, when disinfecting drinking water an excess of organic matter can foster microbiological growth or indicate the presence of other undesirable byproducts.

Measuring TOC levels continuously with online analysers is a highly effective, dependable way to monitor water and wastewater for hazardous contaminants. Depending on the plant, the process and the water quality requirements, plant engineers typically select one of the industry’s two most popular sensing techniques to measure TOC. The ECD Model 3S TOC Analyser measures TOC using the UV persulphate oxidation method with carbon dioxide detection by nondispersive infrared absorption (NDIR). In comparison, the ECD Model UV-6 TOC Analyser measures TOC levels with a UV absorption correlation technique. Both ECD analyser models provide excellent TOC measurement accuracy and repeatability to ensure compliance with water quality standards.

ECD’s Model 3S TOC Analyser measures TOC in liquid samples ranging from 0–5 mg/L to 20,000 mg/L. The UV persulphate oxidation method conforms to EPA, DIN, CE, ASTM, and NAMUR regulations as well as meeting the requirements of ISO and EN directives.

The highly dependable Model 3S TOC Analyser features a fast-loop reservoir with a floating level sensor. If no sample reaches the reservoir for more than a pre-set time, the analyser switches automatically to standby mode. As soon as the sample flow re-starts, the analyser switches back to the analysis cycle automatically. Air bubbles are removed in the reservoir before the sample enters the analyser.

The sample first is acidified and then sparged to remove inorganic carbon. The remaining liquid is mixed with sodium persulfate and digested by two high-performance reactors. The resulting CO2 is then stripped from the liquid and, after drying, its concentration is measured by a NDIR analyser to determine TOC levels.

An alternative TOC analytical method is the UV correlation technique performed by the ECD UV-6 Analyser. The analyser is factory configured by ECD to measure TOC as correlated with the 254 nm wavelength and the final correlated calibration is done by matching the TOC measured to the on-site grab sample TOC value.

The Model UV-6 Analysers are a family of on-line sampling analysers that use UV absorption to perform an analysis. The analysers are configured to perform analysis over a wide range of values for each parameter measured, nitrate, colour, or the correlation at 254 nm wavelength for TOC, COD or BOD.

This technique measures TOC based on the measurement of UV absorption in the sample. The absorbance of the solution or gas is measured though a quartz flow cell at the chosen parameter’s specific wavelength using a long-life xenon light source and photo-detectors. The absorbance level is related to the sample concentration according to the ‘Beer-Lambert Law’.

The UV6 Analyser features an easy-to-use reagent-less design. The UV spectroscopy measuring principle requires no chemical reagent resulting in very low operating and maintenance costs. There are no reagent chemicals to order, store, track and dispose at the end of their useful life, simplifying the entire process.

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