Reagent-free Measurement of COD, Nitrate and Nitrite via Optical Methods in the Lab and Online Directly in the Process

Dr. Ulrich Franke, Susanne Gollor, Xylem Analytics Germany GmbH & Co. KG, WTW

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The chemical oxygen demand (COD), nitrate, nitrite and ammonium are among the central wastewater parameters, in order to ensure safe, optimized and smooth wastewater treatment processes. COD is a sum parameter, whose composition is very plant-specific due to the different contents of the respective wastewaters (the so-called wastewater matrix).

The COD determination in the lab cannot be done in a rush: The DIN standard-compliant COD measurement requires a time period of approx. 2.5 hours. Besides, health-related and environmentally hazardous substances such as potassium dichromate are used, which are also light-sensitive and therefore prone to drifts. As a consequence, in addition to the DIN procedure, the cuvette test with smaller volumes and simple operation has made its mark for self-control purposes at wastewater treatment plants. Yet, even such shortened procedures - not DIN compliant - still require about 30 minutes to obtain the COD value.

The nitrate and nitrite determination by means of cuvette tests is not as time-consuming, but quickly causes significant costs too, when fulfilling "good laboratory practice" (GLP) requirements with double and triple determinations.

Continuous measurement of the parameters COD, nitrate and nitrite to provide control and regulation for process optimization, as well as monitoring of the inlet and effluent of wastewater treatment plants have been used successfully for more than ten years with WTW's IQ Sensor Net online measuring systems.

The measuring principle is based on an absorbance measurement in the UV wavelength range between 200 and 390 nm. In the background, the measured absorption spectrum is evaluated across the entire wavelength range with parameter-specific models. Thus, the concentration is calculated and the reading displayed instantly. This spectral measuring procedure for direct measurement of COD, nitrate and nitrite of the WTW online world has now been transferred to the photometer.

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