A comparison of continuous COD measurement with laboratory analysis

The cheese dairy in Holtsee, Germany
The cheese dairy in Holtsee, Germany

PHOTOMETRIC methods of measuring Chemical Oxygen Demand (COD) are widely used in sectors like the water industry, as a means to determine the quantities of organic compounds (such as pollutants) present.

Envitech, a UK supplier of waste water monitoring equipment, was recently involved in the use of an In Situ Spectral Analyser (ISA), in this case one from Go Systemelectronik, in a cheese dairy in Germany. Here, the company discusses some of the practicalities of using such instruments in this kind of application.
The spectrometer measuring system was used in the dairy for a period of about six months to continuously monitor the COD content in an effluent chamber.
The trial was designed to see if the ISA could provide a continuous measurement of COD which was accurate enough to be used in management of the plant – in other words to see if it could support product loss control and effluent treatment.
The first step was to take samples over 24 hours and analyse the samples in the dairy’s lab. It turned out that the correlation between the lab measurements and the ISA was quite good. However the day to day variability of the COD content was very high. The key feature of this ISA is its ability to add more data and refine the calibration. So more measurements were taken over time to provide a wide range of COD values with which to calibrate the ISA.

The spectrometer measuring head in the effluent chamber
The spectrometer measuring head in the effluent chamber

Comparing online and laboratory measurements
These samples were taken over time and the COD values measured in the dairy’s laboratory using Macherey & Nagel tests and a photometer. The resulting COD values were compared with the spectra measured by the photometer for those samples. The software package (also from Go Systemelectonik) performs the calculations required to do this and produces calibration factors which relate the spectra to the COD content.
This project took place over a few months and the resulting data is shown in Figure 1, below. The graph provides a comparison of the analysis data (lab or photometer) and the data captured by the spectrometer measuring system.
An ISA spectrometer is a useful tool for the detection of product loss events and for protection of the environment.
In real time the COD from milk products can be detected with a response time of 60 seconds. This short response time allows the output signals to be used for triggering alarms, and controlling processes.
This idea is being tested in the UK and we confidently expect to produce similar results and cost benefits.

• For more details contact Envitech on 029 2033 7134 or go to www.envitech.co.uk/multiparameter_spectrometer.htm.