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15.03.2019

Carbon Parameters - an overview

Here you’ll find a short overview of possible measuring methods of the carbon parameters COD and BOD
WTW - Carbon Parameters
WTW - Carbon Parameters

To describe the organic load of water, the standard parameters COD and DOC as well as TOC or BOD are used. The most important standard parameters are the Chemical Oxygen Demand (COD) and the Biochemical Oxygen Demand (BOD), while the „Total Organic Carbon“(TOC) and the  „Dissolved Organic Carbon“(DOC), play a minor role. For TOC measurement, the photometric procedure is more sophisticated and bears a wider tolerance level in the results.

COD

The COD is the main sum parameter of water and the indicator of the sewage purification performance. Therefore COD is used to quote the effluent discharge fee. COD determination can be performed  and documented with cell tests according to the DIN 15705 standard. COD is determined as Oxygen (in mg/l) which would be needed for oxidation of the organic material, if oxygen would be the oxidant. The result is given as COD or O2 in mg/l.

Methodology:

For the determination of COD the water sample will be strongly acidified and heated up with a defined amount of the strong oxidant potassium dichromate  (K2Cr2O7)  in the presence of silver sulfate being a catalyst  and mercury sulfate to mask interfering Chloride ions . In this so-called digestion at 148°C for 2 hours according to DIN 38409-H41, the dichromate is consumed for the oxidation of organic material. Important is the complete cool down of the sample to allow complete settlement of  turbidity particles and thus to avoid faults in the subsequent photometric measurement. So, the photometric COD-determination takes place indirectly by the amount of the remaining dichromate: The consumption of dichromate allows to calculate the  equivalent of oxygen O2 respectively COD from the calibration curve at a defined wavelength and to display the measurement result as O2 or COD in mg/l.

The COD determination is time-consuming and involves the use of hazardous chemicals. This is the reason for today’s wide-spread use of photometric cell tests, offering many advantages: Low amount of reagents and ready-made reagent liquids make the application for users safe, easy and faster.

Due to methodology itself with higher tolerances of several percent in the results together with potential error sources such as sludge flakes in individual samples, a double or triple determination is common and according to GLP. There are many test kits available providing different measuring ranges  for 3 photometer instrument lines: the spectrophotometers photoLab® 7100 VIS and photoLab® 7600 VIS, the portable pHotoFlex® instruments and the filter photometer photoLab® S6 / S12

OptRF: the Optical Reagent-Free measurement of COD – Online and in the Lab

For self-check of sewage plants, a so-called optical reagent-free (OptRF) measurement is offered. The concentrations are not determined via chemical method based on a calibration curve at a defined wavelength as described in the above mentioned method. 

Here, the determination does not take place via a chemical procedure based on a defined wavelength and stored calibration curve like the procedure described above. Instead, the water sample is put directly in a quartz glass cuvette to perform a scan in the UV-VIS range from 200-390 nm and then to output the result directly in mg/l COD. There is no preparation time required.  In the OptRF procedure, the scans are evaluated for COD output on base of complex algorithms using real sample information behind. Applications are sewage plant outlets and often surface or river waters.

An additional optimization of self-check with OptRF can be obtained for the individual  sewage plant or environment: This is called matrix adjustment and is performed with reference measurements by routine analysis with cuvette tests. 

The WTW® online system IQ SENSOR NET  is the well-proven pioneer of this procedure:

The monitoring via such stationary online systems with reagent-fee sensors continuously shows fluctuation and peaks. The photometer photoLab® 7600 UV-VIS has made available these proven optical reagent-free methods (OptRF) for the Lab. Thus, the photometer can be used equally for routine analysis with test kits, for  matrix adjustment of IQ SENSOR NET, the optical reagent-free measurement directly or  a preliminary determination of required measurement range using OptRF. The advantage is besides lower cost also less waste and therefore less environmental impact. 

BOD

By determining the degradation of carbons, the BOD is an important parameter for water management, for the quality determination of water bodies and the efficiency assessment of water treatment plants. 

Using the official method of BOD5 (DIN EN 1899-1) the concentration of D.O. will be measured in diluted samples before and after incubation. When performing the “BOD self-check measurement” according to DIN EN 1899-2 in a closed respirometer the absorption of the carbon leads to a measurable pressure change and allows the calculation of the BOD. In both cases the samples have to be incubated for 5 days at 20 °C.

General tests on bio-degradation, e.g. soil respiration or bio-degradability of new chemicals can conveniently be carried out using OxiTop®-systems.

Up-to-date UV-VIS sensors for constant monitoring of WWTP effluents of municipal waste water are capable of measuring BOD as well. It is an indirect method using the spectral correlation between COD and BOD (adjustment with known BOD values from laboratory measurements) to calculate the current BOD.

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