Online-Analytics of Organic Sum Parameter

Today, due to the multitude of organic compounds, the organic contamination of waste water cannot be defined individually with passable analytical effort. Therefore, composite parameters are used for the assessment of organic pollution in water. In the following, the most important parameters will be described briefly and their application in online analysis will be considered.

BOD, biochemical oxygen demand

The BOD indicates the content of oxygen needed to decompose organic compounds in waste water by bacteria. It enables estimations of performance in the whole plant. For BOD measurement it is naturally essential to work with the biomass of the waste water treatment plant. Only then the results correlate with the real situation of the waste water treatment plant. The BioMonitor by LAR AG works with the named principle and hence enables a reliable optimisation of waste water treatment plants.

COD, chemical oxygen demand

The parameter COD has been defined as the amount of oxygen which is needed when all organic ingredients would be oxidised completely. The internationally dominant method is the so-called dichromate method, but is hardly suitable for online operations, because of the slow analysis duration of two hours as well as the chemicals used. However, alternative methods have been developed by various producers and implemented in online analysers. LAR's COD analyser use the ultra high temperature method at 1,200°C for reliable determination of the real COD. This method is considerably cleaner.

TOD, total oxygen demand

As the name already suggest, this parameter measure the total oxygen demand of water. In the United States it is standardised with the ASTM D6238 and very commonly used. This parameter is a preferable alternative to the COD measurement. Moreover, the technique is equivalent to TOC measurement and hence, it is easy to handle and very suitable for online measurements, especially with the thermal combustion method at 1,200°C.

TOC, total organic carbon

The content of TOC reflects the organic contamination in water. Water and especially waste water contains more or less a lot of particles, whose size as well as character vary considerably. Needless to say, all particles containing organic carbon should be included in the measurement of the TOC. Only this lead to true results.

As a matter of course, all compounds of organic carbon, whether volatile or bound to particles, have to be included to the measurement to satisfy the term TOC. If the composition of the sample or water is changed before the actual measurement, for example by a possible removal of solids containing organic carbon, then, inevitably, the term TOC is out of question. LAR AG measures the TOC according to the German Institute of Standardisation (DIN) and moreover without any filtration or pretreatment resulting in TRUE TOC.

Systems for sample preparation

For determination of significant measurement results the sample preparation is of high importance. The ideal sample preparation system avoid that the sample composition is changing and that the analyser works maintenance-free, without any adverse effects. LAR's FlowSampler is one of the described sampling device that uses the anti-isokinetic sampling technique. Samples are siphoned-off from the sample stream against the main flow direction of the stream. Due to the flow speed, big solid particles will not be sucked into the sampling device. The taken sample corresponds 98% with that of a grabbed sample.

Résumé

Today, the use of composite parameters becomes more important in environmental monitoring, as well as in process control. There are proven online analysers in the market for each sum parameter decribed above. Naturally, it is important to choose the parameters according to their purpose and choose the ideal application-specific online analyser.