The main task of a waste water treatment plant is to reduce the total organic load of waste water in addition to all the progress made in nitrogen and phosphate elimination. Organic compounds consist mainly of the elements carbon and hydrogen. The cleaning process converts them to carbon dioxide and water while consuming oxygen.
To measure the organic load of water, the parameters TOC, DOC, COD or BOD are used. The differences of these parameters (marginal column) show that these measurements are not identical and that the measured values therefore can not be equal.
For an analytical online determination, all these parameters require complex procedures for digestion and the corresponding instrumentation.
The SAC (spectral absorption coefficient) is a parameter that can be determined more easily. Many organic compounds have characteristical UV absorption spectrums. The intensity of the light attenuation can, therefore, be correlated with the organic load.
This correlation is significant in measuring media with low variations of composition concerning color, solids and their optical characteristics. Waste water, however, contains many substances with completely different optical characteristics. For each substance, a different correlation factor concerning the carbon content applies. Measuring at only one wavelength, e.g. at 254 nm for the SAC254, can often show the load only inadequately – especially if the matrix changes.
The CarboVis® and NiCaVis® sensors measure the total spectrum range from ultraviolet to long wave visible light. The measured values are determined from the high information content of the spectral data. The calculation is based on methods and characteristics that were achieved from a multitude of measurements and longtime analyses. The user can, therefore, select algorithms that are adapted to the measuring site (inlet, outlet etc.) having a high correlation with the basic parameter COD. The spectral procedure has an additional advantage: the turbidity of the test sample, which affects optical measurements, is optimally compensated over a wide wavelength range.
