The paper describes on-line and in-situ measurements of the nitrate nitrogen concentration in a sewer. The measurements were carried out to enable automated control of iron nitrate dosing to prevent odour problems. A submersible UV/VIS spectrometer (spectro::lyser by s::can Messtechnik, Vienna) was used for the measurements. It utilises the whole UV/VIS range (200-750 nm) for simultaneous measurement of nitrate, COD, and TSS with just one single instrument. The so-called global calibration for nitrate (based on UV/VIS spectra of municipal waste water of different origin) performed well, no local calibration was necessary. The ex-proof version of the spectrometer was installed directly in the sewer and measured in real-time. An optical path length of 2 mm showed the best results for monitoring NO3-N in sewage water. The instrument is equipped with a very effective auto-cleaning system using pressured air. A maintenance interval of > 4 weeks seems to be sufficient, and only a manual cleaning of the instrument should be carried out.
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