Comparison of and uncertainties in raw sewage COD measurements by laboratory techniques and field UV-visible spectrometry

A comprehensive investigation of the uncertainty of different COD analysis methods (DIN, small tube tests (STT) and UV-visible spectrometry methods) has been carried out on potassium hydrogen phthalate standard solutions and raw sewage samples from a large wastewater treatment plant. The UV-visible method allows estimating COD equivalent concentration from the measured absorption spectra by means of site specific regression and correlation functions. COD_din, and COD_stt methods showed equivalent global results but specific calibration relationships are necessary when high accuracy is required. The COD_stt method is suitable for immediate COD analysis in the field. Sub-sampling is the main source of uncertainty. Spectrometry is able to estimate COD_eq with an uncertainty of the same order of magnitude as the uncertainty in COD_din.