J.-L. Bertrand-Krajewski

Partial Least Squares local calibration of a UV-visible spectrometer used for in situ measurements of COD and TSS concentrations in urban drainage systems.

Recent UV-visible spectrometers deliver on line and in situ absorbance spectra in wastewater or stormwater transported in urban drainage systems. After calibration with local data sets, spectra can be used to estimate pollutant concentrations. Calibration methods are usually based on PLS (Partial Least Squares) regression. Their most important difficulty lies in the identification of the number of both i) the latent vectors and ii) the independent variables. A method is proposed to identify these variables, based on an exhaustive tests procedure (Jackknife cross validation and matrix of prediction indicator). It was applied to estimate TSS (total suspended solids) or COD (chemical oxygen demand) concentrations at the inlet of a storage-settling tank in a stormwater separate sewer system, and compared to three other calibration methods used either for turbidity meters or UV-visible spectrometers. With the available calibration data set: i) the spectrometer gives results with better prediction quality than the turbidity meter, ii) for the spectrometer, local calibration gives better results than global calibration, iii) the proposed PLS method gives results with a similar order of magnitude in uncertainties as the manufacturer local calibration method, but is more open and transparent for the user. Similar results were obtained for a second data set.

Reproducibility and uncertainty of wastewater turbidity measurements.

Turbidity monitoring is a valuable tool for operating sewer systems, but it is often considered as a somewhat tricky parameter for assessing water quality, because measured values depend on the model of sensor, and even on the operator. This paper details the main components of the uncertainty in turbidity measurements with a special focus on reproducibility, and provides guidelines for improving the reproducibility of measurements in wastewater relying on proper calibration procedures. Calibration appears to be the main source of uncertainties, and proper procedures must account for uncertainties in standard solutions as well as non linearity of the calibration curve. With such procedures, uncertainty and reproducibility of field measurement can be kept lower than 5% or 25 FAU. On the other hand, reproducibility has no meaning if different measuring principles (attenuation vs. nephelometry) or very different wavelengths are used.

Précision et reproductibilité du mesurage de la turbidité des eaux résiduaires urbaines sur échantillons

Measuring the turbidity of urban sewage is interesting because in raw waste water, pollutants are mainly particulates and/or bound to particulates. Moreover it is a real time and continuous measurement. This paper deals with knowledge elements and methods for the determination of the components of the uncertainty on turbidity measurement on water samples (principle of measurement, calibration, drift,...). These methods contribute to the use of this measurement technique by operators of urban drainage systems. They were applied for comparing experimental results obtained with various devices and various calibration and sampling strategies. The level of overall short term accuracy reached for turbidity measurement in urban waste water is satisfactory, provided that the non-linearity of some devices to corrected if needed.