Water Qual. Res. J. Canada, 2005 • Volume 40, No. 1, 51–58 Copyright © 2005, CAWQ 51 UV Spectrophotometry as a Non-parametric Measurement of Water and Wastewater Quality Variability Olivier Thomas, 1 * Estelle Baurès 2 and Marie-Florence Pouet 3 1 Observatoire de l’environnement et du développement durable, Université de Sherbrooke, Sherbrooke, Québec J1K 2R1 2 Syndicat Interdépartemental pour l’Assainissement de l’Agglomération Parisienne, 8 rue Villiot, 75012 Paris 3 Département de Génie Civil, Observatoire de l’environnement et du développement durable, Université de Sherbrooke, Sherbrooke, Québec J1K 2R1 The composition of water and wastewater, varying temporally and spatially, depends on factors such as environmental con- text, types of pollution sources, weather conditions leading to dilution or solids transportation, length of sewer network, etc. Because quantitative parameters are often not adapted for the characterization of wastewater quality variability, a non- parametric measurement is proposed, based on comparison of the UV absorption spectra of samples. The presence of isos- bestic points, occurring in the set of spectra either directly or indirectly after normalization, allows quantification of the vari- ability of a given water or effluent. A normalization step is used when dilution exists in the case of a mixture of water types (discharge or rain). Several examples show how to calculate the variability or to estimate the dilution factor from UV spectra data, even without results of physicochemical parameters. Key words: water characterization, wastewater characterization, non-parametric measurement, isosbestic point, variability, UV spectrophotometry, fingerprint Introduction Knowledge of water and wastewater quality variability is important, both for environmental purposes (quality monitoring, treatment plant efficiency, impact of dis- charge) and for industrial purposes (spills, production control). The best way to measure water and wastewater quality variability is to acquire quantitative data, prefer- ably with on-line monitoring systems (Bourgeois 1991; Thomas 2004b). The use of the water “fingerprint” is also proposed in laboratory analytical techniques, such as chromatography (Noij 2003), ICP-MS (Fernandez 2000) or spectroscopy including fluorescence (Baker 2004; Pons 2004). Depending on monitoring needs, a qualitative approach may be envisaged for regulation compliance (Valcarcel 2000). In some cases, knowledge of physicochemical parameters is not indispensable and can be replaced by characterization of properties such as the estimation of qualitative variability or treatability. Obviously, these properties can be estimated from rele- vant parameters (Juanico 1990; Weber 1990; Saint Hilaire 2004), but sometimes, more simply, from the direct use of analytical factors. This last point is the basic principle of the non-parametric measurement (NPM), which, as for a non-parametric statistical test, does not require to be related to a given parameter (respectively, a given statistical law) (Baurès 2002a,b). This means that there exists a qualitative relationship between the analytical factor and the information to be given. Thus, the more relevant analytical techniques which can be envisaged are the ones giving multiple responses that are difficult to exploit without extensive knowledge of the phenomenon to be studied. This is the case for all scanning techniques such as spectroscopic techniques. UV spectrophotometry was chosen based on its numerous and decades-old existing applications for water and wastewater quality monitoring. From the first work (Hoater 1952), many studies based on correlations between one absorbance value (generally 254 nm) and one parameter (e.g., COD, TOC) have been published (Briggs 1968; Mrkva 1969; Dobbs 1972; Thomas 1978). More recently, the development of procedures for the exploitation of the entire spectrum (Gallot 1993; Thomas 1995, 2004a) has led to the simultaneous calcu- lation or estimation of parameters or compound concen- trations (Thomas 1993, 1996). Moreover, indices and simple procedures for water and wastewater characteri- zation have been proposed from UV spectra exploitation (Baures 2004; El Khorassani 1999; Naffrechoux 1991; Muret 2000; Vaillant 2002; Winiarski 1995). In this study, a simple NPM procedure is presented for the measurement of the variability of water and wastewater, which depends on different factors, e.g., environmental context, type of pollution sources, weather conditions leading to dilution or solids trans- portation, length of sewer network. The NPM proce- * Corresponding author; olivier.thomas@usherbrooke.ca