Mesurages en continu des flux polluants particulaires en réseaux d'assainissement urbains: Enjeux, méthodes, exemple d'application

The continuing growth of cities in most countries generates increasing pollutant loads discharged into urban water bodies by sewer systems, with very important environmental stakes. The determination, the control and the reduction of pollutant loads will not be achievable by means of usual discrete sampling and laboratory analyses. Various continuous in situ measurement techniques are now available. Among them, turbidity measurements allow estimating TSS (Total Suspended Solids) and COD (Chemical Oxygen Demand) event and annual loads transferred in urban sewer systems. In order to obtain reliable results and to estimate their uncertainties, appropriate methods have to be applied for i) sensor calibration, ii) application of calibration functions, iii) measurements of TSS and COD in samples with standard methods, iv) detection of possible multivariate outliers in the available data sets, v) specific 1st or 2nd order polynomial regression between TSS (or COD) values and turbidity values accounting for uncertainties in both variables, and vi) application of regression functions to estimate TSS (or COD) loads at event and annual scales. All above steps are described and illustrated with examples. A case study shows the application of the proposed methods to estimate the TSS and COD loads discharged by an overflow structure in a combined sewer system : 30 overflows measured in 2004 discharged approximately 2 100 kg of TSS and 2 900 kg of COD. These loads have a relative uncertainty of less than 5%.