Self-monitoring of water quality in sewer systems using absorbance of ultraviolet and visible light

Continuous pollution measurement is interesting to optimize the operation o f sanitary facilities as well as to minimize the stormwater discharges. An experimental study was carried out for the determination of Suspended Solid s (SS) and Chemical Oxygen Demand (COD) concentrations in combined sewers u sing ultraviolet and visible absorbances. The maintenance of the measuremen t system requires six hours a month for the cleaning of the hydraulic feedi ng system and adjustment of the optical device. The feeding system develope d increased the representativeness and reliability of the pollution measure ment, but needs to be validated on other measurement sites. The determinati on of SS concentrations from visible absorbances requires 2 calibration cur ves for dry and rainy weather respectively. The corresponding accuracies ap pear satisfactory when compared with the results of standard sampling/labor atory analysis. The accuracy of COD determination from ultraviolet absorban ce is less satisfactory, but could perhaps be improved taking into account another parameter. Then the optical measurement of SS and COD is interestin g to determine average or long term pollution loads, for example the yearly impact of urban stormwater discharges. With this kind of continuous and on -line measurement, it is possible to react with short delay to unexpected p henomena which could damage the environment or water treatment efficiency.