Infoscience

Student project

Estimation des rejets urbains de zinc et de cadmium au moyen d'une analyse de flux de substance. Application à la commune de Lausanne

This work is focused on the problem of the contamination of surface water and sediments by heavy metals, more particularly by cadmium and zinc, contained in runoff water and wastewater. To bring to light the urban cycle of cadmium and zinc, we have used the methodology of the substance flow analyses (SFA) and have elaborated a model. This model consists of various sources, flows, processes and stocks. The zone of study of this work is the municipality of Lausanne, situated at the edge of the Lake Geneva in Switzerland. This city possesses a bay; the bay of Vidy, which knows an important problem of pollution of sediments. Indeed, the values of concentration in heavy metals of sediments, in particular in cadmium and zinc, are superior to the limit values TEC and PEC (cf. chapter 6). The sewer system is of combined type and of separated type. Waters of the combined system are brought to the STEP of Vidy to be treated. On the other hand, waters of the separated system are sent without treatment to surface water. We have identified several sources of cadmium and zinc as domestic wastewater, car traffic, rail traffic, roofs and sidings of buildings, the dry and wet deposition. We have quantified the sources and coefficients of transfer of our model with data of the literature and hypotheses. We have put these values in SimBox, a program developed by the EAWAG, which allows to model the flows of substance and make analyses of sensibility and variation of the parameters. We have obtained the following results for the municipality of Lausanne. The contributions in sediments are 6.996 kgCd/year and 2961.11 kgZn/year. The contributions in surface water are 2.328 kgCd/year and 987.49 kgZn/year. The major sources of cadmium in surface water and sediments are the street furniture (70%), domestic wastewater (14%), roofs (8%) and sidings (3%). The major sources of zinc in surface water and sediments are the street furniture (24%), sidings (20%), roofs (19%), the tires of vehicles (16%) and domestic wastewater (14%). We have proposed several technical solutions to reduce the contributions of metals in surface water and sediments as the infiltration of runoff water in grounds or their discharge in wastewater (combined sewer), roads cleaning, pretreatment followed by a discharge in surface water. We have talked about the last two examples more specifically; roads cleaning and pretreatment followed by a discharge in surface water. The roads cleaning allows a reduction in sediments of 2.6% for cadmium and 3% for zinc. As for the filtering socks placed in the garbage dumps of roads, the reduction is 29% for cadmium and 15% for zinc.

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