Search

L’urbanisation croissante, cause de la surcharge des réseaux d’assainissement, implique de repenser la gestion des eaux pluviales. L’utilisation de techniques dites alternatives permet de gérer l’eau à l’amont en se rapprochant du cycle naturel de l’eau via des ouvrages de stockage-restitution ou stockage-infiltration. L’effet de cette gestion amont des eaux pluviales sur la dynamique des écoulements et sur la contamination en métaux lourds du ruissellement est étudié pour deux événements pluvieux de caractéristiques différentes. Le fonctionnement d’un bassin versant de référence, en assainissement séparatif classique, est comparé à celui de trois autres bassins versants équipés de diverses techniques alternatives. Les résultats mettent en évidence un important lissage des écoulements par les techniques alternatives : écrêtement des pointes de débits (jusqu’à un facteur 20) et étalement des rejets d’eau dans le temps (temps de vidange compris entre 2 heures et 7 heures). De plus, une réduction significative des volumes rejetés, par rapport à ceux qui auraient été produits par un réseau séparatif classique (d’un facteur 1,2 à 2) a été observée. L’apport d’eau au réseau est donc limité lors des pluie intenses, évitant de surcharger le réseau, déjà fortement impacté. Pour les métaux lourds, les concentrations mesurées montrent une réduction du cuivre et du plomb à l’aval des techniques alternatives. Pour les autres métaux, une analyse plus détaillée de l’occupation du sol est nécessaire avant de pouvoir conclure. Dans tous les cas, la diminution des volumes d’eau rejetés implique une diminution des flux de contaminants dans les eaux pluviales.

Afin de répondre aux objectifs de la directive cadre sur l’eau (DCE) fixés par la réglementation européenne, la majorité des études se sont concentrées sur l’évaluation de la qualité des rejets industriels et des eaux usées traitées. Rares sont les études réalisées sur les eaux pluviales (EP) urbaines. Or, ces dernières peuvent être rejetées sans traitement dans les cours d’eau et donc avoir un impact sur l’écosystème aquatique. Ainsi, l’objectif de cette étude est de vérifier la présence de 88 polluants (28 substances prioritaires, 17 substances dangereuses prioritaires et 43 autres contaminants urbains) dans les EP collectées à l’exutoire de trois bassins versants situés dans Paris et sa banlieue proche. Par ailleurs, leurs concentrations sont évaluées à la fois dans les phases dissoute et particulaire. Les résultats montrent que les eaux pluviales urbaines contiennent 43% des polluants prioritaires, 41% des polluants dangereux prioritaires et 63% des autres contaminants urbains recherchés.

An experimental catchment area was set up in the centre of Paris (France) so as to follow up the quality of wet weather flows from the entry to the exit of a combined sewer network. The distinctive characteristic of this site is its location in a town centre and the extent of the equipment used to monitor the water pollution over the whole length of its course through the catchment area. The results obtained show a change in quality between the runoff entering the sewer network and the combined storm water flow at the sewer's outlet, which cannot be explained only by the mixture with domestic wastewater. In particular, an increase was observed in the concentrations of suspended solids (SS), VSS, COD, BOD and Cu, in the proportion of pollutants linked to particles and in the characteristics of the particles. A calculation of the total masses going in and out of the sewer network during a rainfall event shows that the erosion of in-sewer pollution stocks is the main source of particles and of organic matter in wet weather flows, whereas heavy metals loads originated from roof runoff, due to the corrosion of metallic roofs. Particles eroded from the sewer sediments during rain events were found to be quite different from the particles of type A deposits and organic biofilms. Nevertheless, they have mean organic and metallic loads that are of the same order of magnitude as the particles of the organic layer at water sediment interface. A change in the chemical form of heavy metals was noticed during the transport in the sewer and it is suspected that a fraction of the dissolved metals from the runoff is adsorbed on sewer sediments.

Sewer deposits play a significant role as a source of pollution in combined sewer overflows. The objective of this paper is to identify, using a direct approach on a sewer length, the main source of organic solids among the different deposits and their erosion processes. The different deposits studied were large inorganic solids at the bottom of the sewer, biofilms on the walls of the sewer and organic solids at the water–bed interface. The method applied consisted firstly of analysing very precisely (mass and pollutant loads) the solids in the three types of deposits along a sewer line with a length of 150 m. Then, the deposits were eroded by in situ flushing experiments using tap water. Finally the nature and the mass of particles eroded along the length of the sewer were compared with the nature and the mass of solids in each type of sewer sediment in order to identify the main source of organic solids in the sewers. This comparison indicates that the organic layer at water–bed interface is the main source of eroded solids. An erosion test carried out under continuous observation confirms that the organic layer is actually eroded.

The pollutant loads conveyed by street cleaning waters, by street runoff and the maximum pollutant load that can be removed from a street surface by water were measured for three streets in central Paris. Suspended solids (SS), chemical oxygen demand (COD), biological oxygen demand 5 (BOD5) and heavy metal loads were measured as well as the pollutant contents of transported particles and settling velocities. The contribution of street cleaning to dry weather flow pollution in combined sewers and its effect on wet weather flow pollution are discussed. Street cleaning waters are a minor source of dry weather pollution – with the exception of lead – and of dry weather flow volume. The pollutant load removed on a daily basis from street surfaces by street cleaning waters was found to be similar to that removed during one rainfall event, for SS and organic matter. However, it was five times lower for heavy metals. This load is nevertheless far inferior to the total mass of pollutants stored on the street surface. Thus, the effect of street cleaning on abating runoff pollution seems limited. An unexpected effect of street cleaning is that it induces sediment erosion inside the sewer during dry weather periods, thus reducing the stock of pollutants available for wet weather flow.

An experimental urban catchment has been created in the centre of Paris, in order to obtain a description of the pollution of urban wet weather flows at different levels of the combined sewer system, and to estimate the contribution of runoff, waste water and sewer sediments to this pollution. Thirty-six rainfall events were studied from May to December 1996. Dry weather flow was monitered for one week. Roof, street and yard runoff, total flow at the catchment outlet and waste water were analysed for SS, VSS, COD and BOD5, on both total and dissolved fraction. Results show an evolution in the characteristics of wet weather flow from up to dowstream: concentrations increase from the catchment entry to the outlet, as well as the proportion of particle-bound pollutants and the part of organic matter. A first evaluation of the different sources of pollution establishes that a major part of wet weather flow pollution originates from inside the combined sewer, probably through erosion of sewer sediments.