Les zones de rejet végétalisées Planted Discharge Area

Les zones de rejet végétalisées sont des espaces aménagés entre la station d’épuration et le milieu récepteur, censés contribuer à la réduction de l’impact des rejets sur le milieu naturel. Leur fort développement observé en France depuis une dizaine d’années souligne l’importance de lever les diverses incertitudes concernant la pertinence des objectifs visés et les limites réglementaires, afin d’encadrer concrètement la mise en uvre de ces nouveaux ouvrages. Planted Discharge Areas (PDA) are placed between the outlet of the WasteWater Treatment Plant (WWTP) and the receiving body. They may receive treated wastewater, stromwater or the both. Today, the French Authorities don’t assign any efficiency requirements to a planted discharge areas. The expected objectives from such landscape laying-out are various: i) reduction of the volumes discharged into the river, ii) qualitative improvement of some parameters (nitrogen, phosphor, micro-polluting, faecal contamination), iii) production of biomass, iv) landscape integration and environmental benefits, … Mechanisms involved in a PDA are multiple and concern 3 compartments: free surface water, soil, vegetation. It’s necessary to analyse each of these compartments according to the local conditions and on-site measurements are necessary. The “soil” compartment is the most important and needs to be study (pedological, geological and hydrogeological properties) before any construction of PDA.This article also presents the results of a national survey carried out in 2011. They confirm the explosion of the number of these systems over the last 5 years. This survey also underlines the diversity of the situations (WWTP capacity, area and hydraulic loads, design, expected objectives) without allowing to establish a simple link between the various parameters. Today, it is not possible to assert that PDAs always have a beneficial effect regarding the protection of the surface receiving body.