The treatment plant (STEP) in Sous-Bois, with a nominal capacity of 1800 EH, was commissioned in 1896. The installation suffers from recurring hydraulic and organic overloads. Exceedances of discharge standards are often observed. The collection network is prone to parasitic water from meteoric origins disrupting its operation. These overflows in rainy weather lead to leaching and consequently sludge departures. Corrective measures must be implemented as soon as possible. The Community of CAP NORD, in charge of the sanitation of the municipality of Lorrain, must undertake the reconstruction of this STEP in order to extend the pururatory capacity to 2,200 equivalent inhabitants (EH) taking into account future connections (+ 400 EH) on the collection network. The project therefore includes – the reconstruction work of the STEP – work on the reconstruction of a back-up station.