The water discharged from the vertical filter is directed by a flow pipe upstream of the horizontal filter and flows by gravity from upstream to downstream. The effluent is distributed over the entire width and height of the filter by a distribution system located at one end of the basin; it then flows in the horizontal direction through the substrate. The water level, initially set during implementation in the box downstream of the filter by the height of the outlet pipe, is about 10 cm below the surface of the substrate. As a result, the bottom layer of the substrate is permanently saturated with water. Thus, anaerobic (deep parts of the filter) and aerobic (non-flooded surface layers, rhizosphere) zones are simultaneously in action. While passing through the filter, the solids still present in the water are retained by the root system of the plants, which allows the microorganisms of the substrate to degrade them effectively. Plants absorb soluble substances and accumulate them in their biomass. Microbial degradation and plant assimilation are thus more complete in this step than in the vertical filter. In addition, the anaerobic conditions that are predominantly present are favorable to denitrification.