The vertical filter basin is filled with layers of gravel of different superimposed granulometry (gravel-gravel-sand), in which are planted macrophytes (Typha latifolia and Phragmites mauritianus and Canna indica).
The effluents resulting from the pretreatment are thus discharged at the surface of the filter and percolate gravitarily through the substrate. It flows through it undergoing physical (filtration), chemical (adsorption, complexation, etc.) and biological (biomass-supported) treatment.Most of the suspended solids (MES) are retained on the surface of the filter. by mechanical filtration of the substrate Organic matter is degraded and mineralized by microorganisms A part of soluble substances and nutrients are assimilated by plants to constitute biomass The presence of plants enhances the mechanical filtration of the substrate via a root system dense, and stimulates biodegradation activities.