The main objective of this study is the design of an ecological urban wastewater treatment plant in the region of Gabes (southern Tunisia). The treatment process by vertical flow reed filters (VFPR) was chosen. This is an extensive aerobic biological purification technique with fixed cultures on a granular support. This process is perfectly suited to small and medium-sized communities and provides good purification performance. Its principle consists in developing an aerobic biomass, fixed on filter beds, capable of ensuring the oxidation of organic and nitrogenous matter carried by the effluent to be treated and the physical retention of suspended matter. The reeds play an essential role in the purification process and in the management of clogging. The dimensioning of the said plant was carried out taking into account the quality and quantity of the wastewater to be treated and the arid conditions of the Tunisian south.