Treatment of Domestic Wastewater from Small Cities on Vertical Flow Constructed Wetlands (VFCWs)

To improve denitrification and phosphorus removal in VFCWs, the French company SCIRPE has developed a patented process called Azoé^® which comprises biological pre-treatment on a trickling filter followed by two stages of partially saturated vertical flow filters planted with reeds. Phosphorus removal is carried out by addition of FeCl₃ at the outlet of the biological trickling filter. Phosphorus thus precipitated accumulates mainly on the surface of the first filter in the form of a sludge layer. After several years of operation, this surface layer represents a stock of phosphorus potentially mobilizable as a function of the biophysicochemical conditions of the environment. However the fate of organic and mineral micropollutants, (pharmaceutical residues, pesticides, metals, etc.) in the system is poorly known. In the present study, the performance of these systems was studied by monitoring an Azoé^® plant in operation. The results confirmed the very good efficiency in carbon and nitrogen treatment, the good stability of phosphorus retention, and a high rate of removal or retention of most micropollutants analyzed.