The use of vertical flow constructed wetlands (VFCWs) is well developed in France and other countries for the treatment of wastewaters from small communities. The patented Azoé® process has been developed by a French company, SCIRPE, in order to improve denitrification and phosphorus removal as compared to classical VFCWs. It includes a biological trickling filter pretreatment followed by two stages of partially flooded VFCW. The performances of partially flooded VFCW are well demonstrated for the removal of organic matter and nitrogen. The system is now being considered for phosphorus removal as well. In this article, sludge and granular materials sampled from the filters of a municipal plant where the Azoé® system has been operated for 8 years were analyzed in order to provide data that may contribute to a better understanding of the dynamics of phosphorus retention. Elemental analyses showed that phosphorus was predominantly captured in the sludge layer accumulated at the surface of the first stage. The progressive mineralization of the sludge over time was also clearly highlighted. The phosphate phases were mainly associated with iron and calcium. The transport of phosphorus via the migration of fine particles through the porous medium in the first stage was also observed.

The use of Vertical Flow Constructed Wetlands (VFCW) is growing rapidly in Europe for domestic wastewater treatment in small communities. In order to improve denitrification and dephosphatation as compared to classical VFCW, the Azoé-NP ® process has been developed. The process line consists of: a biological aerobic trickling filter as a primary treatment stage, ferric chloride (FeCl 3) addition for phosphorus (P) treatment and two stages of partially saturated VFCW. A municipal wastewater treatment plant using Azoé-NP ® process has been monitored during eight years through 44campaigns of 24h time-proportional inlet-outlet sampling followed by analyses of TSS, BOD 5 , COD, TKN, NO 3-N and TP concentrations. The results revealed good performances of the overall treatment. To better characterize the performance of each treatment step, five additional 24h monitoring campaigns were performed with samples taken from four different points along the treatment line. Results showed a good performance in dissolved carbon removal and nitrification by the trickling filter. The main part of the treatment was found to be done by filtration throughout the first filtration stage. Nitrate removal was achieved principally at the second filtration stage. Phosphorus migration through the first stage and its slight retention at the second stage was observed.

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