Recirculation on a single stage of vertical flow constructed wetland: Treatment limits and operation modes

Classical French vertical flow constructed wetlands (VFCWs) plant comprises two stages of treatment which the first one nearly respects standard outlet levels. It is therefore attractive to use recirculation on a single vertical stage to reduce its footprint when outlet levels required are not too severe regards to nitrification. The present study aims at evaluating performances and limits of a full-scale experimental plant during one year and a half. The monitoring has been done measuring continuously hydraulic fluxes and treatment performances in different operating conditions. Results showed good performances of the recirculating VFCW according to BOD, COD and SS parameters: mean outlet levels of 14, 73 and 19 mg L−1, respectively. Besides, nitrification is strongly dependant on recirculation rate and seasons (temperature effect). Recirculation over a single stage of VFCW can improve nitrification efficiency while enhancing carbon and SS removal.