Enhanced Denitrification by a Hybrid HF-FWS Constructed Wetland in a Large-Scale Wastewater Treatment Plant

The municipal centralized treatment plant (a classic activated sludge technology) receiving the wastewater produced by the municipality of Jesi (Ancona province, Central Italy), was upgraded in 2002. A new “nitrification–denitrification” compartment and a hybrid constructed wetland (CW) were added to the system to enhance denitrification and provide final treatment polishing. The whole system treats about 19,000 m³ day^–1 (more than 60,000 PE) and a part of the effluent is reused in a nearby industrial area. All the new sections have been provided with an online monitoring system, in order to reduce as much as possible the energy consumption for the denitrification process, and relying more on the final wetland stage whenever it obtains sufficient treatment levels. The upgraded wastewater treatment plant has been operating since January 2003, and after the first year of operation, good development of the macrophyte communities has been observed. The hybrid CW system consists of an initial sedimentation pond (volume of 2,000 m³), a 1 ha horizontal subsurface flow system (HF) second stage, and 5 ha free water system (FWS) as a final component. It has taken almost 18 months since the CW start-up for denitrification to occur at considerable levels. Nitrates removal rates ranged from 25% to 98% during the hottest period of the year.