Title: Optimization of a Duplex constructed wetland for nitrogen removal through nitrification and denitrification

Author: Violet Margaret Namakula

Supervising Institution: IHE Delft  - Institute for Water Education

Year: 2014



Nitrogen removal in constructed wetlands (CWs) is mainly through nitrification and denitrification processes. The occurrence of these two processes in a single stage of CWs is difficult due to their conflicting dissolved oxygen requirements. To enhance nitrogen removal from wastewater and at the same time reduce the area occupied by the CW, a Duplex CW was designed. It consists of two stages, a vertical flow CW (VFCW) on top of a horizontal flow filter (HFF). Wastewater is fed first to the VFCW and after 1 day hydraulic retention time (HRT) it is drained to the HFF for 3-4 days HRT.

This study aims at improving nitrogen removal by enhancing nitrification through aeration and recirculation, also to improve denitrification by providing extra carbon source through a bypass to the HFF. Three configurations of the Duplex CW were used. (i) Aerated Duplex CW- artificial aeration was applied to only the VFCW, (ii) Recirculating Duplex CW- after the 1 day HRT of the VFCW, the effluent was recirculated 445 % between the HFF and the VFCW thereafter kept stagnant for 2-3 d in the HFF and, (iii) Control Duplex CW-Waste water was simply kept in the VFCW (1 day HRT) and then drained to HFF (3-4 d HRT).

The potential nitrification rate for the VFCWs ranged between 0.13 (Control Duplex CW) to 0.45 mg/kg/h (Recirculating DuplexCW). Potential denitrification rate in the HFF ranged from 27-58 mg/kg/h implying that the HFF of the Duplex CW has a capacity to treat wastewater with very high nitrate concetration.The control Duplex CW performed better in nitrogen removal with 2.03 g/m2/d followed by aerated Duplex CW and lastly recirculating Duplex CW.

Keywords: Aeration, Constructed wetlands, Denitrification potential, Nitrification potential, Nitrogen, Recirculation.