Evaluation of the performance and space requirement by three different hybrid constructed wetlands in a stack arrangement

•We propose a Duplex-constructed wetland (CW) for domestic wastewater treatment.•The Duplex-CW combines a vertical flow (VF) CW on top of a horizontal flow filter.•The VF CW contributed the most to the overall treatment.•Artificial aeration did not significantly improve the treatment efficiency.•A Duplex-CW is a compact system with an area requirement of 2.6–3.4 m2 PE−1.

Lack of space for a wastewater treatment plant is a common problem in many areas, especially in dense cities. Constructed wetlands (CWs) are efficient natural systems; however, they require large areas. The aim of this study is the development of a compact CW design for the treatment of domestic wastewater, the Duplex-CW: a hybrid system with a stacked design (vertical flow CW (VFCW) on top of a horizontal flow filter (HFF)). The performance of three different configurations of Duplex-CW, called fill and drain, stagnant batch and free drain, was compared. The VFCWs operated differently with the intention of creating different oxygen conditions, whereas the HFFs were operated identically. The Duplex-CWs were subjected to three different wastewater strengths, corresponding to designs of 7.9, 3.4 and 2.6 m2 PE−1. The highest strength was treated with and without artificial aeration of the VFCW of each configuration. The contribution to the total removal of each compartment (VFCW and HFF), the effects of the use of artificial aeration, the solids accumulation, above- and below-ground biomass and the footprint requirements of the three configurations tested were determined. The fill and drain configuration performed better than the other two, the VFCW compartment being more active in the treatment than the HFF. It achieved an area of 2.6–3.4 m2 PE−1 and it needed 2–3 times lower area than what a single VFCW would have needed to reach similar total nitrogen effluent concentrations. The Duplex-CW did not contribute to the footprint reduction, for other parameters (e.g. COD, TSS and total phosphorus).