Performance of ornamental plants in mesocosm subsurface constructed wetlands under different organic sewage loading

The aim of this study was to evaluate the performance of ornamental plant species in mesocosm subsurface constructed wetlands (CWs) to treat sewage under different organic loading rates (OLRs). The effect of ornamental plant species was evaluated in four mesocosm CWs with: (a) Iris pseudacorus (MCW-Ip), (b) Eichornia crassipes (MCW-Ec), (c) Tulbaghia violacea (MCW-Tv), and (d) Cyperus papyrus (MCW-Cp). The OLRs were evaluated as follows: (a) 0.47 gCOD/(m2 d) (Phase I), (b) 0.84 gCOD/(m2 d) (Phase II), and (c) 1.94 gCOD/(m2 d) (Phase III). Inside the four mesocosm CWs during the three phases showed oxidation-reduction potential and dissolved oxygen below −50 mV and 0.7 mg/L, respectively, which are characteristic values of anoxic-anaerobic environments. The 5-day biological oxygen demand (BOD5), chemical oxygen demand (COD) and total suspended solids (TSS) removal efficiencies were above 50% for the four mesocosm CWs in the three phases. In the cases of NH4+-N and PO4−3-P, removal efficiency was above 70% only for MCW-Cp during the three phases. When the OLR increased, BOD5, COD, TSS, NH4+-N, PO4−3-P and fecal coliform (FC) effluent concentrations from the four mesocosm CWs were affected in a significant way (α < 0.05) (except PO4−3-P in MCW-Cp). The species of ornamental plant showed non-significant (α > 0.05) effect on COD and BOD5 effluent concentrations for Phase I, II and III. Conversely, Tulbaghia violacea and Cyperus papyrus had a significant influence (α < 0.05) in the NH4+-N, PO4−3-P and FC, effluent concentrations during Phase II and III. Therefore, these two species of ornamental plants have greater potential to treat sewage under different OLRs, even though Tulbaghia violacea is a plant that has not been used previously in CWs.