Performance of ornamental plants in monoculture and polyculture horizontal subsurface flow constructed wetlands for treating wastewater

The aim of this study was to evaluate the effect of two ornamental plants in monoculture and polyculture horizontal subsurface flow (HSSF) constructed wetlands for treating wastewater. Two pilot-scale HSSF systems each with a surface area of 4.5 m2 were operated over two years (2015 and 2016); a) one was planted with a mixture of Cyperus papyrus and Zantedeschia aethiopica (HSSF-Cyp/Zant), and b) the other was planted only with Cyperus papyrus (HSSF-Cyp). To compare the performance between monoculture and polyculture systems, in situ parameters and organic matter (chemical oxygen demand (COD) and biological oxygen demand (BOD5)), total suspended solids (TSS), nutrients (total nitrogen (TN) and total phosphorus (TP)) and pathogens (fecal coliform (FC) and total coliform (TC)) removal efficiencies were evaluated. Moreover, growth characteristics, biomass production and nutrients uptake of different plants used in HSSF systems were also compared. The removal efficiencies of organic matter, suspended solids, nutrients and pathogens during the operational years were above 60%, 90%, 10% and 1.8 Log most probable number (MPN)/100 mL, respectively, without significant differences between HSSF-Cyp/Zant and HSSF-Cyp. The biomass production and the density of both HSSF systems fluctuated between 19.7 and 21.5 kg dry weight (DW)/m2 and 454-684 individuals/m2. Regarding the nutrient content of different plants used, Zantedeschia aethiopica, which was planted in the polyculture system, had the highest TN and TP content in all plants tissues (59.6 g N/kg·DW and 8.28 g P/kg·DW, respectively). However, TN and TP mass balances determined that the effect of monoculture and polyculture systems was not significant. Despite these results, polyculture CWs represent a good alternative of treatment system because they provide social benefits to the community such the improving of the system landscape and a better habitat quality. Moreover, some authors reported that polyculture system enhance the resistance to environmental stress and disease and the system landscape.