Plants used in constructed wetlands with horizontal subsurface flow: a review

The presence of macrophytes is one of the most conspicuous features of wetlands and their presence distinguishes constructed wetlands from unplanted soil filters or lagoons. The macrophytes growing in constructed wetlands have several properties in relation to the treatment process that make them an essential component of the design. However, only several roles of macrophytes apply to constructed wetlands with horizontal subsurface flow (HF CWs). The plants used in HF CWs designed for wastewater treatment should therefore: (1) be tolerant of high organic and nutrient loadings, (2) have rich belowground organs (i.e. roots and rhizomes) in order to provide substrate for attached bacteria and oxygenation (even very limited) of areas adjacent to roots and rhizomes and (3) have high aboveground biomass for winter insulation in cold and temperate regions and for nutrient removal via harvesting. The comparison of treatment efficiency of vegetated HF CWs and unplanted filters is not unanimous but most studies have shown that systems with plants achieve higher treatment efficiency. The vegetation has mostly a positive effect, i.e. supports higher treatment efficiency, for organics and nutrients like nitrogen and phosphorus. By far the most frequently used plant around the globe is Phragmites australis (Common reed). Species of the genera Typha (latifolia, angustifolia, domingensis, orientalis and glauca) and Scirpus (e.g. lacustris, validus, californicus and acutus) spp. are other commonly used species. In many countries, and especially in the tropics and subtropics, local plants including ornamental species are used for HF CWs.