Plants Used in Constructed Wetlands With Horizontal-Review

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Transcript of Plants Used in Constructed Wetlands With Horizontal-Review

Hydrobiologia DOI 10.1007/s10750-011-0738-9

WETLAND RESTORATION

Review Paper

Plants used in constructed wetlands with horizontal subsurface ow: a reviewJan Vymazal

Springer Science+Business Media B.V. 2011

Abstract The presence of macrophytes is one of the most conspicuous features of wetlands and their presence distinguishes constructed wetlands from unplanted soil lters 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 ow (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 efciency of vegetated HF CWs

and unplanted lters is not unanimous but most studies have shown that systems with plants achieve higher treatment efciency. The vegetation has mostly a positive effect, i.e. supports higher treatment efciency, 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. Keywords Constructed wetlands Horizontal ow Macrophytes Phragmites australis

Role of plants in constructed wetlands Guest editors: Dominik Zak, Robert McInnes, Jorg Gelbrecht / Restoration, biogeochemistry and ecological services of wetlands J. Vymazal (&) Department of Landscape Ecology, Faculty of Environmental Sciences, Czech University of Life Sciences in Prague, Prague, Czech Republic e-mail: vymazal@yahoo.com J. Vymazal ENKI, o.p.s, Trebon, Czech Republic

The presence of macrophytes is one of the most conspicuous features of wetlands and their presence distinguishes constructed wetlands from unplanted soil lters 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 (Brix, 1997). However, only several roles of macrophytes apply to constructed wetlands with horizontal subsurface ow (HF CWs) (Table 1). The plants used in

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Hydrobiologia Table 1 Summary of the major roles of macropyhtes in constructed treatment wetlands (modied from Brix, 1997) Macrophyte property Aerial plant tissue Role in treatment process Light attenuationreduced growth of photosynthesis Inuence of microclimateinsulation during winter Reduced wind velocityreduced risk of resuspension Aesthetic pleasing appearance of the system Storage of nutrients Plant tissue in water Filtering effectlter out large debris Reduced current velocityincreased rate of sedimentation, reduced risk of resuspension Excretion of photosynthesis oxygenincreased aerobic degradation Uptake of nutrients Provision of surface for periphyton attachment Roots and rhizomes in the sediment Stabilizing the sediment surfaceless erosion Prevention of the medium clogging in vertical ow systems Provision of surface for bacterial growth Release of oxygen increases degradation (and nitrication) Uptake of nutrients Release of antibiotics, phytometallophores and phytochelatins Roles important in HF CWs in italics

constructed wetlands 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 nutrient removal via harvesting (Czkova-Koncalova et al., 1996; Kvet et al., 1999). Insulation of the ltration beds Insulation that the plant cover provides during winter, especially in temperate and cold climatic regions, is very important for a steady performance of the system (Smith et al., 1996; Mander & Jenssen, 2003; Vymazal & Kropfelova, 2008a). When the standing litter is covered by snow, it provides a perfect insulation and helps keep the substrate free of frost (Brix, 1998). The litter layer helps in protecting the soil from freezing during winter, but on the other hand, it also keeps the soil cooler during spring (Haslam, 1971a, b; Brix, 1994). Insulation of ltration beds is relevant only under temperate and cold climatic conditions, i.e. under

freezing conditions during winter. As a consequence, the processes responsible for pollutant removal do not slow down substantially, and the treatment performance is steady during the year. The steady treatment performance of HF CWs, with the partial exception of ammonia and total nitrogen removal, was reported from Switzerland (Zust & Schonborn, 2003), Norway (Giver, 2003; Mhlum & Jenssen, 2003), Germany (Kern, 2003; Steinmann et al., 2003), Czech Republic (Vymazal, 2001, 2009; Vymazal et al., 2003), USA (Dahab & Surampalli, 2001; Hill et al., 2003), Italy (Gorra et al., 2007) or Korea (Ham et al., 2004). Belowground parts as substrate for bacterial growth The rhizosphere is known to harbour a great diversity of bacterial forms (Paul & Clark, 1996) and it is generally assumed that planted wetlands outperform unplanted controls mainly because the plant rhizosphere stimulates the microbial community (Gagnon et al., 2006). It has been suggested that plant rhizosphere enhances microbial density and activity by providing root surface for microbial growth, a source of carbon compounds through root exudates and a micro-aerobic environment via root oxygen

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Hydrobiologia

release (Gersberg et al., 1986; Brix, 1997). Higher microbial densities in planted systems were reported, for example, by Hatano et al. (1993) and Munch et al. (2005). Several authors have shown plants to differ in root surface area available for bacterial growth (Hatano et al., 1993; Vymazal et al., 2003, Kyambadde et al., 2004; Gagnon et al., 2006) and Paul & Clark (1996) pointed out that the plants inuence the specic rhizosphere population. This phenomenon was observed in constructed wetlands by Collins et al. (2004) or Li et al. (2008). Plants can also affect microbial species composition and diversity by releasing exudates and oxygen into the rhizosphere that in turn indirectly affects enzyme activity (Singh & Kumar, 2008). Oxygen release to the rhizosphere of wetland plants The primary difference between water-saturated and well-drained soils is the availability of oxygen for root respiration, microbial respiration and chemical oxidation processes (Brix, 1993). In well-drained soils, the pore spaces are lled with air showing a relatively high-oxygen content. Microorganisms living in the soil and roots of plants growing in the soil are therefore able to obtain oxygen directly from their surroundings. As the soil pore spaces are connected with the atmosphere above the soil, the oxygen in the pore spaces is replenished by rapid diffusion and convection from the atmosphere (Brix, 1993). In a watersaturated soil, the pore spaces are lled with water. The rate of diffusion of oxygen through water is some 104106 times slower as it is through air, principally due to the smaller diffusion coefcient in water, but also because of low solubility of oxygen in water (Greenwood, 1961; Drew, 1979). Consequently, water-saturated soils become anaerobic (oxygen-free or anoxic) except for a few millimetres at the surface (Jackson & Drew, 1984). Due to absence of oxygen in waterlogged soils, the roots and rhizomes of plants growing in water-saturated substrates must obtain oxygen from their aerial organs internally through the air spaces in the plants (e.g. Laing, 1940; Coult, 1964; Teal & Kanwisher, 1966; Armstrong, 1978, 1979; Dacey, 1980; Studer & Brandle, 1984; Brix, 1993). The main anatomical feature of wetland plants is the presence or development of air spaces in different parts of the leaves, stems, rhizomes and roots (Gopal &

Masing, 1990; Brix, 1998; Tiner, 1999). The presence of aerenchyma (air-lled) tissue in many wetlands plants enables these plants to grow in anaerobic or anoxic soils. The extensive lacunal systems which normally contain constrictions at intervals to maintain structural integrity and to restrict water invasion into damaged tissues may occupy up to 60% of the total tissue volume (Studer & Brandle, 1984). Many studies have demonstrated an increase in aerenchyma in plants subjected to ooding and to stronger anaerobiosis (Seliskar, 1988; Burdick & Mendelssohn, 1990; Kludze & DeLaune, 1996). Various gas transport mechanisms in wetland plants have been reviewed, for example, by Brix (1993). Wetland plants tend to minimize their oxygen losses to the rhizosphere but they do, nevertheless, leak oxygen from their roots (Armstrong & Armstrong, 1988; Brix, 1989). Oxygen release rates from roots depend on the internal oxygen concentration, the oxygen demand of the surrounding medium and the permeability of the root-walls (Sorrell & Armstrong, 1994). Rates of oxygen leakage are generally highest in the sub-apical region of roots and decrease with distance from the root apex (Armstrong, 1979). The oxygen lea