Macrophytes and Rural Biodiversity

2.4.4 Macrophytes and Rural Biodiversity

2.4.4.1 Macrophyte Types and Characteristics

The range of macrophytes that are planted in constructed wetlands is wide and varied. They are intrinsic to the use of constructed wetlands and play a vital role in nutrient removal (Brix 1994; Hunt and Poach 2001). The USEPA (1988) guidance manual refers to cattails, reeds, rushes, bulrush, and sedges, all of which have different ranges of pH tolerance. For example, cattails usually tolerate pH values of between 4 and 10, while other aquatic plants such as rushes and sedges have much narrower tolerance margins. The USDA (1991) guidebook on constructed wetlands lists several species that have been identified as being suitable for use in constructed wetland systems in North America. The guidebook also states that not all wetland plants are suitable for treatment systems, since they should be able to tolerate continuous flooding and exposure to high nutrient concentrations in the influent (USDA 1991). Tanner (1996) summarized a list of properties that wetland plants should have:

• ecological acceptability (no significant weed or disease risks or danger to the ecological or genetic integrity of surrounding natural ecosystems); • tolerance of local climatic conditions, pests, and diseases; • tolerance of pollutants and hypertrophic waterlogged conditions; • ready propagation and rapid establishment, spread, and growth; and • high pollutant removal capacity.

The principal functions that macrophytes provide are numerous: stabilization of the beds, provision of physical filtration, prevention of clogging of vertical sys- tems, insulation against frost in winter, and provision of a large surface area for microbial communities, which are vital to successful wastewater treatment (Brix 1994; Scholz 2006a). In addition to supporting the treatment processes in con- structed wetlands, macrophytes also serve highly underrated functions in tradi- tional civil engineering design by promoting natural aesthetics and landscape integration.

Furthermore, planting of the most suitable and often native species is important in the ICW concept to improve the biodiversity of the vicinity around the structure (Harrington et al. 2005; Scholz et al. 2007b). The predominantly aquatic plants provide habitats for wildlife such as mammals, birds, and insects. The biodiversity of macroinvertebrates has been shown to be extremely high in certain ICW in

84 2 Wetland Case Studies

Ireland, e.g., some wetland systems have up to 60% of the country’s native species of aquatic macroinvertebrates present. The adaptation of wetland plants to live in anaerobic soils is important as their root structures provide aerobic areas that help to sustain nitrifying bacteria (Brix 1994). As well as providing oxygen for bacte- ria, they also provide oxygen to the anaerobic substrate and thus help to stimulate aerobic decomposition.

2.4.4.2 Toxicity Tolerance Thresholds

The toxicity tolerance thresholds and the corresponding uptake rates of pollutants by wetland plants have been researched (Brix 1994; Harrington 2005; Hill et al. 1997; Hubbard et al. 1999). However, these studies have usually examined the more common genera used in constructed wetlands (Brix 1994; Clarke and Bald- win 2002; Hubbard et al. 1999). For example, Clarke and Baldwin (2002) tested common species such as softrush (Juncus effuses L.), broadleaf arrowhead (Sagit- taria latifolia Willd.), softstem bulrush (Schoenoplectus tabernaemontani C.C. Gmel.), lesser bulrush (Typha angustifolia L.), and common bulrush (Typha latifo- lia L.) at varying ammonia concentrations and water depths.

Other studies have assessed more genera growing in temperate or tropical cli- mates (Tanner 1996). However, findings concerning the effect of ammonia on plants are not fully conclusive. The preference to pretreat wastewater prior to it entering the wetland system or the recycling of effluent suggests that the aquatic plants studied were most likely susceptible to ammonia toxicity, although some studies suggest that the plants are more tolerant than is commonly reported in the literature, stating that there was no apparent effect on some plants due to relatively high ammonia concentrations (Hill et al. 1997).

Comparisons between different plant species have been undertaken to examine their uptake rates (Hubbard et al. 1999; Poach et al. 2003; Tanner 1996). Brix (1994) reported on the uptake rates of common emergent, free-floating, and sub- merged plant species in wetlands. For example, bulrush (Typha latifolia L.) had an impressive nitrogen uptake rate for relatively small planted areas, but low phos- phorus uptakes for considerably larger areas. The nutrients are, however, bound in the biomass but could be removed by harvesting.

With plants being an important integral part of constructed wetlands, attention has been paid to the opportunity to use them for additional purposes. Therefore, cash crops such as soybean and rice have been assessed in terms of their use in wastewater treatment (Humenick et al. 1999; Szogi et al. 2000, 2003, 2004).

These research studies indicate that such plants are able to grow in treatment wetlands receiving swine wastewater. The potential yield from such cash crops could make constructed wetlands that use these plants more attractive, particularly in developing countries. Constructed wetlands could be used for the treatment of wastewater and also to yield a steady food supply or income. This would be par- ticularly beneficial for small-scale farmers, because they would be able to produce their own feed while treating their own wastewater at the same time.

2.4 Integrated Constructed Wetlands for Treating Swine Wastewater 85

Alternative methods of using aquatic plants are not limited to cash craps. The most common macrophytes planted on floating mats in anaerobic lagoons treating swine wastewater have been assessed by Hubbard et al. (2004). The nutrient up- take rates were relatively high. Less commonly used plant species native to certain regions have been examined as well. For example, vetivergrass (Vetiveria zizani- oides Nash) was used in Thailand (Kantawanichkul et al. 2003). This grass was suitable for tropical hydraulic and organic loading rates.