The terrestrial plants used in land treatment systems described in Chapter 8 of this book provide the major pathway for removal of nutrients in those systems. In those cases, the system design loading is partially matched to the plant uptake capability of the plants and the treatment area is sized accordingly. Harvesting then removes the nutrients from the site. The emergent aquatic plants used in wetlands also take up nutrients and other wastewater constituents. Harvesting is not, however, routinely practiced in these wetland systems due to problems with access and the relatively high labor costs. Studies have shown that harvesting of the plant material from a constructed wetland provides a minor nitrogen removal pathway as compared to biological activity in the wetland. In two cases (Gearheart et al., 1983; Herskowitz, 1986), a single end-of-season harvest accounted for less than 10% of the nitrogen removed by the system. Harvesting on a more frequent schedule would certainly increase that percentage but would also increase the cost and complexity of system management. Biological activity becomes the dominant mechanism in constructed wetlands as compared to land treatment systems, partially due to the significantly longer HRT in the former systems. When water is applied to the soil surface in most land treatment systems, the residence time for water as it passes from the surface through the active root zone is measured in minutes or hours; in contrast, the residence time in most constructed wetlands is usually measured in terms of at least several days.
In some cases, these emergent aquatic plants are known to take up and transform organic compounds, so harvesting is not required for removal of these pollutants. In the case of nutrients, metals, and other conservative substances, harvesting and removal of the plants are necessary if plant uptake is the design pathway for permanent removal. Plant uptake and harvest are not usually a design consideration for constructed wetlands used for domestic, municipal, and most industrial wastewaters.
Even though the system may be designed as a biological reactor and the potential for plant uptake is neglected, the presence of the plants in these wetland systems is still essential. Their root systems are the major source of oxygen in the SSF concept, and the physical presence of the leaves, stems, roots, rhizomes, and detritus regulates water flow and provides numerous contact opportunities between the flowing water and the biological community. These submerged plant parts provide the substrate for development and support of the attached microbial organisms that are responsible for much of the treatment. The stalks and leaves above the water surface in the FWS wetland provide a shading canopy that limits sunlight penetration and controls algae growth. The exposed plant parts die back each fall, but the presence of this material reduces the thermal effects of the wind and convective heat losses during the winter months. The litter layer on top of the SSF bed adds even more thermal protection to that type of system.
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