Some emergent macrophytes are capable of forming floating mats (Figure 2b), even though their individual plants are not capable of such existence. Typha spp. (cattails),
Glyceria maxima (giant sweetgrass, mannagrass), Phragmites australis (common reed), Cyperus papyrus (papyrus), Alternanthera philoxeroides (alligator weed), or Hydrocotyle umbellata (pennywort) are all capable of growing in mats. Treatment systems have been operated in this fashion; however, in some cases the floating mats developed unintentionally.
CW with floating mats of G. maxima was used to treat meat-processing effluent rich in nitrogen in New Zealand in the late 1980s. Sections of mat of G. maxima were excised from a mature wetland and floated on the wastewater to rapidly establish a plant cover on the wetland. No soil or sediment was placed at the bottom of the system. Results of this study indicated that in the summer the removal of total nitrogen increased when it reaches 53%. The active denitrification sites in the floating wetland system were the plant mat and the sediments (which developed during the operation of the system), both of which supply organic carbon and anaerobic sites for denitrification.
In the 1990s, CWs with floating mats of emergent vegetation were used, for example, in the United Kingdom or Uganda, and recently this system was used in Belgium. In the United Kingdom, this system with floating mats of P. australis is used to treat stormwater runoff waters including de-icing waters during the winter at the Heathrow International Airport in London. However, in order to keep the plants in the upright position, quite expensive stainless frames were used. In Belgium, floating mats-based CWs are used to treat municipal combined sewer overflow (Figure 13). In the system a mixture of plants is used with the dominance of Sparganium sp., Juncus sp., Carex sp., Iris pseudacorus, and Schoenoplectus lacustris. The plants are kept in position with a simple coconut fiber network. In Uganda, the FWS CW for the treatment of municipal wastewater was planted
with C. papyrus and Phragmites mauritianus. The growth of both plants is so vigorous that there is no need to keep the plants in upright position. The advantage ofthese systems is that both above- and belowground biomasses could be harvested and the amount of nutrient removed via harvesting is much higher as compared to other CWs which use emergent vegetation.
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