In natural wetlands, most of the nutrients required for plant growth are obtained from the soil by emergent aquatic plants. Cattails, reeds, and bulrushes will grow in a wide variety of soils and, as shown in the SSF wetland concept, in relatively fine gravels. The void spaces in the media serve as the flow channels in the SSF wetland. Treatment in these cases is provided by microbial organisms attached to the roots, rhizomes, and media surfaces. Because of the relatively light loading in most SSF wetlands, this microbial growth does not produce thick layers of attached material such as typically occur in a trickling filter, so clogging from this source does not appear to be a problem. The major flow path in FWS wetlands is above the soil surface, and the most active microbial activity occurs on the surfaces of the detrital layer and the submerged plant parts.

Soils with some clay content can be very effective for phosphorus removal. As described in Chapters 3 and 8, phosphorus removal in the soil matrix of a land treatment system can be a major pathway for almost complete phosphorus removal for many decades. In FWS wetlands, the only contact opportunities are at the soil surface; during the first year of system operation, phosphorus removal can be excellent due to this soil activity and plant development. These pathways tend to come to equilibrium after the first year or so, and phosphorus removal will drop off significantly. Soils have been tried in Europe for SSF wetlands, primarily for their phosphorus removal potential. This attempt has not been successful in most cases, as the limited hydraulic capacity of soils results in most of the applied flow moving across the top of the bed rather than through the subsurface voids so the anticipated contact opportunities are not realized. The gravels used in most SSF wetlands have a negligible capacity for phosphorus removal. Soils, again with some clay content, or granular media containing some clay minerals also have some ion exchange capacity. This ion exchange capability may contribute, at least temporarily, to removal of ammonium (NH4) that exists in wastewater in ionic form. This capacity is rapidly exhausted in most SSF and FWS wetlands as the contact surfaces are continuously under water and continuously anaerobic. In vertical-flow SSF beds, described in Chapter 7, aerobic conditions are periodically restored, and the adsorbed ammonium is released via biological nitrification, which then releases the ion exchange sites for further ammonium adsorption.

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