Wetland Treatment Units

Wetlands are defined as land where the water table is at (or above) the ground surface long enough to maintain saturated soil conditions and the growth of related vegetation. The capability for wastewater renovation in wetlands has been verified in a number of studies in a variety of geographical settings. Wetlands used in this manner have included preexisting natural marshes, swamps, strands, bogs, peat lands, cypress domes, and systems specially constructed for wastewater treatment.

The design features and expected performance for the three basic wetland categories are summarized in Table 1.2. A major constraint on the use of many natural marshes is the fact that they are considered part of the receiving water by most regulatory authorities. As a result, the wastewater discharged to the wetland has to meet discharge standards prior to application to the wetland. In these cases, the renovative potential of the wetland is not fully utilized.

Constructed wetland units avoid the special requirements on influent quality and can also ensure much more reliable control over the hydraulic regime in the system; therefore, they perform more reliably than natural marshes. The two types of constructed wetlands in general use include the free water surface (FWS) wetland, which is similar to a natural marsh because the water surface is exposed to the atmosphere, and a subsurface flow (SSF) wetland, where a permeable medium is used and the water level is maintained below the top of the bed. Detailed descriptions of these concepts and variations can be found in Chapters 6 and 7. Another variation of the concept used for sludge drying is described in Chapter 9.

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