Wetland systems can effectively treat high levels of BOD, total suspended solids (TSS), and nitrogen, as well as significant levels of metals, trace organics, and pathogens. Phosphorus removal is minimal due to the limited contact opportunities with the soil. The basic treatment mechanisms are similar to those described in Chapter 3 and Chapter 4 and include sedimentation, chemical precipitation and adsorption, and microbial interactions with BOD and nitrogen, as well as some uptake by the vegetation. Even if harvesting is not practiced, a fraction of the decomposing vegetation remains as refractory organics and results in the development of peat in wetland systems. The nutrients and other substances associated with this refractory fraction are considered to be permanently removed.
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