The removal of settleable organics is very rapid in all wetland systems and is due to the quiescent conditions in FWS systems and to deposition and filtration in SSF systems. Similar results have been observed with the overland flow systems described in Chapter 8, where close to 50% of the applied BOD is removed within the first few meters of the treatment slope. This settled BOD then undergoes aerobic or anaerobic decomposition, depending on the oxygen status at the point of deposition. The remaining BOD, in colloidal and dissolved forms, continues to be removed as the wastewater comes in contact with the attached microbial growth in the system. This biological activity may be aerobic near the water surface in FWS systems and at the aerobic microsites in SSF systems, but anaerobic decomposition would prevail in the remainder of the system. Removals of BOD in FWS constructed wetlands are presented in Table 6.1.
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