For engineering purposes, wetlands have been described in terms of the position of the water surface. The free water surface (FWS) wetland is characterized by a water surface exposed to the atmosphere. Natural marshes and swamps are FWS wetlands, and bogs can be if the water flows on top of the peat. Most constructed FWS wetlands typically consist of one or more vegetated shallow basins or channels with a barrier to prevent seepage, with soil to support the emergent macrophyte vegetation, and with appropriate inlet and outlet structures. The water depth in this type of constructed wetland might range from 0.2 to 2.6
ft (0.05 to 0.8 m). The design flows for operational FWS treatment wetlands range from less than 1000 gpd (4 m3/d) to over 20 mgd (75,000 m3/d).
The biological conditions in these wetlands are similar, in some respects, to those occurring in facultative treatment ponds. The water near the bottom of the wetland is in an anoxic/anaerobic state; a shallow zone near the water surface tends to be aerobic, and the source of that oxygen is atmospheric reaeration. Facultative lagoons, as described in Chapter 4, have an additional source of oxygen that is generated by the algae present in the system. In a densely vegetated wetland, this oxygen source is not available because the plant canopy shades the water surface and algae cannot persist. The most significant difference is the presence, in the wetlands, of physical substrate for the development of periphytic attached-growth microorganisms, which are responsible for much of the biological treatment occurring in the system. In FWS wetlands, these substrates are the submerged leaves and stems of the living plants, the standing dead plants, and the benthic litter layer. In subsurface flow (SSF) wetlands (see Chapter 7), the substrate is composed of the submerged media surfaces and the roots and rhizomes of the emergent plants growing in the system. Many of the treatment responses proceed at a higher rate in a wetland than in facultative lagoons because of the presence of the substrate and these periphytic organisms, and the response in SSF wetlands is typically at a higher rate than in FWS wetlands because of the increased availability of substrate in the gravel media.
In addition to a higher rate of treatment than FWS wetlands, the SSF wetland concept offers several other advantages. Because the water surface is below the top of the gravel, mosquitoes are not a problem as the larvae cannot develop. In cold climates, the subsurface position of the water and the litter layer on top of the gravel offer greater thermal protection for the SSF wetland. The greatest advantage is the minimal risk of public exposure or contact with the wastewater because the water surface is not directly, or easily, accessible; however, the major disadvantage for the SSF concept is the cost of the gravel media. The unit costs for the other system components (e.g., excavation, liner, inlets, outlets) are about the same for either SSF or FWS wetlands, but the cost of gravel in the SSF system adds significantly to project costs. For design flow rates larger than about 50,000 gpd (190 m3/d), the smaller size of the SSF wetland does not usually compensate for the extra cost of the gravel. Because of these costs, the SSF concept is best suited for those smaller applications where public exposure is an issue, including individual homes, groups of homes, parks, schools, and other commercial and public facilities. It will be more economical to utilize the FWS concept for larger municipal and industrial systems and for other potential wetland applications. The FWS concept also offers a greater potential for incorporation of habitat values in a project. An example of a FWS wetland is shown in Figure 6.1.
The treatment processes occurring in both FWS and SSF wetlands are a complex and interrelated sequence of biological, chemical, and physical responses. Because of the shallow water depth and the low flow velocities, particulate matter settles rapidly or is trapped in the submerged matrix of plants or gravel. Algae are also trapped and cannot regenerate because of the shading
effect in the densely vegetated portions of the wetland. These deposited materials then undergo anaerobic decomposition in the benthie layers and release dissolved and gaseous substances to the water. All of the dissolved substances are available for sorption by the soils and the active microbial and plant populations throughout the wetland. Oxygen is available at the water surface and on microsites on the living plant surfaces and root and rhizome surfaces so aerobic reactions are also possible within the system.
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