Because of the continuous inundation, the soils or the media in a SSF wetland are anaerobic, which is an environment not well suited to support most vegetative species; however, the emergent plant species described previously have all developed the capability of absorbing oxygen and other necessary gasses from the atmosphere through their leaves and above-water stems, and they have large gas vessels, which conduct those gasses to the roots so the roots are sustained aero-bically in an otherwise anaerobic environment. It has been estimated that these plants can transfer between 5 and 45 g of oxygen per day per square meter of wetland surface area, depending on plant density and oxygen stress levels in the root zone (Boon, 1985; Lawson, 1985). However, current estimates are that the transfer is more typically 4 g of oxygen per square meter (Brix, 1994; Vymazal et al., 1998).
Most of this oxygen is utilized at the plant roots, and availability is limited for support of external microbial activity; however, some of this oxygen is believed to reach the surfaces of the roots and rhizomes and create aerobic microsites at these points. These aerobic microsites can then support aerobic reactions such as nitrification if other conditions are appropriate. The plant seems to respond with more oxygen as the demand increases at the roots, but the transfer capability is limited. Heavy deposits of raw sludge at the head of some constructed wetlands have apparently overwhelmed the oxygen transfer capability and resulted in plant die-off. This oxygen source is of most benefit in the SSF constructed wetland, where the wastewater flows through the media and comes in direct contact with the roots and rhizomes of the plants. In the FWS wetland, the wastewater flows above the soil layer and the contained roots and does not come into direct contact with this potential oxygen source. The major oxygen source for the FWS wetland is believed to be atmospheric reaeration at the water surface. To maximize the benefit in the SSF case, it is important to encourage root penetration to the full depth of the media so potential contact points exist throughout the profile. As described in Chapter 7, the removal of ammonia in a SSF wetland can be directly correlated with the depth of root penetration and the availability of oxygen (Reed, 1993).
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