fox sedge, Carex vulpinoidea) are generally considered to offer superior levels of nutrient removal, with ideal levels of root-zone penetration (typically 0.6 m) and high levels of root mass and surface area. Conversely, although cattails (Typha latifolia) are often observed in natural wetland settings, they are generally regarded as a nuisance plant in constructed wetlands since their shallow-rooting tendency leads to relatively low treatment properties (i.e., much of the wastewater passing through an SSF bed would pass beneath the cattail root zone).
Duckweed and water hyacinths (see the insert image on the top of Figure 16.34) are commonly used in free-water systems. Duckweed has a far smaller root mass than that of hyacinth, and it extends far less deeply into the FWS system water depth, but even then it is often considered to be an important contributor to free-water systems, either by itself or as a secondary plant form. Water hyacinths are even more favored in free-water
systems given the sizable extent of their root systems, which extend deeply into the depth of these ponds. Hyacinths can also sustain high nutrient uptake rates, and as such they are also used in applications for tertiary-level wastewater polishing wetlands. Large, rapidly growing plants such as hyacinths, though, can clog a free-water system unless they are harvested on a regular basis, and in turn this residual plant mass could constitute a sizable waste material. Even then, however, these waste plants may still be put to beneficial use; for example, Disney World in Florida mulches their waste hyacinth plants and digests the residual organics to produce methane gas for use as an energy-rich fuel.
One of the key uncertainties with wetland systems, though, is that of the true mechanism^) behind which contaminants are removed within wetlands, and whether the responsible agent be that of plant- or microbial-based pathways. Much the same argument could also be raised for phytoremediation systems (see Section 16.2.4), in that our understanding of these processes bears a similar degree of lingering fuzziness.
Undoubtedly, plants can contribute directly to the observed uptake and degradation of contaminants, using a complementary range of plant-based sorption, uptake, accumulation, volatilization, and degradation mechanisms. Furthermore, the medium itself will provide some measure of complementary treatment, through either simple filtration or more complex physical-chemical mechanisms (e.g., sorption, ion exchange). Plants will also play a significant role in the metabolic passage of water through a wetland, and in some instances their rates of water loss through evapotranspiration will prove to be the dominant path for water movement, even to the point where the wetland may have little, if any (e.g., 5% or less), actual free-flowing discharge.
However, microbes extensively colonized onto and around plant root surfaces will clearly play an important, perhaps even dominant role with contaminant removal in plant-based wetlands. Figure 16.39 depicts this sort of extensive bacterial attachment, using an acridine orange cell stain to highlight living cells against the darker, unstained root surface.
Wetland plants nurture this sort of microbial colonization actively on several accounts. First, plant root and root tip surfaces inherently offer an extensive surface area for this sort of microbe growth. Second, plants will pump photosynthetically generated oxygen
through these same roots, such that this release will nurture the growth and colonization of aerobic microbes (i.e., largely bacteria plus fungi) within the root's immediate vicinity. Third, plants actively release an organic-rich mix of simple sugars from their root, known as root exudates, which will again sustain and promote active microbial growth.
On a collective basis, therefore, these plants will promote the extensive growth of microbial biomass within the immediate vicinity of their root structures and, in turn, these microbes will contribute to the wetland system's overall treatment efficiency. Cyclic oxygen release and uptake by wetland plants will actually occur on a diurnal cycle commensurate with the metabolic swing between daytime photosynthesis and nighttime respiration. FWS systems may consequently experience daily shifts in oxidizing and reducing conditions within the bulk wetland fluid, which intermittently promotes aerobic, anoxic, and anaerobic bacterial metabolism. For SSF systems, though, these same shifts in day-night plant behavior will similarly affect microbial growth in the root rhizosphere zone. Microbes immediately adjacent to root surfaces will probably be capable of sustained aerobic activity during daytime periods, but farther back from this oxygenated zone, the soil and its microbes will routinely follow a more anaerobic life-style. As such, there both aerobic and anaerobic microbial mechanisms will be involved in the degradation process, as well as a variety of interrelated precipitation, sorption, ionexchange, filtration, and even volatilization reactions.
Regional climactic conditions will represent another significant design factor, both in terms of selecting an SSF vs. FWS design strategy and in selecting the types of plants to be used. Whereas free-water wetlands are not suited to geographical areas in which prolonged winter weather would freeze much of the system, SSF operations have proven to be useful in cold-weather regions extending even into the Arctic Circle. During the winter, an ice layer will develop across the top of SSF wetlands, extending perhaps as much as 20 cm or more in depth. However, overlying ice and snow layers act as an insulating cover so that the underlying medium and roots remain at a temperature above freezing. Granted, at these ambient air temperatures the plants themselves will have shifted from an active to a dormant state, but their remaining vascular stalks are believed to act as physical channels or conduits that carry oxygen through the ice and into the medium where degradative metabolism is still maintained by an active microbial consortia. The reduced temperature, however, results in slower metabolic rates that must be taken into account by providing longer hydraulic retention times. Conversely, summertime wetland operations not only enjoy considerably faster rates of microbial degradation, but in many instance the plants involved will maintain far higher rates of evapotranspiration. In fact, depending on which plants are used, there may even be periods during summer season operation at which a wetland's incoming flow is largely released into the atmosphere rather than being released as a conventional discharge.
Many of the emergent plant species used in SSF systems tend to be fairly tolerant of cold-weather exposure. Conversely, hyacinths used in FWS applications are not hardy (able to tolerate freezing weather), and therefore their use is limited to warm climates. Duckweed, on the other hand, is able to withstand colder climates, but even then this plant form is still not widely used in these areas, given the general reluctance to use free-water systems in cold-weather localities.
Notwithstanding the apparent biological benefits of constructed wetlands, negative factors can be associated with these operations that must be addressed. For example, free-water systems, particularly those without supplemental aeration, have been known to encounter troublesome problems with mosquito growth as well as occasional odor generation. In both instances, though, these types of problems can be alleviated or avoided with either aerated free-water systems or subsurface-flow operations.
Annual maintenance for wetlands is, again, a rather inexact science. Harvesting is practiced infrequently with plants grown within subsurface systems. Instead, carefully controlled burning is used more commonly to remove the burden of dead, senescent plants and to help with eradicating invasive plants that have moved into the wetland during the past growth season. Dandelions and goldenrod are two of the more common invasive species.
With free-water wetlands, the plants maintained within lowly loaded operations will probably have to be thinned out with harvesting on an annual basis. Commensurate with increased loading levels, these harvesting rates will then have to be increased, to the point where monthly or biweekly plant thinning may be necessary with the more highly loaded (e.g., aerated) systems.
As for the regulatory requirements and monitoring policies currently stipulated by various states for constructed wetlands, there is considerable variation. For example, different stipulations may be placed on the location of flow monitoring equipment, but in most instances this equipment is situated at the effluent end of the last wetland cell. In some instances, a second flow monitoring device (i.e., typically a flow totalizer) is installed at the head end of the system, by which the operations personnel can then quantify the level of water loss maintained within the overall operation due to direct evaporation plus plant-based evapotranspiration.
FWS and SSF systems both typically exhibit fairly high level BOD and SS removal efficiencies, ranging from 750 to 90%, albeit somewhat below the performance levels that might be achieved with either fixed-film or activated sludge processes. For many wetland applications, particularly those faced with tight surface or subsurface discharge limits, nitrification will probably represent one of the most critical issues, and the relative rates, and efficiencies, of removing nitrogenous contaminants in these wetlands can vary immensely with time and from one site to the next in relation to plant type, organic loading rate, hydraulic loading, and climate. It is certainly possible for nitrification to occur in these wetlands, but the results quantified to date regarding wetland nitrification have been rather erratic, probably due to the fact that the temperatures and dissolved oxygen levels in these wetlands can vary quickly. One recent upgrade on this account has been that of using parallel SSF cells maintained in a flip-flop sequence of cyclic fill-and-draw stages, where these periodic, diurnal-type drain steps would hopefully provide increased oxygenation of nitrifiers working within the rhizosphere region.
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