Wetland systems have played an important role in the process of cleansing surface waters since the original evolution of Earth's hydrological and biological ecosystems eons ago. However, our own efforts with extrapolating this natural concept into controlled treatment systems for remediation of wastewater and stormwater runoff have been far shorter, roughly covering only the past half-century.
The involved processing strategy is that of densely clustering water-loving plants within shallow (^60 to 75 cm deep) basins and then allowing the incoming waste or runoff flow to experience a prolonged period of contact with these plants and their extensive root matrixes. Rather naturally, the public's qualitative perception of these types of environmentally "green" plant-based systems inherently tends to be more positive than is the case with traditional, concrete-intensive wastewater treatment options, and as such, these systems have recently drawn considerable appeal (Cole, 1998).
Indeed, the purported benefits of green wetland systems, compared to the conventional approach of hardware-intensive wastewater treatment, are as follows (U.S. EPA, 2000):
• They can be constructed in highly attractive, environmentally friendly configurations (see, e.g., Figure 16.35).
• They may offer lower capital costs (at least for smaller applications).
• They will almost certainly have lower operating costs.
• They should require relatively less operating attention.
Clearly, a primary benefit of constructed wetlands, in terms of their engineered design, construction, and operation, is that of their simplicity. Wetland cells are typically constructed to use a gravity-based flow scheme that has few, if any, moving parts; most systems use only solar energy; and their demands on operator attention and care are quite low (being limited to occasional flushing of the influent and effluent lines to avoid plugging, and seasonal removal of dead, senescent plant debris).
However, these systems can also have a number of related disadvantages that must be recognized, particularly in terms of performance. One key issue is that their best levels of effluent quality (e.g., BOD, TSS, ammonia) are seldom realized in a matter of weeks or months after a new facility is installed, but instead, will probably require a year-plus period for system maturation. In addition, these systems are also apt to experience a higher degree of seasonal variability than what is typically the case with conventional wastewater systems, and these seasonal variations will be difficult to control. At the same time, these natural systems also tend to be susceptible to problems of a sort that would otherwise not be an issue with standard wastewater treatment operations. For example, the performance of the plants could be partially disrupted or completely curtailed by plant infestation and disease. Given the inherent plug-flow mode of wastewater passage through these units, an incoming slug of potentially harmful household chemicals (e.g., paint thinner, lawn care chemicals, bleach) could also kill a substantial front-end area of this biological operation for an extended period. In the particular case of wetlands with which their waste stream is open to the atmosphere, they have been known to encounter troublesome problems with mosquito growth as well as occasional odor generation.
These constructed wetland designs can be roughly subdivided into two major options, including those that rely on emergent plants and those that use submerged plants either alone or in combination with emergent plants. The names given to these two options are, respectively, that of a subsurface flow (SSF) and a free water surface (FWS) constructed wetland. In either case, it should be noted that some form of pretreatment (e.g., using a septic tank) is routinely provided ahead of wetlands receiving wastewater flows, to remove setttleable solids and floating oils and grease that might otherwise pass into the wetlands, where they might degrade system performance. In general, submerged-flow wetland systems tend to require somewhat less operational attention than free-water systems, and they may also have a slight advantage in terms of required land areas for comparable flows.
The SSF options (see Figure 16.36) are commonly constructed with one or more cells filled with various forms of a porous media, such as coarse and fine gravel washed previously to remove fine solids that would otherwise reduce necessary void volume, and planted with various forms of emergent wetland plants. Larger-diameter media (i.e., 4- to 5-cm coarse gravel) is normally placed across the head-end face of the cell to spread laterally and distribute the incoming waste uniformly, and then again spread across the back of the cell to recollect the effluent and direct it smoothly to the system's outlet piping. Within the wetland itself, a small-diameter medium (e.g., typically a washed, 0.75- to 1.5-cm-diameter river gravel) is used to sustain plant rooting. Waste-water introduced into these cells subsequently flows horizontally through the open, void space of the porous, subsurface medium at a relatively low velocity, during which it comes into contact with both the plant's extensive root system and its affiliated rhizo-sphere consortia of microorganisms.
The FWS option (depicted schematically in Figure 16.37) for constructed wetlands is built much the same as those in the SSF mode, with single or serial open cells that are similarly lined with much the same barrier material. These free-water cells, however, are only partially filled (10 ! 15 cm deep) with soil or a small-diameter (0.5- to 1-cm) gravel medium to support the plant root systems, such that the remaining depth is then filled with water and with floating plus rooted plant mass. Both emergent and submergent plant varieties can be grown in these cells; emergent plants will root themselves in the bottom medium, while the submerged plants can either exist in a free-floating mode whose roots dangle downward or are again embedded into the bottom medium. When given a means of supplemental aeration (e.g., using compressed air diffusers), the depths used with free-water systems are sometimes extended somewhat beyond 1-m depths.
Hydraulic residence time (HRT) is a primary design factor for all of these constructed wetlands, with minimal recommended values of 5 to 7 days (Hammer, 1989). Conservative designers may opt for considerably longer times, and many systems are built with HRTs extending into multiweek periods. However, when provided with supplemental aeration, or in the case of most subsurface flow designs, these residence times are typically reduced downward to values around 1 week.
Example 16.4: Preliminary Constructed Wetland Design Having completed their three prior preliminary designs, one final process design overview is requested for the Deer Creek, Illinois, community, in terms of a constructed wetland system. Given the prolonged subfreezing winter temperatures typically encountered at this northern midwest U.S. community, a subsurface-flow (SSF) constructed wetland will be sized preliminarily to handle a daily average wastewater flow of 605.7 m3/day and its commensurate total BOD loading of 121.2 BOD/day (Note: Although these constructed wetlands are commonly built with preliminary septic tank units, this example design will be based on the community's full BOD load as a conservative measure in light of potential septic tank upset conditions).
Preliminary Design Details
Hydraulic loading, system sizing, and residence times
Design hydraulic residence time (HRT): 7 days
Projected wetland liquid depth: 0.46 m (i.e., 18 in.)
(Note: Typically, only the lower two-thirds of the media depth is saturated.)
Expected media porosity = 0.4
Projected wetland volume:
Projected wetland surface area: (10,600 m3)/(0.46 m) = 23,043 m2 (-1.9 ha)
Projected wetland total media volume: (23,043 m3)/(0.61 m) = 14,046 m3
(Note: The latter volume would include both coarse inlet and outlet media plus smaller 0.75- to 1.5-cm river gravel.)
Projected organic loading:
(121.2 kg CBOD/day)/(10,600 m3) Projected hydraulic loading:
1. As with the stabilization lagoon option, a constructed wetland system will need to be considerably larger than the activated sludge or trickling filter designs for this community, in both surface area and volume.
2. Construction costs for constructed wetlands can escalate rapidly with larger facilities (e.g., beyond a daily flow of -500 m3/day) due to the capital cost of the media (e.g., washed coarse and fine gravel) and its on-site delivery.
3. Constructed wetlands are also apt to require less operational attention, but their cold-weather ability to secure nitrification may require additional treatment steps depending on regulatory requirements for effluent ammonia.
In the case of a free-water-surface operation, the determination of minimal hydraulic retention time is based simply on the ratio of the system's open water volume to the expected peak incoming wastewater flow. For subsurface-flow designs, however, this calculation must take into account the available pore volume of the medium through which the wastewater will be flowing, as follows:
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