Nature’s filter: Constructed wetlands
In 1985, Orlando, Fla., began construction of the Orlando Easterly Wetlands (OEW), an approximately 1,220-acre complex that the city figured would provide critical wildlife habitat for central Florida. To simulate natural wetlands, the city planted 2.2 million plants, including cattails, bulrushes, duckweed and water lilies.
It has worked like a charm. “We have recorded 170 different bird species, such as bald eagles, egrets and heron. We also have noticed deer, bobcat, otter and turkey [at the site],” says Mark Sees, wetlands analyst for OEW.
OEW obviously serves an aesthetic purpose. But beneath the surface of its critter-laden waters is a hidden function — the one for which it primarily was constructed: OEW’s vegetation and water help Orlando treat its wastewater. The wetlands “polish” about 20 mgd of wastewater from the Iron Bridge Regional Water Pollution Control Facility before discharging it into the St. John’s River.
Releasing wastewater into a wetland might sound like an absurd idea, but more and more cities and counties are finding that the practice is providing a cost-effective addition or alternative to mechanical treatment systems. In fact, in 1999, more than 200 communities in the United States — ranging from large cities to small towns — reported using constructed wetlands for wastewater treatment. They are learning that constructing wetlands to help treat wastewater by mimicking the processes of natural wetland areas improves treatment, enhances the environment and reduces costs.
Wetlands host a number of biological processes that serve to clean pollutants from water. Wetland plants trap sediment in their roots and branches, where microbes digest nutrients, breaking them down into harmless compounds. The process removes suspended solids, pathogens, organic matter, and sediment-attached nutrients and metals. Wetlands also provide a natural setting for disinfection; the sun’s rays destroy the cellular walls of some viral and microbial pollutants in a process known as photolysis.
The fact that wetlands consist of stable — as opposed to rushing — water is a key factor in their ability to perform those processes. “The most basic [process in a wetland] is the impeding of the water flow, which results in quiescent water zones that promote the settling of particulate matter,” says Rob Pearson, program manager for the Colorado Governor’s Office of Energy Management and Conservation (OEMC).
When the treatment process is over, the resulting effluent consistently meets discharge requirements, according to Pearson. Because of that, many communities in Colorado use constructed wetlands to improve and/or replace traditional mechanical systems with a history of not complying with state and federal pollution control regulations. In many towns, the constructed wetlands got noncomplying wastewater facilities back on track.
For example, the Dove Creek Wastewater Treatment Facility’s mechanical system was not meeting the rate of removal requirements for total suspended solids (TSS), until the town constructed wetlands in 1999. The wetlands helped to lower the amount of TSS enough to help the plant meet monthly goals.
In Orlando, OEW has reduced the amount of phosphorous discharged into the St. John’s River since it went online in July 1987. “The St. John’s River has phosphorous levels of about 100 parts per billion,” Sees notes. “Our wetlands’ effluent discharge is usually around 50 parts per billion. In terms of phosphorous loading, [the river] isn’t as susceptible to algal blooms and fish kills because of the dilution.”
According to the U.S. Environmental Protection Agency, two types of constructed wetlands treat wastewater effectively:
Free water surface flow (FWS) wetlands use a combination of emergent aquatic plants (such as cattails, bulrushes and reeds), floating plants (such as duckweed and water hyacinth) and submergent aquatic plants (such as sago pondweed and widgeon grass) to treat dirty water. FWS wetlands exhibit complex aquatic ecology and serve as habitats for aquatic and wetland birds.
Vegetated submerged bed (VSB) wetlands use emergent plants that are rooted in gravel. The wastewater travels through the gravel rather than over the surface of the wetland’s floor. According to EPA, the most current data shows that VSB systems perform just as well without plants as they do with plants, and, therefore, wetland ecology is not as critical in those treatment systems as it is in FWS systems.
With either type, wetlands can serve as more than just a treatment system, with the addition of amenities such as nature trails and viewing sites. According to a Colorado study of constructed wetlands, current designs include more natural wetland shapes (rather than the simple rectangular cells) with more attention to the habitat and the aesthetic value of borders, islands and plant diversity.
The team of biologists and engineers that conducted the Colorado study found a high correlation between the biological value of wetlands and their value as treatment systems. Constructed wetlands that scored high biologically also scored high from an engineering standpoint. Conversely, those wetlands that had low biology value also tended to function poorly from an engineering perspective.
The low costs involved with the operation and maintenance of constructed wetlands can make up for the additional aesthetic expenses. “Constructed wetlands are widely used instead of conventional treatment systems [because of] their low cost, minimal maintenance and their relatively low technology character,” says Tom Sturgis research biologist for the U.S. Army Corps of Engineers. “In some cases, the capital costs of such systems are generally only a bit lower than other alternatives, but operating costs are much lower.”
The lower costs usually are attributable to the “passive” nature of wetlands treatment. Constructed wetlands use naturally occurring processes, and, consequently, the required operator skill level is relatively low. “Mechanical treatment plants use unit processes that contain highly concentrated levels of bacteria that must be frequently checked and adjusted, requiring a highly skilled operator to obtain consistently reliable performance,” says James Watson, environmental engineer specialist for the Tennessee Valley Authority.
Attention to details
Despite the advantages, constructed wetlands are not equally effective everywhere. According to Watson they are most appropriate for small rural communities where land is available and relatively cheap.
According to EPA, the land area required for a constructed wetland depends on the quality of influent. As a general rule, if a constructed wetland receives highly pretreated wastewater, it requires less area than one that accepts wastewater with little pretreatment. Constructed wetlands normally range from fewer than two acres to 200 acres per million gallons of influent to be treated each day.
Some cities have expressed concern about the effectiveness of constructed wetlands in cold climates. However, EPA has found wetlands to be just as effective in cold weather as in warm weather. “In cold climates, the biological processes slow down,” says Bob Freeman, natural treatment systems expert for EPA. “You just need a bigger area so that the wastewater spends a longer amount of time in the wetlands to give the microbes time to work.”
“In northern climates, many, but not all, wetland treatment systems are run on a seasonal basis, with winter storage of water,” says Sturgis. “Also, in an average winter, a northern wetland may not freeze at all. The vegetation holds up the ice, which, in turn, supports a blanket of snow that insulates the system from the extreme weather,” he says.
Of course, constructed wetlands are not maintenance-free — regardless of their location. Organic material builds up in the system’s cells during the course of 10 to 15 years, reducing the quality of the effluent and allowing it to funnel through the system forming a sort of quasi-river. When that occurs, officials must take steps to rehabilitate the affected wetlands. However, EPA maintains that the bulk of the accumulated solids occurs at the influent side of the system. As a result, only a portion of the system (10 percent to 25 percent of the total surface area) may have to be cleaned rather than the entire system.
Some critics charge that the exotic plants and animals that find their way into wetlands can destroy native plants and the natural balance of the system. Plant and animal control is important to the maintenance and health of the system. “Nutria (a large non-native rodent that is wreaking environmental havoc in the Louisiana swamps) are in northern Florida, and, if they get to our wetlands and start destroying all of our plants, we could have a huge problem,” Sees says.
Communities thinking about implementing a constructed wetland system should state the design objectives and goals of the project as clearly and specifically as possible, Sturgis says. “Identify an existing water quality problem and target water quality or degree of improvement to be achieved. The simpler the design objectives, the more likely the success,” he says.
The project team of the Colorado study advises communities to:
Think natural. The more a constructed wetland looks and acts like a natural wetland, the better it will perform its natural water purification processes.
Be intimately involved with the wetland. Wetland operation is more similar to traditional land-man relationships (think farming or ranching) than traditional wastewater facility operation.
Aim to attract wildlife while ensuring that it is not destructive. Ideally, wildlife should be able to live in harmony with a wetland. Unfortunately, some wildlife can damage the wetland and impair wastewater treatment. Consideration should be given discouraging overpopulation of wetlands by potential pests.
Control water depth. The ability to control the depth of water in the wetland has been shown to be an important control for aerating cells, discouraging pest infestations, easing piping repair and general operation of the wetland.
While wetlands are a natural and effective way to approach wastewater treatment, their success depends on planning and patience. Even after they are designed and constructed, it may take up to a year for them to become operational. But, for many cities and counties, the benefits that constructed wetlands provide far outweigh the possible problems that can occur.