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Lending Mother Nature A Hand

The engineering community typically has not been involved in conservation, but now engineers are collaborating with biologists, ecologists, and other scientists to solve some of the world's water problems.

- By Corinna Wu

Last summer, the Purdue University Research Farms in West Lafayette, Ind., found themselves in some deep doo-doo. A machine that automatically sprays fertilizer over crops failed to turn itself off, dumping a huge amount of liquid manure over the field. By the time the accident was discovered, excess manure had begun flowing into the farms' drainage ditch, threatening to contaminate the Wabash River downstream.

It could have turned into a major incident, but luckily, Purdue engineering students had recently completed constructing a sort of wastewater treatment facility right next to the ditch: an artificial wetland. The heavily contaminated water was pumped into the wetland and emerged clean enough to swim in.

Mention wetlands and most people conjure up an image of a humid swamp edged with cypress trees, buzzing with mosquitoes, and maybe supporting an alligator or two. But natural wetlands are found in all kinds of climates and are variously known as swamps, bogs, fens, sloughs, or marshes, depending on the kind of vegetation found there. Water can saturate it just part of the year or all year round. And like Purdue, many communities in the United States and around the world are actually building artificial wetlands to solve wastewater treatment problems.

Often referred to as the “kidneys of the world,” wetlands clean water by filtering out contaminants and concentrating them. Water moves through a wetland slowly, giving plants and microorganisms a chance to ingest and break down pollutants. Compounds containing nitrogen and phosphorus might make water unfit to drink but are the very nutrients that plants and microbes need to thrive. Some types of vegetation can even take up and store heavy metals—a property that has prompted environmental scientists to consider plants as a low-cost way to remediate sites contaminated with industrial waste. After moving through a wetland, the now-clean water usually makes its way to a stream or deep into the ground.

Constructed wetlands serve the same purpose as the natural ones—to treat contaminated water. In some parts of the world, such as Africa, a wetland might be the only such treatment available, says Thomas Crisman, director of the Center for Wetlands at the University of Florida, Gainesville. “It's raw sewage going in.” And like at the Purdue Farms, a constructed wetland can act as a buffer zone for agricultural runoff, intercepting fertilizers and pesticides before they trickle into the environment.

But in urban areas, constructed wetlands usually act as the last step in the filtration process. “We see them as a means of retrofitting old sewage treatment plants in small municipalities,” Crisman says. “They don't have the ability to upgrade their facilities, yet they're dumping out into a local stream. And so a small town could put a wetland at the end of their sewage treatment plant to polish it off.”

In the town of Gilbert, Ariz., located southeast of Phoenix, wetlands don't do any clean-up at all. About 15 years ago, Gilbert decided that it would reuse 100 percent of its water supply and built a treatment plant to handle the wastewater it generated. At the same time, the town decided to funnel part of that water back into the ground through a group of 11 ponds on a 72-acre site—a wetland in the middle of the desert.

The water is treated before being sent to the ponds and then drains through the sandy soil, eventually making its way to wells where it can be pumped out again for use. About a third of the 6 to 7 million gallons of wastewater the town processes daily makes it into the basins, says Scott Anderson, director of the Riparian Institute, the organization that promotes and develops Gilbert's wetland preserves. It takes only a week for the water to completely percolate into ground, he adds, and only 5 percent is lost to evaporation—not bad for Arizona's sunny, dry climate. Anderson says that since the first wetland site at Neely Ranch was built in 1990, the ponds have indeed succeeded in raising the water table.

The construction of the basins also had an unintended effect. “We were inadvertently attracting a lot of different kinds of birds, especially migratory birds during particular seasons,” says Anderson. This happened even though they had not planted any vegetation to encourage birds to use the ponds as a pit stop. So Anderson and his colleagues decided to go all out and develop the site as a riparian habitat, a waterside landscape characterized by trees like cottonwoods and willows. Riparian areas only occupy 1 percent of Arizona's landscape but support more than 60 percent of its wildlife. “Years ago, there used to be a lot of water running through this valley,” says Anderson. “But we've since dammed those rivers, so that's taken away a lot of the riparian areas.” And more of them are in danger of disappearing as water is siphoned off.

Now, Neely Ranch and a newer preserve at Water Ranch have become favored places for birdwatchers; the Audubon Society has identified 140 different species that make appearances at the sites. Circled with paths and picnic areas, the ponds are now part of recreational areas for the town—oases in the desert. Some of the water is diverted to a fishing lake, stocked with trout, bass, and catfish.

Waste Not, Want Not

Crisman says that since natural wetlands serve many purposes, constructed wetlands are a way for engineers to achieve several goals at once. “The engineering community [traditionally] has not been involved in conservation. Now here's the ability to engineer a system that can serve a conservation purpose.” For example, he and his colleagues are trying to identify ways of squeezing value out of wetlands beyond their traditional benefits. “If you consider wastewater as a resource, then wastewater can be used to create products,” he says.

Working with the Technical University in Darmstadt, Germany, for example, he's looking at bamboo as a potential product. A strong, flexible, fast-growing grass, bamboo can be chipped into particle board or used as a building material. “We're trying to create a product that's part of the maintenance of the system,” he explains. That way, regular maintenance of the wetland would yield an additional source of income, which would be especially welcome in developing countries.

For students, constructed wetlands offer a way to look at old problems in new ways. As an example, Crisman cites a project that was initiated a few years ago by students at the University of Florida. They redesigned a planned stormwater retention basin on campus into a functional wetland, securing all the necessary permits and getting the support of the university and local water management authority. Called the Stormwater Ecological Enhancement Project, it's now used as a teaching facility. “We're really trying to do those things at the interface between classical engineering and ecology and conservation,” Crisman says. “Those sort of hybrid programs are where it's going to go in the future.”

At Purdue, the constructed wetland next to the research farms is part of the Engineering Projects in Community Service (EPICS) program funded by the National Science Foundation. EPICS pairs student engineering teams with community service organizations, allowing the students to get real-world experience as technical consultants. “The constructed wetlands [project] is atypical of the normal program because we don't have a true social service agency that we're working for,” says William Oakes, co-director of EPICS at Purdue. “In a sense, we're working for the environment.”

Many students find these experiences useful, even if they never work on another constructed wetland for the rest of their careers. “In all areas of civil engineering, environmental impacts are becoming more and more important,” says Geoff Henggeler, a junior civil engineering student who's worked on the constructed wetland at Purdue for about two years. “I don't think that there's a single civil engineering project that you could be involved with that doesn't involve environmental engineering at least to a minor degree.”

Wetlands projects at universities draw students from all different areas of science and engineering. Twenty different majors are represented in the EPICS project, Oakes says. The opportunity to collaborate with a diverse group of people is valuable training, since wetlands engineers often rely on the expertise of biologists, ecologists, and hydrologists to figure out the needs of a particular site.

As an engineering solution, constructed wetlands serve multiple purposes. Some of those goals can be measured in dollars, but others are harder to assign a value. In Gilbert, what started as a municipal water project has become a point of pride for the town. “Communities here in the valley have their own identity,” says Anderson. “Gilbert's kind of identified this as their own little niche, creating these habitats, these areas that are really unique. And it's been very successful.” And for engineering students finding their way to a career, working on a constructed wetland project is one way to put their classroom skills to a real-world test.

 

Corinna Wu is a freelance writer based in suburban Washington, D.C.
She can be reached at cwu@asee.org.