Nature’s nitrogen removal solution may provide an important tool at the end of the onsite treatment train.
Wastewater treatment usually involves a combination of pumps and power, but for millions of years it has been done with plants and sunlight. It is this nature-based plan that William Strosnider follows in his search for wastewater engineering solutions.
Strosnider is an associate professor of environmental engineering and director of the Center for Watershed Research & Service at St. Francis College in Loretto, Pennsylvania. For some years he has been experimenting with floating wetlands to clean polluted water, and his latest piece of research on this appeared in the January issue of the Journal of Environmental Quality. He and colleagues at the University of Oklahoma built a few floating wetlands in some unused aquaculture ponds at the
University of Oklahoma and measured how well they worked. Pumper asked him to talk about his research and where floating wetlands can fit among the options for wastewater professionals.
Pumper: What does your research center do?
Strosnider: We’ve been running for about five years, and we have two goals. One is to get students out in the field, primarily to help nonprofit and government partners. The other is to undertake research projects that are actionable and close to the ground. We’ve done a lot of work on improving stream quality in western and central Pennsylvania.
The big, interesting idea in our work is figuring out what nature already can do to improve water quality and seeing how we can engineer that for our purposes. It obviously is possible. We just need to be creative, and maybe a bit more patient to let things grow.
Pumper: What is a floating wetland, and why use such a simple approach?
Strosnider: A floating wetland is just a raft of vegetation that isn’t anchored to the shore or the bottom of a pond. Cattails form floating rafts naturally. They grow out from the edge of a pond, and at some point this raft of plants will detach and float around the pond in a stable state.
It really isn’t a simple process because nature isn’t. But traditionally we’ve met wastewater treatment engineering goals by using a lot of power or refined chemicals. We’re trying to achieve the same goals with processes that require fewer inputs. Usually these solutions require more land area — treating water with a wetland requires maybe 10 times the land area of a machine-based solution — but could require very little ongoing maintenance and no pumping.
When we get this figured out, it is a solution that you could use in, for example, a detention pond in a subdivision.
Pumper: How did your experiment work?
Strosnider: Ultimately the goal is to engineer a raft of vegetation that renews itself with sunlight and nutrients from the water. Plants do that naturally under the right conditions, but it turns out it’s pretty tricky for us to do the same thing.
One issue in Oklahoma was the wind. It’s serious wind, and it blew over our young plants quite a few times, so the plants had to send up new shoots. That requires a lot of energy from the roots, and plants can do that only so many times. We lost quite a few plants that way.
We also found we needed a better substrate for the plants to root in. Or they need to be started in a greenhouse first so they can establish a good root structure before they’re put into a pond.
Pumper: Did you use special plants for this?
Strosnider: No, we planted the cattails and bulrushes you find in any North American wetland.
The water we put them in had somewhat elevated concentrations of nitrogen, the kind of water you would find in a pond collecting runoff from a farm field.
These floating wetlands also create habitat for animals, and as far as we could find, no one has cataloged all the animals that will use floating wetlands. I was attacked repeatedly by this red wing blackbird that nested on one of our rafts. There were also other birds, amphibians, snails and spiders. The cattail rafts were home to many more creatures than the open pond we put next to our experimental pond as a comparison.
Pumper: Why haven’t people looked at this technique before?
Strosnider: They have for about two decades, but it’s been done in fits and starts, and much of this work seems to have been driven by the interests of individuals. Now it seems formal funding is increasing for research into such alternative technologies.
People already use aerators to introduce more oxygen and knock down algae blooms in ponds. Floating wetlands could be a yet softer solution. We’re trying to figure out what we could do with a series of floating wetlands on a pond, how much coverage would you need to make a difference in water quality.
People in the wastewater industry are already using this technology. Wetlands are being increasingly applied for municipal wastewater treatment across the globe, especially in Europe. Over the last three decades, an increasing number of treatment wetlands have come online, addressing everything from raw wastewater treatment to the polishing of final effluent. There are a few folks out there promoting this technology, but although we know theoretically what will happen, we can’t plug data into a formula and calculate a recommendation for a customer because the basic research hasn’t been done yet.
For example, we don’t know how nitrate removal happened in our pond. We’re pretty sure it happens in or around the root zone, but to use floating wetlands as a solution we need to know what the mechanism is and, by extension, how many plants we need to bring about a given reduction in nitrogen.
Pumper: Why look at floating wetlands when we’re already building fixed wetlands for water treatment?
Strosnider: That’s one of the big questions: Can floating wetlands compete with the conventional ones? Research hasn’t answered this question yet, and without it you can’t do a cost-benefit analysis for customers. One fact about current floating wetland systems is they’re very expensive, and part of our project is trying to find an inexpensive combination of materials.
Pumper: How can wastewater professionals use this technology?
Strosnider: They could be particularly useful for nitrogen removal. Wetlands are most commonly used as the final stage in municipal wastewater treatment systems to address residual nitrogen, phosphorus, pathogen, or organic concentrations. Since nitrogen is often in its oxidized form (nitrate) by the time it reaches the final stage of a wastewater treatment plant, high-coverage floating treatment wetlands might be able to out-perform traditional surface flow wetlands in nitrogen removal by more effectively providing the anaerobic conditions necessary for nitrate processing. And the same goes for use in an onsite installation.
Pumper: What advice do you have for people who want to experiment with floating wetlands on their own before the research is done?
Strosnider: First of all, I would design for the winds and waves you would expect from a typical storm in your area. You need something that will be stable in the elements.
Next, give the structure a head start first in a greenhouse or some other protected area. Let the plants get comfortable and intertwined with the substrate you’re using, and then put them outside.
In any situation where land is cheap and you have less access to a power grid, chemicals, complicated equipment, and maybe expertise, cost-benefit calculations shift toward passive approaches like floating wetlands.