Does Your Wastewater Treatment Plant Need A Manmade Wetland?

There is truth to the notion that nature got it right the first time around. For all the tinkering that we do to perfect our environment, there is still much we can learn from the natural world around us.

One stubborn wastewater treatment problem, that of nutrient removal, may have found a new solution courtesy of recent research that looked toward Mother Nature and the use of floating treatment wetlands (FTWs).

An FTW is a buoyant structure, usually made from plastic piping, meshing, and Styrofoam, upon which a collection of plants can root and grow. By simulating the presence of a natural wetland, wastewater treatment plants could harness their nitrogen-reducing power for their own purposes.

The 28-month study compared water columns beneath FTWs in two test ponds to that of an open-air pond, each fed with nitrate-rich water. The researchers found that the water under the FTWs had lower dissolved oxygen, sulfate, nitrate, and pH than that of the control pond. They concluded that this is a promising start for potential wastewater treatment applications.

“Our results indicated that [FTWs] may be able to enhance nitrogen removal by creating good circumstances for denitrification,” said Dr. William Strosnider, an associate professor at Saint Francis University and researcher for the study. “So, it could be possible to apply them after an aerobic step where wastewater is nitrified.”

That being said, Strosnider stressed that there is much left unknown about how FTWs remove nitrogen and how effective this could really be for treatment plants struggling with nutrient influent.

“Further studies are needed to quantify rates of removal, the coverage of FTWs needed to achieve those rates, and how those rates would vary in different types of waters,” he said. “So, there’s a lot more to do.”

For the study, the research team built four different designs to explore which features would yield the best treatment results.

“The features that we found to be important are those that enhance stability for firm rooting of the plants, like finer meshed materials to support the plantings, and those that keep the plant in fairly shallow water, 20 centimeters deep or less,” Strosnider said.

A big focus of the study, and one that will be key to the affordability and sustainability of FTWs, was finding an alternative to the plastics typically used for their construction.

“One aspect of our study was to try to figure out how to minimize these non-renewable, non-biodregadable materials and maximize the ability of wetland plants to grow stable floating wetland structures themselves,” said Strosnider. “In special circumstances, wetland plants like the cattail we used in our study can form stable productive rafts of floating vegetation. We were trying to figure out a way to jump-start and guide the shape of this phenomenon for water quality improvement and habitat creation purposes.”

Solving this problem will be the focus of Strosnider’s future work and may be the key to installing FTWs and wastewater treatment ponds all over the country.

“I’m most interested in figuring out how to best get FTWs to essentially grow themselves into stable structures,” Strosnider said. “They could be a good-looking, sustainable tool for water quality improvement that also provides valuable habitat.”

However, until the material issues and price point are figured out, it seems that FTWs are not quite ready for widespread commercial use at wastewater treatment plants.

“Right now, conventional floating treatment wetland structures are relatively expensive when considered for high-coverage use,” said Strosnider. “I actually would have trouble recommending a wastewater treatment plant install floating wetlands before more research is done so that designs could be guided by reliable sizing equations.”