Aquatic plant may help remove contaminants from lakes

An interdisciplinary team of South Dakota State University researchers is collaborating with Roger Foote, coordinator for the Upper Big Sioux River Watershed Project, to find out whether this is a viable option to help remove phosphorus, nitrates, nitrites and even heavy metals from South Dakota lakes and streams.

The project, which began last year, initially sought to determine whether duckweed can grow in South Dakota and whether livestock will eat it. The answer to both questions is yes, according assistant professor Lin Wei of the Department of Agricultural and Biosystems Engineering. Wei, assistant professor Joseph Darrington, who recently joined the project, and animal science professor Julie Walker are working on the nearly $20,000 project now.

Former associate professor Erin Cortus, now at the University of Minnesota, led the project during its first year. Three graduate students have been involved in the research, which is supported by the U.S. Geological Survey through the Water Resources Institute with nonfederal matching funds from South Dakota Agriculture Experiment Station.

Foote was operating the old water treatment plant in Watertown as a phosphorus removal facility when he encountered duckweed. "My intent was to use algae to remove phosphorus, but the processing plant became contaminated with duckweed. I was removing hundreds of pounds of duckweed every week," he recalled. "It's very prolific and can compete with algae. It will grow until it runs out of space and prefers warm, slow-moving water."

The researchers collected samples of duckweed from Foote's indoor phosphorus removal facility and from the Big Sioux River.

"Duckweed can grow in South Dakota and even in the North Central region, but there are limits based on the season," Wei reported, with the best growth occurring in the summer. "Duckweed is also easy to harvest and process." The primary challenge is to control the water content, which is 80 to 90 percent in fresh duckweed.

"There's no issue with feeding it," Walker said, but it must be at least partially dried. "It won't require anything fancy, perhaps just laying it out in the sun like is done with hay."

In terms of protein content, duckweed is about 16 percent crude protein, Walker explained. "That's relatively high, so we could potentially use it as a supplement with a lower quality forage to bring up the protein content."

Walker dried the duckweed and made silage in small amounts, around seven ounces each, and then incubated the samples for 60 and 90 days. "We saw a slight decrease in crude protein and an increase in ammonia as a protein source when we went to 90 days," she reported. Walker did not identify any harmful toxins in the tested samples; however, she cautioned the types of contaminants in the water will make a difference.

The next step will be to see how contaminant uptake, including heavy metals, will affect the feed quality, as well as figuring out cost-effective ways to reduce water content.