What if hog farmers could reduce the chances of lagoon overflow while also generating extra income?

North Carolina is the nation’s third-largest hog-producing state, and a big challenge hog farmers face is lagoon overflow on their farms. A new approach by an NC State University professor aims to make animal producers’ waste management practices more sustainable and less stressful. Ryan Sartor’s idea involves leveraging domesticated duckweed (Lemna) varieties in wastewater lagoons to prevent overflows on farms and harvesting the duckweed as a by-product.

Sartor, an assistant professor with a joint appointment in the departments of Molecular and Structural Biochemistry and Biological and Agricultural Engineering in the College of Agriculture and Life Sciences, has been working to domesticate duckweed since 2022. The North Carolina Department of Agriculture and Consumer Services has awarded two grants to Sartor to help support the research.

“Breeding is one of the essential things that has to happen to make a new crop,” Sartor says, noting that one of the key advantages from domestication and breeding is yield stability. “The ability to predict what our yield’s gonna be, is super valuable in agriculture.” 

Other groups have attempted to use duckweed in similar ways, but Sartor’s research began with a new approach. “The biggest thing we’re doing that no one’s ever done is breeding, so we’re presumably going to be able to generate much more productive varieties than anyone’s ever used,” he says.

Preventing Overflows

One of the standout uses for duckweed is its ability to absorb nutrients like nitrogen and phosphorus from wastewater. 

When there is heavy rain, farmers take extensive measures to ensure their lagoons don’t overflow and impact the surrounding environment. 

To avoid this, farmers use irrigation methods that pump wastewater onto designated sprayfields to fertilize crops while simultaneously reducing wastewater in the lagoons. Due to regulations, producers are only permitted to pump a certain amount of nitrogen and other nutrients onto the fields. 

These nutrients are fine in small amounts, but farmers follow strict nutrient management plans to limit excess and prevent groundwater pollution and runoff. The higher the nutrient content in the wastewater, the less farmers can apply onto sprayfields.

This is where Sartor hopes duckweed can make an impact. 

Growing duckweed directly in the lagoons allows those nutrients to be absorbed and stored in the plant. “We think the conservative estimate, in a system that’s producing pretty optimally, is we’ll be able to remove at least half of the nutrients from that water,” Sartor says. 

The remediated wastewater will have less nutrients, so more water can be sprayed onto fields. “The farmer would be able to spray twice as much water onto those fields legally,” Sartor says. 

From Lab Tests to Field Trials

Sartor’s plan to develop duckweed as an effective waste management crop began in the lab. He and his team focused on breeding different varieties to produce duckweed with the desired traits such as heat tolerance, high ammonia tolerance and increased starch content.

The selected varieties were then moved to the greenhouse where the next round of breeding took place. This hydroponic greenhouse system tests 90 duckweed varieties in triplicate, measuring 270 samples at a time. These samples are grown in submerged baskets in water that can be automatically weighed several times per day to measure growth rate.

The top performing samples are then moved to the field site at the university’s Lake Wheeler Road Field Laboratory, where they are grown in above-ground pools of diluted hog wastewater. “This field system is a final validation step, so we pick the top three from the greenhouse, and then see how they thrive in the pond system,” Sartor says.  

The pools have a collection mechanism that filters the water into bags, where the duckweed is then collected and moved to a drying area. 

Collecting the duckweed is an essential step in this process because it ensures the nutrients aren’t rereleased into the wastewater when the plant dies. “If you don’t actually use the duckweed after uptaking nutrients, it’ll sequester some nutrients temporarily, but then they’re all just going to go back once it decomposes, so you have to collect it,” Sartor says.

Waste to Worth

Since duckweed absorbs and stores nutrients, the dried material can be repurposed as a value-added by-product. Sartor imagines a future where a farmer can harvest the duckweed and sell it as animal feed, organic fertilizer or biofuel feedstock.

The varieties Sartor is breeding have roughly 30% protein and high starch content. This makes it a potentially optimal animal feed option. 

Another possible avenue is to use it as an organic fertilizer. “Dried Lemna biomass has at least twice the nutrient concentration as dried cow manure,” Sartor says. 

Pelletizing the harvested duckweed could be a more economical and environmentally sustainable option, as opposed to manure, which is more expensive to transport. It could also release nutrients at a slower rate, essentially reducing excess runoff. 

Lastly, Lemna is a very promising sustainable biofuel feedstock. It can be used to make biogas, ethanol, butanol or even hydrogen. Sartor is currently working to expand on this potential usage in his research. 

Sartor plans to continue developing this closed-loop agricultural system to demonstrate how growing duckweed can promote sustainable farming and provide economic benefits on farms.  He envisions a system that is a win-win for hog farmers. 

“I think the perfect situation would be for a producer to put one of these systems on their farm to potentially de-risk their manure management system, and ideally it would also provide an additional source of income,” Sartor says.

This post was originally published in College of Agriculture and Life Sciences News.