Energizing the food-energy-water nexus: the fortuitous tale of duckweed

UNIVERSITY PARK, Pa. — For Rachel Brennan,  associate professor in the College of Engineering, water has long been important. How interesting that her upbringing in southern New Mexico and a tiny aquatic plant would influence her work in sustainability and the food-energy-water (FEW) nexus.

“I was raised in the desert southwest, and water is very precious there,” Brennan said. “Groundwater was our main source of drinking water, and conservation was critical.”

Her interest in water has led her to develop a University Strategic Initiative proposal that aims to rally the University community around a framework for solving global FEW challenges.

Brennan has worked on both groundwater remediation and surface water projects while at Penn State. She is also the director of Penn State’s Eco-Machine, an ecological wastewater treatment facility that cleans water using diverse microorganisms, plants and other aquatic life.

“I am not a wastewater person,” she said, “but because most of my previous work focused on bioremediation, I thought if there is a biological way to remediate things such as pharmaceuticals, personal care products, and pesticides from wastewater, it would occur in a diverse ecosystem like the Eco-Machine.”

As the Eco-Machine purified the wastewater, Brennan considered ways to salvage the aquatic plant biomass from the system and turn it into valuable products. She briefly contemplated making biofuels from the algae in the system, but discovered it was challenging to work with in this environment. In fact, the algae had started to take over her research site.

“Some of our algae started to grow out of control,” Brennan said. “It started to grow long filaments and clog things.”

Enter the duckweed.

“For some reason, just fortuitously, this tiny plant showed up,” said Brennan, who noted she doesn’t know how the duckweed started to grow, but it flourished with no help from humans.

Historically, duckweed, an invasive species that is now on all continents but Antarctica, has been used as food, and for good reason. Duckweed is rich in protein, comparable to soybeans. It has all 10 essential amino acids, iron, iodine, and vitamin A, all of which are commonly deficient in the diets of malnourished people.

Duckweed is also used as fodder for livestock and fish in some communities.

And if it is not eaten, duckweed is an amazing fertilizer.

“The plant is a hyperaccumulator,” said Brennan. “It sucks up nitrogen and phosphorus out of the water extremely well. When you put it in the soil, it takes a while for the duckweed to break down, so it acts like a slow-release fertilizer.”

According to Brennan, many commercial chemical fertilizers can get washed away by rain — but not duckweed.

“We’ve done studies with it out in fields and in greenhouse tests, and we get comparable crop yields as conventional fertilizers but with much less nutrient runoff,” Brennan said. “We are closing the loop by using duckweed to recover the nitrogen and phosphorus from a waste stream, and then we put it back into the soil.”

Brennan’s research group found that duckweed is an excellent a fertilizer for grains like sorghum, and many types of vegetables. In fact, tomatoes love duckweed as a fertilizer, and growers can have multiple harvests of leafy greens, such as lettuce, with one application of duckweed.

Soil health is also improved by duckweed because of the plant’s carbon content.

“Commercial chemical fertilizers don’t contain organic carbon,” Brennan said, “but the carbon in the duckweed stays in the soil for multiple seasons, enriching the quality of the soil and contributing to the overall health of the soil ecosystem.”

To help with the original research, Brennan said she started collaborating with people who have expertise in plant science and soil microbiology. Now Brennan is also collaborating with experts in food science and science communication as her team looks to validate duckweed as a source of high-quality protein for meat and dairy animals, and eventually humans.

In addition to its uses as a food and fertilizer, duckweed is an exceptional candidate for biofuel.

“Duckweed changes its metabolism depending on the water quality in which it’s grown,” Brennan said. “If you grow it in very nutrient-rich waste, it synthesizes a lot of protein.”

However, if the nutrients to build proteins are in very low amounts, the plant preferentially sequesters starch, which can be easily converted into biofuels such as ethanol and methane, and other commodity chemicals that are eco-friendly.

“For a biofuel to be cost-competitive, you have to be able to use every single component of a crop,” said Brennan. “Even after we produce biofuels, there is a small amount of residual waste from duckweed left over, and that can be used as fertilizer or animal fodder.”

This little plant may even be heading to space. Because the plant takes very little room to grow (just a couple of inches of water) and is plentiful (doubling every couple days), duckweed is ideal for space travel.

“The plant could be used to purify the astronauts’ air and wastewater,” Brennan said. “Or it could be consumed or used as a fertilizer to help grow vegetables. Duckweed has already been tested in the Spacelab, and future experiments to test its ability to survive extended spaceflight missions are underway.”