Duckweed has traditionally been studied because of its inherently rich protein content at 30 percent to 35 percent on a dry-weight basis. The purpose was to explore whether duckweed could be a protein source for animal and human food. A growing interest in sustainable ethanol feedstock development, however, has researchers exploring the plant’s starch content.
North Carolina State University researchers Anne-Marie Stomp, associate professor of forestry, Jay Cheng, professor of biological and agricultural engineering, and Mike Yablonski, post-doctoral research associate, are discovering that duckweed can be used to clean up animal waste at industrial hog farms and could be used to make ethanol. They have determined that duckweed grown on swine wastewater can produce five to six times more starch per acre than corn, according to Stomp, who co-authored the research with Cheng.
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The research, funded by the Biofuels Center of North Carolina, was presented at the annual conference of the Institute of Biological Engineering in March in Santa Clara, Calif.
“The original investigations focused pretty much entirely on the protein side,” Stomp says. “At the time all of that work was being done, there was no compelling economic reason to domesticate this plant because we had plenty of other plant protein sources in grain and legumes. Back then, the prices of those grains and legumes were low and the market was fully supplied.”
The one challenge that has impeded duckweed’s progress in becoming a sustainable, dedicated energy crop for biofuels production or being used as a bioremediator for farm or city wastewater treatment operations is the fact that it wasn’t domesticated. “The trick to domesticating duckweed is going to be how much it will cost per ton to grow this stuff,” Stomp says, adding that data on economic feasibility will be released later this year. “That number provides a threshold for commercial viability,” she adds.
Cheng and Stomp are currently developing a pilot-scale project to further investigate the best way to establish a large-scale system for growing duckweed in animal wastewater, and then harvesting and drying the plant. “We’re actually exploiting a lot of existing technology used in the food industry, because duckweed is like a slurry,” Stomp says. “You can pump it, sieve it and do other things.”
In the meantime, duckweed will remain of interest to scientists as a viable synergistic component to the renewable fuels/energy sectors, possibly even being used with corn in existing ethanol operations, according to Stomp. “We’re not saying we’re going to replace corn,” she says. “It’s just another option out there for ethanol producers. It’s the idea that if we’re going to solve this energy crisis we’re going to need a bunch of ideas. One idea isn’t going to save us.”
Water Purification Potential
Propagated in agricultural and/or municipal wastewater, duckweed naturally extracts nitrogen and phosphate pollutants. This could benefit large-scale hog farms where animal waste is stored in large lagoons for biological treatment. Duckweed’s bioremediation properties allow it to capture pollutants and prevent their release into the air. The plant could save farmers money because they wouldn’t have to purchase expensive desalination equipment for their lagoons. “Duckweed is exquisitely good at recovering low levels of nutrients from water,” Stomp says. “It gets the water clean enough for reuse naturally, and it’s virtually cost-free for farmers.”
Duckweed can also reduce algae growth (by shading), coliform bacteria counts and mosquito larvae on ponds, while concentrating heavy metals, capturing or degrading toxic chemicals and encouraging the growth of other aquatic animals such as frogs or fowl. Additionally, duckweed is one of the fastest growing plant species on the planet. Scientists are also beginning to unlock duckweed’s potential as a player in carbon cycling and carbon sequestration.
Duckweed bioaccumulates about 99 percent of the nutrients contained in wastewater and produces a valuable protein-rich biomass as a byproduct, which can be fed to certain fish and added to poultry feed. Duckweed can also assimilate small hydrocarbons such as glucose and sucrose and, as a result, perform heterotrophic growth from the wastewater. The nutrients can be removed permanently from the system as the plants are harvested.
Due to its high affinity for absorbing pollutants in wastewater, Stomp posed a hypothetical scenario where the use of duckweed by farmers could mutually benefit a city willing to provide wastewater effluent for fresh water reuse. For example, a farmer could pay the city or municipality for its wastewater and have it transported to his farm, a concept that some people refer to as duckweed-based wastewater treatment, Stomp says. The farmer could take that wastewater and mix it with his livestock wastewater to dilute it so that it can be used to grow duckweed, which would clean the water and the farmer could sell it back to the municipality for reuse.
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