Greater Duckweed (Spirodela polyrhiza) — one of the smallest and simplest freshwater plants known — generally gets a bad rap. That’s because the millimeter-sized floating plant thrives on the worst sort of livestock and human wastewater, basically garden-variety sewage. In fact, in the South Pacific, New Zealand and Australia, it’s frequently used to clean such wastewater.
For years, researchers have been trying to commercialize duckweed as a viable source of bioenergy for the production of ethanol, biodiesel, natural gas and steam-generated electricity.
But even now, there’s little agreement on whether duckweed is best suited as a natural option for turning so-called municipal graywater into something clean enough to drink from the tap, or as a renewable biomass. In pelletized form, it can also be used to feed tilapia, shrimp or poultry, and is even co-fired with coal.
Duckweed to bioenergy conversion may ultimately work best when done in tandem with some sort of ongoing wastewater cleanup.
The plant itself is composed of only a single kidney-shaped leaf, connected to the water on, which it floats by only a few thin underwater roots. However, duckweed advocates point to the fact that, in warm climates, it can basically grow anywhere and at all altitudes.
A shallow big pond full of effluent from secondary treatment is just like liquid duckweed fertilizer, says Anne Marie Stomp, a retired North Carolina State University plant molecular biologist.
“You dump duckweed on top and every three days you take half of it away and the rest keeps growing,” says Stomp.
Duckweed also has an advantage over algae biomass; it is large enough that it can be easily separated from water, it is very easy to air dry and, like hay, is also easy to store.
It may soon become more prevalent in the U.S. if the Environmental Protection Agency (EPA) places more stringent requirements on wastewater discharge permits, and small towns could be forced to comply with tertiary wastewater treatment options.
“Small towns could be forced to put in $100 million chemical tertiary treatment plants which they can’t afford,” said Stomp. “However, duckweed is fantastically good at tertiary wastewater treatment.”
Duckweed bio-engineering could also make it even more attractive as a bioenenergy feedstock.
A paper published earlier this year in the journal Nature Communications provides new and comprehensive details of the duckweed’s genome.
“If used for ethanol or electricity, any improvement in its BTU [output] would require that you improve the carbon allocation of the organism,” said Joachim Messing, Director of the Waksman Institute of Microbiology at Rutgers University in New Jersey.
Messing, the paper’s senior author, says that, it requires knowing the duckweed’s gene content.
“We can bio-engineer the organism so that it has a better carbon output — in some form of carbon, either sugar, protein or oil — to potentially make kerosene, gasoline or diesel,” said Messing.
Duckweed, however, is already capable of doubling its population in as little as 48 hours, a fact that hasn’t eluded police officer Sam Licciardello, CEO of Biomass Alternative Power in Mantua Township, New Jersey. He is heading up a group that is investing $40 to $60 million to use duckweed to generate both electricity and natural gas by late 2015.
“I will be growing duckweed in a 15-acre, gutter-connected greenhouse site,” said Licciardello. “It will accumulate steam from the gasifiers that will run two turbines which will create electricity and supply the grid with 12 MW. While making steam, it will also create natural gas from the duckweed which will be stored, then tested processed and released in a metered [grid] system.”
But Stomp remains skeptical.
“No one wants to fund research to figure out a high-value product from duckweed because there is no mass source or cropping system for the plant,” said Stomp. “So, nothing but futile attempts at commercialization get started, usually by people who are passionate but have limited business sense.”
Yet Licciardello couldn’t disagree more.
For his own operation, Licciardello explains that the duckweed will be automatically harvested from six separate greenhouse sections before being screened and dried in a process that removes 75 percent of its water. From there, it will move into a patented convection system that will use a furnace-like closed loop process to heat and burn the duckweed to create both natural gas and steam.
Duckweed is dried down to 25 percent moisture before being put into three rows of 11 gasifiers that are fired up to 1,600 degrees Fahrenheit. The gasifiers will be automatically fed with duckweed. The steam created from the process will travel back to the turbines.
Licciardello says a component inside the gasifiers actually separates the natural gas from the steam. The natural gas component is then pumped into a holding tank before being fed into the natural gas grid.
Steam from the process will be fed into one of the two Siemens-built turbines at 12 MW of capacity. Biomass Alternative Power plans on selling its electricity to Florida’s NextEra Energy. The New Jersey start-up’s natural gas is to be purchased by British Petroleum (BP) for possible transport to California via cross-country pipeline.
When up and running, Licciardello says Biomass Alternative Power will become the only commercial duckweed-to-bioenergy conversion operator in North America.
“The production of ethanol and biogas from duckweed still cannot compete against petroleum products (gasoline and natural gas) economically,” said Jay Cheng, an agricultural and biological engineer at North Carolina State University.