Duckweed is cleaning up ponds, lagoons

CorrespondentApril 27, 2014 

  • Duckweed dynasty?

    You’d never know by looking at it, or by how hard some people try to get rid of it, but science journals are touting duckweed’s many uses. Virginia State University aquaculture development agent Louis Landesman says, “It might be the most promising plant of the 21st century.”

    Nirmala Rajbhandari, who volunteers as an educator at the N.C. Museum of Natural Sciences’ Micro World Investigate Lab and who cultured species of duckweed as a scientist at N.C. State University, says duckweed’s benefits go well beyond its pollutant-killing properties. After it has cleaned wastewater, duckweed can be skimmed off the surface and used to feed livestock, she says. High in protein, it can also be used to feed wildlife and some fish.

    But its greatest potential use is the fact that starch coming from duckweed can quickly be fermented into ethanol.

    In 2009, research by Dr. Jay Cheng and Dr. Anne-Marie Stomp of N.C. State found that growing duckweed on hog waste could produce five to six times more starch per acre than corn. Furthermore, starch from duckweed can be converted into ethanol using the same facilities used for corn.

    Given these benefits, it may be surprising that duckweed hasn’t been heavily marketed or that farmers haven’t utilized it more. Most chalk that up to skepticism about new discoveries or the notion that every worthwhile agriculture crop has already been discovered.

    Rajbhandari said duckweed can even be added to salads. It tastes like watercress – fresh and kind of peppery.

You’ve almost certainly seen this tiny flowering green plant with the oval shape in ponds, lakes and streams, but you might not be certain what it is. That could change soon as the water-cleaning properties of duckweed grow on the public.

Duckweed – eaten by a duck, looks like a weed – is the subject of an experiment at the N.C. Museum of Natural Sciences that shows how the plant virtually eliminates pollution by removing nitrogen and phosphates from water. This quality has the potential to combat the rising levels of sewage runoff from modern farming and industrial byproducts in U.S. waterways, an ongoing environmental health issue.

Bob Alderink, coordinator for the Natural World Investigate Lab in the Nature Research Center at the museum, says the bioremediation experiment (using living organisms to clean a toxic environment) has provided consistent results during its few months.

“We’ve been using a tiny little practice pond – and when I say pond, I’m talking about a 9- to 10-inch dish that’s about 3 inches deep,” he said. “We’ve got several of those little ponds, and we overfertilize them to where their ammonia content and nitrogen content is just way spiked, really too high.

“Then we monitor it, doing water testing every few days, and watch how the duckweed grows very rapidly and continually pulls up the ammonia. Finally, at the end of three weeks – sometimes four weeks – by using our water-testing kits we can’t even determine any ammonia or nitrite or nitrate.”

He said the testing kit is the kind you would buy to test for ammonia, nitrite and nitrate in fish tanks and aquariums. “Fish and amphibians can’t tolerate any ammonia,” he said. “They start to suffer from even very low levels of ammonia in the water. So if this (testing kit) is used in an aquarium to say, ‘Yes, this is free of ammonia, your fish are fine,’ I would take that as a pretty good indicator.”

More useful than algae

Many consider duckweed – the world’s smallest flowering plant – a nuisance because it can prevent sunlight from reaching deeper parts of the water. That limits the ability of plants beneath the surface to photosynthesize and produce oxygen, which can cause fish kills.

Duckweed has five genera – Wolffiella, Spirodela, Lemna, Landoltia and Wolffia – and about 35 species, with thousands of varieties of those species. It has one to four leaves and ranges in size from 2 mm or less in diameter to about 20 mm in diameter, approximately the size of a fingernail or thumbnail. Because it’s green and appears to form mats on a pond’s surface, it’s often mistaken for algae.

To tell the difference, the general rule of thumb is to dip your hand in a pond that has the green mass. If the mass separates, it’s probably duckweed. If it doesn’t, it’s probably algae.

Duckweed improves water quality, while algae causes taste and odor problems in water and can plug water treatment filters. Duckweed also grows very fast, which enables it to out-compete algae in the same body of water.

Its ability to grow rapidly allows it to gobble up large quantities of contaminants including ammonia, lead and arsenates. “Some species can double every 36 hours, which is pretty remarkable for a plant,” Alderink said. “And it is everywhere. On my way home, as I’m driving over this bridge, I can just peer over the side and see duckweed.”

He added there’s one caveat in the plant’s role as a water cleaner: “You have to remove it after a certain period of time or it does no good. If it dies, you’re returning the nitrogen and phosphates right back to the water again.”

Help for hog farms

Alderink said he’s not an expert on duckweed, just a coordinator for the experiments. He said he’s grateful to have learned a lot about duckweed from Nirmala Rajbhandari, who recently began volunteering as an educator at the museum’s Micro World Investigate Lab.

Though Rajbhandari didn’t work on the wastewater cleaning experiment, a great deal of duckweed research has been conducted at N.C. State University, where she specialized in duckweed tissue culture and genetic engineering. She said duckweed’s ability to remove nitrogen and phosphorus pollutants can be a major asset at the estimated 2,500 hog farms in North Carolina – where, according to Washington, D.C.-based Food & Water Watch, more than 10 million hogs are being raised on farms that use antibiotics, artificial hormones and other chemicals to speed the hogs’ growth.

“Hog farmers are drastically increasing in the southeastern region of the U.S.,” she said. “As a result, more wastes are produced. Hog farmers usually use anaerobic lagoons for treatment and temporary storage of animal wastewater. The effluent is used to irrigate cropland and recycling to flush manure out of the hog house.

“Hog farmers in the southeastern U.S. have very limited cropland, so the problem is big for releasing harmful gases and polluting the environment. It is very important now for an alternative nutrient management system.”

Noting that farmers are often reluctant to try unproven methods, she says more research money should be allocated in order to show more concrete results. “This way, farmers will agree to take a chance and be willing to apply the new method,” she said. They also need to be convinced of the practicality of the results, especially in terms of economic value.

She suggests an outreach program regarding the uses of duckweed in cleaning wastewater, the uptake of nutrients and using it to produce economically viable biomass; providing incentives for using the new method; and finding a way for farmers to move to larger land so they can practice the new method.

Duckweed’s properties and uses are valued around the world, including in developing countries. Alderink said that in places such as Bangladesh or India, “they pay people to wade out in these ponds and take baskets and skim duckweed off the surface. They can turn around and feed it back to herbivore-ish fish they grow.”

In some ponds and lakes, duckweed is used for removing heavy metals such as mercury or different kinds of toxins. “In that case, you wouldn’t be feeding it back to a living thing; you’d have to move it to a designated spot like a toxic waste dump,” he said.

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