Duckweed: cleaning water at the grassroots.

As waste streams from human communities and industries continue to swell, many environmental scientists foresee a time when conventional treatment systems will be simply unable to handle them. These high-tech systems rely on thousands of miles of sewage collection pipes and channels, emergency outlets, huge waste processing tanks, turbine engines, and chemical monitoring systems, as well as a continuous supply of energy to operate each facility at rates of millions of gallons of wastewater per day.

Rising energy and technology costs have made these systems increasingly expensive, and the complex technical and legal issues associated with water systems have made it difficult even to repair a sewer system in a wealthy country, let alone construct a new system in an impoverished one.

Faced with these and other significant problems, water resource planners and engineers arc looking for other options, among them a surprisingly natural alternative using duckweed, a common aquatic plant. Known in Thailand as the "poor man's food" or khai-nam, this tiny, floating water weed is an abundant source of protein and nutrition that Thai families have been harvesting for generations.

Duckweed treats waste by breaking down and converting certain types of waste to nutrient-rich biomass and to clean water. It can process high amounts of nutrients through its plant and root structures to create a rapid, regenerating growth cycle that is easy to start up and maintain. This natural system may seem inadequate and even backwards in comparison to modern treatment methods, but its low-tech nature is in fact its most attractive attribute: it is inexpensive and demands much less energy than conventional treatment plants. Duckweed and other water weeds, including water hyacinth, the common reed phragmite, bulrush, and hydrilla, typically form green tangled mats of slime that take over canals, ponds, streams and even hydroelectric plants. Although duckweed has a lower rate of nutrient removal than other water weeds, it is more tolerant of cold weather, has a higher nutritional value, is easier to handle and grows faster and thicker than other plants. It can withstand temperatures ranging from 7 to 34 degrees Celsius (44 to 93 degrees Fahrenheit), and with more than 40 different varieties, it is native to many regions of the world. Under optimal conditions, a planting the size of a thumb will grow almost half a hectare (1.2 acres) in 55 days and can be harvested every four to five days along the way.

Duckweed flourishes in quiescent, shallow water bodies where it grows into a dense mat that reduces the availability of oxygen and turns the water into an anaerobic environment. The mat also prevents sunlight from penetrating through the water surface, suppressing photosynthesis and inhibiting larvae and mosquito breeding and algae growth, which competes for many of the same nutrients as duckweed. In the presence of excessive amounts of nutrients, algae can cause huge algal blooms that slowly suffocate and kill water life by cutting off air and sunlight below the surface waters in the water column. Duckweed does not cause the same "blooming" effect because it takes in nutrients much faster and grows only on the water surface.

Conventional sewage treatment takes place in two or more stages. A "primary" treatment involves sedimentation, filtration, and screening to remove floating objects, sand, and stones, and "secondary" treatment uses biological methods, including aeration, oxidation and filtration, so the sludge interacts with air and bacteria. "Tertiary" (or "advanced") treatment involves chemical methods such as chlorination, distillation, and, depending on what the water is being used for, significant levels of chemical inputs to neutralize pollutants. These high-tech systems generate huge amounts of sludge. While some of the treated sludge can be used to fertilize agricultural land, most sludge requires immediate disposal to avoid rapid accumulation. In contrast, a typical duckweed system consists of a series of ponds and lagoons with screens, grit chambers, grease traps, and either natural or artificial water channels. Existing wetlands or marshes can be used as a buffer between two lagoons, and often a stage of chlorination is added to disinfect the discharge. Because a single duckweed plant is only about the size of a thumbtack and has only a single root or, in some cases, no roots, it is easily windblown. This problem inspired Viet Ngo, the founder of St. Paul, Minnesota-based Lemna Corporation, to design a floating frame system to protect duckweed from being blown into piles. The frames stabilize plant growth and reduce susceptibility to wind and waves. A Lemna system costs $20 to $40 per person, compared to the $300 to $600 per person for a conventional sewage treatment, according to Ngo's estimates.

The minimal root system has its advantages as well, making it easy to skim duckweed off the water's surface, dry it out in the sun, and use it as food, feed or fertilizer. It does not need structural tissue for leaves or stems, so most of the plant tissue is metabolic and can be used as a feed or food product: at least 25 percent (and up to 35 to 40 percent according to some estimates) of the dry mass is protein. Duckweed is actually twice as high in protein, fat, nitrogen and phosphorus as water hyacinth, and its protein content is also higher than alfalfa (17 percent), a popular livestock feed.

The smallest species of duckweed, the rootless Wolffiella, is edible, and cultivated as a vegetable in Burma and Laos, as well as in Thailand. In Bangladesh, it is a popular fertilizer for fish farming, dragged from duckweed ponds directly into nearby fish ponds, where it is fed to tilapia, a fast-growing, valuable freshwater fish.

Duckweed systems are particularly applicable in developing countries, where wastewater treatment may be nonexistent and the nutrient-rich byproducts can be reused. Raw sewage often flows freely in street drains, and in areas where no water system infrastructure exists, duckweed treatment ponds can be a way to initiate at least a basic level of waste treatment. In Campo De Carlos, Mexico, on California Baja Sur, Lemna Corporation is building a system that will accommodate a daily flow of 687,000 gallons per day and benefit local cropland, which will be irrigated with the resulting effluent. By treating the wastewater with duckweed, the people of Campo De Carlos are embracing a more holistic approach to waste than conventional treatment, integrating waste input and output to create useful by-products. Even the World Bank has endorsed duckweed, committing funds during the past year for construction of duckweed systems in Tunisia and Morocco.

Duckweed is also playing a successful role in some systems that combine plants and animals. The Solar Aquatic System developed by John Todd at the New Alchemy Institute in Cape Cod, Massachusetts, uses 200 plants and more than 20 fish species that work together to clean up wastewater. Flowing through a series of tanks and vegetative ponds, the wastewater and sludge are recycled into fish food by photosynthesis and microbial processes. In Piracicaba, Brazil, a natural waste treatment system that includes duckweed is used to purify polluted water and make it potable.

In addition to serving as a substitute for conventional waste treatment facilities, duckweed can be used to supplement and improve such systems where they already exist. Russell Clayshulte, principal water resources planner for Denver, Colorado's Regional Council of Governments, reports that duckweed is being used as an inexpensive method to reduce levels of phosphorus in treated wastewater, rather than the more expensive option of refitting the existing main facility. Using matching grant money available through the United States' new Clean Water Act, regional water resource agencies are building diversion structures downstream from the Plum Creek advanced wastewater treatment facility to elevate stream water into a duckweed holding pond.

Although the U.S. Environmental Protection Agency has endorsed duckweed as an "alternative and innovative" technology, the weed does have major limitations. Duckweed cannot process heavy metals, trace pesticides, or toxic substances, so the plants need to be tested before being used for feed. They must also have large, shallow bodies of water in order to flourish, so they are difficult to launch in an urban area or mountainous region where open, flat land is scarce and expensive. Researchers at the Ben-Gurion University's Blaustein Institute for Desert Research in Israel estimate that one to two square meters of mat is necessary for each person served by a duckweed treatment pond, which translates to 20 hectares (50 acres) for a city of 100,000 people. In cold weather, an even larger surface area is needed to purify the same volume of water because duckweed growth rate slows.

The Denver project has come up against another complication in putting its supplemental system in place: engineers must "repollute" the water that comes from the advanced treatment facility upstream at Plum Creek. In this case, duckweed is being used to remove a small but significant quantity of phosphorus--small because much of the phosphorus was removed by the conventional plant upstream, but significant because the quantity removed is enough to bring the phosphorus concentration down to permissible levels. Duckweed needs more phosphorus than is left in the water that comes from Plum Creek in order to grow a complete covering, so fertilizer must be added to the pond. With enough duckweed growing on the pond, nearly all of the phosphorus will be removed, leaving the water taken from the pond cleaner than it was when it left the conventional system upstream. But even with the necessary "repollution," the duckweed system is more economical than any conventional alternative. Although khai-nam has been around for centuries, modern societies are just beginning to realize its potential for waste treatment and aquaculture. Whether it be in conjunction with existing wastewater treatment plants, as a single plant system, or as one species in an integrated aquatic system, duckweed may prove to be a feasible green-tech answer to our growing waste problem. The waste stream doesn't stop at the local sewage treatment plant, and neither should our search for solutions.