Duckweed: cleaning water at the grassroots.

• Author: Platt, Ann E.

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...