Seminar Abstract: Duckweed is an aquatic plant that can be used to recover nutrients from wastewaters. The grown duckweed can be a good resource of starch and utilized for the production of fuel ethanol. In the last eleven years we have been working on growing duckweed on anaerobically treated swine wastewater and utilizing the duckweed for fuel ethanol production. Duckweed strains that grew well on anaerobically-pretreated swine wastewater were screened in an in-vitro laboratory experiment. The selected duckweed strains were then tested for nutrient recovery under laboratory and field conditions. The rates of nutrient uptake by the duckweed were determined in the study. The mechanisms of nutrient uptake by the duckweed and the growth of duckweed have been studied. A modified Monod model has been developed to describe nitrogen transport in a duckweed-covered pond for nutrient recovery from the swine wastewater. Nutrient reserve in the duckweed biomass has been found the key to the kinetics of duckweed growth.
Duckweed has been found an alternative starch source for fuel ethanol production. Spirodela polyrrhiza from North Carolina, grown on anaerobically treated swine wastewater was found to have a starch content of 45.8% (dry weight). Enzymatic hydrolysis of the duckweed biomass with amylases yielded a hydrolysate with a reducing sugar content corresponding to 50.9% of the original dry duckweed biomass. Fermentation of the hydrolysate using yeast gave an ethanol yield of 25.8% of the original dry duckweed biomass. These results indicate that the duckweed biomass can produce appreciable quantity of starch that can be readily converted into ethanol. Duckweed proliferates through clonal, vegetative budding of new fronds and accumulates biomass faster than field crops, almost 28 times faster than corn. Annual starch production from a duckweed system can be 5-6 times higher than corn per acre.