Lemnaceae or duckweed is an aquatic plant that can be used to recover nutrients from wastewaters. The grown duckweed can be a good resource of proteins and starch, and utilized for the production of value-added products such as animal feed and 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 the swine wastewater were screened in laboratory and greenhouse experiments. The selected duckweed strains were then tested for nutrient recovery under laboratory and field conditions. The rates of nitrogen and phosphorus uptake by the duckweed growing in the laboratory and field systems were determined in the study. The mechanisms of nutrient uptake by the duckweed and the growth of duckweed in a nutrient-limited environment have been studied. When there are nutrients (N and P) available in the wastewater, duckweed takes the nutrients from the wastewater to support its growth and to store the nutrients in its tissue. When the N and P are completely removed from the wastewater, duckweed can use its internally stored nutrients to keep its growth for a significant period of time. A modified Monod model has been developed to describe nitrogen transport in a duckweed-covered pond for nutrient recovery from anaerobically treated swine wastewater. Nutrient reserve in the duckweed biomass has been found the key to the kinetics of duckweed growth. Utilization of duckweed for value-added products has a good potential. Using duckweed to feed animals, poultry, and fish has been extensively studied with promising results. Duckweed is also an alternative starch source for fuel ethanol production. Spirodela polyrrhiza 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 significant quantities of starch that can be readily converted into ethanol.

Growing Duckweed to Recover Nutrients from Wastewaters and for Production of Fuel Ethanol and Animal Feed

http://www.bashanfoundation.org/gmaweb/pdfs/luztbioreme.pdf

Treatment of recalcitrant wastewater from ethanol and citric acid production using the microalga Chlorella vulgaris and the macrophyte Lemna minuscula.

Management and control methods of invasive alien freshwater aquatic plants: a review

Summary Duckweeds have received research attention because of their great potential to remove mineral contaminants from waste waters emanating from sewage works, intensive animal industries or from intensive irrigated crop production. Duckweeds need to be managed, protected from wind, maintained at an optimum density by judicious and regular harvesting and fertilised to balance nutrient concentrations in water to obtain optimal growth rates. When effectively managed in this way duckweeds yield 10-30 ton DM/ha/year containing up to 43% crude protein, 5% lipids and a highly digestible dry matter. Duckweeds have been fed to animals and fish to complement diets, largely to provide a protein of high biological value. Fish production can be stimulated by feeding duckweed to the extent that yields can be increased from a few hundred kilograms per hectare/year to 10 tonnes/ha/year. Mature poultry can utilise duckweed as a substitute for vegetable protein in cereal grain based diets whereas very young chickens suffered a small weight gain reduction by such substitution. Pigs can use duckweed as a protein/energy source with slightly less efficiency than soyabean meal. Little work has been done on duckweed meals as supplements to forages given to ruminants, but there appears to be considerable scope for its use as a mineral (particularly P) and N source. The protein of duckweeds requires treatment to protect it from microbial degradation in the rumen in order to provide protein directly to the animal. The combination of crop residues and fresh duckweeds in a diet for ruminants appears to provide a balance of nutrients capable of optimising rumen microbial fermentative capacity. These diets can, therefore, be potentially exploited in cattle, sheep and goat production systems particularly by small farmers in tropical developing countries.

Duckweed - a potential high-protein feed resource for domestic animals and fish

Estimation of biological control prospects

Biological Control of Weeds: Southeast Asian Prospects

Information on reproductive requirements of grass carp (Ctenopharyngodon idella Val.) was obtained from published accounts, from a visit to rivers in the USSR where introduced grass carp have reproduced, and from discussions with Soviet fishery workers. Turbulent areas at the confluence of rivers or below dams are the focal points for reproduction. Successful spawning occurs only in large rivers or canals where water velocity exceeds 0.8 m/s and volume is roughly 400 m3/s. The eggs are carried 50 to 180 km, depending on water temperature and current speed. The larvae hatch 1 day after spawning and make their way to vegetated lagoons, impoundments, or lakes closely connected to the river. They begin feeding on rotifers at 2 to 4 days and change to larger zooplankton in about a week. Temperatures required for stimulation of sexual maturation, egg incubation, and survival of young range from 19 to 30 C, with an optimum of about 23 C. Because requirements for each factor must be found in juxtapositio...

Reproductive Requirements and Likelihood for Naturalization of Escaped Grass Carp in the United States

Phosphorus Removal from Static Sewage Effluent Using Duckweed 1

Growth of Spirodela polyrhiza in static sewage effluent

Biological control is a cheap, permanent, and nontoxic method of controlling aquatic weeds. Insects and fish are the most effective biological agents. The use of herbivorous fish in the United States to control the growth of aquatic vegetation has been limited because of the susceptibility of most of these fish to low temperature. However, the amur is an herbivorous fish that can tolerate temperature extremes. The amur has a voracious appetite and consumes many aquatic plants. It has been successfully introduced in other countries of the world, and, in the future, may be used to mitigate some of the aquatic weed problems prevalent in the United States. Many fishes, snails, manatees, and other organisms can be expected to be effective only in tropical or subtropical waters. Their usefulness in temperature regions will be sharply limited by minimum temperatures. The use of viruses, fungi, bacteria, and other pathogens offers a potential for biological control of aquatic weeds. Low growing aquatic plants may prevent the establishment of some problem plants.

David L. Sutton

Papers

Reproductive Requirements and Likelihood for Naturalization of Escaped Grass Carp in the United States

Phosphorus Removal from Static Sewage Effluent Using Duckweed 1

Growth of Spirodela polyrhiza in static sewage effluent

Biological Control of Aquatic Weeds

Grass carp (Ctenopharyngodon idella Val.) in North America