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RESEARCH THEMES

Aquatic macrophytes have been suggested as a low cost option for the purification of wastewater and production of plant biomass (Araujo, 1987; Brix and Schierup, 1989; Gijzen, 1996; Oron, 1994; Reddy and DeBusk, 1987; Skillicorn et al., 1993). Water hyacinth (Eichhornia crassipes), pennywort (Hydrocotyle umbellata), water lettuce (Pistia stratiotes) and duckweed (Lemnaceae) have been reported for the efficient removal of nutrients (Gijzen, 2001b). Water hyacinth has been used most widely, due to its high nutrient uptake capability, but no attractive application of the plant biomass has been identified so far. The use of duckweed in wastewater treatment is not well documented, nor are design criteria available. The scarce studies reported so far suggest that this technology holds great promise as low cost wastewater treatment.

What is duckweed?

Duckweed is a floating aquatic macrophyte belonging to the botanical family Lemnaceae which can be found world-wide on the surface of nutrients rich fresh and brackish waters. The Lemnaceae family consists of four genera (Lemna, Spirodela, Wolffia and Wolfiella) and 37 species have been identified so far.

Compared to most other plants, duckweed has a low fiber content (about 5 %), since it does not require structural tissue to support leaves and stems. The nutrients taken up by duckweed are assimilated into plant protein. Under ideal growth conditions protein content of more than 40 % on dry weight basis may be achieved (Oron et al., 1985; Landolt, 1986; Skillicorn et al., 1993). Duckweed protein has relatively high concentrations of the amino acids lysine and methionine resembling animal protein. Trace minerals and pigments are also present in high concentration, which make duckweed favorable as nutritious animal feed. The efficient uptake of nutrients such as N and P from the water body could be applied for removal of nutrients from wastewater. Different studies have shown that duckweed systems are capable of treating wastewater (Alaerts et al., 1996; Reddy and DeBusk, 1985).

Duckweed ponds for sewage treatment

Wastewater treatment by duckweed covered ponds has attracted the attention of researchers in various parts of the world. Duckweed-based pond (DBP) systems have been applied at full-scale in Taiwan, China, Bangladesh, Belgium and the USA (Edwards, 1980; Zirschky and Reed, 1988; Alaerts et. al., 1996). Solids and organic materials in the wastewater are removed via sedimentation and bacterial liberalization, while the plants are active in nutrient removal. DBP systems might have the following advantages over waste stabilization ponds (WSPs):

· The duckweed cover restricts sunlight penetration into the water body, and this limits algae development. Markedly lower TSS levels are therefore expected in the final effluent (van der Steen, 1998).

· Evapotranspiration is lower than evaporation from an open water surface under the same meteorological conditions (Oron et al., 1985; Oron, 1990).

· Duckweed system may generate economic return via the commercialization of biomass for fodder and effluent for irrigation (Skillicorn et al., 1993).