Lemna minor: reducing N, P and other nutrients in waste water

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In order to minimize the environmental impact of nutrient leaching when waste water is discharged, the waste water stream can be inoculated with and eventually treated by duckweed. Apart from the removal of N and P, duckweed also partially removes metals. Another benefit is that algae growth is reduced due to the blocking of the entry of light.
At this moment, there are no Lemna cultures for sale. A wild culture (from nature) usually consists of a mixture of species, Lemna minor, and Lemna major, and can easily be obtained at places where polluted surface water is gathered.
Table 1 summarizes the range of water parameters which allow for growth; however, interactions between various parameters are not taken into account. A closed cultivation system can avoid growing problems due to biological and physical factors, but brings higher costs with it compared it an open system.
Eventually, duckweed must be harvested if you want to remove nutrients from the water. This can happen either by skimming the surface of the water or by pumping the water and duckweed over a mesh.
Duckweed grown on sewage or animal wastes normally does not contain toxic pollutants and can be fed to fish or to livestock, or spread on farmland as a fertilizer.  If the duckweed is to be fed to animals, a retention period in clean water will be necessary to ensure that the biomass is free of water-borne pathogens.

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Elise Vandewoestijne

Provinciaal Proefcentrum voor de Groenteteelt (PCG), Belgium
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