Duckweeds are small, fragile, free floating aquatic plants that grow ubiquitously on fresh or polluted water throughout the world. When conditions are ideal, it’s biomass can double every 1-2 days. The diluted slurry from biogas digesters was found to be a good medium for the growth of duckweed. Duckweeds were also being used by some farmers as a protein rich food supplement for cattle, poultry and fish. In aquaculture, it can assist in controlling the algae growth, which denies oxygen for fishes. Duckweed is eaten by people also, mainly in Thailand. Some of their species are being used for medicine is countries like China.
Advantages of duckweed compared to soybean
* Duckweeds are as good as or better than soybean as cattle, poultry, aquaculture feed.
* Duckweeds give many times more protein yield per hectare compared to soybean.
* Duckweed protein has higher concentrations of the essential amino acids, lysine and methionine, than most plant proteins and more closely resembles animal protein in that respect.
* Duckweeds can be grown in wastewater and can be harvested daily.
Another important use for duckweed is in waste processing. Duckweeds are being used in water treatment plants as they have the potential to purify both domestic and industrial wastewater that too at minimal cost of operation. They also help in recovering mineral nutrients which would otherwise be lost with the wastewater.
There is widespread research interest on generating bio-fuels from duckweeds. We have made advances in production of biogas from aquatic plants like water hyacinth and duckweeds can be easily harvested compared to algae or other aquatic plants. Until better techniques are developed for generating ethanol or biodiesel from duckweeds, generating biogas from duckweed will be the attractive utilisation (in fact biogas is being used in buses, cars and trains in Linköping, Sweden)
So I am envisioning decentralised waste treatment plants in which wastes (organic wastes & wastewater) are being fed into a biogas plant where the wastes are subjected to anaerobic decomposition resulting in odour free, germ free slurry plus of course methane gas. This slurry shall be fed to ponds where duckweeds are grown may be along with fish. Duckweeds should be harvested daily from the pond and shall be fed into biogas plant after pre-treatment.
Picture from http://www.fractalnature.com/duckweed.html