A cheap treatment method could be to implement duckweed-based constructed wetlands in RAS. Duckweed is a free-floating water plant that is mostly not larger than a few millimetres to 1 centimetre. Lemna minor is a species consisting of just 2 to 4 leaves and one root. Other duckweed species are similar but have a different amount of roots and can be smaller or larger. The plant is most famous for its growth, and is therefore sometimes considered as a plaque. In 2004, its explosive growth was even visible from space, as the lake Maracaibo (Venezuela) was largely covered with the plant.
This ability gives the plant several key advantages. First, to sustain its growth, the plants takes up a high amount of N and P. Additionally a duckweed lagune also removes N and P because (de-)nitrifying bacteria that attach to the plant biomass and the still water which is ideal for sedimentation.
Besides its explosive growth, duckweed contains a high protein content of up to 45%. Additionally, the moderate amount of fibre makes it readily digestible for monogastric animals and many species of fish. Especially carps and tilapia have been shown to eat the plant.
Recent results gathered at Inagro
All these characteristics have made duckweed already for years an interesting plant to be implemented in fish farms. Skillicorn wrote for example in 1993 an extensive report on aquatic farming in developing countries with duckweed. However, the research and advise centre, Inagro wants to investigate if duckweed has also potential in a European context. Therefore, we monitored the growth of duckweed in Flanders in outdoor conditions on 1m² large pilots. We supplied the duckweed with the effluent of our pikeperch production during a whole growing season.
It could quickly be concluded that temperatures and light intensity inhibit the growth during winters, however, an average productivity of 9,1 t dry weight per hectare per year was still observed. Furthermore, a protein content of 29% was found. This could be considered very productive, as it results in a protein production per hectare that is almost triple the size of soy bean cultivated in Brazil.
The effluent of the pikeperch production was also intensively monitored throughout the season. Some processes like ‘grading’, could alter the growing medium. During ‘grading’, salt is added to reduce stress that the fish experience. Salt could be harmful for plants, but duckweed shows to be indifferent to the added concentrations.
Ambition for the future!
In the future we want to increase the knowledge on duckweed cultivation. But research is also needed on the formulation of the produced biomass in fish feed. For some fish like tilapia and grass carp, the fresh addition could be feasible, but other species need more processed forms of duckweed and higher protein contents. However, if formulation succeeds for a reasonable price, then the waste of aquaculture can lead to fish feed. In that way, nutrient circles are closed and the impact of aquaculture on the environment is minimalised.
If you are interested in what we have done, or you want to collaborate on this research, please contact the corresponding author: firstname.lastname@example.org
This research was funded by The Flemish government within the framework of Circular Flanders project: “Waardeketen Eendenkroos”, grant number: OC-SO-2018 201, and the H2020 project Nutri2Cycle, grant number: 773682.