Duckweed and it’s uses as a fodder supplement, a biological cleaning agent and its effects on wastewater treatment systems.
Note: There are many species within this genus, Lemna minor is the most commonly encountered and cultivated species. The plants classed as duckweeds fall into 3 genera: Lemna, Wolffia, and Spirodela.
Duckweed species tend to be small aquatic plants that float on the surfaces of water bodies, they can grow in thick mats on the water’s surface and tend to thrive best in fresh or brackish waters that are slow moving and are high in nutrients (Leng et al., 1995).
Due to being aquatic plants and not needing to support upright structures such as stems or leaves above the surface, they have low levels of fibre (5%) (see use as feed).
This aquatic plant has a high annual yield, and with a bi to tri weekly harvest it may yield a dry mass of up to 55 ton/ha/annum with a protein content of around 30% (Oron, 1994). Other sources state a dry mass yield of 10-30 ton/ha/year (Leng, et al., 1995), see Table 1.
The fastest reproduction of this genus of plant tends to occur in warmer water temperatures, with temperatures 21 °C or above, however survival occurs over a wide range of temperatures; 6-33 °C (Leng et al., 1995).
These species are relatively hardy and can tolerate pH ranges from 5-9, however survive best around a neutral pH, they are salt tolerant and can survive in salt concentrations of up to 4000mg/litre TDS (Leng et al., 1995).
Fast growing duckweed that is regularly harvested has benefits over long growing colonies that tend to have higher fibre, ash and lower protein content. It is therefore important if possible to harvest frequently and provide the colony with protection from wind (which causes piling up on water body walls or shores) and results in self shading.
Duckweed has a very high reproductive value. It can double its mass in two days under ideal conditions, this growth pattern is more similar to the exponential growth of unicellular algae than the typical growth of higher plants. Daughter fronds bud out of mature fronds and one mature frond may produce up to 10 generations of progeny over 10 days, the older a frond gets, the higher its fibre and mineral content becomes (Journey et al., 1993).
The yield and growth of these species is typically variable, however optimum yields tend to occur under conditions of high nutrients, nutrient balance, low competition, high levels of light and moderate pH. See Table 1 attached from (Leng, et al.,1995).
Use in wastewater treatment
Duckweed is prominent in the treatment of sewage and waste water, it thrives in water with high concentrations of nitrogen (N), potassium (K) and phosphorus (P) (Leng, 1999). It tends to concentrate these minerals and converts them to relatively high levels of protein, a substance typically lacking in agricultural feeds.
As the result of the nitrate cycle present within sewage treatment systems the conversion of ammonia to nitrite and nitrate via anaerobic and aerobic bacteria can be utilised. By taking advantage of this knowledge a high protein food source can be obtained from waste products such as ammonia present in sewage waste (Oron, 1994).
This is also particularly useful in negating excessive algal growth prevalent in sewage ponds and settling pools, it blocks a large proportion of the sunlight and removes nutrients such as nitrate and phosphates. It may also play a role in reducing turbidity in settling ponds (Caicedo et al., 2000) as there will be less surface movement of water because of air movement as well as other factors such as algae. It also significantly reduces evaporation when compared to a water surface void of duckweed.
It also has been found that the use of certain species of duckweed (Lemna gibba) functioned not only to improve sewage water quality, but under successful operating conditions allowed this water to be of a sufficiently high quality to meet irrigation criteria (Oron, 1994).
There has been research in the use duckweed as a wastewater treatment system (Caicedo et al., 2000). They suggest anaerobic pre-treatment of sewage, followed by water holding in duckweed filled ponds. This particular method was suggested as the anaerobic pre-treatment tended to reduce the biological oxygen demand; however this had no negative effect on nutrient concentrations and bacterial counts, as such this seems a feasible method of sewage waste treatment (Alaerts et al., 1996; Caicedo, et al., 2002). It would also reduce the need for extensive oxygenation regimes and as such may reduce waste processing costs. The biological and chemical oxygen demands in systems utilising duckweed are also said to be similar to conventional stabilisation ponds (Bonomo et al., 1997). For more regarding these systems and the effects of ammonia, nitrogen and pH on growth rates see Caicedo et al., (2000) According to Hammouda et al., (1995) duckweed also has an excellent capacity for heavy metal absorption in waste treatment plants.
Interestingly, the presence of duckweed species on sewage ponds may reduce the dissolved oxygen content of these sludge dams. This is beneficial as anaerobiosis is key in the breakdown of biological material into mineral components such as nitrogen, carbon and phosphorous (Hammouda et al., 1995). There have also been studies that suggest using multiple duckweed species in conjunction to improve waste uptake ability of nutrients and heavy metals (Culley and Epps, 1973).
The two largest drawbacks to using duckweed in sewage or wastewater treatment would likely be:
- The possible need for aeration in order to obtain optimum growth of minor, however the need for this is debated.
- There are also certain environmental and market regulations in some countries that state that duckweed grown in sewage systems need to be disposed of and treated as sewage sludge (Bonomo, et al., 1997).
Use as fodder
The low fibre content of duckweed is beneficial when using this as a source of fed as it is easily digested by most stock animals and when compared to traditional feeds, typically reaching up to 50% fibre content i.e. rice, maize or soy beans. This allows monogastric animals (non-ruminants) to also utilise duckweed species as a food source.
It can be used to feed all manner of vertebrates, from fish to Aves, ruminants and monogastric species, as such it is not difficult to imagine a potential feed market for stock animals or fish, particularly in a country with low rainfall. This would be beneficial as duckweed covered water bodies have a higher water quality, lower nutrient levels and a reduced amount of evaporation.
Duckweed can be used as a soy and fish meal substitute when dried in poultry feeds, this can be used for broilers, layers and chicks (Journey et al., 1993). Layers fed with duckweed produced more and higher quality eggs than control groups in a test conducted in Peru South America (Journey et al., 1993).
It has also been found that duckweed grown in solutions containing 50 and 100% sewage tended to have almost 3 times as much protein as duckweed grown on pond water (Hammouda et al., 1995). Similar results were also obtained by Culley and Epps, (1973) Duckweed grown on waste water has frequently been found to be of a higher feed value then duckweed grown in ponds without high nutrient and mineral levels.
The typical component analysis can vary as a result of growth conditions and area, however the values of 3 samples were analysed and can be observed in this Figure 2 from Journey et al., (1993).
There are many benefits of using duckweed supplements in animal feeds, examples of such benefits are high mineral concentrations, high protein content, presence of essential amino acids such as methionine and lysine. Duckweed is second only to blood meal in providing these amino acids, however it can be produced on a much larger scale and much more economically. See Figure 3 from Journey et al., (1993).
Duckweed as a human food source
Surprisingly little has been published in academic journals regarding the use of duckweed as a food source for human consumption. One study said that due to the high concentrations of calcium found in many water bodies led to high levels of calcium within duckweed species and subsequently calcium oxalate crystals. These can cause kidney stones and the like when consumed in high concentrations, however the calcium concentrations in water bodies can be controlled and this can be managed in order to prevent this from occurring. Dry heat has been shown to break down calcium oxalate in certain foods, so it could possibly also be used in duckweed grown in areas with a high calcium concentration.
Duckweed has been eaten in salads, soups and as substitutes for lettuce, spinach, water cress and alfalfa. If grown for human consumption, it must be grown in aerobic conditions. If grown in anaerobic conditions with a layer of duckweed too thick on the water’s surface anoxic conditions may develop with no photosynthesis occurring, this may lead to the growth of anaerobic bacteria. This can be negated by growing the duckweed in aerobic conditions, harvesting regularly to avoid thick duckweed mats or by lacto-fermentation, which would remove potential pathogens.
Duckweed is the way forward as both a source of food for humans, a source of feed for animals and an effective management tool for sewage and wastewater plants. It has many attributes that make it the ideal candidate for these applications and could be a crop of major importance in the future.
Alaerts, G. J., Mahbubar, R., & Kelderman, P. (1996). Performance analysis of a full-scale duckweed-covered sewage lagoon. Water Research, 30(4), 843-852.
Bonomo, L., Pastorelli, G., & Zambon, N. (1997). Advantages and limitations of duckweed-based wastewater treatment systems. Water Science and technology, 35(5), 239-246.
Caicedo, J. R., Espinosa, C., Andrade, M., & Gijzen, H. (2002). Effect of anaerobic pretreatment on environmental and physicochemical characteristics of duckweed based stabilization ponds. Water science and technology, 45(1), 83-89.
Caicedo, J. R., Van der Steen, N. P., Arce, O., & Gijzen, H. J. (2000). Effect of total ammonia nitrogen concentration and pH on growth rates of duckweed (Spirodela polyrrhiza). Water Research, 34(15), 3829-3835.
Culley Jr, D. D., & Epps, E. A. (1973). Use of duckweed for waste treatment and animal feed. Journal (Water Pollution Control Federation), 337-347.
Hammouda, O., Gaber, A., & Abdel-Hameed, M. S. (1995). Assessment of the effectiveness of treatment of wastewater-contaminated aquatic systems with Lemna gibba. Enzyme and Microbial Technology, 17(4), 317-323.
Journey, W. K., Skillicorn, P., & Spira, W. (1993). Duckweed aquaculture. A new aquatic farming system for developing countries. The World Bank, Washington DC.
Leng, R. A. (1999). Duckweed: A tiny aquatic plant with enormous potential for agriculture and environment. (http://www.fao.org/Ag/againfo/resources/documents/DW/Dw2.htm)
Leng, R. A., Stambolie, J. H., & Bell, R. (1995). Duckweed-a potential high-protein feed resource for domestic animals and fish. Livestock Research for Rural Development, 7(1), 36.
Oron, G. (1994). Duckweed culture for wastewater renovation and biomass production. Agricultural water management, 26(1-2), 27-40.