![]() Various aquatic floating plants have been proposed as agents of choice for the bioremediation of wastewaters because of these important features. Growing these plants in wastewater can significantly improve the water quality by accumulating nutrients and heavy metals, and by regulating the oxygen balance. Recently, aquatic plants which rapidly colonize lakes and contaminated wetlands have attracted significant attention because of their high growth rates, high biomass production, bioremediation capacity, easy maintenance and easy harvest. However, one of the major challenges of microalgal biotechnology for biofuel production is the high cost of harvesting. Apart from their high growth rates and substantial lipid/triacylglycerol (TAG) yields, microalgae can grow in wastewater (animal, municipal and mining wastewaters) efficiently removing the primary nutrients (C, N and P), heavy metals and micropollutants, and they do not compete with crops for arable lands. Microalgae have shown obvious advantages in the production of biofuels compared with energy crops. Since the utilization of wastewater is very limited for most of terrestrial crops, attention has been shifted towards the use of aquatic plants and microalgae. The use of wastewater as a source of reclaimed water and key nutrients for growing energy crops would significantly reduce the cost and energy requirement for biofuel production. ![]() The deleterious consequences of the extensive usage of arable lands to produce biofuels have triggered an intensive search for the next generation of energy crops which can grow on marginal lands not used for food production, forestry, or other uses of social value, including nature conservation. The high productivity, the ability to grow on wastewaters and unique chemical composition make Azolla species the most attractive, sustainable and universal feedstock for low cost, low energy demanding, near zero maintenance system for the production of a wide spectrum of renewable biofuels. With a high C/N ratio, fermentation of Azolla biomass generates 2.2 mol/mol glucose/xylose of hydrogen, making this species a competitive feedstock for hydrogen production compared with other bioenergy crops. The ethanol production from Azolla filiculoides, 11.7 × 10 3 L/ha-year, is close to that from corn stover (13.3 × 10 3 L/ha-year), but higher than from miscanthus (2.3 × 10 3 L/ha-year) and woody plants, such as willow (0.3 × 10 3 L/ha-year) and poplar (1.3 × 10 3 L/ha-year). Given the high annual productivity of Azolla, hydrothermal liquefaction can lead to the theoretical production of 20.2 t/ha-year of bio-oil and 48 t/ha-year of bio-char. Two Azolla species, Azolla filiculoides and Azolla pinnata, were used as feedstock for the production of a range of functional hydrocarbons through hydrothermal liquefaction, bio-hydrogen and bio-ethanol. This led to a 2.6-fold reduction in toxicity of the treated wastewater to shrimps, common inhabitants of wetlands. The growth of Azolla filiculoides in synthetic wastewater led up to 25, 69, 24 and 40 % reduction of NH 4–N, NO 3–N, PO 4–P and selenium, respectively, after 5 days of treatment. Together with their evolutional symbiont, the cyanobacterium Anabaena azollae, Azolla biomass has a unique chemical composition accumulating in each leaf including three major types of bioenergy molecules: cellulose/hemicellulose, starch and lipids, resembling combinations of terrestrial bioenergy crops and microalgae. Representatives of Azolla species are some of the fastest growing plants, producing substantial biomass when growing in contaminated water and natural ecosystems. Aquatic plants which can rapidly colonize wetlands are attracting attention because of their ability to grow in wastewaters and produce large amounts of biomass. The quest for sustainable production of renewable and cheap biofuels has triggered an intensive search for domestication of the next generation of bioenergy crops.
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