Abstract
In the last few decades, microalgae have attracted attention from the scientific community worldwide, being considered a promising feedstock for renewable energy production, as well as for a wide range of high value-added products such as pigments and poly-unsaturated fatty acids for pharmaceutical, nutraceutical, food, and cosmetic markets. Despite the investments in microalgae biotechnology to date, the major obstacle to its wide commercialization is the high cost of microalgal biomass production and expensive product extraction steps. One way to reduce the microalgae production costs is the use of low-cost feedstock for microalgae production. Some wastes contain organic and inorganic components that may serve as nutrients for algal growth, decreasing the culture media cost and, thus, the overall process costs. Most of the research studies on microalgae waste treatment use autotrophic and mixotrophic microalgae growth. Research on heterotrophic microalgae to treat wastes is still scarce, although this cultivation mode shows several benefits over the others, such as higher organic carbon load tolerance, intracellular products production, and stability in production all year round, regardless of the location and climate. In this review article, the use of heterotrophic microalgae to simultaneously treat wastes and produce high value-added bioproducts and biofuels will be discussed, critically analyzing the most recent research done in this area so far and envisioning the use of this approach to a commercial scale in the near future.
Highlights
Each year, the European Union generates 2.5 billion tons of waste, or five ton per person
These microorganisms show several benefits over the autotrophic microalgae to treat waste streams, such as: (a) they can grow in cheaper bioreactors, requiring less sophisticated equipment, and are scaled-up; (b) they do not require light to grow, which reduces the equipment requirements and costs; (c) the cultures attain higher, denser cell concentrations and intracellular product productivities than autotrophic cultures; (d) the algal biomass composition can be tailored by changing the type of organic substrate in the medium; (e) heterotrophic microalgae can remove organic carbonaceous, nitrogen, and phosphorus compounds from the wastes more efficiently than autotrophic growth [19,20,21]
Heterotrophic microalgae show many advantages over autotrophic microalgae to treat food wastes and municipal/domestic wastewater, as the former has the ability to grow on wastes with high COD loads, simultaneously producing microalgal biomass, biofuels, and high value-added products such as lipids, poly-unsaturated fatty acids (PUFA), and carotenoids, with different commercial applications
Summary
The European Union generates 2.5 billion tons of waste, or five ton per person. Microalgae are considered robust and versatile microorganisms, as they can have autotrophic, mixotrophic, or heterotrophic metabolisms These microorganisms can be used to treat different types of waste streams, simultaneously producing valuable products (i.e., polyunsaturated fatty acids, pigments, proteins, animal feed, biofertilizers, and biofuels) that may be used in diverse industries, such as food, feed, pharmaceutical, nutraceutical, cosmetic, and chemical, as real biorefinery alternatives. According to Wang et al [6], the total feedstock costs can achieve up to 80% of the total process costs when using glucose as a carbon source In this way, zero or low-cost substrates such as wastes (including wastewater and industrial byproducts and residues) should be used as a source of nutrients for heterotrophic microalgae growth, to reduce the overall process costs [15] and to accomplish the EU Circular Economy Action Plan 2020, known as The European. The most recent research done in this area will be critically analyzed, and the use of this approach to a commercial scale in the near future is envisioned
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