Microalgae are efficient photosynthetic microorganisms, capable of capturing CO2 in the presence of light and assimilate nutrients to produce biomass rich in proteins, carbohydrates and lipids. They represent a promising alternative to produce biofuels not only to replace fossil fuels, but also conventional crops that still have many negative environmental impacts such as deforestation and technical difficulties of processing lignocellulosic biomass. However, microalgal biomass production is often uncompetitive against these mature technologies and the environmental appeal itself is not sufficient for large-scale production. Aiming to improve economic assessments, higher value molecules from primary metabolites have been considered for industrial production. Several species accumulate lipids rich in omega-3 and − 6 fatty acids, whose consumption is associated with health benefits (Swanson et al., 2012). Consequently, commercialization of such lipids is more advantageous due to their higher nutritional value and because they represent a vegan option to replace fish consumption, for example, as a source of proteins and essential fatty acids. Additionally, innovative methods of cultivation, harvesting and dewatering, drying, and lipid extraction from microalgal biomass could help reduce installation and operational costs for sustainable large-scale production (Ahmad et al., 2022).
Different growth systems have been proposed for cultivating microalgae (Ríos Pinto et al., 2020), generally divided into open ponds and closed photobioreactors. Open pond cultivation is the oldest and simplest, and therefore cheapest, way of growing microalgae, as it can be easily constructed outdoors, using sunlight and possibly CO2 supplementation for photosynthesis. However, it is susceptible to invasion by other microorganisms and climate variation. Photobioreactors, on the other hand, are more expensive but able to control growth conditions and achieve significantly higher biomass productivities. Flat plate photobioreactors (FP-PBR), for example, are a configuration often associated with increased production of polyunsaturated fatty acids (PUFAs) (Ferreira et al., 2019a). In addition, they can yield 5–20 times more biomass than other PBRs (Vo et al., 2019). Multicriteria analysis also ranked FP-PBR the second-best microalga cultivation system, based on experimental results at laboratory and pilot scales comparing plate, tubular (horizontal/helical) and plastic bag PBRs to raceway/circular ponds (Unay et al., 2021). Recently, adaptations in temperature control (Nwoba et al., 2020) and mixing (Xu et al., 2020) were suggested to improve this system. Assessing the use of either synthetic medium or wastewater, a FP-PBR was able to provide simultaneous increase of biomass productivity and lipid content by using a cheaper and more sustainable medium (Kumar et al., 2019). Although FP-PBRs are associated with high costs, studies show economic viability, as in the case of a marine microalgae biomass produced under non-sterile conditions and using centrate as nutrient source (Romero-Villegas et al., 2018).
Microalgae growth in wastewater has been progressively studied as an effort to reduce operational costs with nutrients, which comprise the majority of input expenses (Chandrasekhar et al., 2022). This wastewater can be divided in municipal, agricultural and industrial (You et al., 2022). Several agricultural effluents have been tested, especially vinasse (Falconí et al., 2021) and piggery slurry (Moheimani et al., 2018), as well as domestic wastewater (Drira et al., 2016). Their properties pose various challenges to cultivation, such as toxicity due to heavy metals and high organic demand (Abinandan and Shanthakumar, 2015) and vulnerability to contamination. Nonetheless, these studies have shown that microalgae have the ability to assimilate these nutrients, which could improve biomass productivity while providing wastewater treatment. Additionally, they are able to remove different contaminants of emerging concern such as pharmaceuticals (García-Galán et al., 2021) and vital nutrients such as phosphorous (Rezania et al., 2021). In the circular bioeconomy concept, the valorization of residues is essential to obtain a sustainable process aiming at zero waste and providing coproducts.
This study evaluated Desmodesmus sp. growth in FP-PBR using gas, liquid and solid residues from sugarcane biorefining (CO2, vinasse, and bagasse biochar) as nutrient and carbon sources. Desmodesmus is similar to the genus Scenedesmus, whose species are considered a robust microalga, with greater potential to withstand environmental variation. The aqueous effluent and biochar were previously processed to prepare them to be used in the cultivations. Finally, the effect of light intensity on biomass and lipid production was evaluated together with CO2 concentration in multifactorial experiments. Both concentrated CO2 gas and intense light supply are known strategies for lipid accumulation and this study approached these two variables for the first time using Desmodesmus sp. grown mixotrophically. In this context, microalgae cultivation is coupled with wastes valorization since vinasse is the leading waste generated by a proportion of 12–15 L to each liter of bioethanol (de Godoi et al., 2019) and 250–270 kg of bagasse are produced for each ton of sugarcane in Brazil (de Souza, 2020). Also, although some information is already available for cultivation using vinasse, most studies lack the evaluation of wastewater/effluent-grown microalgae concerning their fatty acid profiles. The results shown here support future studies and industrial applications that aim at the integration of bioremediation and high-value microalgae production.
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