## 1 Introduction
Above all, microalgae are widely recognized as a valuable source of renewable biofuels, including bio-oil obtained through hydrothermal liquefaction (HTL) of raw wet biomass [18; 20], hydrogen-rich biogas generated through photobiological processes [21], bioethanol produced via carbohydrate fermentation [22], methane produced by anaerobic fermentation of algal biomass [23], and biodiesel derived from lipid transesterification [24]. In many studies [25; 26], integrated production systems for biofuels and various chemicals are highlighted. These systems, when utilizing waste resources, enable the distribution of capital and operational costs across the production of multiple target products. This, in turn, reduces high production costs and increases the competitiveness of these solutions in the market. An example of this is the co-pyrolysis of plastic waste and microalgae biomass, which offers synergistic benefits for both fuel production and the synthesis of value-added products, such as fine chemicals and biofetrilizers [4; 5]. The industrial utilization of by-products derived from algae alignswith the goals of sustainable development and the principles of the circular economy and can significantly contribute to improving the economic efficiency of production and the development of this industry sector [5; 7]. To achieve relatively low cultivation costs for algae used in biofuel production, the photoautotrophic growth mode is most commonly employed. This cultivation approach requires the delivery of adequate light, carbon dioxide, and water enriched with various inorganic compounds, such as nitrogen and phosphorus, to the photobioreactors. The availability and intensity of light play a critical role in influencing algal growth, making light management a key factor in optimizing productivity [27]. The cultivation of algae in the photoautotrophic mode is carried out using both open systems, such as open ponds, and closed systems, such as photobioreactors [28; 29]. Depending on the applied culture medium and cultivation conditions--such as light intensity and exposure time, mixing method, and aeration in photobioreactors--algae with diverse physicochemical properties can be obtained [5]. These properties are crucial in selecting the method for algae concentration, influencing dewatering efficiency, and thus the long-term removal of algae from the system. Considering the composition of the algal cell, where the average density of carbohydrates is 1500 kg/m\({}^{3}\), proteins 1300 kg/m\({}^{3}\), and lipids 860 kg/m\({}^{3}\), the density of microalgal cells will be directly dependent on the proportion of these components in their total mass [8; 30]. The lipid content of microalgae exhibits significant variability depending on the species and cultivation conditions [20]. Freshwater species such as _Chlorella vulgaris_ and _Ankitrodesmus falcatus_ demonstrate lipid content ranging from 25 to 42% and 28 to 37%, respectively, while marine species such as _Tetraselmis suecia_ and _Neochloris oleabundans_ exhibit ranges of 18-26% and 36-42%, respectively. Cell density values for all species, as reported in the literature, fall within a narrow range of 1030-1200 g/L across both freshwater and marine environments. These slight differences in cell and water density are considered key factors contributing to the fundamental challenges in achieving effective gravitational separation of microalgal cells from water [8]. It is worth noting that the size of microalgal cells cultivated, for example, for energy purposes, primarily depends on their species and cultivation conditions, and typically falls within the micron range (1-50 \(\upmu\)m). Their concentration in the liquid is generally low, ranging from 0.5 to 3 g/L [31]. Despite the numerous benefits of algae and their expanding range of potential applications, significant technical and economic challenges persist, hindering their global commercialization.