## 3 Research Results and Their Analysis  
In order to determine the size distribution of individual algae cells, their aggregates, and the forming flocs, samples were taken from the photobioreactor during the culture period and subjected to microscopic observations (Figure 2a). The Motic AE2000T inverted microscope, equipped with a MOTIC digital camera (Moticam A5, 5 MP live resolution) and specialized image analysis software (Motic Images Plus 3.0), was used for this purpose. Each measurement was repeated 10 times, allowing for the determination of the average value of the measured quantities and providing high confidence in the obtained results. The size distribution of individual algae cells in the suspension, ranging from 2.8 to 7.2 \(\upmu\)m and corresponding to cell volumes between 11.5 and 195.4 \(\upmu\)m\({}^{3}\), is shown in Figure 2b. The cell size distribution histogram indicates an average size of 4.6 \(\pm\) 0.2 \(\upmu\)m, with relatively few cells smaller or larger than this value. During cultivation, the algal cells grew and, as a result of autoflocculation, aggregated into clusters of various sizes. To precisely determine the sizes of the forming aggregates, separate microscopic measurements were taken for the suspension samples. A representative image of a randomly selected sample is shown in Figure 3. As observed, the sizes of individual microalgae cells differ significantly from those of their aggregates, formed due to autoflocculation. Computerized image analysis revealed that the average size of the formed flocs ranged between 20 and 140 \(\upmu\)m.  
Figure 3: Microscopic view of _Chlorella vulgaris_ BA-167 algae (**a**); cell and aggregate size distribution histogram under autoflocculation, \(C_{a,0}\) = 0.960 g/L (**b**). Figure 2: The algae _Chlorella vulgaris_ BA-167: (**a**) general view of the cells under the microscope; (**b**) cell size distribution histogram. The comparative analysis of floc sizes formed with the coagulant Flokor 1.2A revealed that their size significantly exceeded that of aggregates formed during autoflocculation, reaching over 140 \(\upmu\)m.  
The flocculation-sedimentation process with Flokor 1.2A follows the typical mechanism of metal coagulants commonly used in wastewater treatment to remove suspended solids. This process involves two main mechanisms: charge neutralization and sweep flocculation. In charge neutralization, positively charged metal coagulants (Al\({}^{3+}\)) are attracted to negatively charged colloidal particles (algal cells), leading to the formation of large microfloc agglomerates. This interaction minimizes the repulsive forces between the cells, causing cell collisions and destabilizing them, which initiates the bridging of algae into irregular floc formations, as observed in the microscopic images (see Figure 4). When excess coagulant is added beyond the neutralization point, metal hydroxide precipitates, such as Al(OH)\({}_{3}\), are formed. These precipitates are large, heavy, and sticky, helping to capture colloidal materials. In sweep flocculation, the metal precipitates settle, entrapping the colloidal materials within the formed flocs. Flokor 1.2A (polymerized aluminum coagulant) requires a lower concentration of Al\({}^{3+}\) compared to other metal coagulants like ferric chloride, due to its smaller ionic radius (0.05 nm vs. 0.065 nm for Fe\({}^{3+}\)), resulting in a higher charge density. This higher charge density enables Al\({}^{3+}\) to more effectively bridge and neutralize the surface charge of microalgal cells. Flokor 1.2A requires both rapid and slow mixing stages. Rapid mixing (150 rpm) facilitates the \"coating\" of algal cells with Flokor 1.2A and helps prevent uneven dosing of the coagulant. However, excessive agitation can disrupt intercellular bridges and break down the formed flocs. In contrast, flocculation mixing (at 45 rpm) is slower, allowing optimal interaction and aggregation of the flocs.