## 1 Introduction
The typical flocculation process consists of two stages: rapid mixing (RM) and slow mixing (SM). The RM stage, conducted at 100-300 rpm for 1-5 min, initiates suspension destabilization. Subsequently, the SM stage, performed at 20-50 rpm for 9-20 min, facilitates the growth of larger agglomerates, contributing to effective flocculation [11; 12; 13; 14]. Metallic salts based on iron or aluminum salts are the most common type of inorganic flocculants used to harvest microalgae [12; 15]. Nevertheless, these non-biodegradable flocculants contribute to secondary pollution and generate toxic sludge, resulting in costly and intricate treatment processes [16]. Research indicates that the bioaccumulation of ferric ions surpasses that of ferrous ions across various microalgae, with _Chlorella_ exhibiting the highest resistance [17]. In-depth investigations by other authors focused on the detrimental impacts of ferric chloride and aluminum sulfate on microalgae, revealing that over 95% of the metals were transferred to the biomass [18]. Additionally, some studies highlight the potential harm associated with human exposure to aluminum, establishing a connection with neurodegenerative diseases, notably Alzheimer's disease [19; 20]. These findings underscore the need for considering the environmental and health implications of the chosen flocculants in microalgal harvesting processes. Due to environmental problems associated with the use of chemical flocculants, natural organic flocculants have emerged as alternatives considering their greener production and application [21]. These are biodegradable, non-toxic, produce smaller volumes of sludge, require smaller dosages to work efficiently, and there is no presence of metallic residues in the microalgal biomass. However, they can be more expensive. From the organic flocculants, there has been a growing interest in applying chitosan [14; 22], a linear amine-based biopolymer formed by units of D-glucosamine and N-acetyl-D-glucosamine, obtained by the deacetylation of chitin. Waste from the seafood industry is currently the main natural source of chitosan [16]. While organic polymers have been documented in some studies, little is known regarding their application in _C. vulgaris_ harvesting and process optimization. The main objective of this study is to evaluate and optimize the harvesting efficiency of _C. vulgaris_by chemical flocculation, followed by sedimentation, using chitosan, aluminum sulfate and ferric sulfate for comparison. Preliminary studies aimed to determine the optimal pH for flocculation; that is, the pH at which the formation of primary flocs was achieved with the lowest flocculant dosage. Additional studies were conducted with different flocculant dosages at the optimal pH previously determined to select the optimal dosage range for flocculation. Finally, a 24 full factorial design experiment was conducted to investigate the impacts of flocculant dosage, settling time, and RM and SM time on harvesting efficiency. The objective was to identify optimal conditions for flocculation. Notably, when employing chitosan as a flocculant, the inclusion of two mixing phases is unnecessary. This streamlined approach not only minimizes time and energy expenditures at an industrial scale but also enhances the efficiency of microalgae harvesting through chemical flocculation.