## 3 Results and Discussion
### Coagulation/Flocculation and Sedimentation of the C. vulgaris Culture  
Table 1 shows the harvesting efficiencies after the coagulation/flocculation and sedimentation processes of _C. vulgaris_ using the acid and alkaline _O. cochenillifera_ extracts as coagulants. In these tests, the control assay indicated that microalgal sedimentation naturally occurred after agitation in the jar test without the addition of coagulants. In this assay, a harvesting efficiency of only 31.0% was obtained after 30 min of sedimentation. Regarding the experiments with coagulants, the increase in the coagulant dosages led to lower harvesting efficiencies after flocculation. Moreover, the acid coagulant provided a higher efficiency compared to the alkaline coagulant. However, after sedimentation for 30 min, the opposite was observed. The harvesting efficiencies for the acid coagulant ranged between 72.9% and 80.8%, with the highest value observed for the lowest coagulant dosage (3.5 g L\({}^{-1}\)). Regarding the alkaline coagulant, the optical density was significantly lower after sedimentation compared to the remaining assays. Hence, the harvesting efficiencies were higher in this case, ranging between 99.3% and 99.5%. Moreover, after the sedimentation period, the results were very similar between the different coagulant dosages, allowing for inferring that the homogenous cactus solution was able to agglutinate the microalgal biomass, causing greater sedimentation. In summary, the _O. cochenillifera_ extracts appear to be promising coagulants for microalgal biomass since both acid and alkaline coagulants significantly increased the harvesting efficiency after sedimentation compared to the control assay. Nevertheless, the alkaline extract coagulant appears to be the most suitable option for this purpose, which denotes a greater efficiency of the cactus extract at higher pH ranges after sedimentation. [PERSON] et al. [18] stated that the efficiency of plant coagulants (polymers) is due to variations in charge density and molecular weight, which are able to neutralize the charge and form bridges between algal particles, respectively. The same authors affirmed that high molecular weight cationic polymers might reverse the surface load of the algae and stabilize the suspension, while low molecular weight polymers may not cause flocculation. The high coagulation capacity of the _O. cochenillifera_ extract is attributed to the presence of mucilage which is a viscous, cationic complex carbohydrate whose particles adsorb more in an alkaline solution than in an acid or salt solution. Mucilage is a slimy fluid that appears directly when the cactus is cut. This fluid is the active coagulant agent since it has the ability to precipitate ions and particles from aqueous solutions through bridging mechanisms, in which numerous negatively charged microalgae cells are adsorbed or neutralized onto the _Opuntia_ chains [20,33]. Moreover, during flocculation, the microalgal cells in solution, being destabilized or not, collide and aggregate due to Van der Waals forces, forming flocs [12]. According to [PERSON] et al. [12], the polymer size and charge density may influence this process. In this study, the polymer in the alkaline solution might have led to the destabilization of the surface charge of microalgae allowing the cells to agglomerate and settle with a higher harvesting efficiency than the acid coagulant. The pH is also an important factor that influences the surface charge of the coagulants and the stability of the microalgae cells in suspension, as well as the flocculation efficiency [34]. In this study, adding the acid and alkaline extract coagulant solutions to the microalgal suspension caused a shift in the culture's pH to 7.9 and 9.1, respectively. However, there was no variation in the hydrogenation potential during the coagulation/ flocculation tests and after the biomass sedimentation. Therefore, the microalgal harvesting process occurred within these pH values. High pH can favor microalgal flocculation.