| Summary: | Microalgae harvesting by coagulation can use coagulant agents such as alum, synthetic polymers or biocoagulants. Biocoagulants have attracted the attention of researchers because they are natural, biodegradable, and promote high microalgal harvesting efficiencies. This study aims to optimize the harvesting of <i>Chlorella vulgaris</i> based on the dosage of the <i>Opuntia cochenillifera</i> extract and the choice of eluent for biopolymer extraction. The outdoor cultivation of <i>C. vulgaris</i> achieved a specific growth rate of 0.455 d<sup>−1</sup> and a maximum biomass concentration of 1.28 g<sub>DW</sub> L<sup>−1</sup>. In order to harvest the microalgal biomass, the polymer present in the mucilage of <i>O. cochenillifera</i> was extracted using NaOH and HCl. Coagulation and sedimentation assays were performed with different coagulant dosages: 3.5, 5.9, and 8.2 g L<sup>−1</sup>. The maximum harvesting efficiencies using the acid and alkaline extract coagulant solutions were 80.8% and 99.5%, respectively, with a dosage of 3.5 g L<sup>−1</sup>. According to the results, the <i>C. vulgaris</i> biomass can be harvested with the mucilage from <i>O. cochenillifera</i> in acid and alkaline eluents. The application of this biocoagulant constitutes a sustainable solution for microalgal harvesting.
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