Here, we report on the harvesting of microalgae cells by coagulation using chitosan and the optimization of this process. Chitosan is a natural and environmentally friendly biopolymer created by the extensive deacetylation of chitin from shrimp, crab and crawfish. Although conventional chemical coagulants such as alum may have negative impacts on human health, the use of chitosan as coagulant is suitable to harvest live microalgae. In this paper, the jar-test method was used to identify the best chitosan conditions (including concentration dosage, mixing time, mixing rate and sedimentation time), for harvesting the Chlorella sp. from their cultures. The effect of mixing time, mixing rate and sedimentation time on microalgae removal were analyzed using the optimized chitosan dosage. The mechanism of coagulation using chitosan appears to consist of adsorption and charge neutralization. Chitosan not only acts as an adsorbent, but also spontaneously coagulates to agglomerate the microalgae cells. This two-in-one process makes the chitosan a good coagulant, allowing removal of microalgae cells, even at low concentrations. Chitosan successfully removed 99.0 +/- 0.4% of the microalgae cells at the following optimal parameters: chitosan concentration of 10 ppm, mixing time of 20 min, mixing rate of 150 ppm and sedimentation time of 20 min. An increase in chitosan concentration resulted in restabilization of the microalgae cultures, reducing the efficiency of the process. (C) 2011 Elsevier B.V. All rights reserved.