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article i nfo Coagulation-flocculation is an efficient, economic, and scalable process suitable for harvesting microalgae. The study tested the effectiveness of 11 agents (chitosan and 10 commercial, industrially available reference prod- ucts) forharvestingChlamydomonas reinhardtii,a modelmicroalga.The highest efficiencies(N95%)with the low- est dose (b30 g/kg of biomass for 2 of the 4 polymers tested) were achieved for highly charged cationic polyacrylamides with a high molecular weight. Mineral coagulants were less efficient. Efficiencies above 80% were attained for 2 out of 4 flocculants with doses ranging from 40 to 80 g/kg of dry biomass. Chitosan appeared tobea goodalternative to polyacrylamides; itattainedan efficiency of over90%fora 35g/kgdoseof drybiomass. The study further investigated a reuse of the supernatant resulting from the solid-liquid separation, an essential prerequisite to scale up the process. Two types of cultivation tests were performed for three selected—chitosan, polyaluminium chlorosulfate, and an 80% cationic polyacrylamide: 1) using various ratios of supernatant to fresh medium and 2) using various concentrations of flocculant in a fresh medium. These preliminary experiments showedthat flocculantresiduesalterthegrowth ofC. reinhardtii indifferent waysdepending onthe chemicalna- ture and concentration of the considered flocculant. No lethal toxicity was observed.