The presence of microalgae affects water quality and beneficial uses of surface freshwaters. Chitosan with proven potential for harvesting specific microalgae species from their culture medium to produce biofuels and bioproducts also appears promising for efficient removal of mixed microalgae species from surface freshwaters. The main focus of the present study was on removal of mixed microalgae species using chitosan as a coagulant to help improve water quality. Important operational parameters were optimized for economical microalgae removal. The microalgae cells, chlorophyll-a, total nitrogen, and total phosphorous removal efficiency were 82.06%, 89.90%, 69.32%, and 44.01%, respectively, at an optimum chitosan dose of 10 mg/L, pH 8, slow mixing time 7 min, slow mixing rate 30 rpm, and settling time of 15 min. The results show that chitosan coagulation efficiently removed mixed microalgae species from surface freshwater with significant improvement in water quality and recovery of algal biomass for other beneficial applications. Key words: chitosan, microalgae harvesting, flocculation, water quality improvement, microalgal biomass. La presence de microalgues a une incidence sur la qualite de l'eau et les utilisations benefiques des eaux douces de surface. Le chitosane, avec un potentiel confirme en matiere de recolte d'especes precises de microalgues a partir de leur milieu de culture pour la production de biocarburants et de bioproduits, semble egalement prometteur pour l'elimination efficace des especes mixtes de microalgues des eaux douces de surface. L'objectif principal de la presente etude etait d'eliminer les microalgues mixtes en utilisant le chitosane comme coagulant pour ameliorer la qualite de l'eau. Des parametres operationnels importants ont ete optimises pour l'elimination economique des microalgues. L'efficacite d'elimination des cellules de microalgues, de chlorophylle-a, d'azote total et de phosphore total etaient 82.06, 89.90, 69.32 et 44.01 %, respectivement, a une dose optimale de chitosane de 10 mg/L, pH 8, temps de melange lent 7 min, vitesse de melange lente 30 rpm et temps de sedimentation de 15 min. Les resultats ont montre que la coagulation du chitosane a permis d'eliminer efficacement les microalgues mixtes des eaux douces de surface, ce qui a permis d'ameliorer considerablement la qualite de l'eau et de recuperer la biomasse algale pour d'autres applications benefiques. Mots-cles : chitosane, recolte de microalgues, floculation, amelioration de la qualite de l'eau, biomasse de microalgues., 1. Introduction Excessive growth of microalgae, especially cyanobacteria, in surface freshwaters forms unsightly surface scum, causes severe oxygen depletion and fish mortalities upon die-off, releases algal toxins, taste and odour, [...]