Characterizing Microalgae (Nannochloris oculata) Harvesting by Aluminum Flocculation



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Recent progress in algae biotechnology indicates that microalgae have the potential of becoming a significant source for food, feed proteins, nutraceuticals, and lipids for biofuels. Typically low concentrations of microalgae cultures (less than 2 g/L) make harvesting of algae biomass one of the key economic bottlenecks for microalgae production of biofuels and bioproducts. Among the various biomass harvesting options currently under consideration, flocculation appears to be the least expensive and most flexible method for harvesting and initial concentration of dilute algal cultures. In addition to initial biomass concentration, processing factors that could also affect harvesting efficiency include culture pH, flocculant dosage, and media ionic strength (conductivity). This thesis reviews challenges of harvesting and concentration of green microalgae and examines the effect of pH, flocculant dosage, and culture conductivity on charge neutralization and flocculation of Nannochloris oculata by aluminum chloride.

N. oculata flocculation was studied by manipulating the culture pH and ionic strength before the addition of aluminum chloride. The removal efficiency, concentration factor, settling rate, and zeta potential of the culture were measured to assess the effect of processing variables and understand mechanisms that govern N. oculata flocculation by aluminum chloride. Flocculation tests conducted with culture concentrations of 10^7 cells/ml revealed that AlCl3 concentration of 0.05 g/L and flocculation pH of 5.3 were optimal conditions for achieving 100% removal efficiency and a twentyfold algae concentration. At flocculant concentrations greater than 0.05 g/L, removal efficiencies were equally good but resulting concentration factors decreased with increasing AlCl3 dosage. Zeta potential measurements were correlated with flocculation dosage, initial cell concentration, medium pH, and aluminum solubility curves to conclude that densely charged multi-valent aluminum hydroxide species were responsible for the efficient flocculation at pH 5.3 with 0.05 g/L AlCl3.