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.

This thesis builds around the idea of a biofuel production process that is comprised of biomass production, biomass gasification, gas cleaning, and fuel production. In this work, we specifically looked into H2S removal as a part of cleaning the producer gas and flocculation of microalgae which is involved in the harvesting of microalgae after biomass production. One of the impurities to remove from the producer gas is hydrogen sulfide which can be removed by using a packed bed of zinc oxide. Despite the regular use, it was only recently shown that during reaction with H2S, nano-size particles of ZnO exhibit void formation and outward growth. In this work, a micro-scale model was introduced to describe the void formation and outward growth. On the macro-scale, the simulations captured pore clogging of pellets due to the outward growth. The pore clogging prevents the full conversion of pellets and consequently leads to shorter breakthrough times of beds. The second problem investigated here deals with the flocculation of microalgae. Microalgae is produced in relatively low concentrations in the incubator liquid medium and during the harvesting, the concentration is increased to an acceptable level. The harvesting process includes a flocculation followed by a filtration or centrifuge unit. During flocculation, microalgae are stimulated to aggregate and form clusters. The experiments showed that the mean size of clusters formed during flocculation increases with time to a maximum and then starts decreasing, resulting in an overshoot in the mean size profile. The size of clusters influence the efficiency of the afterward filtration or centrifuge, thus it is of interest to carefully track the size evolution of clusters, making the studying of overshoot a crucial research topic. In this work, the possible mechanisms behind this overshoot were investigated. / <p>QC 20170330</p>