Development and intensification of a foam flotation system in harvesting microalgae for biofuel

Abstract

Due to their photosynthetic efficiency, microalgae tend to have high lipid content and growth rates, hence their importance to the biofuel sector. However, the viability of microalgae-derived biofuel is hindered by high capital and/or operating costs required for cultivation, harvesting, and drying. Harvesting the microalgae cells represents a substantial process cost, accounting for an estimated 30% of the total cost of production particularly because of the low concentration of microalgal biomass relative to water in the algae culture. Foam flotation can be utilised as an energy-efficient harvesting and enriching technique for microalgae biomass with the potential to significantly reduce the production cost of algal fuel. In this thesis, foam flotation was used for the first time in a continuous mode to harvest freshwater and marine microalgae species in an attempt to overcome the trade-off between recovery efficiency and enrichment in batch and semi-batch foam flotation. The influences of cell surface characteristics on flotation performance were investigated by quantifying hydrophobicity, zeta potential, and contact angle. Fractional factorial and response surface designs of experiment were used to determine the best operating conditions to achieve an effective combination of a high recovery efficiency (for greater biomass removal from the growth medium) and concentration factor (to lower downstream dewatering and drying costs). Tubular setups of different smooth-successive contraction and expansion ratios (foam riser) were used for the first time to enhance foam drainage. A recovery efficiency of 91% was obtained for Chlorella vulgaris with a concentration factor of 722. Foam flotation demonstrated a much lower power consumption (0.052 kWh m-3 of algae culture) in comparison to other flotation techniques including dissolved air flotation and electro-flotation. The algal biomass harvested by foam flotation was processed directly using hydrothermal liquefaction (HTL) without extra stages for dewatering and drying or intermediate storage. Thus, it can offer precise investigations on the process feasibility and it also represents a more realistic scenario for the application of HTL. The fate of surfactant in harvested microalgae and its effects on the HTL product yield and distribution were also investigated. HTL of C. vulgaris recovered by foam flotation demonstrated that the surfactant had additional benefits on HTL product yield, distribution, and composition. Overall, foam flotation is an effective, rapid, low cost, media (and arguably species) independent, scalable harvesting system which is able to operate continuously. Foam flotation also delivers algal biomass having additional advantages for biofuel production