Microalgal Cultivation and Utilization in Sustainable Energy Production

Abstract

Microalgae are a promising feedstock for biofuel and bioenergy production due to their high photosynthetic efficiencies, high growth rates and no need for external organic carbon supply. However, microalgal biomass cultivation for energy production purposes is still rare in commercial scale. Further research and development is needed to make microalgal derived energy sustainable and economically competitive. This work investigated cultivation of fresh water microalga Chlorella vulgaris and marine microalga Dunaliella tertiolecta and their utilization in production of hydrogen, methane, electricity, butanol and bio-oil after bulk harvesting the biomass. Growth of the two microalgae was studied in five different photobioreactor (PBR) configurations especially concentrating on the quantification and characterization of heterotrophic bacteria in non-axenic microalgal cultivations and microalgal utilization of different nitrogen sources. Anaerobic cultures used for the energy conversion processes were enriched from a mesophilic municipal sewage digester separately for production of H₂, CH₄ and electricity from the two microalgal species. After culture enrichment, energy conversion yields of microalgal biomass to the different energy carriers were compared. Anaerobic microbial consortia utilizing microalgal biomass were characterized based on the 16S rRNA gene sequence analysis. H₂ and CH₄ production potentials were tested in anaerobic serum bottles and electricity and butanol production in fed-batch operated two-chamber microbial fuel cells (MFCs). All the PBR configurations tested were amenable to C. vulgaris and D. tertiolecta biomass production. Highest biomass concentrations and productivities by C. vulgaris and D. tertiolecta were 3.8 and 3.2 g L⁻¹, and 0.60 and 0.83 g L⁻¹ d⁻¹,, respectively. They were obtained in bubble column PBRs at 12% CO₂, 10 mM NO₃⁻ and 350 μmol photons m⁻² s⁻¹. Static mixers used in the flat plate PBRs did not generally enhance the growth of either C. vulgaris or D. tertiolecta at the low light intensities (50 μmol photons m⁻² s⁻¹) used. However, the low light intensity resulted in high growth rates at the early stages of growth. The highest specific growth rates were obtained in the flat plate PBRs and were 2.0 and 1.4 d⁻¹ for C. vulgaris and D. tertiolecta, respectively. Bacterial growth occurred simultaneously with microalgal growth and correlated generally well with algal exuded dissolved organic carbon (DOC) concentrations. The bacterial communities were relatively stable and reproducible in both C. vulgaris and D. tertiolecta cultivations. However, algal associated bacterial communities were vastly different in C. vulgaris and D. tertiolecta cultures due to different growth medium salinity (