Convergence of direct-transesterification and anaerobic digestion for improved bioenergy potentials of microalgae

Direct-transesterification is an effective and economical method to extract fatty acid methyl ester (FAME) from microalgal biomass by omitting lipid separation step, but it still generates glycerol residue requiring further treatment. To minimize the waste, this study investigates the synergistic coupling of direct-transesterification and anaerobic digestion (AD). Four types of catalysts (HCl, H₂SO₄, NaOH, and K₂CO₃) were tested for better direct-transesterification followed by the AD of lyophilized Botryococcus braunii. Results indicate that base catalysts can extract more FAME than acid ones. The ranges of FAME yield were recorded as 6.0–9.2% of the inoculum biomass, equivalent to >50% of FAME contained in inoculum biomass. Fourier transform infrared spectroscopy (FT-IR) spectra reveals that the FAME production from microalgae can be inhibited by existing proteinaceous soluble microbial products (SMP) or extracellular polymeric substances (EPS). The AD demonstrates a good methane potential of residual by-product, glycerol. The biochemical methane potential (BMP) test show that 90% of organic matter was converted to methane, and prove that the direct-transesterification contributes to following anaerobic digestion without inhibition. This result indicates that additional >22% of total energy recovery is possible by the AD. Overall results imply that this convergence of direct-transesterification and AD offers an economic and efficient way to control the bioenergy recovery from microalgal biomass.