Improving the Biodegradability of Scenedesmus obtusiusculus by Thermochemical Pretreatment to Produce Hydrogen and Methane

Hydrolysis can be considered as the bottleneck for biofuel production from microalgae due to the recalcitrance of this type of biomass. Thermochemical pretreatment is an effective and widely used process to solubilize different types of biomass. Nevertheless, the variability of the cell wall composition, among different microalgae species, hinders the development of a suitable pretreatment. In this work, the most relevant factors for thermochemical pretreatment to solubilize organic matter from microalgal biomass (Scenedesmus obtusiusculus) were analyzed and identified. The aim was to obtain hydrolysates, rich in soluble organic matter, that allow attaining higher hydrogen and methane yields than with raw microalgae, using a factorial design to identify the significant factors of the thermochemical pretreatment. The results showed that temperature (100 °C) and HCl concentration (3%) were the most significant factors to achieve carbohydrates solubilization close to 100%. Further, reaction time (1.7 h) and biomass concentration (30 g total solids/L) were also important to achieve up to 60% chemical oxygen demand (COD) solubilization. Overall, the microalgal hydrolysates produced 48 NmL H2/g VS and 296 NmL CH4/g VS, which is 1.7- and 1.3-fold higher hydrogen and methane yields, respectively, than raw microalgae. These results are meaningful because high soluble carbohydrates and COD would allow the application of high organic loading rates in continuous reactors for biofuel production using renewable biomass such as microalgae.