Hydrogen and methane production from microalgal biomass hydrolyzed in a discontinuous reactor inoculated with ruminal microorganisms

Abstract

Hydrolysis is considered the limiting stage of the anaerobic digestion of microalgae due to the recalcitrance of this type of biomass. Hydrolysis of microalgae with ruminal fluid, a microbial culture with high hydrolytic capacities, is a promising pretreatment method. In the present work, the long-term hydrolysis process of a native consortium of microalgae (98% Scenedesmus) was studied in a sequential batch reactor inoculated with ruminal fluid. The influence of the hydraulic retention time (HRT) was evaluated in such a way to obtain a hydrolysate rich either in sugars or volatile fatty acids (VFA) and then to produce, in a subsequent stage, hydrogen or methane. Results evidenced that it is possible to maintain the hydrolytic activity for long periods when ruminal fluid is used as the inoculum. Using an HRT of 2 days a hydrolyzate rich in sugars was obtained. The fermentation of this hydrolysate produced 1.7 times more hydrogen than the amount produced with raw microalgae. When the HRT was adjusted to 8 days, the hydrolysis efficiencies increased to 60% and a hydrolysate rich in VFA was obtained. The methanization of this effluent yielded about 81% of the theoretical value. The most abundant bacteria present in the culture were Pseudomonas, Bacteroides, Clostridium, Acinetobacter and Proteocatella. The gaseous biofuel production was improved by hydrolyzing the microalgal biomass with ruminal fluid (up to 2.6 times more than the raw biomass), and it was possible to recover up to 16.8 kJ per gram of volatile solids.