Improvement of methane production at alkaline and neutral pH from anaerobic co-digestion of microalgal biomass and cheese whey

Over the past decades, biofuels from microalgae have gained increasing attention. In the present work, methane production from mono-digestion of thermochemical microalgal hydrolysate and co-digestion with cheese whey was evaluated in an anaerobic sequencing batch reactor. The reactor was fed with 5 g total chemical oxygen demand (TCOD)/L and operated either at alkaline or neutral pH. The average methane content in the biogas was higher at alkaline pH (83 %) than at neutral pH (61 %), whereas the methane yield from co-digestion was similar at alkaline (233 N mL/g TCOD) and neutral pH (226 N mL/g TCOD), being 1.8-fold higher than the monodigestion of the microalgal hydrolysate. After increasing stepwise the organic load up to 12 g TCOD/L, the methane production rate reached peaks around 1000 N mL/L-d, being between 200-800 mL/L-d in those stages with lower organic loads. The methane production from the particulate fraction of the microalgal hydrolysate and its co-digestion with cheese whey demonstrated that the particulate fraction was indeed used to produce methane, but co-digestion did not improve methane yield. The results showed that microalgal hydrolysate could be efficiently co-digested with a carbon-rich substrate enhancing methane production.