Controlling Metabolic Flux in Vacuum-Assisted Fermentation: The Interplay of pH and Operating Mode in the Valorization of Glucose to Volatile Fatty Acids and Biofuels

Abstract

Over the last two decades, anaerobic digestion has been increasingly implemented for organic waste treatment and production of biogas and electric energy which can be exploited beyond the plant boundaries. However, this process has limitations including high capital costs, low revenues from energy recovery, and the generation of nutrient-rich streams which require further treatment. Hence, anaerobic digestion was modified to go beyond energy recovery through the production and recovery of higher-value products via anaerobic fermentation. A newly proposed process called IntensiCarb™ (IC) applies vacuum-assisted fermentation to enable process intensification, enhance the resource recovery and implement circular economy policies. However, a critical knowledge gap exists regarding the influence of key operational parameters on its performance outcomes. This research addressed this uncertainty by systematically investigating the effects of pH (5.5, 7.0, and 9.0) and vacuum operating mode (sequential evaporation and intermittent evaporation) on the fermentation of glucose in lab-scale, semi-continuous reactors.

The results demonstrated that pH regulated metabolic flux, inducing a shift from hydrogen- and butyrate-producing pathways to ethanol- and acetate-dominant fermentation as pH increased from 5.5 to 9.0. The vacuum-enhanced modes intensified the process, operating at double the organic loading rate of the conventional system. However, performance—evaluated based on COD-normalized product yields—was highly dependent on the interaction between pH and operating mode. At neutral and alkaline pH, a clear performance hierarchy was established against the baseline reference points: the intermittent evaporation-fermentation (IEF) mode yielded more total volatile fatty acids (TVFA) and hydrogen than both the sequential evaporation-fermentation (SEF) mode and conventional fermentation (CF). For instance, at pH 9.0, the IEF mode achieved a maximum TVFA yield of 63.0 ± 1.1%, outperforming both the SEF (60.1 ± 1.7%) and CF (56.7 ± 1.2%) baselines. This superior performance was attributed to the IEF mode’s ability to alleviate thermodynamic limitations (e.g., inhibitory hydrogen partial pressure) through more frequent, in-situ product removal compared to the end-of-cycle removal in SEF.

Critically, this trend reversed under acidic conditions (pH 5.5), where CF produced a higher TVFA yield (64.6 ± 2.3%) than either IEF (53.2 ± 1.0%) or SEF (51.7 ± 1.0%). This antagonistic interaction was attributed to heightened product inhibition from the accumulation of undissociated VFAs under the intensified IC conditions. These findings reveal that while the IC process is a powerful platform for targeted chemical production, its performance is dictated by the interplay between pH and process intensification, which must be carefully managed to avoid inhibitory effects and maximize resource valorization.