The accumulation of volatile fatty acids and phenols through a pH-controlled fermentation of olive mill solid waste

Highlights: • Olive mill solid waste was fermented at acidic, neutral and alkaline conditions. • More than 90% of the organic matter was biomethanized at neutral pH. • Volatile fatty acids reached a maximum concentration of 3.69 g COD/L at pH 9. • The highest phenolic compounds accumulation occurred at pH 5. -- Abstract: This work aims to compare the use of olive mill solid waste as substrate in pH-controlled fermentation at acid (pH = 5), neutral (uncontrolled, pH ≈ 7) and alkaline (pH = 9) operating pH levels. The results obtained in this study indicate that operating pH strongly affected the anaerobic microorganisms and, hence, different target compounds could be obtained by adjusting the operating pH. Fermentation at neutral pH resulted in the conversion of 93.5% of the fed chemical oxygen demand to methane. However, fermentations at pH 5 and 9 resulted in the inhibition of the methanogenic activity. At pH 9, volatile fatty acids reached a maximum concentration of 3.69 g O₂/L, where acetic acid represented up to 79.3% of the total volatile fatty acids. Unlike volatile fatty acid production, an optimal operation of fermentation at pH 5 could allow the recovery of phenols such as vanillin.