The Effects of the Most Important Phenolic Constituents of Olive Mill Wastewater on Batch Anaerobic Methanogenesis

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Abstract
The most important phenolic constituents of Olive Mill Wastewater (OMW), i.e. tyrosol, oleuropein and caffeic, p-hydroxybenzoic and protocatechuic acids, were added at various concentrations to aliquots of domestic anaerobic sludge in Hungate serum bottles and these were incubated at 35°C. The concentration of methane in the headspace gas was monitored to determine if the phenolics were fermented to methane or if they inhibited the anaerobic process. Only tyrosol and p-hydroxybenzoic acid were fermented to methane. At 600 mg l−1 (but not at 400 mgl−1) oleuropein reduced the rate and the amount of methane produced. The ortho-diphenols (caffeic and protocatechuic acids) were inhibitory at 1000 mg l−1 but not at 500 mg l−1. In cultures supplemented with acetic and propionic acids (VFA), and in unsupplemented cultures, tyrosol at concentrations up to 600 mg l−1 was fermented to methane. Between 900 and 1300 mg l−1 tyrosol methane production was neither enhanced nor inhibited relative to control cultures containing no tyrosol. Inhibition of methane production was evident when tyrosol was present at ≥ 2000 mg l−1. In similar experiments with p-hydroxybenzoic acid enhanced methane production was observed at concentrations of ≤ 500 mg l−1; no enhancement or inhibition was observed at 700 mg l−1; and inhibition was noted when p-hydroxybenzoic acid was present at ≥ 1000 mg l−1.

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