The effects of the most important phenolic constituents of olive mill wastewater on batch anaerobic methanogenesis

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