Anaerobic digestion of olive mill wastewater

Anaerobic digestion of olive-mill wastewater (OMW) was carried out in a continuously fed
mode bioreactor. The bioreactor was operated at different hydraulic retention times (HRTs),
using OMW, either raw or pre-treated with white-rot fungi. Two different kinds of feed were
tested in this process, one which was thermally treated and subjected to sedimentation, in
order to remove the solids contained and the other without any physicochemical treatment
(raw OMW). Thermally pretreated OMW did not allow a stable operation even at an HRT of
30d. Further pretreatment of the OMW with a white-rot fungus for removal of the contained
phenolics, allowed a stable operation at an HRT of 30 d. On the other hand, simple dilution of
the raw wastewater, without any solids removal, lead to a stable operation at an HRT of 30d
and was accompanied by higher production of biogas. The presence of the solids in the OMW
proved to be a determining factor for the stability of the process and could be attributed to a
possible adsorption on the solids of hydrophobic compounds, such as long-chain fatty acids
that are toxic to methanogens.