Anaerobic digestion treatment of olive mill wastewater for effluent re-use in irrigation

Anaerobic digestion of olive mill wastewater (OMW) plus piggery effluent was carried out in an up-flow anaerobic filter type. The digester influent had up to 83% (v/v) of crude OMW with no need for chemical correction. The process converted 70-80% of the influent COD (20-60 kg COD m-3), produced 1-3 m3 m-3 d-1 (65-75% CH4) of gas and a more stabilised effluent with a neutral/basic pH was produced. Nevertheless, as some polymerised phenolic compounds are not readily biodegradable, no significant decrease in the OMW black colour was achieved. However, since those lignin residues are innocuous and oxidable into humic matter, the produced effluent appears as environmentally safe and suitable for agricultural irrigation.

OMW could be digested successfully by combining it with a complementary effluent, as original or degraded piggery effluents, with no need for chemical correction or dilution with water. Volumes of around 80% of OMW in the influent can be considered a limit in terms of filter efficiency. At this inlet composition, the digestion of OMW with PD (F2 unity) allowed olive oil mill effluents more concentrated than OMW with PE (F1 unity). Around 70–80% of the influent COD, ranging from 20 to 60 kg COD m-3 could be converted into biogas. Volumes of 1–3 m3 m-3 d-1 containing approximately 65– 75% of methane were obtained. The process provided the neutralization of influent, as well as the conversion of 50–70% of its phenol contents. The remaining substrate colour is mainly due to the presence of highly polymerized phenolic compounds that are not readily biodegradable. However, since these residues are not toxic and oxidable into humic matter, the produced effluent appears as environmentally safe and suitable for agricultural irrigation, provided that potassium accumulation in soil is avoided.