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 m(3) 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.