The co-composting of the solid residue and wastewaters from the olive-oil production process has been studied as a new method for the treatment of wastewater containing high organic and toxic pollutants. The experimental results for a demostration plant using solid residue from olive extraction as bulking material and olive-oil-processing effluents as continuously fed wastewater are reported. Composting temperature was controlled between 45 and 65 degrees C by air supply and the wastewater addition was fed mainly in order to keep the moisture in the range of 45 to 60% and secondly to replace the carbon substrate. During 23 days of operation in the thermophilic region, the system was fed with 263 m(3) wastewater in total, which means an average rate of 11.4 m(3)/day wastewater or 2.9 kg wastewater per kg solid residue. The total bioenergy production was estimated to be about 90 000 000 kcal. Then followed a 3 months stabilization period in the mesophilic region until the final product reached ambient temperature. Copyright (C) 1996 Elsevier Science Ltd.