BACKGROUND: The study focused on the biodegradation of pretreated olive mill wastewater (OMW) properly diluted in and mixed with a biodegradable substrate and/or domestic sewage stream. It utilized the permeate of a chemically conditioned two-stage ultrafiltration/nanofiltration system with COD of 5700mgL(-1) and a total phenol content of 100mgL(-1). Peptone mixture with similar characteristics to domestic sewage was adopted as the biodegradable synthetic substrate. The study relied on two sets of microbial culture; one acclimated to peptone and the other to a mixture of peptone and domestic sewage. Respirometric analyses were conducted on pretreated OMW diluted in the selected substrate using corresponding biomass seeding. RESULTS: Model evaluation of the oxygen uptake rate profiles indicated that the pretreated OMW was fully consumed in the respirometric tests. There were no inhibitory/toxic impacts of OMW dosing on the biomass. The entire biomass participated in the utilization of OMW/substrate with no metabolic adjustment problems. CONCLUSION: Experimental results provided conclusive evidence that co-treatment with a domestic sewage stream is a viable option for OMW management, after a chemically conditioned two-stage filtration used as the pretreatment step. Pretreatment removes the recalcitrant nature of the wastewater, which becomes fully biodegradable under appropriate dilution and mixing conditions. (C) 2016 Society of Chemical Industry