A study was undertaken to evaluate the efficiency of applying olive oil mill wastewater as a non-nitrogenous external carbon source in the second anoxic stage of a five stage modified Bardenpho system for nutrients removal in order to assure consistently very low concentrations of total nitrogen (well below 3 mg/l) in the treated effluent. Addition of olive oil mill wastewater was found acceptable only up to 50 mg sCOD of mill waste/l of wastewater fed to the system because at higher additions color problems in the treated effluent were encountered. The required dosage of olive oil mill wastewater was found to be in the range 4.6-5.4 mg sCOD/mg N-NO3 removed. Operation with olive oil mill wastewater effects at the same time higher removal of phosphorus. Addition of physicochemically pretreated olive oil mill wastewater with lime to the second anoxic tank at a rate of 22-45 mg sCOD/l of municipal type wastewater fed (ratio of volume of the mill waste added to the volume of the municipal type wastewater fed 1:1000-1:2000) resulted in a treated effluent with total nitrogen below 3 mg/l and soluble phosphorus well below 1 mg/l.