Free cells of Aspergillus niger were grown on olive mill wastewater (OMW) supplemented with rock phosphate (RP) in an air-lift bioreactor in batch and repeated-batch processes. The fungus grew well and reduced the chemical oxygen demand of the waste by 35% and 64% in the batch and repeated-batch (fourth batch) processes, respectively. Total sugar content was consistently reduced (ca. 60%) in both processes while reduction of total phenols was minimal. RP was solubilised and maximum soluble P was 0.63 and 0.75 g l(-1) in the batch and repeated-batch (third batch), respectively. Several types of OMW +/- RP, microbially-treated or not, were tested in a greenhouse for their fertilising ability on a soil-wheat (Triticum durum Desf.) model system. Beneficial effects were highest using OMW treated by the repeated-batch process. The treated plants showed an increase in seed biomass, spike number, and kernel weight. Harvest index was highest (0.49 +/- 0.04) after treatment with OMW from the repeated-batch process. (C) 2003 Elsevier Ltd. All rights reserved.