Removal of phosphate ions (PO4-P) from aqueous solution by means of fly ash was investigated in a crossflow microfiltration system. Batch experiments prior to membrane filtration were conducted to determine PO4-P removal capacity of the fly ash. The effect Of PO4-P concentration, initial pH of solution and the fly ash dosage on the PO4-P removal was studied. It was found that if the initial pH of solution is about 6, maximum PO4-P removal is obtained and as the fly ash dosage increase the percentage Of PO4-P removal rises due to an increase in solubility of calcium ions and final pH in water. In membrane filtration, PO4-P treated with the fly ash was separated from water with crossflow microfiltration technique. The effect of the fly ash dosage, PO4-P concentration, transmembrane pressure drop (DeltaP) and membrane type on the membrane fluxes (J*) and PO4-P rejections (R-p*) were investigated. Under certain conditions, 100% R* could be achieved depending on the fly ash dosage. It was seen that J* and R* increase with increasing of the dosage. When the concentration Of PO4-P in the feed solution is increased, R* reduces because of the low final pH and inadequate calcium ion concentration. Furthermore, it was also found that results obtained for J* and R* were better for cellulose nitrate (CN) membranes compared with cellulose acetate (CA) membranes. The effect of transmembrane pressure drop (DeltaP), the fly ash dosage and PO4-P concentration on steady state permeate fluxes and rejections have been explained by specific cake resistances (alpha). It has been seen that separation of insoluble PO4-P compounds by crossflow membrane filtration seems to be advantageous to classical batch separation in respect of efficiency. (C) 2003 Elsevier B.V. All rights reserved.