Polyaluminium chloride (coagulant) is a very complex compound. The change in the ratio HCO3/CO2 during coagulation cause a change in pH in the subsequent RO unit (because CO2 is not rejected by the RO membrane), and it was found that this change is constant for any given recovery, e.g. at 75% recovery, the pH change between the RO feed water and concentrate water is 0.6. The simulation of aluminium solubility and concentration along RO systems were performed assuming the presence of three aluminium species, Al-13(7+), Al(OH)(4)(-), Al(OH)(2+). The 13 4 simulation of aluminium solubility was performed at two temperatures, 20 degrees C to represent the summer period and 5 degrees C to represent the winter period. Results showed that for winter (5 degrees C) an RO feed water pH of at least 7.2 and for warm weather (20 degrees C), a feed water pH of at least 6.7 are required respectively, to avoid scaling of aluminium on the RO membranes. In conclusion, theoretical simulation of aluminium solubility and results of deposition factor showed that aluminium more than likely plays a role in fouling of the RO membranes and could be a reason for the frequent cleaning.