With an ever-growing world-wide demand for water and decreasing availability, emerging technologies such as ultrafiltration (UF) hold the key to future water treatment. Two applications to use unconventional sources for water production are described. The reuse of effluent of wastewater treatment plants (WWTP) for high-quality water production will certainly be an interesting application of UF in the years to come. The technical feasibility for this application has been shown and operational costs have been estimated on the basis of three pilot plant trails. At 0.09 to 0.22/m(3), depending on the WWTP effluent quality, this may be considered to be economically attractive. Another application of UF is the pre-treatment of seawater reverse osmosis (SWRO) plants. These plants, of which a number are in operation in the Middle East, are currently being operated at a fraction of their original design capacity due to poor performance of their conventional pretreatment. UF can provide a feed water quality, which provides SDI values well below 2, thus enabling the SWRO plant to perform at its original design capacity. Again, operational costs of this pretreatment are very attractive at 0.07 Euro to 0.09 Euro/m(3).