Ultrafiltration (UF) has established itself as one of the key technologies in the water treatment industry over the last decade, providing superior filtrate water quality regardless of fluctuations in the feed water quality and protecting downstream treatment steps.UF has demonstrated its advantages as seawater reverse osmosis (RO) pre-treatment for desalination application and also in the treatment of municipal and industrial tertiary wastewater to produce water mainly for irrigation,aquifer replenishment or as a pre-treatment to a RO step.In-depth knowledge of polymeric chemistry is mandatory for developing, manufacturing and operating new membranes.On-site evaluation of membrane performances on real water is also essential.Seawater properties can vary significantly depending on location and region.Depending on the quality of the secondary effluent in regard of potential membrane foulants (e.g.dissolved or colloidal organic substances), operating parameters and required chemicals for the pre-treatment (e.g.coagulants) and different cleaning processes can have a significant impact on the design and also on the capital and on the operational costs.This paper presents investigations and optimization of different operating strategies depending of the water to be treated to ensure a proper UF system design while ensuring reliable operational parameters.The study describes differences in membrane behavior and shows that system optimization yields stable and long term operation.