A hybrid process, consisting of ozonation and membrane filtration, followed by activated carbon filtration, was studied in demonstration scale for the drinking and service water production from surface water in Thailand. The main focus of this study centred on the evaluation of the entire process, especially on the membrane reactor technology under tropical conditions. To this end a demonstration plant with a capacity of about 5 m(3)/h in continuous operation was designed and build up in a 40 ft sea container. Submerged ceramic micro-(M F) and ultrafiltration (UF) membranes with a cut-offs of 200 and 80 nm were used for the separation process inside the ozone reactor. Pilot tests with different kinds of surface water were carried out in Thailand, as a typical tropical country. The tests have shown that during ozonation the permeate fluxes of the submerged membranes have constant fluxes on a high level for UF in a range of 60 to 80% and for MF in a range of 35% of the pure water flux. The low amount of concentrate generated by this process contains a high amount of mineralised elements and is harmless for discharging. The comparison of the different raw water qualities with the qualities of the produced freshwater have shown, that the process is suitable for the decentralised production of drinking and service water in tropical countries. Finally a first evaluation of the economic efficiency of this process concept is given as an outlook.