The new process for process water production from river water is divided into three stages: prefiltration, ultrafiltration and reverse osmosis. The latter technique has been state-of-the-art in the preparation of drinking water, boiler feed water and ultrapure water from conventionally pretreated raw water for many years. On the other hand, ultrafiltration is new to this sector and has only recently been used on the industrial scale. It is used as a single-stage process to purify drinking and process water as well as surface water as an alternative to conventional treatment pro cess es (e.g. ozonization-precipitation-flocculation-coagulation-chlorination-gravel filtration). Multistage, fairly complex processes are employed in the conventional pretreatment of river water. The use of different chemicals necessitates special safety measures and careful harmonization and control of water chemistry in view of the requirements of downstream reverse osmosis. By contrast, processes based on membrane technology enable a simply designed plant to be used with several advantages. Axiva in cooperation with the water supply company for the Hochst Industrial Park have developed and successfully tested an efficient and cost-effective ultrafiltration process for river water in pilot-scale operation over several years. During this trial period low-energy systems based on different filtration concepts (cross-flow, dead-end), efficient hydrophilic membranes and a specific operating and backflushing technique specially designed for the application were employed. In this way reliable plant operation with high availability was obtained and suspended particles and microorganisms were removed from the river water (River Main) without any problem. The permeate quality was very high throughout the trial period (<0.05 NTU) and met the requirements for feed water to the RO plant. Stahlwerke Bremen, a major steel company, in cooperation with the Institute of Environmental Process Engineering of the University Bremen and Axiva are building a large-scale pilot plant for the treatment of water from the River Weser by the combination of direct ultrafiltration and reverse osmosis (total capacity 36 m(3)/h RO-permeate). The aim of this joint cooperative venture is to acquire operating experience with the pilot plant over an extended period. In three years' time the experience gained can be used in a full-scale plant replacing existing conventional plant technology to produce ultrapure water with the new, modern treatment method. The pilot plant is being started up in July 2000.