Membrane based processes for conventional production of demineralized water often consist of ion exchange and/or reverse osmosis. An innovative alternative process is using rainwater followed by low-pressure micro-/ultrafiltration offering potential benefits concerning the environment, energy consumption and costs, A project is carried out to determine the benefits of the alternative process for the production facility of S.Search b.v., a company that develops and produces membranes. The project is partially funded by Novem b.v., the Dutch organization for energy and environment. Data of rainfall is collected and attuned to water consumption using a inhouse developed spread sheet, It appears that at average annual rain fall of 850 mm there is no shortage of demineralized water at a water consumption of 1 m(3)/d discontinuous. Membrane performance on rainwater is currently tested, but is expected to show high flux together with low frequency of flushing and cleaning, Rainwater quality after treatment by micro-/ultrafiltration seems sufficient for S.Search b.v. Flexibility of the process using rain water in terms of water quality and quantity however is less compared to the conventional process. Therefore a cost calculation and comparison is carried out to determine when use of rainwater followed by micro-/ultrafiltration is attractive compared to processes using reverse osmosis and ion exchange.