Membrane technology has enormous potential in water preparation and the treatment of waste water. However, conventionally constructed membrane plants are relatively expensive for decentralised applications. A cheaper solution for the preparation of drinking water is gravity ultra-filtration under low pressure conditions. This does not require any pumps or regulating systems, which leads to a reduction in costs and also makes the application independent of electricity or other energy resources. Tests were carried out in the laboratory and on a pilot scale with a series of different natural waters. In doing so it was discovered that the flow rate stabilised after a few days and remained stable for several months. This new technology means that the costs for water preparation using ultra-filtration are also acceptable for use in developing countries.