The ultrafiltration system removes even the smallest undissolved compounds from the WWTP effluent. The UF cassettes, which contain a range of strong membrane screens, are immersed in the dirty water. These screens feature holes with a diameter of 0.04 micrometres. With a pump, the water is sucked through the screens, out of the container. The dirt, including undissolved compounds and bacteria, stick to the outside of the membrane screens. On the outside of the screens, air is blown into the water from below, causing the screens to move, so that the dirt cannot cake. Every 10 minutes, the direction of the pumps is reversed in order to force clean water through the screens from the inside, removing any dirt on the outside.