Systems for buffering and purifying extremely polluted water should be sealed from the ground by means of a film. This prevents direct infiltration. These systems are often used to pre-purify rainwater running off from busy roads and car parks. These systems have a ground filter that offers a good purification yield, and always feature an overflow that, thanks to the pre-purification, can be connected to surface water or an infiltration system. Only if no surface water is available in the immediate vicinity and infiltration is impossible should the overflow be connected to the sewer system.