Abstract: The present invention relates to a system which makes it possible to remove undesirable liquids, generally an aqueous phase, which precipitate to the bottom of tanks for storing liquid products. Use is made of a drainage device (30) which comprises a concave body (31), a seat (26), and an interface float (8) which has a plug portion (25). This interface float (8) is designed to sink into the product and to float on the aqueous phase. As the level of the aqueous phase drops, the plug portion (25) settles sealingly on the seat (26), halting the drainage operation since all the accumulated aqueous phase has already been drained off. The drainage system may be automatic or manual. In the former case, use is made of a sensor (12) for detecting the presence of an aqueous phase and commanding the process of opening a shut-off valve (11) and allowing drainage of the accumulated aqueous phase.