The invention relates to a method and a water treatment system (2) for treating and/or keeping clean water (1), in particular from swimming facilities (4), whereby the water (1) is pre-cleaned in a first step in at least one mechanical cleaning device (5, 8) and in a second step is put through a biological cleaning process in at least one biological cleaning device (12, 13). The water (1) is transported between the cleaning steps by force of gravity and the cleaned water (1) is optionally returned to the swimming facility (4).