Abstract: The invention relates to a water treatment system (1) and a method for treating water (2) located in a water reservoir (3). The water treatment system (1) comprises a recirculation means (4) and a membrane filtration means (9) with a plurality of filter modules (10) disposed in the recirculation means (4). The water treatment system (1) particularly comprises a gas supply means (24) by which gas can be introduced into the filter modules (10) to clean the membranes of the filter modules (10) of the membrane filtration means (9) or by which gas supply means (24) gas can be introduced into the water reservoir (3) at one or more locations, periodically or when required, to circulate and mix up the water (2).