Water treatment system and method for treating water located in a water reservoir

Method for treating water (2) located in a water reservoir (3), for example in a swimming pool, pond or aquarium, in particular for cleaning and disinfecting the water (2), comprising:

• removing a pre-settable quantity of water (2) per unit of time from the water reservoir (3) via one or more extraction line(s) (6) of a recirculation means (4);
• filtering the removed partial quantity of water (2) by means of a membrane filtration means (9) disposed in the recirculation means (4), the membrane filtration means (9) comprising a number of filter modules (10) fluidically connected in a parallel arrangement;
• returning the water to the'water reservoir (3) via one or more return line(s) (7) of the recirculation means (4);
• cleaning the membranes of the filter modules (10), periodically or when required, by backwashing with a backwashing liquid by reversing the direction of flow through the filter modules (10) compared with that of the filtration operation and discharging the dirty liquid occurring during backwashing via a discharge (17),
• wherein in order to clean the membranes of the filter modules (10) of the membrane filtration means (9), gas is introduced by a gas supply means (24) into the filter modules (10) on the intake side or the gas is introduced into the water reservoir (3) at one or more locations by the gas supply means (24), periodically or when required, to circulate the water (2) in the water reservoir (3), the gas supply means (24) being used either to flush the filter modules (10) with gas or to circulate and mix the water (2) in the water reservoir (3).