An apparatus for treating water or liquids in general, including a reverse osmosis filtration device provided with at least one inlet associated with a line for supplying the liquid to be treated, at least one outlet of the permeate associated with a line for dispensing the treated liquid and at least one concentrate outlet associated with a line for discharging the waste. The particularity of the present invention resides in that it includes, downstream of the filtration device, at least one tank that forms inside it two adjacent chambers with a mutually variable volume, in which one chamber is connected to the delivery line while the other chamber is connected to the discharge line.