A water purification apparatus and method for optimizing efficiency of the water purification apparatus comprising a fluid circuit including a Reverse Osmosis, RO, unit (3), providing a permeate flow, and an electrically controlled deionization unit (4) downstream the RO unit (3) receiving at least part of the permeate flow. The method comprises obtaining (S1) a value indicative of power consumption by the electrically controlled deionization unit and determining (S2) whether the obtained value indicative of the power consumption meets at least one criterion. The method further comprises controlling recirculation of reject water produced by the water purification apparatus, based on a result of the determining (S2), in order to optimize efficiency of the water purification apparatus.