A two-vessel liquid separation apparatus includes a pump and its motor submersed in a one vessel and a reverse osmosis membrane in the other vessel. The vessels are separated by a contiguous dividing wall. The membrane and the pump are in fluid communication with one another through the dividing wall so as to allow liquid to pass from one vessel to the other in accordance with a given geometry. The pump receives liquid from the membrane containing vessel through holes in the dividing wall, the pump creating a highly turbulent flow caused by a cyclonic and co-axial effect, directly at the lower end of the membrane; the connexion of the pump outlet at the dividing wall is very close to the membrane thus providing an optimal efficiency of turbulence and flow at critical locations of bacteria collection. The injection of new liquid to be concentrated at the inlet of the pump causes an immediate mixture of liquid to be concentrated with concentrated liquid, which mixture circulates rapidly in a close circuit between the first and second vessels.