Method for concentrating fresh raw milk, comprising the steps of: - supplying fresh raw milk to a liquid treatment system, the system comprising: - an inlet configured to receive fresh raw milk; - a pressure vessel connected to the inlet, wherein the pressure vessel is provided with a pressure system configured to realize a pressure of at least 10 bar in the pressure vessel, wherein the pressure system comprises a compressor configured to supply air and/or a supply container configured to supply an inert gas to the pressure vessel in order to thereby pressurize the pressure vessel; - a reverse osmosis membrane (8, 108) for filtering the raw milk; - a pump for pumping the raw milk under pressure out of the pressure vessel and through or along the reverse osmosis membrane; and - a permeate outlet (10, 110) for the separated water and a milk outlet for the concentrated milk, - wherein the pressure system is further configured to realize a pressure of at least 10 bar in the liquid treatment system (2, 102); - pressurizing the liquid treatment system at a substantially constant pressure with a pressure of at least 10 bar by supplying a gas, wherein substantially the whole liquid treatment system is placed under an overpressure; - carrying the fresh raw milk through the liquid treatment system with the pump; - separating water from the fresh raw milk with the reverse osmosis membrane; and - separately discharging separated water and the concentrated milk. - supplying fresh raw milk to a liquid treatment system, the system comprising: - an inlet configured to receive fresh raw milk; - a pressure vessel connected to the inlet, wherein the pressure vessel is provided with a pressure system configured to realize a pressure of at least 10 bar in the pressure vessel, wherein the pressure system comprises a compressor configured to supply air and/or a supply container configured to supply an inert gas to the pressure vessel in order to thereby pressurize the pressure vessel; - a reverse osmosis membrane (8, 108) for filtering the raw milk; - a pump for pumping the raw milk under pressure out of the pressure vessel and through or along the reverse osmosis membrane; and - a permeate outlet (10, 110) for the separated water and a milk outlet for the concentrated milk, - an inlet configured to receive fresh raw milk; - a pressure vessel connected to the inlet, wherein the pressure vessel is provided with a pressure system configured to realize a pressure of at least 10 bar in the pressure vessel, wherein the pressure system comprises a compressor configured to supply air and/or a supply container configured to supply an inert gas to the pressure vessel in order to thereby pressurize the pressure vessel; - a reverse osmosis membrane (8, 108) for filtering the raw milk; - a pump for pumping the raw milk under pressure out of the pressure vessel and through or along the reverse osmosis membrane; and - a permeate outlet (10, 110) for the separated water and a milk outlet for the concentrated milk, - wherein the pressure system is further configured to realize a pressure of at least 10 bar in the liquid treatment system (2, 102); - pressurizing the liquid treatment system at a substantially constant pressure with a pressure of at least 10 bar by supplying a gas, wherein substantially the whole liquid treatment system is placed under an overpressure; - carrying the fresh raw milk through the liquid treatment system with the pump; - separating water from the fresh raw milk with the reverse osmosis membrane; and - separately discharging separated water and the concentrated milk.