Abstract: A self-cleaning reverse osmosis liquid treatment system, method of cleaning a reverse osmosis membrane, and a dual configuration flow check valve for inlet and outlet heads of such a system. The system includes at least two membrane vessels each having at least one reverse osmosis membrane contained therein. One vessel processes intake liquid such as sea water, ground water or other fluids or juices into product liquid in the normal way, having a waste liquid discharge, while the other vessel simultaneously undergoes cleansing. Product liquid discharging from the vessel processing the product liquid is redirected at lower pressure back through an additional inlet in the inlet head having one check valve in its normally closed configuration positioned in the inlet head of the vessel being cleansed.