What is a flow restrictor and why should I use it? Reverse osmosis units work by creating a back pressure on the exiting water to force the pure water through membrane. This back pressure is created by adding a flow restrictor on the waste pipe and unless this is fitted you will never achieve good quality water. The new generation of D-D RO units now have a flow restrictor with a tap on the top of it. This tap allows the user to take the restrictor out of line allowing a greater water flow to periodically flush the membrane and help to keep it clean, which in turn enhances the life of the membrane and aids water quality. To flush the membrane simply open the tap so that it is in line with the restrictor. Once the flushing has been done (flush for several minutes) then the tap must be closed. The tap will be closed when it is turned 90 degrees to the direction of flow and will begin to cause back pressure again. This flushing action should be carried out every 2 weeks or so to keep the membrane in top condition.