Abstract: A liquid purification system is disclosed. In this system water is pumped into a filter cartridge. A reverse osmosis filter membrane in the filter cartridge removes impurities from the water, but not all of the water passes through the membrane. Water passing through the membrane exits the filter cartridge through a first outlet port, and is sent to a storage tank. Water not passing through the filter membrane exits the filter cartridge through a second outlet port. This water then passes through a pressure relief valve. The pressure relief valve can be adjusted to vary the water pressure and flow rates in the system. Some of the water exiting the pressure relief valve passes through a flow restrictor valve and then to drain. The flow restrictor maintains a constant flow rate at variable pressures. In this system, the flow restrictor is operable to maintain the same flow rate through it to drain and through the first filter outlet port.