A laboratory scale water purification system for producing up to 300 1/h deionized type 2 pure water from tap water, said system comprising: a feed medium flow path (C,E) including a pump for elevating the pressure of the feed medium and supplying the feed medium under pressure to a feed inlet of a reverse-osmosis device, wherein the reverse-osmosis device is adapted to produce a permeate flow and a concentrate flow from the feed medium and has a permeate outlet and a retentate outlet; an electro-deionization device having an inlet in fluid communication with the permeate outlet of the reverse-osmosis device, and a purified water outlet; a first retentate flow path in fluid communication with the retentate outlet of the reverse-osmosis device, for removing retentate from the system, said first retentate flow path including a first flow rate regulator adapted to be remote controlled; a second retentate flow path in fluid communication with the retentate outlet of the reverse-osmosis device for recirculating retentate to the feed medium flow path at an upstream position of the pump, said second retentate flow path including a second flow rate regulator adapted to be remote controlled; a first flow meter downstream of the permeate outlet for detecting the permeate flow rate produced by the reverse-osmosis device; a first conductivity cell provided in the first retentate flow path for detecting the conductivity (ion concentration) of the retentate flow; a second conductivity cell provided in the feed medium flow path for detecting the conductivity (ion concentration) of the feed medium flow; and an automatic controller for controlling the first and second flow rate regulators (3,4) based on the detection results from the first flow meter and the first and second conductivity cells (15,16) such that a predetermined target recovery rate and a predetermined target permeate flow rate are controlled for the reverse-osmosis device. a feed medium flow path (C,E) including a pump for elevating the pressure of the feed medium and supplying the feed medium under pressure to a feed inlet of a reverse-osmosis device, wherein the reverse-osmosis device is adapted to produce a permeate flow and a concentrate flow from the feed medium and has a permeate outlet and a retentate outlet; an electro-deionization device having an inlet in fluid communication with the permeate outlet of the reverse-osmosis device, and a purified water outlet; a first retentate flow path in fluid communication with the retentate outlet of the reverse-osmosis device, for removing retentate from the system, said first retentate flow path including a first flow rate regulator adapted to be remote controlled; a second retentate flow path in fluid communication with the retentate outlet of the reverse-osmosis device for recirculating retentate to the feed medium flow path at an upstream position of the pump, said second retentate flow path including a second flow rate regulator adapted to be remote controlled; a first flow meter downstream of the permeate outlet for detecting the permeate flow rate produced by the reverse-osmosis device; a first conductivity cell provided in the first retentate flow path for detecting the conductivity (ion concentration) of the retentate flow; a second conductivity cell provided in the feed medium flow path for detecting the conductivity (ion concentration) of the feed medium flow; and an automatic controller for controlling the first and second flow rate regulators (3,4) based on the detection results from the first flow meter and the first and second conductivity cells (15,16) such that a predetermined target recovery rate and a predetermined target permeate flow rate are controlled for the reverse-osmosis device.