PURPOSE: To contemplate an economical effective use of water with saved natural source consumption, by producing pure water from water by-produced in the synthesis of gasoline by a reverse osmosis method after treatments such as neutralization, ditillation, etc.
CONSTITUTION: Gas generated from a reactor 1 for synthesizing an aromatic hydrocarbon mixture from methanol as a raw material, is cooled and condensed in a cooler 2, and separated into an oil phase and a water phase by a separator 3. The separated generated water, after neutralized, is carried into a distilling column 5, where methanol, acetone, etc., having lower boiling points than that of water, are separated from the water, and then into a reverse osmosis device 9. In the device 9, the water is pretreated, e.g. sterilized, pH-adjusted, etc., and then passed through a reverse osmosis membrane to obtain pure water. Thus, the effective use of water can be done.