Desalination apparatus and desalination method

A desalination apparatus comprising:

• a high-pressure first reverse osmosis membrane device having a first reverse osmosis membrane that removes a salt content from raw water supplied with predetermined pressure to produce first permeated water;
• a low-pressure second reverse osmosis membrane device having a second reverse osmosis membrane that removes a salt content in the first permeated water supplied from the first reverse osmosis membrane device to produce second permeated water;
• a first flow regulation valve that is mounted in a discharge line for discharging first concentrated water supplied from the first reverse osmosis membrane device and regulates flow rate of the first concentrated water;
• a second flow regulation valve that is mounted in a first return line for returning second concentrated water supplied from the second reverse osmosis membrane device to a preceding stage of the first reverse osmosis membrane device and regulates flow rate of the second concentrated water;
• and
• a control device that measures a supply temperature of the raw water by a thermometer, and controls the first flow regulation valve and the second flow regulation valve depending on the measured temperature, wherein
• when the measured temperature of the raw water is lower than a set temperature, the control device regulates the first flow regulation valve to maintain a discharge amount of the first concentrated water constant and regulates the second flow regulation valve to reduce the flow rate of the second concentrated water to be returned so as to perform operation such that supply flow rate for the first reverse osmosis membrane device is controlled to be lower than supply flow rate applied at an increased temperature and predetermined product water recovery ratio is maintained.