Seawater desalination system and energy recovery apparatus

An energy recovery apparatus for exchanging pressure energy between concentrated seawater discharged from a reverse-osmosis membrane-separation apparatus and a part of seawater to be treated by the reverse-osmosis membrane-separation apparatus in a seawater desalination system for producing fresh water from the seawater by supplying the seawater to the reverse-osmosis membrane-separation apparatus to separate the seawater into fresh water and concentrated seawater, the energy recovery apparatus comprising:

• a plurality of chambers configured to pressurize the seawater by pressure energy of the concentrated seawater by supply and discharging the concentrated seawater and the seawater;
• a first flowmeter used for integrating a flow rate of the seawater or the concentrated seawater which flows into the chamber;
• a second flowmeter used for integrating a flow rate of the seawater or the concentrated seawater discharged from the chamber;
• at least one switching valve provided in each of the plurality of chambers and configured to switch the inflow of the concentrated seawater to the chamber and the discharge of the concentrated seawater from the chamber;
• and
• a controller configured to control the respective switching valves of the plurality of chambers on the basis of integrated flow rate of the chamber, the integrated flow rate of the chamber being obtained on the basis of the flow rate of the first flowmeter and/or the second flowmeter;
• wherein the controller controls the respective switching valves of the plurality of chambers by a trapezoidal waveform signal whose signal value varies from −100% to +100% with time, valve openings of the respective switching valves being determined by inputting signal values of the trapezoidal waveform signal to predetermined formulas of the relationship between signal values of the trapezoidal waveform signal and valve openings of the respective switching valves.