Two stage nanofiltration seawater desalination system
The present invention is directed to a method and apparatus for desalinating seawater utilizing a two stage seawater desalination system, a first stage including at least one high performance nanofiltration membrane to receive seawater feed pressurized by a first stage pump sufficiently and to produce a first permeate, and a second stage including at least one high performance nanofiltration membrane to receive the first permeate pressurized by a second stage pump to between about 200 psi and about 300 psi to produce potable water.
1. A method for desalinating seawater, comprising:
pumping filtered seawater having a total dissolved solids content of not less than 35,000 mg/l feed through a first stage desalination operation consisting of one or more nanofiltration membranes to produce a first permeate and a first retentate, where the first permeate exiting the first stage desalination operation has a total dissolved solids content of no more than twelve percent compared with a total dissolved solids content of seawater immediately prior to entering said first stage desalination operation; and
pumping said first permeate having a salinity approximately the same as that which exited the first stage desalination operation through a second stage desalination operation consisting of one or more nanofiltration membranes to produce a second permeate and a second retentate, where the second permeate exiting the second stage desalination operation has a total dissolved solids content of no more than 500 mg/l.
2. The method for desalinating seawater of claim 1 where the first and second stage desalination operations employ no more than five nanofiltration membranes for each stage.
3. The method for desalinating seawater of claim 1 wherein the pumping of the filtered seawater through the first stage desalination operation occurs at a pressure of less than 600 psi.
4. The method for desalinating seawater of claim 1 where at least a portion of the second retentate is added to the filtered seawater prior to said filtered seawater being pumped through said first stage desalination operation.
5. The method for desalinating seawater of claim 1 where energy is recovered by depressurizing the first retentate.