Desalination method and apparatus utilizing nanofiltration and reverse osmosis membranes

A method of desalinating seawater comprising the steps of:

• locating a reverse osmosis (RO) element upstream of a nanofiltration (NF) element for series flow in a single pressure vessel having an inlet and a concentrate outlet;
• pumping a seawater feed containing monovalent ions and divalent ions in series flow through the RO element and then through the NF element in said pressure vessel at a pressure which is below the osmotic pressure of seawater so that substantially no permeation occurs through the RO element, but which is such that the NF element creates a first concentrate stream and a NF permeate stream having a highly reduced concentration of divalent ions and also a reduced concentration of monovalent ions;
• collecting and temporarily storing said NF permeate stream;
• and
• interrupting said pumping of said seawater feed and instead pumping said temporarily stored NF permeate through said pressure vessel inlet at a pressure above said NF permeate's osmotic pressure to cause permeation to occur through the RO element while blocking permeate flow within the NF element which creates an RO permeate stream of potable water and a second concentrate stream the entire volume of which flushes the feed side of the NF element.

Apparatus for producing potable water from an aqueous feed containing high concentrations of monovalent and divalent ions, which apparatus comprises:

• a pressure vessel having inlet and outlet ends and containing at least one nanofiltration (NF) element, which selectively rejects divalent ions, and at least one reverse osmosis (RO) element located upstream of said NF element, which rejects a high percentage of monovalent and divalent ions;
• said RO element including a membrane which rejects at least about 98% of dissolved salt from a feed containing about 2,000 TDS at a pressure between about 150 and 225 psig and provides a flow rate of at least about 30 gallons per square foot per day and said NF element being formed with membrane which rejects at least about 95% of divalent ions and about 50 to 55% of monovalent ions in an aqueous feed containing about 2,000 ppm of magnesium sulfate and about 2,000 ppm of sodium chloride at a pressure of about 70 to about 100 psig and provides a flow rate of at least about 25 gallons per square foot per day;
• energy recovery pumping means;
• a pump for delivering the aqueous feed through said energy recovery pumping means to said inlet end of said pressure vessel at a pressure below that at which permeation occurs through said RO element;
• means connecting said pump to a source of said aqueous feed so as to cause flow thereof through said pressure vessel sequentially first to said RO element and then to said NF element, whereby said NF element separates said aqueous feed into a first concentrate stream and an NF permeate stream which is substantially reduced in divalent ion content;
• an intermediate liquid storage tank;
• means for directing said NF permeate stream to said intermediate storage tank;
• means for
• (a) interrupting the flow of said aqueous feed from the source;
• (b) connecting said pump so as to draw liquid from said intermediate storage tank and thus pump said NF permeate back through said pressure vessel, and
• (c) blocking permeate flow within said NF element when said pump draws liquid from said intermediate storage tank,
• so that said pumped flow of said NF permeate from said intermediate storage tank through said pressure vessel flushes said NF element and results in said RO element separating said NF permeate into a potable water permeate stream and a second concentrate stream;
• and
• means for directing each said concentrate stream exiting said pressure vessel to said energy recovery pumping means to power same.