Low energy reverse osmosis process
We provide a system and method for reverse osmosis treatment of water, including seawater and brackish water. Methods and systems of embodiments of the invention may include, for example, ultrafiltration followed by biofoulant removal, both of which precede reverse osmosis. In preferred embodiments the system is run at a low flux. For example, a flux of 6-8 GFD may be used with seawater. Additional embodiments may provide the above process in conjunction with a reverse-osmosis membrane cleaning system. The membrane cleaning system is a “clean in place” system that includes use of the natural pressure differential in the reverse osmosis system to remove biofoulants and their precursors.
1. A method for desalination by reverse osmosis and reverse osmosis membrane cleaning, comprising:
providing a feed stream of water to be desalinated;
filtering the feed stream through an ultrafiltration membrane, thereby removing turbidity including bacteria and viruses from said feed stream;
filtering the feed stream through positively charged media, thereby removing biofoulants, bacteria and viruses from said feed stream, wherein the step of filtering the feed stream through an ultrafiltration membrane is conducted prior to the step of filtering the feed stream through the positively-charged media; and
removing salt from the feed stream by reverse osmosis, wherein the reverse osmosis is conducted at an operating flux, wherein the operating flux results in an energy consumption, and wherein conducting the reverse osmosis at a flux lower than the operating flux does not reduce the energy consumption by more than 10%; and
cleaning at least one reverse osmosis membrane used in removing salt from the feed stream by reverse osmosis, comprising, for a reverse osmosis membrane having a feed side and a permeate side:
maintaining a concentration differential by adding reverse osmosis concentrate to the feed side of the reverse osmosis membrane, thereby allowing the reverse osmosis concentrate to remain in the feed side and causing a flow of a permeate water from the permeate side to the feed side due to the concentration differential.
2. The method of claim 1 , wherein filtering the feed stream through the ultrafiltration membrane reduces SDI of the feed stream to less than 3-5 SDI.
3. The method of claim 2 , wherein filtering the feed stream through the ultrafiltration membrane reduces turbidity of the feed stream to less than 0.1.
4. The method of claim 1 , wherein filtering the feed stream through the positively-charged media reduces turbidity to less than 0.08 NTU.
5. The method of claim 1 , wherein said cleaning is conducted based on a preset increase in differential pressure or number of hours of operation of the reverse osmosis membrane.
6. The method of claim 1 , wherein said method excludes addition of chlorine upstream of the ultrafiltration membrane, and wherein the method may include addition of chlorine during a backwash to avoid ingress of chlorine in feed steam to be desalinated.
7. The method of claim 1 , wherein said feed stream is seawater, and wherein said operating flux is between 6-8 GFD.
8. The method of claim 7 , wherein the TDS of the seawater is between 35,000 and 45,000 TDS.
9. The method of claim 1 , wherein said feed stream is brackish water, and wherein said operating flux is less than 10 GFD.
10. The method of claim 1 , wherein said feed stream is recycled waste water, and wherein said operating flux is less than 8 GFD.
11. The method of claim 1 , wherein conducting the reverse osmosis at a flux lower than the operating flux does not reduce the energy consumption by more than 5%.
12. The method of claim 1 , further comprising, after removing salt from the feed stream by reverse osmosis, sending a reverse osmosis reject to a thermal evaporator in a zero liquid discharge process.
13. A method for desalination of a feed stream by reverse osmosis, comprising:
preconditioning a feed stream of water for desalination by treatment in at least one member of the group consisting of a clarifier, a filter, and a dissolved air flotation unit to a turbidity of less than 8 NTU;
treating the feed stream by passage through ultrafiltration membranes;
backwashing the ultrafiltration membranes with ultrafiltration permeate water;
treating the feed stream by passage through positively-charged media for biofoulant removal, wherein the step of treating the feed stream by passage through ultrafiltration membranes occurs before the step of treating the feed stream by passage through the positively-charged media;
feeding the feed stream to a reverse osmosis membrane unit having an operating flux, wherein the operating flux results in an energy consumption, and wherein conducting the reverse osmosis at a flux lower than the operating flux does not reduce the energy consumption by more than 5%; and, producing a desalinated reverse osmosis permeate stream and a reverse osmosis reject stream;
recovering pressure from the reverse osmosis reject stream;
cleaning at least one reverse osmosis membrane used in removing salt from the feed stream by reverse osmosis, comprising, for a reverse osmosis membrane having a feed side and a permeate side:
further cleaning comprising reverse osmosis concentrate on the feed side and reverse osmosis permeate on the permeate side, and regulating flow to the feed side of the reverse osmosis membrane, thereby allowing the reverse osmosis concentrate to remain in the feed side and causing a flow of reverse osmosis permeate from the permeate side to the feed side due to a concentration differential maintained by adding reverse osmosis concentrate to the feed side.
14. The method of claim 13 , comprising enhancing said backwashing of the ultrafiltration membranes by addition of chlorine and caustic soda.
15. The method of claim 13 , wherein said feed stream is seawater, and wherein said flux is between 6-8 GFD.
16. The method of claim 13 , wherein said feed stream is brackish water, and wherein said operating is less than 10 GFD.
17. The method of claim 13 , wherein said feed stream is recycled reverse osmosis reject water, and wherein said operating flux is less than 8 GFD.