Multi-Valent Ion Concentration Using Multi-Stage Nanofiltration

Abstract

A system and method for producing from saline source water a product containing an increased ratio of multi-valent ions to mono-valent ions, which includes multiple nanofiltration units arranged to selectively remove mono-valent ions from the water fed into each nanofiltration stage in the nanofiltration permeate stream while retaining multi-valent ions in the nanofiltration reject stream. The rate at which the increase in the multi-valent ion- to mono-valent ion ratio is obtained may be enhanced by introduction of lower salinity water into the nanofiltration reject between stages, and by recirculating a portion of downstream nanofiltration reject flow into an upstream nanofiltration unit. The enhanced multi-valent ion product is suitable for multiple uses, including irrigation of plants and remineralization of desalinated water. The relative concentrations of the multi-valent ions in the product may be adjusted, for example by selection of nanofiltration membrane technologies which have higher or lower rejection for specific multi-valent ions.

What is claimed is: 
     
         1 . A multi-valent ion concentration system, comprising:
 a plurality of nanofiltration units,   wherein
 at least one of the plurality of nanofiltration units is configured to
 reject in a nanofiltration reject stream multivalent ions from a feed water supplied to the at least one of the nanofiltration units, and 
 pass in a nanofiltration permeate stream mono-valent ions from the feed water supplied to the at least one of the nanofiltration units, 
 
 a first one of the plurality of nanofiltration units is configured to receive saline source water as the feed water to the first nanofiltration unit, 
 the plurality of nanofiltration units are interconnected such that the nanofiltration reject stream from the first nanofiltration unit is the feed water to a second one of the plurality of nanofiltration units, and 
 the plurality of nanofiltration units are interconnected such that the nanofiltration reject stream from at least one of the plurality of nanofiltration units is diluted by lower salinity water having a salinity lower than a salinity of the saline source water. 
   
     
     
         2 . The multi-valent ion concentration system of  claim 1 , wherein
 between the first one of the plurality of nanofiltration units and a last one of the plurality of nanofiltration units, each pair of upstream and downstream ones of the plurality of nanofiltration units are interconnected with the nanofiltration reject of the upstream nanofiltration unit being the feed water for the downstream nanofiltration unit.   
     
     
         3 . The multi-valent ion concentration system of  claim 2 , wherein
 each pair of the upstream and downstream ones of the plurality of nanofiltration units are interconnected such that the nanofiltration reject stream of the upstream nanofiltration unit is diluted by the lower salinity water.   
     
     
         4 . The multi-valent ion concentration system of  claim 3 , wherein
 an amount of dilution by the salinity water is such that a predetermined scaling risk limit is not exceeded.   
     
     
         5 . The multi-valent ion concentration system of  claim 5 , wherein
 the predetermined scaling risk limit is 250% or more.   
     
     
         6 . The multi-valent ion concentration system of  claim 1 , wherein
 the nanofiltration reject stream from a last one of the plurality of nanofiltration units has a ratio of multi-valent ions to mono-valent ions that is increased by a predetermined percentage above the ratio of multi-valent ions to mono-valent ions in the saline source water.   
     
     
         7 . The multi-valent ion concentration system of  claim 5 , wherein
 the nanofiltration reject stream from a last one of the plurality of nanofiltration units has a ratio of multi-valent ions to mono-valent ions that is increased by a predetermined percentage above a ratio of multi-valent ions to mono-valent ions in the saline source water.   
     
     
         8 . The multi-valent ion concentration system of  claim 6 , wherein
 the predetermined percentage of ratio increase is 200%.   
     
     
         9 . The multi-valent ion concentration system of  claim 7 , wherein
 the predetermined percentage of ratio increase is 200%.   
     
     
         10 . The multi-valent ion concentration system of  claim 1 , wherein
 at least a portion of the nanofiltration reject stream from a downstream one of the plurality of nanofiltration units is recirculated into the feed water of an upstream one of the plurality of nanofiltration units   
     
     
         11 . The multi-valent ion concentration system of  claim 10 , wherein
 the nanofiltration reject stream portion is recirculated into the saline source water that is the feed water to the first nanofiltration unit.   
     
     
         12 . The multi-valent ion concentration system of  claim 10 , wherein
 the at least one of the plurality of nanofiltration units which is the source of the recirculated portion of NF reject stream is the last of the nanofiltration units.   
     
     
         13 . The multi-valent ion concentration system of  claim 11 , wherein
 the at least one of the plurality of nanofiltration units which is the source of the recirculated portion of NF reject stream is the last of the nanofiltration units.   
     
     
         14 . The multi-valent ion concentration system of  claim 11 , wherein
 a last one of the plurality of nanofiltration units is interconnected with a downstream concentration unit configured to concentrate the nanofiltration reject stream from the last nanofiltration unit.   
     
     
         15 . The multi-valent ion concentration system of  claim 1 , wherein
 the plurality of nanofiltration units are interconnected such that the nanofiltration permeate stream from at least one of the plurality of nanofiltration units is the feed water to a downstream one of the plurality of nanofiltration units, and   
     
     
         16 . The multi-valent ion concentration system of  claim 15 , wherein
 the at least one of the plurality of nanofiltration units whose nanofiltration permeate stream is the feed water of the downstream one of the plurality of nanofiltration units is the next-to last nanofiltration unit.   
     
     
         17 . The multi-valent ion concentration system of  claim 15 , wherein
 the at least one of the plurality of nanofiltration units whose nanofiltration permeate stream is the feed water of the downstream one of the plurality of nanofiltration units is the first nanofiltration unit.   
     
     
         18 . The multi-valent ion concentration system of  claim 15 , wherein
 the plurality of nanofiltration units is interconnected such that at least a portion of the nanofiltration reject stream from the next-to last nanofiltration unit is recirculated into the feed water of at least one upstream one of the plurality of nanofiltration units.   
     
     
         19 . A method for increasing a ratio of multi-valent ions to mono-valent ions in a multi-stage nanofiltration system, comprising the acts of:
 providing a plurality of nanofiltration units, at least one of the plurality of nanofiltration units being configured to
 reject in a nanofiltration reject stream multivalent ions from a feed water supplied to the at least one of the nanofiltration units, and 
 pass in a nanofiltration permeate stream mono-valent ions from the feed water supplied to the at least one of the nanofiltration units; 
   receiving at a first one of the plurality of nanofiltration units a saline source water;   increasing in the first nanofiltration unit a ratio of multi-valent ions to mono-valent ions by nanofiltration;   diluting a nanofiltration reject from the first nanofiltration unit with a lower salinity water having a salinity lower than a salinity of the saline source water;   receiving at a second one of the plurality of nanofiltration units the diluted first nanofiltration unit nanofiltration reject; and   increasing in the second nanofiltration unit the ratio of multi-valent ions to mono-valent ions by nanofiltration.   
     
     
         20 . A method for increasing a ratio of multi-valent ions to mono-valent ions in a multi-stage nanofiltration system, comprising the acts of:
 providing a plurality of nanofiltration units, at least one of the plurality of nanofiltration units being configured to
 reject in a nanofiltration reject stream multivalent ions from a feed water supplied to the at least one of the nanofiltration units, and 
 pass in a nanofiltration permeate stream mono-valent ions from the feed water supplied to the at least one of the nanofiltration units; 
   receiving at a first one of the plurality of nanofiltration units a saline source water;   increasing in the first nanofiltration unit a ratio of multi-valent ions to mono-valent ions by nanofiltration;   diluting a nanofiltration reject stream from an upstream one of the plurality of nanofiltration units with a lower salinity water having a salinity lower than a salinity of the saline source water;   receiving at a one of the plurality of nanofiltration units downstream from the upstream one of the plurality of nanofiltration units the diluted nanofiltration reject stream of the upstream one of the plurality of nanofiltration units; and   increasing in the downstream one of the nanofiltration units the ratio of multi-valent ions to mono-valent ions by nanofiltration.