An improved high recovery single stage double pass membrane process for the economical operation of semi-permeable reverse, osmosis (RO) membranes which are used to purify low flowrates of water containing low concentrations of soluble and sparingly soluble inorganic compounds and achieve high recoveries of very high purity water in the range 67%-99.9% of the Influent Water without precipitation of sparingly soluble inorganic scale compounds on the surface of the membrane, comprising:
- (a) pretreating the Influent Water using a suitable pretreatment means to separate all suspended solids, oil and grease and iron and produce pretreated water;
- (b) blending said pretreated water, containing soluble and sparingly soluble inorganic compounds, with a large, softened, membrane concentrate recycle stream containing a high concentration of Total Dissolved Solids (TDS), with a recycled purified water permeate from the low pressure side of the first pass membranes of said single stage membrane processed and with a relatively low TDS second pass RO membrane concentrate to produce a blended feed stream;
- (c) preconditioning said blended feed stream by adding acid and antisicalant in order to produce a preconditioned blended feed stream having a low scale forming tendency;
- (d) introducing said preconditioned blended feed stream into the high pressure side of the first pass membranes of said single stage double pass membrane process;
- (e) pressurizing said preconditioned blended feed stream on said high pressure side of said first pass membranes of said single stage double pass membrane process to a pressure in the range 100-3000 lb/square inch (psig), to produce purified water permeate on the low pressure side of said first pass membranes substantially free of said inorganic compounds;
- (f) removing an RO membrane concentrate stream containing pre-concentrated soluble and sparingly soluble inorganic compounds from said high pressure side of said first pass membranes of said single stage double pass membrane process without precipitation of said pre-concentrated sparingly soluble inorganic compounds on said first pass membranes;
- (g) recovering said purified water permeate on said low pressure side of said first pass membranes at a rate in the range 67%-99.9% of the rate of introducing said Influent Water containing said soluble and sparingly soluble inorganic compounds to said high pressure side of said first pass membranes of said single stage double pass membrane process;
- (h) introducing said RO membrane concentrate stream into an ion exchange water softening resin selected from a group consisting of chelating resins, weak acid cation exchange resins, strong acid cation exchange resins in their sodium form and a combination of fixed bed and continuous cation exchange systems which separate and retain the hardness due to said sparingly soluble inorganic compounds to provide a softened, high TDS RO membrane concentrate stream;
- (i) recycling said softened, high TDS RO membrane concentrate stream and blending it with said pretreated water, said recycled purified water permeate and said relatively low TDS second pass RO membrane concentrate to produce said blended feed stream which is preconditioned and introduced into said high pressure side of said first pass membranes of said single stage double pass membrane process;
- (j) removing a small RO membrane reject stream from said RO membrane concentrate stream at a rate in the range 0.1% to 33% of the rate of introducing said Influent Water to said high recovery single stage double pass membrane process in order to control the concentration of said soluble inorganic compound and hence the osmotic pressure;
- (k) disposing of said small RO membrane reject stream to drain or via other suitable waste disposal means;
- (l) recycling a fraction of said purified water permeate from said low pressure side of said first pass membranes of said single stage double pass membrane process to said high pressure side of said first pass membranes at a rate in the range 10%-100% of the rate of introducing said Influent Water to said high recovery single stage double pass membrane process, in order to produce high purity water on the low pressure side of said first pass membranes;
- (m) adjusting the pH of said high purity water to the neutral range 6-8 to produce pH-adjusted high purity water;
- (n) introducing said pH-adjusted high purity water into the high pressure side of second pass RO membranes of said single stage double pass membrane process;
- (o) pressurizing said pH-adjusted high purity water on the high pressure side of said second pass RO membranes to provide a very high purity water permeate on the low pressure side and to provide a relatively low TDS second pass RO membrane concentrate on the high pressure side of said second pass RO membranes;
- (p) removing said relatively low TDS second pass RO membrane concentrate from the high pressure side of said second pass RO membranes at a rate sufficiently high to ensure very high purity of said very high purity water permeate;
- (q) recovering said very high purity water permeate having an electrical resistivity >2 mega ohms from said low pressure side of said second pass RO membranes at a rate in the range 50%-90% of the rate of introducing said pH-adjusted high purity water into said high pressure side of said second pass RO membranes;
- (r) recycling said relatively low TDS second pass RO membrane concentrate from said high pressure side of said second pass RO membranes to said high pressure side of said first pass membranes of said single stage double pass membrane process to achieve further purification and further increase the recovery of said very high purity water permeate.