Patent ID: 6113797
Filing Date: 2000-09-05
Classification: B01D,C02F

Abstract:
An improved two-stage single-pass high recovery membrane process for economical operation of semi-permeable reverse osmosis (RO) membranes which are used to purify water containing soluble and sparingly soluble inorganic compounds and achieve high recoveries of purified 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) blending the Influent water, containing soluble and sparingly soluble inorganic compounds which has been pretreated by a suitable filtration means to separate all suspended solids, with a small fraction of recycled ""softened"" and ""suspended solids' free"" second stage RO (i.e. RO-2) membrane concentrate stream containing high Total Dissolved Solids (TDS) from a second stage membrane system (i.e. RO-2) to produce a blended feed stream;(b) preconditioning the said blended feed stream by adding acid and antiscalant and then introducing the preconditioned blended feed stream into the high pressure side of a first stage RO membrane system (i.e. RO-1);(c) pressurizing said blended feed stream on said high pressure side of said RO-1 membrane to produce purified water permeate on the low pressure side substantially free of said inorganic compounds;(d) removing RO-1 ""concentrate"" containing preconcentrated soluble and sparingly soluble inorganic compounds from the high pressure side of said RO-1 membrane without precipitation of said preconcentrated sparingly soluble inorganic compounds on said RO-1 membrane;(e) recovering the bulk of purified water on the low pressure side of said RO-1 membrane at a rate at least 67% of the rate of introducing said water containing soluble and sparingly soluble inorganic compounds to the high pressure side of said RO-1 membrane;(f) blending the said RO-1 concentrate containing said pre-concentrated soluble and sparinglysoluble inorganic compounds with a large fraction of the said recycled ""softened"" and ""suspended solids' free"" RO-2 membrane concentrate stream to produce a blended RO-1 concentrate stream;(g) introducing said blended RO-1 concentrate stream to the high pressure side of the RO-2 membrane system;(h) pressurizing said blended RO-1 concentrate stream on said high pressure side of said RO-2 membrane system to produce purified water on the low pressure side substantially free of said inorganic compounds;(i) removing RO-2 membrane concentrate containing concentrated soluble and ""under-saturated"" sparingly soluble inorganic compounds from the high pressure side of said RO-2 membrane system without precipitation of said sparingly soluble inorganic compounds on the surface of the membrane;(j) recovering the remaining purified water on the low pressure side of said RO-2 membrane at a rate at least 5% of the rate of Influent water to be treated;(k) passing said RO-2 membrane concentrate through a suitable hardness-removal means to provide a ""softened"" and ""suspended solids' free"" RO-2 membrane concentrate stream containing high TDS;(l) splitting said ""softened"" and ""suspended solids' free"" RO-2 membrane concentrate into a small stream, which is recycled and blended with the pretreated Influent water and the said blended feed stream introduced to the high pressure side of said RO-1 membrane, and a large stream which is recycled and blended with said RO-1concentrate and said blended RO-1 concentrate introduced to the high pressure side of the said RO-2 membrane;(m) combining the purified water from said RO-1 membrane with purified water from said RO-2 membrane to provide the final purified water stream at a rate ranging from 67%-99.9% of the rate of the Influent water;(n) removing a small RO-2 membrane reject stream from said RO-2 membrane system in order to control the concentration of soluble inorganic compounds and hence the osmotic pressure.