US 6,113,797 A · High water recovery membrane purification process

Abstract

Disclosed is a two-stage high pressure high recovery process utilizing two reverse osmosis membrane systems intended to provide very high overall water recoveries from contaminated inorganic scale-containing water in an economical manner while preventing scale formation on the membrane and prolonging the useful life of the membrane. The first stage of the process involves using a low pressure membrane system to preconcentrate scale compounds while purifying the bulk volume of the scale-containing water (using antiscalants with pH control) and combining the influent water with a recycle stream of concentrate from the second stage membrane process in order to prevent scale formation. This first stage membrane system is followed by a second stage membrane system wherein the concentrate from the first stage membrane system is treated further at higher pressure in order to provide purification of the remaining preconcentrated stream and achieve very high overall water recoveries. Scale formation in the first stage and second stage membrane systems is prevented by the recycle of "softened" water containing a very low concentration of scale-forming compounds and containing a high concentration of soluble salts to both stages in a specific proportion that represents an economically viable process, while ensuring adequate permeate quality. The second stage membrane concentrate stream is "softened" whereby hardness compounds such as calcium, magnesium, carbonate, sulphate, silica and other sparingly soluble compounds are removed by suitable device before it is recycled to the two membrane purifying stages.

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.

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High water recovery membrane purification process

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High water recovery membrane purification process

Life of this patent

Abstract

Claims40