Liquid purification or separation – Processes – Liquid/liquid solvent or colloidal extraction or diffusing...
Reexamination Certificate
2000-02-22
2002-10-08
Kim, John (Department: 1723)
Liquid purification or separation
Processes
Liquid/liquid solvent or colloidal extraction or diffusing...
C210S638000, C210S641000, C210S650000, C210S651000, C210S660000, C210S806000, C210S702000, C210S723000, C210S724000, C210S749000, C210S805000
Reexamination Certificate
active
06461514
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to the economical purification of water containing soluble and sparingly or partially soluble inorganic compounds using a single-stage membrane process with a unique recycle of ion exchange “softened” membrane concentrate stream.
Hardness compounds such as barium, calcium, magnesium, iron, silica, carbonate and bi-carbonate, fluoride and sulfate are commonly found in surface water supplies such as lakes and rivers as well as groundwater supplies such as water wells and aquifers and in aqueous industrial effluents and landfill leachates. Water containing hardness compounds is frequently purified by using water softeners and demineralizers in the form of “ion exchange resins, IX”, chemical softeners using the cold lime or hot lime softening process, reverse osmosis (RO) membranes, nanofiltration (NF) membranes and/or distillation. Industry needs purified water containing low to very low concentrations of hardness compounds and of soluble inorganic compounds in order to supply their cooling towers, low-pressure and high pressure boilers, heat exchangers and various process uses. On the other hand, the pharmaceutical and electronics industries as well as hospitals and laboratories require high purity water that is almost completely free from inorganic compounds.
The conventional water treatment processes listed above are not suitable because, in the case of DX, the process involves the inefficient transfer of soluble and “sparingly soluble” water impurities to a resin bed which must be regenerated and/or disposed of at high cost. In the case of lime softening, large quantities of chemicals are added and large chemical waste volumes are generated. If conventional RO or NF membranes are used, substantial volumes of RO or NF membrane concentrates will be generated because the permeate recovery (percentage) from these processes is normally limited to approximately 70%-75% and the concentrates must therefore be treated further or disposed of at a large cost. Finally, the very high capital and/or operating costs associated with the direct application of distillation processes normally precludes the use of distillation as a single-step treatment method.
Although membrane filtration processes such as reverse osmosis (RO) and nanofiltration (NF) provide effective and economically viable methods for purifying water, these membrane processes are currently limited in the percentage of purified water produced (known as the permeate recovery or product recovery), by scale formation. Most of the soluble and scale-forming compounds are separated by the membranes and concentrated into a smaller volume, typically 20-30% of the volume of the original water stream. This membrane concentrate volume is normally too large and too costly to dispose of, except in the case of seawater desalination applications where the concentrate stream is returned to sea or where there are no regulatory limits on the concentration of inorganic compounds in the effluent. The main reason why further recovery of purified water from “conventional” RO and NF membranes is not possible is the tendency of inorganic scale such as calcium carbonate and silica to form on the surface of the membranes as the concentration of these compounds is increased beyond their saturation values. Deposition of such compounds frequently results in the loss of purified water production (also known as loss of permeate flux through the membrane) and the eventual need for costly replacement of the membranes.
The use of chemical additives in the water supply such as acids to reduce the pH and inorganic or organic scale inhibitor compounds is practiced in the water treatment arid membrane industry in order to provide some improvement in the water recovery and prevent scale formation. However, such improvement is of limited extent since no scale inhibitor is effective for all the contaminants nor for all permeate recovery ranges and therefore they do not represent an economically viable option for the treatment of the entire water stream.
A survey of prior art shows the following patents:
U.S. Pat. No. 4,000,065 discloses the use of a combination of reverse osmosis (RO) and ultrafiltration (UF) to separate organic material from the aqueous stream. The contaminated aqueous stream is circulated from the high pressure compartment of an RO unit to the high pressure compartment of a UF unit, then to the low pressure compartment of the UF unit and then back to the high pressure compartment of the RO unit.
Japanese Patent 57-197085 discloses a filtration apparatus that comprises connecting UF apparatus and RO apparatus in series so as not to deposit scale on the RO membrane.
U.S. Pat. No. 3,799,806 discloses purification of sugar juices by repeated ultrafiltration and reverse osmosis purification steps.
U.S. Pat. No. 4,083, 779 discloses a process for treatment of anthocyanate extract by ultrafiltration and reverse osmosis treatments.
U.S. Pat. No. 4,775,477 discloses a process for extraction of cranberry presscake wherein the presscake is ground and subjected to microfiltration to remove colloidal high molecular weight compounds followed by reverse osmosis to recover a red-colored solution.
U.S. Pat. No. 5,182,023 discloses a process for removing arsenic from water wherein the water is first filtered to remove solids then passed through an ultrafilter, followed by a chemical treatment to adjust pH to a range from about 6 to 8. Thereafter, scale-inhibitors and anti-fouling materials are added before subjecting the water to reverse osmosis to provide a stream having less than about 50 ppb of arsenic.
Japanese Patent 53025-280 discloses the separation of inorganic and organic compounds from a liquid by first using a reverse osmosis membrane and then using a second reverse osmosis membrane having a more permeable membrane such as a microporous or ultrafiltration membrane. Part of the contaminated liquid obtained from the first membrane is processed through the second membrane.
U.S. Pat. No. 5,501,798 discloses a high recovery water purification process involving the use of reverse osmosis followed by chemical precipitation of hardness compounds from the RO concentrate followed by microfiltration to separate precipitated solids and recycling of the “suspended solids' free concentrate” back to the RO.
The above-referenced patents and other available literature have aimed at preventing precipitation of inorganic scale and other membrane fouling compounds as the water is treated by reverse osmosis membranes. As the concentration of scale and other inorganic and organic fouling compounds build up on the surface of the RO membranes, the purified water permeation rate deteriorates with eventual irreversible loss of productivity and need for costly membrane replacement. Prior art teaches acidification (i.e. pH reduction) as a means of reducing the potential of calcium carbonate scale formation. Prior art also teaches the addition of scale inhibitors such as polyacrylic acids and sequestering agents such as ethylene diamine tetracetic acid (EDTA) and sodium hexametaphosphate (SHMP) in order to reduce the scale formation potential due to barium sulfate, calcium fluoride, calcium and magnesium carbonate and sulfate and silica. However, these scale inhibitor compounds are not sufficiently efficient to allow very high water recoveries and concentration factors to be achieved. Maximum recoveries in the presence of scale inhibitors may be in the range 70%-75%, based on the treatment of hard “well-water”. The above-referenced patents also teach the separation of suspended solids existing originally in natural water sources and industrial effluents or the separation of chemically-precipitated compounds using ultrafiltration or microfiltration before reverse osmosis treatment. While removal of suspended solids by membrane filtration will prevent fouling of the RO membranes, it does not prevent concentration and eventual deposition of the initially soluble scale compounds, as the recovery of purified water is increased using
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