WO2000000273A1

WO2000000273A1 - Method of removing sulfate and/or metal ions from waters or wastewaters

Info

Publication number: WO2000000273A1
Application number: PCT/US1999/014809
Authority: WO
Inventors: Dennis H. Green; Jeff Meuller
Original assignee: Hw Process Technologies, Inc.
Priority date: 1998-06-29
Filing date: 1999-06-29
Publication date: 2000-01-06
Also published as: AU5086599A

Abstract

The present invention is directed in one embodiment to a process in which a feed (5) containing dissolved multivalent contaminants is subjected to membrane filtration (10). The first retentate (14) is subjected to a precipitation process (18) to remove a large portion of the contaminants. The supernate (30) is subjected to further membrane filtration (34). The second retentate (38) is recycled to the first retentate (14), and the first and second permeates (46) and (42) are discharged.

Description

METHOD OF REMOVING SULFATE AND/OR METAL IONS FROM WATERS OR WASTEWATERS

FIELD OF THE INVENTION The present invention is related generally to a process for purifying effluents and specifically to a process for treating an aqueous effluent containing one or more dissolved multivalent contaminants.

BACKGROUND OF THE INVENTION There is growing interest in many countries to enact legislation placing strict controls on metal and/or sulfate emissions in aqueous effluents. By way of example, the Environmental Protection Agency or EPA proposed, in December, 1994, a Sulfate Rule, or Safe Drinking Water Act standard, requiring a maximum of a 500 mg sulfate/L discharge. Research on this Rule must be completed by January 1999, so interest in finding a viable solution to purifying effluents to this standard or BAT, "best available technology", is keen. In addition, metals discharge levels for arsenic, as well as lead, copper, cadmium, and nickel are low, as mandated by EPA regulations. World Bank and World Health Organization environmental guidelines also recommend low metal and sulfate discharge levels. Moreover, many industrial processes require the use of low sulfate water. By way of example in gold and other metal leaching operations, recycling or recovery of contaminated process water is not possible without a cost-effective sulfate removal technology.

The most prevalent technology to remove metal ions from water and wastewater is precipitation. Through addition of lime, soda ash, sodium hydroxide, or other such reagents, metals are precipitated as insoluble hydroxides, carbonates, sulfides, and sulfates. Through careful control of reagent addition, mixing, flocculation, settling, and residence time, low metal discharge levels can be obtained. However, since traditional (least expensive) hydroxide (lime) and carbonate (limestone or soda ash) alkaline reagents form metal complexes that are less soluble than sulfate-metal complexes, a portion of the sulfate ions remain in the precipitation system discharge water (supernate). For example, the contribution of sulfate from theoretical calcium sulfate solubility is commonly about 1,500 mg sulfate/L. This is a limiting factor for minimum sulfate concentration when neutralizing acidic wastewaters by precipitation. Typically, sulfate remaining in a high sulfate water or wastewater after limestone, lime, and/or soda ash precipitation is in the 1 ,200 - 10,000 mg SO₄ L range, depending on incoming water or wastewater quality and the amount of precipitant added. This fundamental failure to meet a (500 mg/L) sulfate discharge standard with conventional precipitation technology has generated growing interest in new technologies for sulfate ion removal.

New technologies under consideration for sulfate ion removal include precipitation with more expensive reagents, ion exchange, biological oxidation, and membrane technology. Each of these processes fail to provide a low cost, simple, and/or effective solution to sulfate contamination. Precipitation for sulfate removal may be practiced using reagents which have more insoluble sulfate complexes than the reagent complex being added. For example, addition of barium carbonate reagent will result in precipitation of barium sulfate. This method of sulfate removal is typically not used due to the high cost of the reagent and the large increase in sludge volume to be disposed. For example, barium carbonate costs ~ $0.61/lb, and removal of 5 g/L sulfate in a 1000 gpm feed water represents a ~$36.00 /

1,000 gallon treatment cost in reagent alone. In addition, it is difficult to get barium carbonate into solution for the reaction to take place. Hot water and rapid mixing are required . Other sulfate removing precipitation reagents may include strontium carbonate or, at high pH (12-13), aluminum sulfate or calcium aluminate. Sulfate removal using calcium aluminate ("the Walhalla process") has been successful in limited applications.

The process works best with sulfate levels of 0.5 - 5 g SO₄/L, with sodium concentrations of less than 500-1000 mg/L. The Walhalla process is a three-step process involving lime precipitation, calcium aluminate precipitation at pH 11.2 with lime consumption, and post- precipitation carbonation/calcium carbonate precipitation with carbon dioxide. Operating costs are ~ $5.00/1000 gallons for 1000 gpm of a water with 5 g SO₄ /L and less than 500 mg/L sodium. The majority of the cost is the $0.22/lb for the calcium aluminate reagent.

Ion exchange technology for removal of sulfate via an anion exchange resin is also expensive. The operating cost is estimated at > $5.00 / 1,000 gallon treatment cost using conventional anion exchange resins for removal of 5 g SO₄/L from a wastewater. In addition, competing ions, such as chloride, and organic fouling of the weak base resins can lower ion exchange capacity for the sulfate ion when processing many waters. A new, continuous ion-exchange process, the GYP-CDC process, utilizes patented cation and anion ion-exchange resins and has a published operating cost of ~$2.00 / 1,000 gallons. The technical feasibility of this process is somewhat in doubt, due to the necessity of handling ion exchange resins in a gypsum environment (the calcium sulfate fouls/plugs the resin beads).

Biological oxidation technology such as PAQUES's THIOP AQ® sulfate reducing and hydrogen sulfide oxidizing reactors has been successfully demonstrated on pilot plant and commercial scale to remove metals and sulfates to below discharge limits. Typically, sulfate is reduced by bacteria utilizing hydrogen in an anaerobic sulfate reducing reactor. The reduction produces hydrogen sulfide and water. The hydrogen sulfide is used to precipitate heavy metals, while excess hydrogen sulfide is oxidized to form elemental sulfur. Published operating costs are $330 per ton of sulfate removed. This is about $7.00/1000 gallons for conversion and precipitation of a 5 g sulfate/L wastewater at 1000 gpm. The THIOP AQ® operating cost is directly proportional to the amount of sulfate to be converted, as hydrogen gas is added one-to-one for sulfate. For example, the

THIOP AQ® operating cost would be ~$70.00 / 1,000 gallons for conversion of 50 g SO4/L. THIOP AQ® plant footprint size, handling hydrogen and hydrogen sulfide gases, and sustaining the bacteria in toxic metal and cold weather environments are additional factors that must be considered when evaluating this technology. Membrane technology, particularly thin film nanofiltration membranes have been shown to be very effective in sulfate and metal ion removal. Pilot plant and commercial scale installations have demonstrated that sulfate and metal ions can be separated into a retentate stream and a clean permeate stream, even in harsh mining water environments containing high metal and sulfate-ion levels. Typically, about 98-99% of the sulfate ions are rejected by nanofiltration membranes. However, such membranes do not obviate the need to dispose of aqueous waste solutions containing sulfates. Membranes simply reduce the volume (and increase the sulfate concentration) in the aqueous waste solutions.

SUMMARY It is an objective of the process of the present invention to provide a purification process for aqueous effluents that inexpensively, simply and or effectively removes sulfates and other multivalent contaminants to levels low enough to comply with pertinent environmental regulations. Related objectives include providing a purification process that removes sulfates using inexpensive reagents at ambient temperature, that does not use ion exchange resins in a gypsum environment, that can be performed using a plant having a small footprint size, that does not use toxic and dangerous reagents such as hydrogen and hydrogen sulfide gases, that is effective on biologically toxic solutions, and that removes the sulfates in the form of a sludge or solid.

These and other objectives can be addressed by the process of the present invention. In one embodiment, the process includes the steps of: (a) filtering an aqueous effluent containing one or more dissolved multivalent contaminants to form a permeate and a retentate, the retentate containing at least most of the one or more dissolved multivalent contaminants in the aqueous effluent;

(b) contacting the retentate with a precipitant to precipitate a dissolved multivalent contaminant and form a treated retentate; (c) removing at least most of the precipitated multivalent contaminant, preferably in the form of a solid or sludge; and

(d) thereafter further filtering the treated retentate to form a second permeate and a second retentate containing at least most of the multivalent contaminants in the aqueous effluent. This process synergistically combines the advantages of membrane filtration and contaminant precipitation technologies to provide surprising and unexpected results.

Specifically, the retentate experiences enhanced precipitation due to higher metal-ion concentrations and coprecipitation, reducing reagent consumption in the precipitation process and lowering precipitation operating costs. The membrane permeate stream(s) are discharged into the environment or re-used as process water. This process can effectively solve the problem of meeting stringent sulfate and metal ion discharge regulations or industrial process water reuse requirements.

The dissolved multivalent contaminant can be any number of metal ions, compounds or complexes incorporating metals in the multivalent state. Exemplary metals include group 11(A) metals, transition metals including without limitation nickel, copper, cobalt, zinc, cadmium, iron, manganese, chromium and silver, group III(A) metals (e.g., thallium), group IV(A) metals (e.g., lead) as well as metalloids/semimetals within groups V(A) and VI(A) including without limitation arsenic and selenium. Preferably, the contaminant is a sulfate compounded with any of the previously described metals such as one or more of copper sulfate, ferric sulfate, ferrous sulfate, zinc sulfate, nickel sulfate, cobalt sulfate, cadmium sulfate, lead sulfate, manganese sulfate, aluminum sulfate, magnesium sulfate, and calcium sulfate.

The aqueous effluent can be derived from any source, such as industrial processes, mining processes, acid rock drainages, landfill drainage, and general wastewaters. The effluent typically has dissolved multivalent contaminant concentrations from as low as 500 ppm to as high as 250,000 ppm and a pH ranging from about pH 1 to about pH 14.

The filters in steps (a) and (d) are preferably a membrane filter and more preferably a nanofilter, a reverse osmosis filter, or an ultrafilter, with a nanofilter being even more preferred. The preferred nanofilter is described in U.S. Patents 5,476,591 to Green, 5,733,431 to Green et al., 5,310,486 to Green et al., and 5,279,745 to Jeffers et al.; copending U.S. Patent Application Serial Nos. 08/871,176, filed June 9, 1997, and

09/183,683, filed October 30, 1998, and PCT US98/11476, filed June 4, 1998, all of which are incorporated herein by reference in their entireties. The membrane filter preferably has a negative charge to reject multivalent anions and their associated cations or a positive charge to reject multivalent cations and their associated anions. In one embodiment, discrete filters are used in steps (a) and (d) and can be substantially identical.

The filtration in the various steps is preferably performed so as to cause at least about 50% by volume of the aqueous effluent to be in the permeate and less than about 50% by volume of the aqueous effluent to be in the retentate.

The filtration can be highly effective in separating multivalent contaminants into the retentate. Preferably, the permeate contains no more than about 5% of the one or more dissolved multivalent contaminants in the aqueous effluent while the retentate contains at least about 95% of the one or more dissolved multivalent contaminants in the aqueous effluent.

The precipitant can be any compound capable of forming a precipitate with the one or more of the ionic components of the dissolved multivalent contaminants, such as the sulfate anion and/or the multivalent cation associated with the sulfate anion. Preferred precipitants include at least one of a hydroxide, an oxide other than a hydroxide, a carbonate, a sulfide, an aluminate, or a metal sulfate different from the metal sulfate to be precipitated. More preferred precipitants include metal hydroxides, metal carbonates, and metal aluminates, with alkaline metal hydroxides and carbonates being even more preferred. These precipitants typically precipitate the multivalent contaminant as a metal hydroxide, carbonate, sulfate, and/or an aluminate.

In a particularly preferred embodiment, the precipitation step includes two substeps. In the first substep, a sufficient amount of a hydroxide precipitant, preferably calcium hydroxide, is added to precipitate at least about 50% and more preferably at least about 75% of the sulfate anions in the effluent. Preferably the amount of hydroxide precipitant added to the effluent is about 100% and more preferably about 75% and most preferably at least about 50% of the stoichiometric ratio relative to the amount of the sulfate anion in the effluent. For most hydroxide precipitants, the sulfate anion and associated metal cation will both be precipitated. By way of example, for a calcium hydroxide precipitant, the calcium cation forms a precipitate with the sulfate anion while the metal cation associated with the sulfate anion forms a precipitate with the hydroxide anion. In the second substep, a carbonate precipitant, preferably sodium carbonate, is contacted with the purified solution from the first substep (from which the precipitates have been substantially removed) to reduce the calcium content of the solution to no more than about 100 ppm. As will be appreciated, the calcium cations in the purified solution will form a precipitate with the carbonate anion. The reduction in calcium content of the purified solution will inhibit the formation of calcium precipitates or deposits on the second membrane filter. Such deposits on the membrane filter can adversely affect the separation performance of the filter. Preferably, the amount of carbonate precipitant contacted with the purified solution is about 150%, more preferably about 100%, and most preferably about 75% of the stoichiometric ratio relative to the amount of calcium in the purified solution.

To further enhance precipitation, pH, reagent addition, mixing, flocculation, settling, and residence time are preferably carefully controlled. PH can be suitably controlled using an acid such as sulfuric acid, hydrochloric acid, or carbon dioxide

(carbonic acid) and/or a base such as sodium hydroxide, calcium hydroxide, or sodium carbonate. Preferably, the pH is maintained during precipitation in the first substep in the range of about pH 9 to about pH 12 and more preferably in the range of about pH 9 to about pH 10 and in the second substep preferably in the range of about pH 9 to about pH 11 and more preferably in the range of about pH 9 to about pH 10. The precipitant can be added in the reactor or in-line upstream of the reactor. Mixing is preferably continuously performed during precipitation in-line or in a continuous stirred reactor such as a thickener. A flocculent can be used to assist precipitation/settling. Preferred flocculents include natural or anionic polymers or inorganic coagulants such as alum and preferably are used in a concentration ranging from about 1 ppm to about 10 ppm. The residence time in each substep is a sufficient period of time for all or substantially all of the precipitant to react with the target anion and/or cation and preferably ranges from about 20 to about 200 minutes and more preferably from about 30 to about 60 minutes. The temperature can be ambient temperature and, for certain precipitants such as barium carbonate, the temperature should be elevated to at least about 30°C but no more than about 50°C.

The removing step can be performed by any suitable technique for removing a solid from a liquid. By way of example, the step can be performed using a thickener, a filter, a cyclone, a filter press, an inclined plate settler and/or a microfiltration membrane. In one embodiment, the removing step includes filtering the treated retentate with a filter having a larger pore size than the filter in step (a). A preferred filter is a membrane filter such as an ultrafilter or a microfilter.

In another process configuration, the precipitating and removing steps can be replaced with an absorbtion/desorbtion process, such as the ion exchange GYP-CLX process or with a biological oxidation and/or reduction process such as a PAQUE'S THIOPAQ® sulfate reducing and hydrogen sulfide oxidizing reactor or with a solidification/vitrification/encapsulation process. The filter in step (a) provides a highly concentrated retentate which can significantly improve the effectiveness of such processes.

In another embodiment of the present invention, a method is provided that includes the steps of: (a) filtering an aqueous effluent to form a permeate and a retentate, wherein the aqueous effluent contains one or more dissolved multivalent contaminants and the retentate contains at least most of the one or more dissolved multivalent contaminants in the aqueous effluent;

(b) contacting the retentate with a precipitant to precipitate a dissolved multivalent contaminant and form a treated retentate; (c) removing the precipitated multivalent contaminant from the treated retentate; and

(d) thereafter recycling the treated retentate to the filtering step.

In yet another embodiment of the present invention, a method is provided that includes the steps of: (a) nanofiltering the aqueous effluent to form a retentate containing at least of the one or more dissolved multivalent contaminants and a permeate containing at least most of the aqueous effluent;

(b) contacting the retentate with a first precipitant, preferably at a first pH, to precipitate a first dissolved multivalent contaminant as a first precipitate and form a first treated retentate;

(c) removing the first precipitate from the first treated retentate to form a partially barren retentate;

(d) contacting the partially barren retentate with a second precipitant, that is different from the first precipitant, preferably at a second pH different from the first pH, to precipitate a second dissolved multivalent contaminant, that is different from the first dissolved multivalent contaminant, as a second precipitate and form a second treated retentate;

(e) removing the second precipitate from the second treated retentate to form a barren retentate; (f) filtering the barren retentate to form a second permeate and a second retentate, the second retentate containing at least most of the remaining at least one or more dissolved multivalent contaminants in the barren retentate; and

(g) recycling the retentate to contacting steps (b) and/or (d). The various embodiments discussed above can provide numerous benefits over the prior art. The process can use relatively inexpensive and nontoxic reagents, be simple to operate, and effectively remove contaminants such as sulfates to low enough levels to comply with pertinent environmental regulations. The process is able to realize these benefits while using a plant configuration having a small footprint size. In one embodiment, for example, the process uses simply a first and typically a second filter with one or more continuous stirred reactors or thickeners located directly after the first filter. The process can effectively remove contaminants not only at elevated but also at ambient temperatures. The process may, but is not required to, use ion exchange resins. The process is versatile in that it effectively purifies not only biologically nontoxic solutions but also biologically toxic solutions having both low and high contaminant concentrations. Finally, the process can produce a solid or sludge waste containing the contaminants for ease of disposal.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a flow schematic of an embodiment of a process according to the present invention.

DETAILED DESCRIPTION In one embodiment, the present invention relates to the removal of sulfate and/or metal-ions from water or wastewater, utilizing one or more membrane systems to provide high quality discharge water and a concentrated sulfates and metals stream for an enhanced precipitation step with reduced reagent consumption. This process provides a novel and viable method for removing sulfates along with metals. The process may be particularly useful to achieve proposed stricter EPA regulations for sulfate and metal ion discharge standards. A sulfate and/or metal-ion containing liquid, i.e. "wastewater", is passed through a first membrane system to produce a concentrate or retentate rich in sulfates and/or metals, and a permeate with reduced sulfates and/or metals. The concentrate or retentate is sent to a precipitation process for gross sulfate and metal-ion removal. The supernate from the precipitation process (after precipitate removal) may be sent through a second membrane system to produce a concentrate or retentate rich in sulfates and/or metals, and a permeate with reduced sulfates and/or metals. The concentrate or retentate is recycled to the head of the precipitation process for further precipitation of sulfates and metal-ions. In the first membrane filtering step, it is desired to have at least most of the effluent volume in the permeate. Preferably, the permeate constitutes at least about 40%, more preferably at least about 50%, and more preferably at least about 75% by volume of the aqueous effluent. The permeate contains preferably no more than about 5 wt% and more preferably no more than about 2 wt% of the one or more multivalent contaminants in the aqueous effluent. In contrast, the retentate constitutes less than about 60%, more preferably less than about 50%, and more preferably less than about 25% by volume of the aqueous effluent. The retentate preferably contains at least about 95 wt% and more preferably at least about 98 wt% of the one or more multivalent contaminants in the aqueous effluent.

The multivalent contaminant typically comprises a sulfate and its associated cation and the precipitation process comprises precipitating the sulfate and/or the associated cation. Preferred precipitants are calcium oxide, calcium hydroxide, sodium carbonate, sodium hydroxide, calcium carbonate, calcium aluminate, barium carbonate, strontium carbonate, iron hydroxide, aluminum sulfate or mixtures thereof. For example, a ferric sulfate contaminant is precipitated using calcium hydroxide (hydrated lime) to form a ferric hydroxide and calcium sulfate precipitate. Preferably, at least about 50% and more preferably from about 75% to about 90% of the multivalent contaminant or ionic component thereof is precipitated from the retentate in the precipitation process. At least about 95% by volume of the precipitates are removed from the retentate as a solid or sludge.

Preferably about 100% and more preferably about 50% of the stoichiometric amount of the precipitant that is required to precipitate the selected contaminants is employed. As in the first membrane filtering step, the second filtering step places most of the volume of the retentate in the permeate. Preferably, the second permeate constitutes at least about 50%, more preferably at least about 60%, and more preferably at least about 75% by volume of the treated retentate. The second permeate preferably contains no more than about 5 and more preferably no more than about 2 wt% of the one or more multivalent contaminants in the treated retentate. In contrast, the second retentate preferably constitutes less than about 50%, more preferably no more than about 40%, and more preferably no more than about 25% by volume of the treated retentate. The second retentate preferably contains at least about 95 and more preferably at least about 98 wt% of the one or more multivalent contaminants in the treated permeate.

In another embodiment, either the first membrane filtering step or the second membrane filtering step are used alone instead of both membrane filtering steps being used together in series; however, at least one membrane filtering step must be used to separate the multivalent contaminants into the retentate.

Membrane filtering in the process can be performed by any suitable membrane filtration device with a nanofilter membrane being most preferred. The preferred nanofilter membrane has a pore size ranging from about 5 to about 100 angstroms and operates at an osmotic pressure ranging from about 100 to about 800 psi. The nanofilter membrane is preferred because it allows passage into the permeate stream of monovalent ions such as sodium and chloride, while rejecting multivalent contaminants such as sulfate and iron into the concentrate stream. This type of membrane filter prevents build-up of monovalent ions from internally recycled streams in the treatment system circuit.

The membrane concentrate(s) experience enhanced precipitation due to higher metal-ion concentrations and coprecipitation, reducing reagent consumption in the precipitation process and lowering precipitation operating costs. The membrane permeate stream(s) are discharged to the environment or re-used as process water. This process effectively solves the problem of meeting stringent sulfate and metal ion discharge regulations or industrial process water reuse requirements.

Bench scale, pilot plant, and commercial scale installations have demonstrated that higher membrane concentration of metal and sulfate ions leads to reduced precipitation reagent consumption due to metal ion solubility and coprecipitation effects. This is particularly true when iron is present in the wastewater due to the surface complexation, adsorption, and electrostatic interaction with the surface of the iron hydroxide precipitate. Principal reagent reductions, usually lime, typically range approximately from a 20 - 30% reagent reduction compared to conventional precipitation processes. If a conventional precipitation plant must use additional precipitation reagents, such as soda ash for softening or ferrous sulfate for arsenic or metal co-precipitation, total reagent reductions realized by using membrane pre-concentration are even higher. For example, membrane concentration before a two step precipitation process for arsenic and metal removal reduced total reagent consumption by about 85%. Membrane concentration before a lime - soda ash precipitation process for metals removal reduced lime reagent consumption by about 20% and soda ash consumption by about 45%. Finally, membrane concentration before a lime precipitation process of very high TDS waters reduced lime reagent consumption by about 20-30%. These reductions were demonstrated on wastewaters with pH's ranging from about pH 1 - 5 and total dissolved solids contents of about 5,000 mg/L to 150,000 mg L.

The combination of membrane technology and precipitation allows removal of the sulfate ion as calcium sulfate via lime addition. Other reagents such as barium carbonate could also be used sparingly for sulfate reduction. Soda ash is then added to remove calcium before the precipitation supernate is sent to the second membrane system. Total operating costs of two membrane systems are in the $1.65 / 1,000 gallon range for 1000 gpm of a 5 g SO₄/L wastewater and in the $1.85 /1,000 gallon range for 1000 gpm of a 50 g SO₄/L wastewater. Lime-soda ash precipitation costs are in the $ 1.50 / 1 ,000 gallon range for 1000 gpm of membrane concentrate from a 5 g SQ L wastewater, with credit taken for reagent savings due to enhanced precipitation. Lime-soda ash precipitation costs are in the $20.00-$35.00 / 1,000 gallon range for 1000 gpm of membrane concentration from an acidic, 50 g SO₄/L wastewater, with credit taken for reagent savings due to enhanced precipitation. Total system operating costs are then ~$2.40 /1000 gallons for the 5 g SO₄/L wastewater, and ~$12.00 /1,000 gallon for 1000 gpm of acidic, 50 g SO₄/L wastewater.

It is also important to note that a single membrane system can treat the wastewater at a total operating cost of $0.85 - $0.95 / 1,000 gallons for a 5 g SO₄/L or a 50 g SO₄/L wastewater.

A single membrane system with or without precipitation may be all that is needed in some cases. For example, a single membrane system may run at high recovery, with the precipitation supernate from the small amount of membrane concentrate blended with a large amount of membrane permeate to meet discharge requirements. Or, if the membrane concentrate stream can be recycled internally in a plant's water balance system, the clean permeate may be discharged to the environment and no precipitation system is needed. In addition, a single, large membrane system can work together with a larger precipitation system, where the membrane concentrate is continually recycled to the head of the precipitation system or the precipitation system supernate is continually recycled to the head of the membrane system. The present invention takes advantage of the membrane separation to provide an improved method of sulfate and metal ion removal to provide a clean permeate stream for discharge, and a reduced reagent consumption for sulfate and metals removal with a precipitation system.

The process improvements claimed in the present invention will result from utilizing one or more membrane systems to process waters and wastewaters, separating metal and/or sulfate ions from water. As shown in Figure 1, the improved process involves passing a water or wastewater 5 containing metal and/or sulfate ions through a first membrane system 10 to produce a first concentrate (or retentate) 14 rich in sulfates and/or metals, and a first permeate 46 with reduced sulfates and/or metals. The water or wastewater 5 is preferably conditioned to have a pH ranging from about pH 1 to about pH 6. Typically, about 90-99% of the sulfates and metals ends up in the membrane concentrate 14. The typical membrane water recovery results in 35-75% by volume of the feed flow reporting as first concentrate flow (about a 1.5 - 4.0 concentrate factor). The limiting factor in membrane water recovery is most often calcium sulfate or calcium carbonate solubility. Acid addition to convert carbonates to CO₂ or anti-sealant addition to immobilize or distort calcium sulfate crystals can be used to increase recovery. Preferably, a sufficient amount of the acid (which is desirably sulfuric acid or hydrochloric acid) is added to react with the calcium carbonate and more preferably at least about 5 mg/L and no more than about 50 mg/L of acid is added to the feed 5. Preferably, the antiscalant is polyacrylic-maleic copolymer or polyacrylic acid, and is added in amounts ranging from about 1 mg/L to about 10 mg/L. Additional methods of increasing recovery may include selective calcium ion removal such as by ion exchange, and placing an enhanced electrical charge on the solution ions such as by a zeta rod device.

The first membrane concentrate 14 may be sent to a lime-soda ash precipitation process 18 for sulfate and metal-ion removal. Lime addition 22 precipitates metals and sulfates as calcium sulfate and metal hydroxides. In the lime addition step, the preferred pH of the concentrate 14 ranges from pH 9 to about pH 12. The amount of lime added is at least about 50% of the stoichiometric amount required to precipitate all of the sulfates. The precipitates are removed from the reactor before the next precipitation step. Soda ash addition 26 lowers the calcium level in the supernate. In the soda ash addition step, carbonate addition precipitates calcium in the supernate from the lime addition step to avoid scaling of the second membrane filter. The pH of the supernate from the lime addition step preferably ranges from about pH 9 to about pH 11 and the amount of soda ash that is added preferably is at least about 75% of the stoichiometric amount required to react with all of the calcium in the supernate. The precipitates are removed from the supernate to avoid clogging or fouling of the second membrane filter.

During both steps, the retentate is stirred continuously. Preferably, the reactors are continuous stirred reactors such as thickeners.

The residence time in each reactor preferably ranges from about 20 to about 200 minutes. The liquid supernate 30 from the precipitation process 18 may be sent through a second membrane system 34 to produce a second concentrate 38 rich in sulfates and/or metals, and a second permeate 42 with reduced sulfates and/or metals. Typically, about 90-99% of the sulfates and metals in the supernate 30 end up in the second membrane concentrate 38. The typical membrane water recovery results in about 25-50% of the feed flow reporting as second concentrate flow (a 2.0 - 4.0 concentration factor). The second membrane concentrate 38, which contains much higher levels of metals and sulfates than the precipitation system supernate 30, is recycled to the head of the aforementioned precipitation process 18 for precipitation of sulfates and metal-ions. This recycle stream also helps the efficiency of the precipitation process in terms of reaction time and lower reactant consumption.

The first and second membrane permeate(s) 42 and 46, which contain about 1- 10% of the metal and sulfate ions in the feed water, are discharged to the environment or re-used as process water.

To prevent build-up of certain water constituents such as chloride, sodium, potassium or other unregulated (monovalent) cations and anions, a bleed stream 50 of precipitation system supernate 30 may by-pass the second membrane system 34 and be blended in with first and second membrane permeate 46 and 42 from both membrane systems. The addition of supernate 30 will be low enough so that the blend of supernate and membrane permeates will always achieve water discharge standards. Preferably, the bleedstream 50 represents from about 1 to about 10% by volume of the discharged liquid 60.

Typically, the overall clean water recovery from the membrane - precipitation - membrane system will be 90% or greater. In addition, the first and second membrane systems 10 and 34 provide a mechanism for unregulated monovalent cation and anion removal. The preferred membrane in both membrane systems, a nanofiltration membrane, poorly rejects monovalent ions, while highly rejecting multivalent ions. The contaminants of interest, multivalent metals and sulfate, are held in the membrane concentrate stream, while monovalent ions freely pass to the membrane permeate stream. As will be appreciated, some of the monovalent will be in the retentate in amounts based on the relative volume of the retentate to the membrane feed. The first membrane system 10 would process about 100 - 20,000 gallons per minute of water or wastewater, with about 35-75% of the feed flow becoming permeate product. Typical reverse osmosis, nanofiltration, or ultrafiltration membranes used in both membrane systems would be AG, SE, SG, DK, DL, GE, and GH series elements from OSMONICS/DESAL of Vista, CA. These spiral wound elements use polysulfone, polyarimid, polyethersulfone, and/or other polymeric membrane materials as base membranes or polymeric surface modifications to base membrane materials. The described membranes span the reverse osmosis to ultrafiltration categories, with the key being at least some (> 5%) but not complete (< 99%) total dissolved solids, metals, and/or sulfate rejections, i.e. a molecular weight cut-off of about 50 - 5,000 MWCO and pore sizes of about 0.0005 - 0.005 microns, i.e. high divalent ion rejections (98-99%) and low monovalent ion rejections (< 50%).

Example 1 A typical system would process about 1,000 gpm of wastewater through about 258 each 8 inch spiral wound DK nanofiltration membrane elements. Acid may be injected into the feed water to eliminate carbonates and bicarbonates, or resolublize components such as iron. Anti-sealant may also be injected to keep sparingly soluble complexes such as calcium sulfate and calcium carbonate is solution. The membrane system, operating at about 400 - 600 psi, would split the feed flow into about 500 gpm of permeate and about 500 gpm of concentrate. The concentrate may be sent to a lime- soda ash precipitation system for metal and sulfate-ion removal as insoluble sludge. The permeate, meeting low metal and sulfate discharge regulations, would be sent to the environment or re-used as plant water.

The precipitation system would process about 25-15,000 gallons per minute of membrane concentrate, with about 90% or greater of the feed flow becoming supernate. A typical system would consist of a one or two step neutralization with limestone or lime to about pH 7-10, a thickener to remove calcium sulfate and metal hydroxide sludge, a soda-ash softening step to about pH 9-10, a thickener to remove calcium sulfate, calcium carbonate, and metal carbonate sludge, and sludge filtration. The filtered sludge would be sent to a tailings pond or landfill disposal site, the filtrate and the precipitation system supernate would be sent to a membrane system or blended with clean permeate water for discharge.

Example 2 A typical lime-soda ash system includes a 700 gpm membrane concentrate flow into a reagent addition and mixing tank with about 20 minutes of retention time (15,000 gallons), where from about 5 to about 100 lbs. and more preferably about 30 lbs of hydrated lime are added to about 1000 gallons of membrane concentrate to achieve a final pH ranging from about pH 9 to about pH 12 and more preferably about pH 9. The addition can be made in line or in a reactor. The slurry would then be sent to a thickener with about 1.0 gpm/ft2 of settling area (700 ft2 circular thickener), where preferably from about 0.1 to about 5.0 and more preferably about 0.5 lbs of flocculent is added to 1000 gallons of membrane concentrate to produce an underflow of preferably from about 1 to about 15% by volume and more preferably about 10% by volume solids for a filter press. The overflow would be sent to a reagent addition and mixing tank with preferably from about 5 to about 40 and preferably about 20 minutes of retention time, where preferably from about 5 to about 50 lbs. and more preferably about 8 lbs of soda ash is added / 1000 gallons of thickener #1 overflow. The slurry would then be sent to a second thickener with 1.0 gpm/ft2 of settling area (700 ft2 circular thickener), where preferably from about 0.01 to about 1.0 and more preferably about 0.1 lbs of flocculent is added to produce an underflow of 10% solids for filtration. The overflow supernate, containing less than about 100 mg calcium/L, would be sent to a membrane system or blended with membrane permeate for discharge.

The second membrane system would process from about 25 - 15,000 gallons per minute of softened precipitation system supernate, with about 35-75% of the feed flow becoming permeate product. Typical reverse osmosis, nanofiltration, or ultrafiltration membranes used would be AG, SE, SG, DK, DL, GE, and GH series elements from Osmonics/Desal of Vista, CA. These spiral wound elements use polysulfone, polyarimid, polyethersulfone, and/or other polymeric membrane materials as base membranes or polymeric surface modifications to base membrane materials. The described membranes span the reverse osmosis to ultrafiltration categories, with the key being at least some (> 5%) but not complete (< 99%) total dissolved solids, metals, and/or sulfate rejections, i.e. a molecular weight cut-off of about 50 - 5,000 MWCO and pore sizes of about 0.0005

- 0.005 microns.

Example 3

A typical system would process about 650 gpm of wastewater through 190 each 8 inch spiral wound DK nanofiltration membrane elements. Acid may be injected into the feed water to eliminate carbonates and bicarbonates, or resolublize components such as iron. Anti-sealant may also be injected to keep sparingly soluble complexes such as calcium sulfate and calcium carbonate in solution. The membrane system, operating at about 400 - 600 psi, would split the feed flow into 450 gpm of permeate and 200 gpm of concentrate. The concentrate would be sent to the lime-soda ash precipitation system for metal and sulfate-ion removal as insoluble sludge. The permeate, meeting low metal and sulfate discharge regulations, would be sent to the environment or re-used as plant water.

Having herein described preferred embodiments of the present invention, it is anticipated that suitable modifications and variations may be made thereto by individuals skilled in the relevant art which nonetheless remain within the spirit and scope of the invention as set forth herein and in the following claims. By way of example, any contaminant removal process besides those set forth above can be used to remove the contaminants from the retentate. More than one membrane filter can be located upstream or downstream of the contaminant removal step. In the former case, the retentate or permeate of the first membrane filter would be passed to a second or later membrane filter and the retentate of the second membrane filter used in the contaminant removal steps.

In the latter case, the purified stream from the contaminant precipitation and removal steps is subjected to membrane filtration and the permeate or retentate subjected to further membrane filtration. The retentate from the further membrane filtration step can be recycled to the feed stream upstream of the first membrane filter and/or the contaminant removal step. Nothing in the Specification shall limit the invention as claimed below and the present invention set forth in the following claims shall therefore be construed as broadly as the prior art permits.

Claims

What is claimed is:

1. A method for treating aqueous effluents containing one or more dissolved multivalent contaminants, comprising:

(a) filtering the aqueous effluent to form a permeate and a retentate, the retentate containing at least most of the one or more dissolved multivalent contaminants in the aqueous effluent;

(b) contacting the retentate with a precipitant to precipitate a dissolved multivalent contaminant and form a treated retentate;

(c) removing at least most of the precipitated multivalent contaminant; and (d) thereafter further filtering the treated retentate to form a second permeate and a second retentate containing at least most of the multivalent contaminants in the aqueous effluent.

2. The method of Claim 1, wherein the permeate constitutes at least about 50% by volume of the aqueous effluent.

3. The method of Claim 1 , wherein the permeate contains no more than about

5% of the one or more dissolved multivalent contaminants in the aqueous effluent.

4. The method of Claim 1, wherein the retentate constitutes less than about 50% by volume of the aqueous effluent.

5. The method of Claim 1 , wherein the retentate contains at least about 95% of the one or more dissolved multivalent contaminants in the aqueous effluent.

6. The method of Claim 1 , wherein the retentate comprises a sulfate and the contacting step comprises precipitating the sulfate.

7. The method of Claim 6, wherein the precipitant is at least one of a hydroxide, an oxide other than a hydroxide, a carbonate, a sulfide, an aluminate, or a sulfate.

8. The method of Claim 1 , wherein the precipitated multivalent contaminant is at least one of a metal hydroxide, carbonate, sulfate, or an aluminate.

9. The method of Claim 1, wherein the removing step includes filtering the treated retentate with a filter having a larger pore size than the filter in step (a).

10. The method of Claim 1, wherein the second permeate constitutes at least about 50% by volume of the treated retentate.

11. The method of Claim 1, wherein the second permeate contains no more than about 5% of the one or more dissolved multivalent contaminants in the treated retentate.

12. The method of Claim 1 , wherein the second retentate constitutes less than about 50% by volume of the treated retentate.

13. The method of Claim 1, wherein the second retentate contains at least about 95% of the one or more dissolved multivalent contaminants in the treated retentate.

14. The method of Claim 1 , further comprising recycling the second retentate to the contacting step.

15. A method for treating aqueous effluents containing one or more dissolved multivalent contaminants, comprising:

(a) filtering an aqueous effluent to form a permeate and a retentate, wherein the aqueous effluent contains one or more dissolved multivalent contaminants and the retentate contains at least most of the one or more dissolved multivalent contaminants in the aqueous effluent;

(c) removing the precipitated multivalent contaminant from the treated retentate; and (d) thereafter recycling the treated retentate to the filtering step.

16. The method of Claim 15, wherein the removing step is performed by at least one of the following devices: a thickener, a filter, a cyclone, a filter press, an inclined plate settler, and a microfiltration membrane.

17. A method for treating an aqueous effluent containing one or more dissolved multivalent contaminants, comprising:

(a) nanofiltering the aqueous effluent to form a retentate containing at least of the one or more dissolved multivalent contaminants and a permeate containing at least most of the aqueous effluent;

(b) contacting the retentate with a first precipitant to precipitate a first dissolved multivalent contaminant as a first precipitate and form a first treated retentate; (c) removing the first precipitate from the first treated retentate to form a partially barren retentate;

(d) contacting the partially barren retentate with a second precipitant, that is different from the first precipitant, to precipitate a second dissolved multivalent contaminant, that is different from the first dissolved multivalent contaminant, as a second precipitate and form a second treated retentate;

(e) removing the second precipitate from the second treated retentate to form a barren retentate;

(f) filtering the barren retentate to form a second permeate and a second retentate, the second retentate containing at least most of the remaining at least one or more dissolved multivalent contaminants in the barren retentate; and

(g) recycling the retentate to contacting steps (b) and/or (d).

18. The method of Claim 17, wherein the first dissolved multivalent contaminant is a sulfate and the second dissolved multivalent is calcium.