COMBINED MEMBRANE AND SORPTION PROCESS FOR SELECTIVE ION REMOVAL

Abstract: A combined membrane/sorption process is used to selectively remove ions from liquid streams. As an example, nitrate ions can be more effectively removed from an aqueous stream also containing sulfate ions. Sulfate ions normally are not needed or desired to be removed from drinking water, but are typically sorbed by the available ion exchange resins and result in inefficient use of the ion exchange capacity. The disclosed process and apparatus use an ion selective membrane (10) to first remove the sulfate ions (12) from the stream and then remove the nitrate ions by ion exchange (20).

Description: This invention relates to the use of a combined membrane and sorption system for selectively removing certain ions from a liquid. It is especially applicable to the removal of nitrates from ground waters which also contain amounts of sulfate ions. In another aspect, the present invention is an apparatus for selective ion removal from liquids. The removal of selected ions from a liquid or solution additionally containing other types of ion is necessary or desired in many areas of technology, such as when the selected ion is or can be converted to a valuable product or the selected ion is undesirable in the use or disposition for which the liquid is intended. For example, ions of valuable metals can be recovered from solutions for further conversion into such metal. The levels of some ions, such as nitrates, need to be reduced in water prior to further use of such water for human consumption. Various processes are currently used to remove and/or concentrate ions for these reasons. Such processes include ion exchange, electrodialysis, reverse osmosis, chemical reduction and various biological techniques. As an illustration of this situation, the high levels of nitrates found in drinking water supplies in some locations are becoming an urgent problem. Adaptations and/or combinations of one or more of the known ion removal systems have been suggested for use in nitrate removal from water. In Wasserwirtschaft 75 (1985), No. 6, Pages 257 to 262 there is a summary of the standard nitrate removal system. German patents 3,301,054 and 3,010,538 discuss strong base anion exchange methods to remove nitrates from water. The problem faced in these and other ion exchange techniques which attempt to remove nitrate ions is the failure of the system to selectively and economically remove the nitrate ions when sulfate ions are present. In has been found that substantial portions of the capacity of the commercially available anion exchange resins are used to sorb sulfate ions and subsequent regeneration is similarly spent ineffectively on such sorbed sulfate ions. In U.S. Patent 4,479,877 there is a further description of the other efforts to remove nitrates by ion exchange methods and the inability to effectively do so due to this problem. In this patent there is taught a specialized strong base anion exchange resin which is suggested for use in nitrate removal from drinking water due to its selectivity of nitrates in preference to sulfates. This type of resin, however, is a very specialized resin and suffers from the various other disadvantages of ion exchange systems. U.S. Patent 4,206,048 discloses a reverse osmosis process for removing a high concentration of total dissolved solids from water. It is further disclosed that some of the nitrate or "nitrate type" ions,such as arsenate, selenate, or borate ions, pass through reverse osmosis membranes, and appear in reverse osmosis product water. This is recognized as a problem, which is solved by passing the permeate stream containing the nitrate and "nitrate type" ions through an ion exchange resin bed to remove the nitrate and "nitrate type" ions. The reject stream from the concentrate brine, which typically contains high levels of nitrate ions, as a typical reverse osmosis membrane rejects between 0 and 81 percent of the nitrate ions. In response to the deficiencies of the prior art process in the selective removal of nitrate or other ions, it is the object of the present invention to provide a sorption process for removal of a first ion from a liquid containing a second ion which competes for removal capacity. Thus, provided is a process for removal of a selected monovalent ion from a liquid further containing bivalent ions wherein both the monovalent and bivalent ions compete for adsorption sites on ion exchange resins; comprising (a) contacting the liquid with an ion selective membrane so as to separate the liquid into a first portion which permeates through the membrane and contains a lower concentration of the bivalent ion and a higher concentration of the monovalent ion than the original liquid stream and a second portion which does not permeate through the membrane which has a higher bivalent ion and lower monovalent ion concentration than the original liquid stream, and (b) contacting the first portion with a sorbant selective for the selected monovalent ion so as to adsorb the selected monovalent ion; characterized by (c) combining all, or part of, the second portion of step (a) with the first portion which has been contacted with the sorbant in step (b) and wherein the ion-selective membrane is a nanofiltration membrane. In another aspect, the present invention is a sorption apparatus for the removal of a first ion from a liquid also containing a second ion which competes for removal capacity, which apparatus comprises: (a) an ion-selective membrane element which separates the liquid into two portions, the first portion comprising a lower concentration of the second ion than the original liquid and (b) a sorption element which receives the first portion from the membrane and removes the first ion, characterized in that the ion-selective membrane is a nanofiltration membrane. The present invention thus provides a process and apparatus for performance of such process which uses known, available elements to accomplish a long sought-after effect accompanied by a number of ancilliary benefits which will be discussed further, below. The present invention can be used for ion removal from many different types of liquids and liquid streams, with primary emphasis on aqueous streams. Although suited for use with ion-containing liquids of many different types, the process and apparatus of the present invention have been found to be particularly well suited for application in the treatment of water for consumption by humans and/or other mammals. The treatment of naturally occurring water for such purposes typically requires at least partial removal of one or more selected ions but must leave certain levels of some ions in the water for reasons of health. The apparatus and process of the present invention can be used in situations where the total dissolved solids (TDS) level in the water is very high, such as for sea water, where the TDS is often greater than 30,000 parts per million (ppm). It is preferred, however, to start with water containing less than 10,000 ppm TDS and more preferably less than 1,000 ppm TDS and remove only the selected ion or ions which are objectionable on health grounds such as, for example, nitrate ions. Water having these lower TDS levels (less than 1,000 ppm) is usually otherwise acceptable in terms of the taste and smell of the water and there generally exists only the health concerns raised by the unacceptably high level of the nitrate ion concentration. In these situations the membrane and ion exchange resin combination used to reject the sulfate ions and remove nitrate ions, respectively, can operate with maximum levels of efficiency. More specifically, it is found that fresh surface and ground water will typically contain less than 1000 ppm TDS, with nitrate levels of up to 200 milligrams per liter (mg/l) and sulfate levels of 500 mg/l (measured for example by ion chromatography). Sometimes there are also low levels of phosphates which will be handled similarly to the sulfates. In these situations, it is really only the nitrate level that needs to be lowered, generally to the range of 20 to 40 mg/l, with the level of total ions, in general, being acceptable if not desirable. The use of various types of sorbants for ion removal by non-catalytic, fluid/solid reactions are well known. Examples of such sorbants include zeolites and hydrocarbon based materials of synthetic and natural origin which exhibit some degree of selective affinity for one or more ionic species. Ion exchange resins are well-known sorbant materials commonly used for ion removal and are preferred sorbants for used in the present invention. The various types of functional groups or moieties associated with the resin either exchange other ions for the ion desired to be removed or chelate or fix the selected ion. In the particular situation where nitrate anions are desired to be removed from aqueous solutions, strong base anion exchange resins are most preferred for this function. The various strong base, anion exchange functional groups and polymer resins with which they are associated, including their relative advantages and disadvantages, are well-known technology. These types of resins are preferred for use in nitrate removal according to the present invention. Such resins are typically feature trimethylammonium or dimethylethanolammonium functional groups, referred to respectively as type 1 or type 2 resins. The various means for employing ion exchange resins and other sorbant materials are also generally well-known in the art. Such systems include packed or fluidized beds of these materials. These beds can be operated in a batch method whereby one bed is used until exhaustion at which time the feed stream is either stopped or diverted to another, regenerated bed. In some systems continuous operation is attempted by continual removal of a portion of the used resin and replacement with fresh resin. In either operation the exhausted sorbant material must be regenerated prior to further use. The sorbant regeneration techniques known in the art are suitable for use with the sorbant material used in the practice of the present invention. In general, one can use the most efficient techniques for the particular sorbant material and bed structure that have been employed. The known regenerates for the strong base anion exchange columns typically used in nitrate removal include solution containing chloride, bicarbonate and/or carbon dioxide. Co-flow regeneration systems can be used with such resins with counterflow regeneration systems being recognized to provide high efficiencies in ion exchange resin regeneration. When using KCl or NH4Cl solutions in the regeneration, the resulting solution is highly suited for use as fertilizer, eliminatiing a problem of disposal of the used regenerating solution. For the particular sorbant material which is used and the selected ion which is desired to be removed, the choice of the membrane element becomes very important. The membrane must be highly selective between the ion which is desired to be removed and the similarly charged, non-selected ions which compete with the selected ions from removal capacity of the sorbant. In most cases this will mean that the membrane will have a high rejection rate for the non-selected ions and a very low, or possibly negative, rejection rate for the selected ions. If a membrane has a high rejection rate for a particular ion, the concentration of the ion in the permeate solution which passes through the membrane is decreased while the concentration of the ion in the reject portion of the liquid which is not passed through the membrane increases. If a membrane has no rejection rate for an ion, the ion concentration in both the permeate and the reject portions will remain essentially the same as in the original liquid. It is also possible for a membrane to have a negative rejection rate for an ion if the ion's concentration in the permeate becomes greater than the ion's concentration in the original liquid. Membranes generally suitable for use in the practice of the present invention are known nanofiltration membranes and are commercially available. These will exhibit the necessary selectivity for the particular ionic species. As used herein the term nanofiltration membrane refers to a membrane having micropores or openings between chains in a swollen polymer network, which micropores are estimated to have an average diameter on the order of 10 angstrom (i.e., one nanometer). The membranes commonly known as reverse osmosis membranes will tend to reject practically all ions and therefore may not effectively separate ionic species. Ultrafiltration membranes, on the other hand, will typically not reject most ionic species but are found to reject higher molecular weight molecules based on their molecular weight. Certain nanofiltration membranes have been found to exhibit very good levels of selectivity between selected and non-selected ions. Illustrative, ion-selective nanofiltration membranes preferred for use in the practice of the present invention are further described in U.S. Patent 4,259,183 which is incorporated by reference and U.S. Patent 4,765,897, also incorporated by reference. Such a nanofiltration membrane can consist of a thin film composite membrane having a polyamide discrimination layer supported on a porous substrate. The polyamide discrimination layer can be prepared by reaction of piperazine and/or a substituted piperazine with one or more acid halides. Preferably the polyamide is prepared from piperazine and trimesoyl chloride and optionally a diacyl halide such as isophthaloyl chloride. Examples of such membranes includes the NF-40 or NF-50 Nanofiltration Elements, commercially available from Film Tec Corporation, Minneapolis, Minnesota. These membrane elements have been found very suitable for the aspect of the present invention where a strong base anion exchange resin is thereafter used to remove nitrates from water. This membrane has been found to be particularly effective in rejecting a high proportion of the sulfate ions which are commonly found in water as well as the other large, divalent anions and cations. The permeate portion from such membrane containing low levels of sulfate ions is then supplied to the strong base anion exchange resin where the nitrate ions are very effectively removed by the resin. The selected type of membrane can be employed in the process according to the present invention in any of the known membrane constructions. Such membrane constructions include spiral wound modules such as the afore-mentioned nanofiltration element, plate and frame constructions, hollow fibers, and arrays of tubular membranes. Spiral wound membrane devices are generally preferred. An embodiment of the process and apparatus according to the present invention is illustrated in Figure 1. In Figure 1 the raw feed liquid (1) contains a first or selected ion which is desired to be removed form the liquid by the sorbant material and a second or non-selected ion which, although not necessary to be removed, is ordinarily removed by the sorbant and thereby reduces the capacity or ability of the sorbant to remove the first or selected ion. This liquid can be in the form of a single charge or a series of periodic charges but is preferably in the form of a stream which can be supplied continuously to the membrane element. The feed liquid (1) can be obtained from many sources including naturally occurring surface or ground waters or chemical processes. Prior to the feeding of the stream to the membrane element, there can be an optional pretreatment element (2) and the optional removal or by-pass of a portion of this raw water feed (3). The various pretreatment techniques which can optionally be used in preparing a raw feed for separation by the membrane element are well-known and include for example softening, pH adjustment, prefiltration, chlorination, anti-scalant addition, flocculation, and/or temperature adjustment. Pretreatment systems are further discussed in U. S. Patent 4,574,049, which is incorporated by reference. The amount and types of any such pretreatment will obviously depend upon the source of the raw feed liquid and its inherent characteristics. The feed stream (4) which is supplied to the membrane element (10) is thereby separated into a first portion (11) and a second portion (12). The essential characteristic of the membrane element (10) is to achieve a lower concentration of the second ion(s) relative to the concentration of the first ion(s) in the first portion (11), to enable this first portion to have the first or selected ions removed more efficiently therefrom. By this process there will be produced the second portion (12) which should contain a relatively higher concentration of the second ions. In the preferred embodiment of the present invention, where nitrate ions are desired to be removed from an aqueous stream containing also sulfate ions, the second portion (12) is the rejected stream from a nanofiltration membrane having a good sulfate ion rejection rate. The first portion (11) is therefore the portion of the stream which has permeated through the membrane and has a lower concentration of sulfate ions. Ideally the membrane element exhibits little or no rejection of the nitrate ions. As is well-known to those skilled in the art of membranes and water treatment, the membrane element may comprise one or more separate membrane elements of the same or different constructions combined in series and/or parallel configurations to produce the desired separation capacity. From the membrane element the first portion (11) of the liquid is then supplied to the sorbant material (20). As mentioned above, the essential characteristic of the sorbant material is its ability to remove from the liquid the selected ion. Examples of sorbants include zeolites and ion exchange resins. if an ion exchange resin is used, it can be one or more of the known commercially available polymeric ion exchange resins with functional moieties which will remove the selected ion and/or exchange for it one or more less detrimental ions. In the preferred embodiment of the present invention, a strong base anion exchange resin is used as the sorbant material to remove nitrate anions from an aqueous stream. Suitable functional groups include the trimethylammonium groups of type 1 resins and the dimethylethanolammonium groups of type 2 resins, or other groups which utilize the quaternized ammonium moiety or similar anion exchange moiety. Ion exchange beds of such resins, although proving somewhat ineffective in the prior art processes where the water contained fairly high sulfate levels have proven very effective in the practice of the present invention to continuously remove a large portion of the nitrate ions in an aqueous sulfate-containing stream with sufficient capacity such that the number of required regenerations for an operating period is minimized. The purified stream from the sorbant material (21) is then available for the desired disposition. In a further embodiment of the present invention, all or part of the second portion (12) produced by the membrane element is added to the purified liquid which is obtained from the sorbant material. In the most preferred aspect of the present invention where it is desired to continuously remove nitrate ions from an aqueous stream, all or part of the rejected portion from the membrane may receive optional treatment (13) and the resultant stream (14) is recombined with the purified aqueous stream (21) after it has left the ion exchange resin to produce the product stream (22). Depending upon the nature of the raw feed (1) it may be possible or desirable to bypass at least a portion (3) with or without the optional pretreatment (2), around the membrane element (10) and the sorbant material (20) and directly or indirectly recombine this stream (3) with the product stream (22). In a preferred embodiment of the present invention the membrane element produces a first permeate portion (11) having removed therefrom essentially all of the divalent ions and impurity molecules found in the raw feed water. The second portion (12) will therefore be the reject portion which has concentrated therein all of these various ions and molecules which may be either desirable or undesirable for the ultimate disposition of the product water (22). This second portion (12) can therefore be very conveniently and efficiently treated by other techniques to remove other undesirable elements of the water stream. For example, activated carbon can be employed in such optional treatment (13) to remove other known pollutants form the stream such as pesticidal or herbicidal chemicals. In the nitrate removal embodiment of the present invention, there are several other surprising advantages according to the present invention. The membrane which has rejected the majority of the sulfate ions has similarly screened or rejected not only the bacteria which are known to collect and grow in ion exchange beds but also the organic molecules produced in the degeneration of cellulosic materials, which organic materials foul the ion exchange resin and are a food source for any bacteria present in the ion exchange resin. A further benefit can be obtained by the partial removal or "bleed-off" (15) of the second portion (12) produced by the membrane. Depending on the content of magnesium and/or calcium in the original water supply which will be concentrated in the second portion (12), this bleed-off (15) may be desired to soften the water. The techniques for regeneration of the sorbant material will vary according to the particular type of sorbant and its method of employment. For example when the sorbant is an ion exchange resin which has been used to remove nitrates from an aqueous liquid, a solution of alternate anions such as chloride or bicarbonate is applied co- or counter-currently and such alternate ions will displace the fixed nitrate ions from the ion exchange resin. This regeneration will produce an amount of regenerate waste brine (23) containing a high concentration of the nitrate ions. This waste brine (23) is then appropriately disposed of and/or treated in an additional process, such as a biological process, which destroys the nitrates. In the latter case it is possible to have a zero discharge operation. To illustrate some of the benefits obtainable according to the practice of the present invention, Table I below, shows the dramatic increase in efficiency which will result when the present invention is utilized in the removal of nitrates from drinking water as compared to the prior art method of using only ion exchange. In this illustration the data is calculated from the data available regarding the individual unit operations. Two raw water feed streams are considered, the first comprising 100 milligrams sulfate ion per liter and 100 milligrams nitrate ion per liter, and a second feed stream comprising 150 milligrams sulfate per liter and 100 milligrams nitrate per liter. Such ion levels can often be found in typical, naturally occurring ground and surface water supplies commonly used to provide drinking water. The calculations are based on the use of about 5380 liters of Dowex® SBR-P brand ion exchange resin, a commercially available strong base anion exchange resin having trimethylammonium functional groups. The membrane system is based on NF40-8040 nanofiltration elements available from Film Tec Corporation which utilize a polyamide barrier layer. The system is composed of 17 pressure vessels each containing 6 spirally wound elements 101.6 cm (40 inches) long and 20.32 cm (8 inches) in diameter. The system is arranged to have a first stage with 11 of the pressure vessels in parallel with the reject portion going to a second stage which has the other 6 vessels in parallel. The feed water is supplied to the membrane element at a rate of 100 cubic meters per hour (m³/h) and under a pressure of 0.6 mega Pascals (MPa). As typically done with a stand-alone anion exchange column, a raw water bypass is used where the ratio of the bypass water flow rate to ion exchanged water flow rate is 0.28. The ion exchange columns with and without the membrane element are run until nitrate leakage through the column exceeds 10 milligrams per liter (mg/l). In the regeneration of both systems 1.5 bed volumes of a 6 percent solution of NaCl is applied counter-currently to the ion exchange resin at a rate of 90 grams NaCl per hour, followed by a co-currently applied rinse of 2 bed volumes of nitrate-free, ion exchanged, permeate water. Table I compares the prior art system to the present invention with the first raw water feed composition and Table II compares the two systems using the second raw water feed composition containing greater sulfate ion concentration. WATER SUPPLY: 100 mg/l Nitrate, 100 m/l Sulfate "Stand Alone" Ion Exchange Combination Membrane/Ion Exchange Ion Exchange Cycle time (h) 12 24.3 Ion Exchange Resin Content Sulfate (eq/l) 0.36 0.07 (approx.) Nitrate (eq/l) 0.28 0.57 (approx.) Ratio of Regenerant Required to Nitrate Removed (eq. Cl per eq. NO₃) 5.5:1 2.70:1 (kg NaCl per kg N) 23.0:1 11.3:1 Waste Brine Produced (m³/day) 37.7 16.3 As can be seen in Table I above, the prior art - process produces 131 percent more waste brine than the process according to the present invention. As recognized by those skilled in this area of technology the reduction in waste brine which must be further process and/or disposed of in some fashion is highly advantageous, both economically and ecologically. Table II below shows that with somewhat higher sulfate concentrations an even greater reduction in waste brine is observed. WATER SUPPLY: 100 mg/l Nitrate, 150 mg/l Sulfate "Stand Alone" Ion Exchange Combination Membrane/Ion Exchange Ion Exchange Cycle time (h) 6.43 23.0 Ion Exchange Resin Content Sulfate (eq/l) 0.49 0.10 (approx.) Nitrate (eq/l) 0.15 0.54 (approx.) Ratio of Regenerant Required to Nitrate Removed (eq. Cl per eq. NO₃) 10.3:1 2.85:1 (kg NaCl per kg N) 43.0:1 11.9:1 Waste Brine Produced (m³/day) 70.2 16.3 With a sulfate concentration of 150 mg/liter, not uncommon in water from which nitrate is needed to be removed, the prior art produces about 330 percent more waste brine per day than the process according to the present invention. As discussed and illustrated above, the present invention provides numerous advantages in the selective removal of certain ions from a liquid. As shown with regard to nitrate removal from drinking water, many distinct advantages over prior art attempts are observed. There is a dramatic reduction in waste brine production due to the high efficiency of nitrate removal by the ion exchange unit. Due to the selective removal of nitrate and the retention of virtually all the more desirable ion in the water that is produced, the process provides improved water quality, particularly regarding health, corrosive and scaling properties. The ultimate use of both membrane permeate and reject portions means that the membrane unit itself is nearly approximately 100% productive. Organic chemical fouling of the ion exchange resin is eliminated. Use of the membrane also provides a filtering effect that prevents bacterial contamination of the ion exchange column and contains bacteria (if any) of membrane surface which may be easier to clean or disinfect than the exchange resin. In addition, this reduces organic food supply to any bacteria present in the ion exchange column. This concentration by the membrane element of micro-pollutants, such as pesticidal and herbicidal chemicals, makes their removal through standard means easier and much more efficient. The possibility of micro-pollutant surges out of the ion exchange column is therefore eliminated. Since the nitrates which have been removed are now more concentrated in the waste brine, denitrification by other means such as biological becomes much easier. List of reference numbers 1 raw feed liquid 2 pretreatment element 3 water feed 4 feed stream 10 membrane element 11 first portion 12 second portion 13 optional treatment 14 stream from optional treatment 20 sorbant material 21 purified stream, aqueous stream 22 product stream

Claims: A process for removal of a selected monovalent ion from a liquid further containing bivalent ions wherein both the monovalent and bivalent ions compete for adsorption sites on ion exchange resins; comprising (a) contacting the liquid with an ion selective membrane so as to separate the liquid into a first portion which permeates through the membrane and contains a lower concentration of the bivalent ion and a higher concentration of the monovalent ion than the original liquid stream and a second portion which does not permeate through the membrane which has a higher bivalent ion and lower monovalent ion concentration than the original liquid stream, and (b) contacting the first portion with a sorbant selective for the selected monovalent ion so as to adsorb the selected monovalent ion; characterized by (c) combining all, or part of, the second portion of step (a) with the first portion which has been contacted with the sorbant in step (b) and wherein the ion-selective membrane is a nanofiltration membrane. The process according to claim 1 wherein the sorption process is an ion exchange and the sorbant is an ion exchange resin. The process according to claims 1 to 2 wherein the first ion is nitrate ion, the second ion is sulfate ion and the liquid is aqueous. The process according to claims 1 to 3 wherein the membrane comprises a thin film composite membrane having a polyamide discrimination layer prepared by the reaction of piperazine and/or a substituted piperazine with one or more acid halides. The process according to claim 4 wherein the polyamide discriminating layer is prepared by reaction of piperazine with trimesoyl chloride and optionally isophthaloyl chloride. The process according to claims 1 to 5 wherein the original liquid has a total dissolved solid of 10,000 parts per million or less. The process according to claims 1 to 5 wherein the original liquid has a total dissolved solid of 1,000 parts per million or less. A sorption apparatus for the removal of a first ion from a liquid also containing a second ion which competes for removal capacity, which apparatus comprises: (a) an ion-selective membrane element (10) which separates the liquid into two portions (11, 12), the first portion (11) comprising a lower concentration of the second ion than the original liquid and (b) a sorption element (20) which receives the first portion (11) from the membrane (10) and removes the first ion, characterized in that the ion-selective membrane is a nanofiltration membrane. The apparatus of claim 8 wherein the first portion (11) is the permeate and the second portion (12) is the reject stream from the membrane (10).

Inventors: GUILHEM, MICHEL
Applicants: THE DOW CHEMICAL COMPANY
Application year: 1987
Main classification: B01D61
IPC codes: B01J41, C02F1, B01D61, C02F9, B01D15