CA1330372C

CA1330372C - Combined membrane and sorption process for selective ion removal

Info

Publication number: CA1330372C
Application number: CA000541689A
Authority: CA
Inventors: Michel Guilhem
Original assignee: Dow Chemical Co
Current assignee: Dow Chemical Co
Priority date: 1986-07-15
Filing date: 1987-07-09
Publication date: 1994-06-21
Anticipated expiration: 2011-06-21
Also published as: AU7503587A; ES2033744T3; DE3781734D1; AU591788B2; JPH0580279B2; JPS6365994A; DE3781734T2; EP0253287B1; EP0253287A1; DE3623796A1; IL83128A0; IL83128A; US4806244A

Abstract

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 to first remove the sulfate ions from the stream and then remove the nitrate ions by ion exchange.

Description

- 1 13~372 COMBINED MEMBRANE AND SORPTION PROCESS FOR SELECTIVE ~ . .
. .....
ION REMOVAL : `"
This invention relateQ ~o the use of a combined membrane and sorption ~ystem for selectively remo~ing certain ionq from a liquid. It is especially ;~
applicable to the removal of nitrates Prom ground water3 which aleo oontain amounts of sulfate ions. In another aspect, the pre~ent invention i~ an apparatus for qelective ion removal ~rom liquidq. `; :
The removal of selected ions from a liquid or ~; ~
10 ~olution additionally containing other types of ion iq ~ - :
neceqqary or desired in many areaq o~ technology, such .
aq when the selected ion is or can be converted to a valuable product or the ~qelected ion i~ undeqirable in; :~
the uqe or diqposition for which the liquid i~ ~ -intended. For example, ions o~ valuable metal~ can be recovered from solutions for further conver3ion into . quch metal The levels of ~ome io~s, such àq nitrates, need to be reduced in water prior to ~urther use of 20 quch water for human con~umption. ~ ~:
VariQus proce~3es are currently used to remove and/or concentrate ions for the~e re:aqons. :Such processes include ion exchange, electrodialysi3, :
- ~
. , .:
: ' 34,969-F

-2 ~3~72 reverse osmosis, chemical reduction and various biological techniques.
A~ an illu~tration of this situation, the high 5 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 systems. German patents 39301,054 and 39010,538 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. It has been found that substantial portions of the capacity of the commercially available anion exchange resins are used to sorb sulfate ions and sub~equent regeneration is similarly qpent ineffectively on such sorbed sulPate 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 thls problem. In thi~ patent there is taught a ~pecialized strong base anion exchange resin which i9 suggested for u~e in nitrate removal from drinking water due to its selectivity of nitrates in preference to sulfates. Thi~ type of resin, however, is a very `~
~pecialized resin and suffers from the various other disadvantages of ion exchange systems.

34,9~9-F -2-

3 ~ ~ 7 2 ~ ~
73402~
In response to the deflciencies of the prior art ;.
processes in the selective removal of nitrate or other ions, the ~:.
present lnventlon is an improved sorption process for removal of .. . .. ;-." ~,~- .~ .
a first ion from a llquld contalnlng a second lon whlch competes for removal capacity.
According to one aspect of the present invention there ~ .
is provlded a process for the removal of a selected monovalent lon from a llquid further contalnlng blvalent ions whereln both the monovalent and blvalent lons compete for adsorptlon sltes on an lon exchange resln; comprlslng~
(a) contactlng the llquld wlth an ion selectlve membrane so as to separate the liquld into a first portion which permeates through the membrane and contalns a lower concentratlon of the .~
blvalent ion and a higher concentratlon of the monovalent ion : ~ -than the orlglnal llquld stream, and a second portion which does not permeate through the membrane which has a higher bivalent ion and lower monovalent lon concentration than the original liquid stream, and (b) contactlng the first portlon wlth a sorbant selectlve for the selected monovalent lon so as to adsorb the selected monovalent lon; characterlzed in that the ion-selectlve membrane ls a nanoflltratlon membrane.
According to a further aspect of the present invention : ~
there is provlded a sorption apparatus for the removal of a first ~ ~ ;
lon from a liquld also containlng a second lon which competes for ~; ' .::

~ `` 133~3~2 removal capaclty, whlch apparatus comprlses:
(a~ an lon-selective membrane element which separates the llquld lnto two portions, the flrst portlon comprlslng a lower ;:
concentratlon of the second lon than the orlglnal llquld, and (b) a sorption element which recelves the flrst portion from the membrane and removes the first lon, characterlzed ln that the :
lon-selectlve membrane ls a nonoflltratlon membrane.
The present lnventlon thus provides a process and apparatus for performance of such process whlch uses known, avallable elements to accomplish a long sought-after effect -accompanled by a number of anclllary benefits whlch wlll be ;
discussed further, below.

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~33~3~2 :

The present invention can be used ~or ion removal from many di~ferent t~pes 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 inv~ntion have been found to be particularly well suited for application in the treatment o~ water for con~umption by humans and/or other mammals. The treatment of naturally occurring water for such 0 purposes typically requires at least partial removal of one or more selected ions but must leave certain levels of ~ome ions in the water for reasons o~ health.
The apparatus and process of the present invention c-an be used in qituations where the total dis~olved solids (TDS) level in the water is very high, such as for sea wate~, where the TDS is often greater than 30,000 parts per million (ppm). It is preferred, however, to start with water containing les3 than 10,000 ppm TDS and more-preferably less than 1,000 ppm TDS and remove only the selected ion or ions which are objeotionable 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 term~ oP the taste and smell of the water and there generally exists only the health concerns raiqed by the unacceptably high level of the nitrate ion concentration. In the~e situations the membrane and ion exchange resin combination used to reject the sulfate lon~ and remove nitrate ions, respectively, can operate with maximum levels of efficiency.
More specifically, it is found that ~resh surface and ground water will typically contain le~s than 1000 ppm TDS, with nitrate levels of up to 200 34,969-F -4_ _5~ 2 milligrams per liter (mg/1) and sulfate levels o~ 500 mg/1 (measured for example by ion chromatography).
Sometimes there are also low levels of phosphates which will be handled similarly to the sulfates. In these ~ituations, it is really only the nitrate level that needs to be lowered, generally to the range of 20 to 40 mg/1~ with the level of total ions, in general, being acceptable i~ 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 ~5 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 ~ituation where nitrate anions are desired to be removed ~rom aqueous solution3, strong base anion exchange resins are most preferred for this function. The various strong base, anion exchange functional group~ and polymer resin~
with which they are a~ociated, including their relative advantages and disadvantages, are well-known technology. These type~ of re~ins are preferred for use in nitrate removal according to the present invention. Such resins are typically feature trimethylammonium or dimethylet~anolammonium functional 34,969-F ~5 ,,C,. ~ , " ",, "

-6- 1 33~372 groups, referred to respectively as type 1 or type 2 resin~.
The various means for employing ion excha~ge resins and other sorbant materials are also generally well-known in the art~ Such systems include packed or ~luidized 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 i~ attempted by continual removal o~ a portion of the used resin and replacement with fresh resin. In either operation the exhausted sorbant material must be regenerated prior to further u-~e.
The sorbant regeneration techniques known in the art are suitable for use with the sorbant material used in the practice o~ the present invention. In general, one can use the mo~t e~ficient techniques for the particular sorbant material and bed structure that ha~e been employed. The known regenerates for the strong base anion exchange columns typically u ed in nitrate removal include solution containing chloride, bicarbonate and/or carbon dioxide. Co-flow regeneration systems can be used with such re~ins with counterPlow regeneration ~ystems being recognized to provide high e~ficiencies in ion exchange resin regeneration. ~hen using KCl or NH4Cl solutions in the regeneration, the resulting solution is highly suited for use a3 ~ertilizer, eliminatiing a problem of ~-disposal of the used regenerating solution. ;~
For the particular sorbant material which is used and the ~elected ion which is desired to be 34,969-F -6~

~ . ~ ., .,, . . . ~ , . .. . .... ..

_7_ ~3~3~

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 Por removal capacity of the sorbant.
- In most cases this will mean that ~he membrane will have a high rejection rate for the non-selected ions and a very low, or possibly negative, rejection rate Por the selected ions. IP a membrane has a high rejection rate Por a particular ion, the concentration of the ion in the permeate solution whic~ passes through the membrane is decreased while the concentration of the ion in the reject portion of the liquid which iq not passed through the membrane increases. I~ 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 alYo possible for a membrane to have a negative rejection rate Por an ion iP the ion's ¢oncentration in the permeate becomes greater than the ion's concentration in the original liquid.
Membranes generally suitable for use in the practice of the pre~ent invention are known and are commercially available. Such membranes can include reverse oqmo~is membranes, nanofiltration membranes an~
ultrafiltration membrane~ which will exhibit the 3 neces~ary selectivity Por the particular ionic species.
PrePerred for use in the present invention are the nanoPiltration membranes, which ~all between the other two types in terms o~ the ~ize of the specie~ which will be pas~ed through the membrane. A3 used herein the term nanofiltration membrane rePerq to a membrane 34,969-F -7-`` 13~372 : `

having mlcropores or openings between chalns ln a swollen polymer network, which mlcropores are estimated to have an average diame-ter on the order of about 10 angstrom (l.e., one nanometer~. The membranes commonly known as reverse osmosls membranes wlll tend to re~ect practlcally all lons and therefore may not effectlvely separate lonic specles. Ultraflltratlon membranes, on the other hand, wlll typlcally not re~ect most lon~c specles but are found to re~ect hlgher molecular welght molecul~s based on thelr mole-cular welght. Certaln nanoflltratlon membranes have been found to exhlblt very good levels of selectlvlty between selected and non-selected lons. Illustratlve, lon-selectlve nanoflltration mem-branes preferred for use ln the practlce of the present inventlon ~ `~
are further descrlbed ln U.S. Patent 4,259,183 and U.S. Patent

4,765,897 lssued on August 23, 1988. Such a nanoflltratlon mem- ~`
brane can conslst of a thln fllm composlte membrane havlng a polyamlde dlscrlmlnatlon layer supported on a porous substrate.
The polyamlde dlscrlmlnatlon layer can be prepared by reactlon of `~
plperazlne and/or a substltuted plperazlne wlth one or more acld halldes. Preferably the polyamlde ls prepared from plperazlne and trlmesoyl chlorlde and optlonally a dlacyl hallde such as lso-phthaloyl chlorlde. Examples of such membranes lncludes the NF-40 or NF-50 Nanoflltration Elements, commercially available from Fllm ;
Tec Corporation, Minneapolis, Minnesota. These membrane elements `~
have been found very suitable for the aspect of the present inven-tion where a strong base anlon exchange resin ls thereafter used to remove nltrates from water. Thls membrane has been found to be ~-partlcularly effectlve ln re~ectlng a ~-33a372 high proportion of the sul~ate ions which are commonly found in water as well as the other large, divalent anions and cations. The permeate portion ~rom such membrane containing low levels o~ 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 prooess according to the present invention in any o~ the known membrane constructions. Such membrane constructions include spiral wound modules such as the afore-mentioned nanofiltration element, plate and frame constru¢tions, hollow fibers, and arrays of tubular membranes. Spiral wound membrane devices are generally preferred.
An embodiment o~ ~he 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 ~n the form of a single charge or a series o~
` periodic charges but is preferably in the form of a qtream which can be supplied continuously to the membrane element. The feed liquid (1) can be obtained from many sources including naturall~ oocurring surface or ground waterq or chemical processe~. Prior to the feeding of the stream to the membrane element9 there can be an optional pretreatment element (2) and the optional removal or by pass of a portion of this raw 34,969-F -9- `

-lo- 133~3~

water feed (3). The various pretreatment techniques which can optionally be used in preparing a raw ~eed 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. The amount and types of any such pretreatments will obviously depend upon the source of the raw feed li~uid and its inherent characteristios.
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 : .
C-34,969-F -10- ~

1, .
~33~2 treatment, the membrane element may comprise one or more qeparate membrane elements of ~he same or different constructions eombined in series and/or parallel conflgurations to produce the desired

5 ~eparation capacity. ;
From the membrane element the fir~t portion (11) of the liquid is then supplied to the sorbant material (20)~ A~ mentioned above9 the essential characteristic o~ the sorbant material is it3 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 re~in~ with functional moieties which will remove the selected ion and~or exchange ~or it one or more le~s detrimental ions. In the pre~erred embodiment of the pre~ent invention, a ~trong ba~e anion exchange re~in is used as the sorbant material to remove nitrate anions ~rom an aqueous stre~m. Suitable functional group~ include the trimethylammonium ~roups of type 1 re~in~ and the dimethylethanolammonium group~
of type 2 resins, or other groups which utilize the quaternized ammonium moiety or similar anion exchange moiety. Ion exchange beds of such re~ins, although proving ~omewhat ine~fective in the prior art processes ~-where the water oontained fairly high ~ulfate levels have proven very effective in the practice of the pre3ent invention to continuou~ly remove a large portion of the nitrate ions in an aqueou~ ~ulfate-containing ~tream with ~ufficient capacity ~uch that the number o~ required regenerations ~or an operating period i~ minimized. The purified ~tream from the 34,969-F -11-~12 133~372 sorbant material (21) is then available for the desired dispoQitionO
In the preferred embodiment o~ 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 ~rom an aqueous stream9 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 ~o produce the product qtream (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 op~ional pretreatment (2), around the membrane element (10) and the sorbant material (20) and directly or indirectly recombine this`
stream (3) with the product ~tream (22).
In a preferred embodiment of the present ` ~:
invention the membrane element produce~ a first permeate portion (11) having removed there~rom 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 variou~ ions and molecules which may be either desirable or undesirable ~: :
for the ultimate disposition o~ the product water (22).
This second portion (12) can therefore be very ~ ~
35 conveniently and efficiently treated by other : .:
technique~ to remove other unde~irable elements of the 34,969-F -12- :

-_13_ 13~3~2 water stream. For example, activated carbon can be employed in such optional treatment (13) to remove other known pollutan~s form the stream such as pesticidal or herbicidal chemicals~
In the nitrate removal embodiment o~ the preqent invention, there are several other surprising advantages according to the present invention. The membrane which has rejected the majority o~ ~he sul~ate ions has similarly screened or rejected not only the bacteria which are known to collect and grow in ion exchange bed_ but al~o the organic molecules produced in the degeneration of cellulo~ic materials, which organic materials foul the ion exchange resin and are a ~ood ~ource for any bacteria present in the ion exchange re~in. 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 o~ magne~ium and/or calcium in the original water -qupply which will-be concentrated in the second portion (12), thiq bleed-off (15) may be desired to ~often the water.

The techniques ~or regeneration of the sorbant material will vary according to the particular type of ~orbant and it~ method oP amployment. For example when the sorbant is an ion exchange resin which has been used to remove nitrate~ from an aqueous liquid, a 30 ~olution oP alternate anions Yuch aq chloride or bicarbonate i~ applied co- or counter-curren~ly and such alternate ion~ will displaoe the fixed nitrate ion~ from the ion exchange resin. This regeneration will produce an amount of regenerate waste brine (23) 35 containing a high concentration o~ the nitrate ions.
Thi~ waste brine (23) iq then appropriately di~posed o~

34,969-F -13_ :~.''. ;`

-14- ~ 3~ 2 and/or treated in an additional process, such as a biological process, which destroys the nitrates. In the latter ease 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-~ ;
10 i3 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 ~eed streams 5 are considered, the first comprising 100 milligrams qulfate ion per li~er and 100 milligrams nitrate ion -per liter, and a second feed tream comprising 150 ``
milligrams sulfate per Iiter and 100 milligrams nitrate 20 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 arve based on the u~e of about ~380 liters B of Dowex SBR-P brand ion exchange resin, a commercially 25 available strong base anion exchange reqin having trlmethylammonium functional groups. The membrane ~ystem is based on NF40-8040 nanofiltration element~
available from Film Tec Corporation whlch utilize a polyamide barrier layer. The ~ystem is composed of 17 3 preqqure vesselq each containing 6 ~pirally wound elements 40 inches long and 8 inches in diameter. The qy~tem is arranged to have a first stage with 11 of the pressure veq~els in parallel with the reject portion 35 going to a second stage which has the other 6 vessels in parallel. The feed water is ~upplied to the c~ ~ .. .

34,969-F -14- ~
. ' ~' ', '.' ~ ,~ ~;' ` -15- 1 33~372 membrane element at a rate of 100 cubic meters per hour (m3/hr) 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 o~
the bypass water flow rate to ion exchanged water Plow 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/1).
In the regeneration of both systems l r 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 5 applied rin e 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 u~ing the second raw water feed compo~ition containing greater ~ulfate ion concentration. :

D .,~

34,969-F -15-...~,....

~3~72 TABLE I
WATER SUPPLYo 100 m~/1 Nitrate, 100 m~/1 Sul~ate - " Combination "Stand Alone* Membrane/Ion Ion Exchange Exchange Ion Exchange Cycle time (hrs) 12 24.3 Ion Exchange Resin Content Sul~ate (eq/1) 0.36 0.07 (approx.) Nitrate (eq/1) 0.28 0.57 (approx.) Ratio of Regenerant Required to Nitrate Removed (eq. Cl per eq. N03 ) .5 . 5 -1 2~70-1 (kg NaCl per kg N) 23.0-1 11.3:1 10 Wa~e Brine Produced (m~/day) 37.7 16.3 *Comparative example, not an example of the present invention As can be 3een in Table I above 9 the prior art process produces 131 percent more waste brine than the process according to the present invention. As recognized by those Qkilled in thls area of technology the reduction in waste brine which must be further process and/or dispo3ed of in some faQhion is highly advantageous, both economically and ecologically. Table II below shows that with Qomewhat higher sulfate concentrations an even greater reduction in waste brine is observed.

TABLE II
WATER SUPPLY:100 mg/1 Nitrate, 150 m~/1 Sulfate "Stand AIone" Membrane/Ion Ion Exchange~ Exchang~

Ion Exchange Cycle time (hr~) 6.43 23.0 Ion Exchange Resin Content Sulfate (eq/1) 0.49 0;10 ~approx.) Nitrate (eq/1) 0O15 0t54 (approx.) 3 Ratio of Regenerant Required to Nitrate Removed (eq. Cl per eq. N03) 1003:1 2.85:1 (kg NaCl per kg N) 43.0:1 11.9.1 Wa~te Brine Produced -(m~/day) 70.2 16.3 ~;
35 ~Comparative example, not an example of the present invention -l6 ~ 33~72 With a sulfate concentration of 150 mg/liter, not uncommon in water ~rom 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 ~bove, 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 waite brine production due to the high efficiency of nitrate removal by the ion exchange unitO 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 quality9 particularly regarding health, corrosive and scaling propertie~. The ul~imate 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 al~o provides a filtering effect that prevents bacterial contamination of the ion exchange column and contains bacteria (if any) to membrane surface which may be easier to clean or disinfect than the exchange resin. In addition, thi~ reduces organic food supply to any bacteria present in the ion exchange column. This concentration i~
by the membran~ element of micro-pollutants, such as pesticidal and herbicidal chemicals, makes their removal through ~tandard meanq easier and mach more 35 ef~icient. The possibility of micro-pollutant surges ~`
out of the ion exchange column is therePore eliminated.
, ~. .

34,969-F -17- ~

18- ~33`~2 .
Since the nitrates which have been removed are now more concentrated in the waste brine, denitrification by ~.
other means such as biological becomes much e~sier~
It will be recogniæed by those skilled in the art that the present invention may be embodïed in other speci~ic forms in addition to those exemplified above -without departing from the spirit or essential characteristics thereof. The pre~ent embodiments are therefore to be considered in a1l aspects as illustrative and not restrictive of the present invention.

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34,969-F -18-

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for the 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 an ion exchange resin; 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 in that the ion-selective membrane is a nanofiltration membrane.

2. The process according to Claim 1 wherein the sorption process is ion exchange and the sorbant is an ion exchange resin.

3. The process according to Claim 2 wherein the second portion of the liquid is the reject stream from the membrane and the first portion is the membrane permeate.

4. The process according to Claim 3 wherein after sorption of the first ion the permeate is combined with the reject.

5. The process according to Claim 4 wherein the first ion is nitrate ion, the second ion is sulfate ion and the liquid is aqueous.

6. The process according to Claim 5 wherein the membrane comprises a thin film composite membrane having a polyamide discriminating layer prepared by the reaction of piperazine and/or a substituted piperazine with one or more acid halides.

7. The process according to Claim 6 wherein the membrane comprises a thin film composite membrane having a polyamide discriminating layer prepared by reaction of piperazine with trimesoyl chloride and optionally isophthaloyl chloride.

8. 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.