Method of ion exchange

The present invention relates to a method of reducing the concentration of contaminant ions, preferably nitrates ions, in impure water. The method comprises steps of: a) passing the impure water through an ion exchange resin to substitute regenerant ions from the resin for dissolved contaminant ions; b) passing a relatively dilute aqueous solution of wash-out ions through said resin to substitute wash-out ions for contaminant ions bound to the resin and; c) passing a relatively concentrated aqueous solution of regenerant ions through said resin to substitute regenerant ions for wash-out ions bound to the resin. The affinity of the resin to wash-out ions diminishes from being greater than that for contaminant and regenerant ions when exposed to a relatively dilute ionic solution to being less than that for said contaminant and regenerant ions when exposed to a relatively concentrated ionic solution. The method allows simplified ion exchange apparatus to be utilized, since the raw water, wash-out solution and regenerant solution may all be passed through the ion exchange resin in the same direction.

DESCRIPTION 
The present invention relates to an improved method of ion exchange and to 
apparatus for carrying out said method. In particular, the present 
invention relates to a method of removing ionic impurities from water, 
using an ion exchange resin to remove the unwanted ions from solution. 
In a conventional ion exchange process, water containing a solution of a 
contaminant first ionic species is passed through a column of ion exchange 
resin, said first ionic species becomes bound to the resin and, the resin 
releases into solution a second ionic species, which had previously been 
bound to the resin. The resin has a greater affinity for the first species 
than the second species and the second species is not considered a 
contaminant when the water is used for its intended purpose. 
Alternatively, the second species maybe hydroxide or hydroxyl Once the 
resin becomes fully loaded with the comtaminant species, it is regenerated 
by passing a concentrated solution of the second, or regenerant species 
through the column. The regenerant species displaces the contaminant 
species under these conditions primarily by mass action, because it is 
present in a very great excess. Thereafter, the resin may be used for 
further removal of the contaminant species. The identity of the first and 
second ionic species may be varied, so long as the resin will bind to the 
first or contaminant species in preference to the second or regenerant 
species, unless the water surrounding the resin has a large excess of the 
regenerant species dissolved therein, as it has during regeneration. More 
than one contaminant species may be present in the water prior to 
treatment and more than one regenerant species may be employed on the same 
column, so long as the resin's affinity for the contaminant species is 
greater than its affinity for the regenerant species. 
The removal of nitrate from water by anion exchange is well known. In such 
a process, the water to be denitrified is passed through a column of anion 
exchange resin. Most raw waters contain dissolved sulphates in addition to 
nitrates and standard ion exchange resins take up anions in the following 
order of preference: Sulphate-nitrate-chloride/bicarboonate (the 
affinities for chloride and bicarbonate do not differ greatly). Other 
resins exhibit different orders of preference; for example so called 
Nitrate selective resins show a reversed order of preference to the 
foregoing. When the nitrate content of the water leaving the column 
exceeds the desired level, the resin is fully loaded and the run is 
interrupted and the column has to be regenerated. 
A column of standard resin may be regenerated with a strong chloride 
solution, normally in the form of sodium chloride. Following regeneration 
with chloride, all the ions removed from the water being treated in the 
subsequent run, are replaced by chloride. This may yield an unacceptably 
high chloride concentration in the final water composition. The ratio of 
dissolved bicarbonate to chloride should be maintained at or above 2:1 
(the bicarbonate being more abundant) to avoid the dezincification of 
brass plumbing fittings by chloride. Therefore, where required, partial 
regeneration with bicarbonate in addition to chloride is used, to improve 
the quality of the product water. The object of regeneration, therefore, 
is to restore the column as far as economically possible to the chloride 
or the chloride and bicarbonate form, before the next product run. 
For chemical economy, only a part of the total available ion exchange 
capacity of a given resin is normally regenerated. However, even under 
such a regime, the regeneration procedure still requires an excess of 
regenerant. It therefore produces a waste which contains the sulphate and 
nitrate removed from the water, plus a substantial amount of surplus 
chloride (and possibly bicarbonate) regenerant. 
This waste of substantial quantities of regenerant raises the costs of the 
process, both with respect to the demand for regenerant chemicals and with 
respect to the disposal of the spent effluent. Disposal can present 
serious difficulties in some locations, where the possiblity of spent 
regenerant being put on the land, or otherwise disposed of to the 
environment is undesirable and may be restricted. It is the high chloride 
content of the spent effluent, which usually prevents disposal of spent 
regenerant into the enviroment. 
The problem is particularly acute with waters containing a high sulphate to 
nitrate ratio. The standard resin removes sulphate before nitrate, which 
reduces the resin's effective capacity for nitrate, shortens production 
runs, increases the regenerant demand and the amount of spent regenerant 
for disposal. Nitrate specific anion resins which have been developed 
specifically to overcome this difficulty only overcome it in part and, 
also show a reduced capacity for nitrate in the presence of large amounts 
of sulphate in the water. 
According to the present invention there is provided a method of reducing 
the concentration of contaminant ions in impure water said method 
comprising the cycle of: 
(a) passing said impure water through an ion exchange resin to substitute 
regenerant ions from the resin for dissolved contaminant ions, thereby 
reducing the concentration of said contaminant ions in the water; 
(b) passing a relatively dilute aqueou solution of wash-out ions through 
said resin to substitute wash-out ions for contaminant ions bound to the 
resin and; 
(c) passing a relatively concentrated aqueous solution of regenerant ions 
through said resin to substitute regenerant ions for wash-out ions bound 
to the resin; wherein the affinity of the resin to wash-out ions 
diminishes from being greater than that for contaminant and regenerant 
ions when exposed to a relatively dilute ionic solution, to being less 
than that for said contaminant and regenerant ions when exposed to a 
relatively concentrated ionic solution 
The term "wash-out ions" is used to define an ionic species having 
properties with respect to any ion exchange resin, regenerant ionic 
species, or contaminant ionic as set out above. This is a well known 
property of polyvalent ions, however it and the present invention are not 
limited to such ions. 
The resin used in the method of the present invention may be a standard ion 
exchange resin, however the invention is not limited to the use of such a 
resin and any form of resin whose properties allow the practice of the 
inventive method, may be employed 
Since the method of the present invention takes advantage of differences in 
the resin's affinity to different ionic species during steps (b) and (c), 
as opposed to relying on a significant excess of regenerant ions to force 
the contaminant ions out of the resin, by mass action, considerably less 
regenerant material is required. This both reduces chemical costs and the 
problem of disposing of the spent effluent. The latter being especially 
important where the regenerant ion is environmentally undesirable, such as 
chloride. 
Preferably, the affinity of the resin to bind both contaminant and 
regenerant ions is substantially insensitive to variations in the total 
ionic concentration in solution about the resin and the water to be 
treated may be contaminated with wash-out ions as a second contaminant 
species, which ions are exchanged for regenerant ions during step (a), the 
purification step. Advantagously, when the wash-out ions are present in 
the contaminated water, step (b), the wash-out step, may comprise a 
continuation of the purification step after the concentration of 
contaminant ion has risen above a predetermined level, above which the 
water would not be suitable for its intended use. 
Advantageously, the solution of wash-out ions is passed through the resin 
in the same direction as the impure water in the purification step (a). 
The solution of regenerant ions also may be passed through the resin 
during the regenerant step (c) in this same direction. Passing all the 
solutions through the resin in the same direction, known as "co-flow" 
operation, allows considerable simplifications to be made in the apparatus 
employed to carry out the method of the present invention. Hence said 
apparatus is more easily constructed and costs less to construct. 
Additionally, operating the resin in co-flow causes the ionic composition 
of the treated water to remain relatively stable over time and thus easier 
to deal with. 
In an embodiment, all said ions are anions and preferably, the wash-out 
ions are poly-or divalent. Furthermore, a plurality of contaiminant ions 
of different species may be present and additional regenerant ions of 
differing species may be empolyed, so long as the affinity of the resin to 
any species of contaiminant ion is greater than its affinity to any 
species of regenerant ion and, the affinity of the resin to the wash-out 
ions varies above and below its affinity to the contaminant and regenerant 
ions in the manner set out above. 
In a most preferred embodiment the contaminant ions are nitrate, possibly 
in admixture with sulphage, and the wash-out ions are sulphate. The 
regenerant ions are preferably chloride, possibly followed by bicarbonate 
or in admixture therewith. 
The methods of the presebnt invention are particularly suited to treating 
water which contains up to 10 mg equivalents perlitre of each specific 
ionic contaminant species. The concentration of wash-out ions is 
preferably up to 40 mg equivalents per litre and more preferably up to 30 
mg equivalents per litre. The concentration of regenerant ions should be 
over 500mg equivalents per litre and preferably over 1000mg equivalents 
per litre. 
In a second aspect, the present invention provides apparatus for carrying 
out the method of the first aspect of the invention. In embodiments of the 
second aspect, apparatus is provided for carrying out the embodiments of 
the first aspect set out herein. 
This invention finds a particular application in the denitrification of 
water, minimises the difficulties otherwise experienced in denitrifying 
water and is especially applicable to those waters containing substantial 
amounts of sulphate. The affinity of ion exchange resins for sulphate ions 
is a function of this ion's bivalent nature, and its high affinity at low 
total ionic concentrations diminishes rapidly at high total ionic 
concentrations. In fact, regeneration with concentrated brine removes 
sulphate quite readily, whereas mono-valent nitrate, whose affinity is 
substantially unaffected by the total ionic concentration, is less easily 
removed. 
The sulphate-containing solution used for the wash-out step may be obtained 
in various ways. Given a raw water with a high sulphate:nitrate content, 
the simplest method is to continue the purification step past its 
determination point, running the product to waste, and using the resin's 
high affinity for sulphate to load it with more sulphate in place of 
nitrate. However, this is wasteful in water and time. In the regeneration 
following the wash-out step, the sulphate loaded resin is so easily 
regenerated, that the early and middle portions of the resulting eluate 
contain almost no excess regenerant and the anionic content is almost 
entirely sulphate. Thus a fraction of this eluate may be recovered as a 
source of sulphate for the wash-out step in the following cycle. 
In one method, this recovered sulphate may be added to the raw water to 
enhance its sulphate content, but at a sufficiently low concentration for 
the resin's affinity for sulphate to remain high. This device reduces the 
waste of both water and time, as compared with using only raw water for 
the wash-out step, a similar result is nevertheless achieved. 
Alternatively, recovery of spent eluate in the manner set out above, in 
some cases may produce such an abundance of sulphate that, even if the 
recovered solution is applied at or near its full strength, the excess of 
sulphate available will be sufficient to load the resin to the desired 
level through the mass action of the excess sulphate. This reduces the 
waste of water and time even further. 
The wash-out step may be applied in co-flow or counter-flow (i.e the same 
direction of flow as the water being denitrified, or the opposite 
direction, respectively). In normal conditions co-flow appears to give 
better results. The wash-out step may be performed by either of these 
methods, provided the supply of sulphate and its concentration are such 
that the resin is converted substantially to the sulphate form. After a 
wash-out step by either of these methods, the desired degree of 
regeneration can be achieved with a much smaller excess of regenerant than 
would have been the case without the wash out step. 
The ease with which sulphate-loaded resin may be regenerated may be 
exploited further. In conventional denitrification by ion exchange, 
counter-flow regeneration is frequently required in order to obtain the 
necessary degree of regeneration at the exit end of the column, in order 
to yield a product water whose residual nitrate content is acceptable. The 
method of the present invention allows the desired product quality to be 
obtained with an economical quantity of regenerant applied in co-flow, 
after the wash-out step, which is simpler and uses a less costly type of 
ion exchange apparatus. 
In cases where water quality considerations call for the resin to be 
regenerated with bicarbonate as well as chloride, the bicarbonate 
conventionally has been applied to the resin separately, after the 
chloride. After a wash-out step in accordance with the present invention 
however, the ease with which sulphate is displaced, makes it possible to 
use a mixed chloride and bicarbonate regenerant, which simplifies the 
procedure and reduces the bulk of waste regenerant for disposal. 
In a most preferred embodiment, the impure water includes nitrate and 
sulphate contaminant ions, the solution of regenerant ions includes 
chloride and bicarbonate ions, and at least a portion of the solution 
derived from the resin in the regenerant step (c), which includes sulphate 
and bicarbonate ions, is used as a source of sulphate regenerant ions. 
This solution may be used as a source of sulphate wash-out ions because 
substantially no chloride ions are passed out of the resin during 
regeneration and, the bicarbonate ions are not taken up readily by the 
resin in the presence of sulphate ions. Once the resin is in the sulphate 
form, after wash-out, the take-up by the resin of bicarbonate is enhanced, 
thus allowing the use of chloride and bicarbonate ions together in the 
regeneration step (c). 
The following example illustrates the advantage of the present invention 
over conventional methods.

EXAMPLE 1 
Water containing 3.0 mg equivalents per litre of sulphate, 1.5 mg 
equivalents per litre of nitrate and 1 mg equivalents per litre of 
bicarbonate and chloride respectively was denitrified using a column 
containing 1 litre of type II macro-reticular anion exchange resin. In all 
runs the regenerant used was 1 litre of 1000 mg equivalents per litre 
sodium chloride solution, applied in counter-flow. All production runs 
were terminated when the nitrate concentration in the product water rose 
to 0.8 mg equivalents per litre. After three cycles of run and 
regeneration, the operation of the resin was considered to have stabilized 
and the volume of water produced was recorded. 
When operated conventionally, with alternate production runs and 
regeneration runs, each production run produced 129 litres of water. 
A second series of trials was then run with an intermediate wash-out step 
carried out at the end of each production run. In each intermediate 
wash-out step 33 litres of untreated water, having a sulphate content 
increased to 26 mg equivalents per litre were passed through the resin. 
With the wash-out step, each production run produced 171 litres of water. 
Furthermore, the spent eluate in the control series, that is without any 
wash-out step, contained 473 mg equivalents of surplus chloride; whereas 
with the wash-out step, the surplus chloride from the regenerant step was 
reduced to 280 mg equivalents per litre. 
EXAMPLE 2 
A water of the composition given below was treated in a column containing 1 
litre of Amberlite IRA 910, a conventional macroporous Type II anion 
exchange resin. The cycle was designed to yield a product of such a 
quality that when mixed in equal proportions with untreated water, the 
resulting blend would always contain less than 0.8 mg equiv/1 of nitrate, 
and whose content of bicarbonate would always be at least twice that of 
chloride, measured in equivalent units. 
To achieve this objective, the exchanger was treated with a washout which 
contained all the spent regenerant from the previous cycle, diluted by 
rinse water, and further diluted with raw water. Regeneration was in two 
stages, a chloride solution being followed by a bicarbonate solution. Both 
washout and regeneration were in co-flow. The column was operated for a 
number of cycles for the resin and solutions to reach a kinetic 
equilibrium. 
The volumes in litres and concentrations in milligram equivalents./litre 
(meg/1, referring to bulked samples) which were obtained in this way are 
given in the table below. 
______________________________________ 
Regenerant 
Bicar- 
Out (dil- 
Raw Chloride bonate 
uted with 
Water Washout Stage Stage rinse) 
______________________________________ 
VOLUME 300 25 0.5 0.75 3.25 
(litres) 
ANION CONCENTRATIONS (meg/l) 
Cl-- 0.85 1.4 1130 0.1 15 
SO4-- 1.21 32 0.1 0.1 255 
NO3-- 1.06 1.0 0.1 0.1 8.3 
HCO3-- 3.28 1.5 0.1 480 13.4 
______________________________________ 
As all the spent regenerant was re-used for washout, the sole effluent for 
disposal from this was 25 litres of washout, with a composition: 
______________________________________ 
ANION meg/l 
______________________________________ 
Cl-- 6.3 
SO4-- 11.8 
NO3-- 8.2 
NCO3-- 9.1 
______________________________________ 
To achieve the quality specification for the product water by conventional 
direct regeneration would require roughly 50% higher regenerant usage, 
calculated in terms of cost to allow for both chloride and bicarbonate. 
EXAMPLE 3 
A water of the composition set out below was treated with a column 
containing 1 litre of Amberlite IRA 910 (see Example 2). A fraction of the 
spent regenerant was retained and diluted with raw water to make up 
washout for the following run. Regeneration was with sodium chloride in 
counterflow, and the column operated in such a way that the product always 
contained less than 0.8 meg/1 of nitrate. The unit was operated for 
several cycles in order to establish a stable working equilibrium, when 
the following quantities and concentrations were observed: 
______________________________________ 
Raw 
Water Washout Regenerant 
______________________________________ 
VOLUME (litres) 
269 33 1 
ANION 
CONCENTRATIONS (meg/l) 
Cl-- 1.16 1.16 1000 
SO4-- 2.2 25.0 2 
NO3-- 1.06 1.1 1 
HCO3-- 0.93 0.93 1 
______________________________________ 
The product water contained on average 0.47 meg/1 of nitrate, so that the 
net removal was equivalent to 0.59 meg/1, i.e. 159 meg for the whole run. 
This nitrate capacity is equivalent to 16% of the NaCl regenerant applied. 
Conventional regeneration to achieve a similar product from a water so 
high in sulphate would take about 50% more NaCl regenerant.