The Basics of Ion Exchange and Water Chemistry

8/3/2019 The Basics of Ion Exchange and Water Chemistry

http://slidepdf.com/reader/full/the-basics-of-ion-exchange-and-water-chemistry 1/4Water Conditioning & PurificationA P R I L 2 0 0 7

The Basics of

Ion Exchange

and

Water Chemistr y

By C.F. ‘Chubb’ Michaud, CWS-VI

Par t 2

I

n Part 1 of this two-part series, we

discussed basic chem istry and ioniza-

tion and the value of the Periodic

Table of Elements to th e wa ter treatmen t

professional. In Part 2, we examine the

proper use of a water analysis and pit-

falls to avoid in deciphering it.

To properly d esign a water treatment

system, particularly with ion exchange

and reverse osm osis (RO), it’s necessary

to first get both a quan titative and qu ali-

t a t ive l i s t ing o f what the in tended

feeds tream contains . This l i s t ing is

known as the water analysis and a p roper

interpretation is a must to assure good

results. Although the purpose of an ion

exchange system is to remove only theof fend ing ion ic component s o f a

feedstream, other factors such as tem-

pera ture, total d issolved solids (TDS), pH

and trace minerals also play a role and

mu st therefore be considered.

Laboratories usually report a water

analys is us ing cer tain approved tes t

method s, which give the results in milli-

grams p er liter (mg/ L). This is conve-

nient because one mg/ L is equal to one

ppm, or part per million. This number,

however, is in units of weight. Ion ex-

changers, on the other hand, don’t deal

with w eight; they d eal with ions, which

are the real chemical compon ents we are

trying to remov e. A milligram of magne-

sium or calcium does not contain the

same nu mber of ions or ionic equivalents

as does sodium or hyd rogen. The conven-

tion comm only used is to convert to ppm

as CaCO3(calcium carbonate). Confusion

arises because both the mg/ L value and

the CaCO3

value can be and often are re-

ported as pp m. A good p ractice would be

to refer to elemental components (the

analysis) as mg/ L and the CaCO3equiva-

lents (the conversion) as ppm .

The convention: CaCO 3 

as ppm and ppm as CaCO 

3 CaCO

3is an arbitrary nam e choice.

It has a formula or molecular weight

(MW) of 100 (comp ared to carbon w ith a

MW of 12). Both the calcium (Ca+2) and

carbonate (CO3

-2) ions are divalent; i.e.,

they have a charge value of +2 and -2,

respectively (comp ared to sod ium at +1)

and , thus, an equivalent w eight of 50.

The equivalent weight of any sub-

stance is equal to its MW divided by its

valence. In the case of CaCO 3, this is 100÷ 2 = 50. It shou ld be noted that n either

Ca +2 nor CO3

-2 have an equivalent weight

of 50, but the combination does. The

equivalent w eight of Ca+2 is 20 (MW = 40

÷ 2 = 20) and the equivalent weight of 

CO3

-2 is 30 (MW = 60 ÷ 2 = 30). We m ust

therefore equate even the Ca and CO3

content of water to the equivalent weight

of CaCO3. We do th is by multiplying by

a conversion factor (wh ich is derived by

dividing the number 50 (the equivalent

weight of CaCO3) by the equivalent

weight of the substance). In the case of 

Ca, th is is 50 ÷ 20 = 2.5. For CO3, it’s 50 ÷

30 = 1.67. Note that for demineralizer

calculations, the CO3

-2 ion will not exist

as a divalent carbonate ion but as a

monovalent bicarbonate ion (HCO3

-1) with

a conversion factor = 0.82). We can

readily see that most common compo-

nents of water have a different molecu-

lar weight, so we will have a variety of 

conver sion factors. Table 1 lists the com -

mon elements and their conversion fac-

tors. A simple w ater an alysis converted

from mg/ L to ppm as CaCO3

is shown

in Table 2.While the total dissolved mineral

content of this water (residu al by evapo-

ration) would measure 432 mg/ L of raw

water (cation = 113.4 + anion 300.4 plus

silica 18 = 431.8), the TDS as CaCO3

is

273.5 pp m for deionization (or DI) pur-

poses. One does not add the cation and

anion valu es together to get total TDS as

CaCO3.

For anion d eterminations, the silica

is quoted as an afterthought: “I have

273.5 ppm water with 15 ppm of silica.”

For mixed bed calculations, this is 288.5

ppm water. Since a grain (of mineral) is

17.1 ppm of TDS as CaCO3, we have 10

grain water (Ca + Mg = 170 ppm as

CaCO3) and for dealkalization, it’s a 10.5

grain water (HCO3

+ CO3

= 184 ppm as

CaCO3). There are 16.0 grains of cations

and 16.9 grains of anions for deionization.

Every ion has a partner Every ion is assumed to have a

counter  ion (as a dancing partner, so to

speak). It should be noted that with ex-

treme pH conditions (i.e. <4 or >10), there

w ill be an excess of cations or a nion s, re-

The Water Analysis

Table 1. Conversion factors for

common water componentsCations Anions

Ca++ 2.50 HCO3- 0.82

Mg++ 4.10 CO3= 0.83*

Na+ 2.18 SO4= 1.04

K+ 1.28 Cl- 1.41

Fe++ 1.79 NO3- 0.81

Mn++ 1.82 SiO2

0.83*

*For ion exchange purposes, it is assumed that carbonatereacts as the monovalent ion. SiO

2is considered to be

weakly ionized H2SiO

3(silicic acid MW= 82. SiO

2has a

MW=60 and is considered to be removed as monovalentSiO

2-1, the conversion is calculated as 50/60 = 0.833).

8/3/2019 The Basics of Ion Exchange and Water Chemistry

http://slidepdf.com/reader/full/the-basics-of-ion-exchange-and-water-chemistry 2/4A P R I L 2 0 0 7Water Conditioning & Purification

spectively. Normally, every cation has an anion (with the ex-

ception of silica) so the total cations shou ld equ al the total an -

ions (without silica). Silica, a weakly ionized acid, is presumed

to exist (for DI pur po ses) as H2SiO

3(silicic acid) and has H + as

its partner. It therefore stands alone as an anion.

Sometimes the water analysis will be incomplete in that

only th e offend ing ions (calcium , magn esium, iron, alkalinity,

sulfate and silica) are reported—sodiu m an d chloride are m iss-

ing. If the analysis appears incomplete, look for the obvious.

You can estimate the p pm as CaCO3

by dividing conductivity

(as micromhos, or mm hos) by 2.5. In Table 2, we show condu c-tivity as 650 um hos. Dividin g by 2.5 gives us a TDS of 260 pp m.

If the totals for cation and anion are not equal, we make

them equal by add ing to the sodium (Na +) or chloride (Cl-) val-

ues. For instance, if the cation total were 15 less than th e anion,

we would add 15 ppm to Na+ as CaCO3

to the cation load . In-

clud e the ppm as CaCO3

values for all m onovalent cations (K+,

NH4

+) as part of the Na+ total and m onovalent anions (NO3- or

F-) as Cl- totals. For DI purposes, iron (Fe+2) can be treated as

Ca +2 after conversion.

We then add silica value to the an ion total to get the total

anion load. This is done after balancing the cation and aniontotals. For the purposes of capacity calculations, it is generally

safe to ignore any item s with valu es below 0.1 ppm . Dividing

these corrected totals by 17.1 converts th e pp m as CaCO3

val-

ues to grains per gallon (gpg) values. Since the ion exchange

capacity is usually deter mined in kilograins (Kgr) per cubic foot,

(one Kgr = 1,000 grains), we can now determ ine the throu gh-

pu t capacity in gallons per cubic foot (gal/ ft3) of resin. Simp ly

divide th e grains of loading into the capacity of the resin.

Traps Values for any given water analysis are not done for the

convenience of the poor engineer w ho is trying to treat the wat er.

They are done by convention. Hard ness (Ca and Mg) and alka-

linity (HCO3

+ CO3

+ OH) are often given as ppm as CaCO3.

Metals, includ ing iron, are often given in micrograms/ L or pp b

(billion) and written as µ g/ L. The µ sym bol is the Greek letter,

mu and it stands for micro (millionth) and not milli (thou-

sandth). Nitrates (and amm onium [NH4]) are often rep orted in

ppm as N (nitrogen). This has to be converted to ppm as NO3

by mu ltiplying by MW ratios. N = 14 and N O3

= 62. Therefore,

10 ppm NO3

as N becomes 10 x 62/ 14 = 44.3 pp m a s ion and

44.3 x 50/ 62 = 35.7 pp m a s CaCO3. SO

4and H

2S may be re-

ported as total sulfur and also must be converted to ion; then to

ppm as CaCO3.

The capacity of DI resin is dep endent up on the w ater analy-

sis, particularly the ratio of sodium to total cation and alkalin-

ity to total anion. First, simp lify the w ater an alysis by grou ping

Table 2. Wate r analysis conversions

as CaCO3

as CaCO3

Cations mg/L ppm Anions mg/L ppmCalcium 60 150 Bicarbonate 220 180

Magnesium 4.9 20 Carbonate 5 4

Sodium 45.9 100 Sulfate 38.5 40

Potassium 2.3 3 Chloride 32.6 46

Iron (ferrous) 0.3 0.5 Nitrate 4.3 3.5

  Subtotal 113.4 273.5   Subtotal 300.4 273.5

Silica 18 15

288.5

Temperature 68°F; pH 7.6; Turbidity 0.5 NTU; Color 35 APHA; Conductivity 650 umhos

8/3/2019 The Basics of Ion Exchange and Water Chemistry

http://slidepdf.com/reader/full/the-basics-of-ion-exchange-and-water-chemistry 3/4Water Conditioning & PurificationA P R I L 2 0 0 7

tem. There is no one-size- fits-all solution.

The ratios of various ions to on e another

will cause the resin capacity to vary as

will the qu ality of effluent one is target-

ing. Flow rate p er cubic foot will also af-

fect capaci ty as wi l l temperature of  

service and regenerant. In addition, the

amount of regenerant is usually deter-

mined by the leakage values (quality)

needed. Effluent water quality is what

sets the whole thing in m otion. Leakage,the background ions that appear to be in-

complete removal of unw anted ions, is a

result of incomplete regeneration. In this

example, the literature tells us that we

can expect a leakage of app roximately 0.5

percent sodium as a percentage of total

cations or (0.005 x 273.5= 1.4 ppm). This

leakage will exist in the product water

as sodium hydroxide (NaOH) because

the an ion resin will convert all anions to

the hyd roxide: H+ converts to HOH and

Na+ converts to NaOH . So wh at does this

mean in water quality? Again, accord-

ing to the literature, NaOH has a resis-

tivity that is about on e-fifth th at of NaCl.

To pu t it another w ay, NaOH is five times

more conductive than an equivalent ppm

of NaCl. 1.4 pp m of N aOH gives a resis-

tivity of 120,000 ohm s/ cm at 25°C (77°F)

or about 8.3 microsiemens (µS).

Sodium leakage is reduced w ith in-

creased regeneration level. In the above

examp le, increasing th e HCl regeneration

level to eight lbs/ cu.ft. would imp rove the

leakage to 0.9 pp m, redu cing the cond uc-

tivity to abou t five µS. Always star t with

the determination of how low you r leak-age has to be (using NaOH as effluent)

which sets the regeneration level. The

regeneran t level sets the capa city and the

capacity sets the volum e of resin needed .

Softener loading There is more to building a softener

than simply m easuring hard ness of the

water and setting th e dial. Your custom er

not only wa nts his or her water softened

today, they want it softened tomorrow,

next mon th an d 10 years from now. This

means the regeneration p rocedure m ust

Table 3. Leaka ge versus total

dissolved solids (TDS) dosage

chart

TDS of feedwater* Pounds of salt

200 2

500 5

800 10

1,200 15

1,500 20

2,000 25

*Expressed as ppm as CaCO3.

the ions to show only Ca, Mg, Na as cat-

ions and H CO3, SO

4, Cl and Silica as an -

ions. Fe ad ds to Ca; K to Na; CO3to HCO

3

and NO3

to Cl. Conversion calculations

can be round ed up to w hole num bers and

percentage calculations can be app roxi-

mate (with the charts you will have to

read, the width of your pencil lead is a

percent or two). Alw ays err in the d irec-

tion of being conservative. Always un-

derstate your capacity. No one has everbeen held liable for a DI system that still

works and delivers after three years. If 

you design your system to just barely

squeak by on Day One, it will not work on

Day Two and all those following. Let’s

give it a try!

 Example: Determine the design and size of 

a DI system that will handle 20 gpm for 12

hours using the above water analysis. Use

six pounds per cubic foot (lbs/ft 3) of HCl for 

regeneration of cation and seven lbs/ft 3 of 

room temperature caustic (NaOH) for the

anion.Condensed water analysis

as CaCO3

as CaCO3

Cations ppm Anions ppm

Calcium 150.5 Alkalinity 184.0

Magnesium 20.0 Sulfate 40.0

Sodium 103.0 Chloride 49.5

Total cation 273.5 Subtotal 273.5

Silica 15.0

Total anion 288.5

Resin capacity determination 

To use most engineering literatureand charts, you first have to break dow n

this analysis in percentages. Our cation

conten t is 37.7 per cent N a, 55 percen t Ca

and 7.3 percent Mg. Our anion is 63.8

percen t alkalinity, 31.0 percen t free min-

eral acid (total of Cl + SO4) and 5.2 per-

cent silica. You can use 40 percent for

sodium and 65 percent for alkalinity, etc.

The literature shows that the cation ca-

pacity (at approximately 40 percent Na

and 65 percen t alkalinity) to be 27.5 Kgr/ 

ft3. Using a 10 percent engineering dow n-

grade, we have a net design capacity of 

24.75 Kgr/ ft3 (27.5 x .9 = 24.75) for the

cation. The anion (use a Type II) will

have a book capacity of 20.3 Kgr/ ft3

(with five percent silica in the influent)

and we will downgrade this by 15 per-

cent for design purposes (multiply by

0.85), wh ich leaves u s w ith 17.25 Kgr/ 

ft3. The engineering downgrade factor

is a safety factor applied to DI calcula-

tions to allow for wear and tear, resin

loss and som e fouling, as well as varia-

tions in the feedstream over the life of 

the resin. It is usua lly 10 percent for cat-

ion resins and 15 percent for anion res-

ins and is deemed to be a three-year pro-

  ject ion. In other words , the sys tem

shou ld still meet capacity specifications

after three year s of cap acity losses.

Since we h ave an anion load of 16.9

gpg, we w ill have to remove 16.9 gr/ gal

x 20 gal/ min x 60 min/ hr x 12 hr/ cycle

= 243,360 grain s/ cycle. Divid ing this by

17.25 Kgr anion capacity, we see we’ll

need 14 cubic feet of anion resin.

Since the cation w ill have to prod ucethe water required to regenerate the an-

ion resin, we must now add that quan-

tity of water to our cation load before

determining the size of the cation ex-

changer. The total gallons are 20 gpm x

60 min/ hr x 12 hr/ cycle = 14,400 gal.

Assuming 75 gallons of water is required

to regenerate each cubic foot of anion

resin, add 1,050 gallons (75 gal/ cu.ft. x

14 cu.ft.). The cation m ust th erefore treat

15,450 gallons (x 16.0 gpg) or 247,200

grains. Dividing this by our cation rat-

ing of 24.75 Kgr, we will need 10 cubic

feet of cation re sin.

The physical design guidelines Standard and acceptable flow rates

for DI design are one to three gpm/ cu.ft.

In our above d esign for a 20-gpm system,

we are at 1.4 gpm / cu.ft. of anion and 2.0

gpm / cu.ft. of cation. Hyd raulic consid-

erations are 4 to10 gpm / sq.ft. of bed area.

If we choose a 30-inch diam eter tank for

the anion, we have 4.0 gpm/ sq.ft. and a

24-inch tank for the cation gives us 6.4

gpm / sq.ft. Most capacities are calculated

on a m inimum of a 30-inch bed depth (thecapacity drops with shorter beds) and a

60-inch maximum. Our 10 cu.ft. of cat-

ion w ill have a bed dep th of 38.2 inches

and our 14 cu.ft. of anion w ill have a bed

dep th of 34.3 inches. It is suggested th at

un der-bedd ing be used in DI systems to

ut ilize the fu ll cap acity of the resin. If this

service requirement of 12 hours is ex-

tended, it may be possible to add resin

to either or both vessels to make up the

extra capacity. If plastic tanks are used,

they are u sually only 72 inches high and

the u sable straight sidew alls are only 13.4

and 20.2 cu.ft. respectively. This means

that the freeboard is only about 35 per-

cent for the cation and 44 percent for the

anion. These are minimal and neither

system can tolerate more resin. To in-

crease the capacity, the user w ould have

to install a duplicate system or a new,

larger system. However, if you started

with 84- or 96-inch sidewall tanks, the

capacities could be increased by 50 to 60

percent simply by adding resin.

Resin capacities are d epend ent on

the water analysis (among other things)

and therefore not constant for every sys-

8/3/2019 The Basics of Ion Exchange and Water Chemistry

http://slidepdf.com/reader/full/the-basics-of-ion-exchange-and-water-chemistry 4/4A P R I L 2 0 0 7Water Conditioning & Purification

also be a rejuven ation procedu re to keep

the u nit operating satisfactorily for ma ny

years. The water analysis can help us

determine how to d o this.

Softener throughput is influenced

not only by hardness, but also by TDS,

iron, temperature, flow rate and regen-

eration level and technique. Since TDS

and iron will generally be part of the

water analysis, we’ll look at those.

Ha rd w ater leakage is caused by re-sidual hardness that is left on the resin

after regeneration and bleeds off during

the service run. Increasing th e salt dos-

age can minimize it. As hard water p asses

through a resin bed, the hardness is ex-

changed for sodium or potassium. The

higher the sodium level (or TDS feed

level), the higher the tendency for the

softened w ater to leach hardn ess back off 

the resin. This reduces the run length

(and thus the capaci ty) between the

baseline leakage and the breakthrough

leakage. Simply kn owing the TDS ahead

of time can allow y ou to avoid costly field

calls to remed y low capacity or leakage

complaints by ad justing the cap acity set-

ting and u sing a h igher salt dose ahead

of time. To achieve five ppm (or less) leak-

age during the run, use the salt settings

from Table 3 for var ious TDS values.

Soluble iron is exchanged onto a cat-

ion exchanger as Fe+2. However, iron

may oxidize on the resin to Fe+ 3 and is

not read ily removed by salt regeneration.

In addition, NaCl usually produces an

alkaline pH brine wh ich w ill precipitate

iron du ring regeneration. By assigning a

higher v alue for iron, we w ill increase the

softener load an d red uce the throughpu t

volum e. This means you will regenerate

more frequently (reducing the p robabil-

ity of iron oxidizing on and in the resin).

To overcome the potential problems of alkaline brine, you can u se a resin cleaner

that autom atically d ump s phosph oric or

citric acid into the brine du ring regenera-

tion or use potassium chloride (KCl),

which general ly produces a s l ight ly

acidic pH .

A good p ractice is to treat each p pm

of iron as on e grain of hard ness. As such,

in our sample water analysis, we have

10 grains loading from hard ness and w e

add 0.3 grains for the iron (total = 10.3).

Soluble iron levels as high as 30 ppm has

been successfully treated w ith a stand ard

softener w ith 10 to 12 pou nd s of salt/ ft3

regeneration level. Citric acid (available

from most chemical suppliers) works

wel l at a level of one pound per 50

poun ds of salt and can be added directly

to the brine tank.

Turbidity

Dirty waters can plug and foul ion

exchang e units, causing chann eling and

capacity loss. Use a pre-filter if the tur-

bidity values are >5 NTU (nephelomet-

ric turbidity un its).

ColorNa tura l organics (such as tannins) or

iron (colloidal, organic or precipitated)

ma y cause color, reported as APHA u nits.

Values for color below 25 APHA are u su-

ally not n oticeable by eye. Again, try to

determ ine what is causing the color and

install proper prefiltration. Softeners d o

not rem ove color. Granu lar activated car-

bon (GAC) and/ or salt regeneration an-

ion resin can often do the job.

Temperature

Ion exchange systems are usually

intended to function w ith water feed tem-

peratures of 10 to 37.77°C (50 to 100°F).

Higher temperatures can be d etrimental

to an ion resins in DI systems. A lab-sup -

plied water analysis may list tempera-

ture, but it is meaningless. Rather, check 

with th e intend ed installation site if an-

ion exchange enters into th e picture. Cat-

ion systems should have n o trouble with

tem per atu res abo ve 121.11°C (250°F) .

Much of the ion exchange process

dep ends up on ions’ ability to d iffuse into

and out of the resin bead m atrix. This is

temperature depend ent and is seriously

slowed by cold-water op erations. Resin

beds shou ld be at least 50 percent larger

in d iameter and 100 percent larger in v ol-

um e to effectively hand le water streams

below 4.44°C (40°F).

Conclusions Obtaining and using a good water

analysis is essential to the proper design

of any water filtration system, particu-

larly an ion exchanger. There is muchvaluable information on a lab analysis

that can help you to avoid d esign errors.

Make sure you understand the water

analysis. Check the math to make sure

the units add up. Make sure the cations

are equal to the anions and then add in

silica to determine total loading. Use a

conservative design with a downgrade

for engineering, even for softeners.

References 1. Dictionary of Chemistry , McGraw-Hill, NewYork, 1994.

2. Kunin, Robert, Ion Exchange Resins, KriegerPu blishing , New York, 1972.

3. Wachins ki, A.M. and J.E. Etzel, Environmen-

tal Ion Exchange, Lewis Pu blishers, New York,1997.

About the author  C.F. ‘Chubb’ Michaud 

is the CEO and Technical

  Director of Systematix

Company, Buena Park,

Calif., which he founded 

in 1982. An active mem-

ber of the Water Quality

 Association, Michaud hasbeen a member of its Board 

and of the Board of Governors and past Chair 

of the Commercial/Industrial Section. He is a

Certified Water Specialist Level VI. He serves

on the Board of Directors of the Pacific WQA

(since 2001) and chairs its Technical Com-

mit tee. A founding member of WC&P’s Tech-

nical Review Committee, Michaud has

authored or presented over 100 technical pub-

lications and papers. He can be reached at 

Syst ematix Inc., 6902 Aragon Circle, Buena

Park CA 90620; telephone (714) 522-5453 or 

via email at cmichaud@ systematixUSA .com

Send Us Your Case Studies

Your products and processes are being used in new

and unique applications across the US and around

the world. Send us your case studies and share the

information with WC&P  readers! Be sure to include

photos, graphs, and all the data – inquiring minds

want to know!

Call (520) 323-6144 or email:

[email protected]

[email protected]