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INDEX
1) RAW WATER & IMPURITIES…………………………………………
2) METHOD OF EXPRESSING DISSOLVED IMPURITIES……………
3) IMPURITIES IN WATER, DIFFICULTIES CAUSED & MEANS OF TREATMENT…………………………………………………………...
4) PRETREATMENT OF RAW WATER………………………………….
5) DEMINERALISATION SYSTEM………………………………………..
5.1 Demineralization process ………………………………………….
5.2 Ion exchange materials…………………………………………….
5.3 Different demineralization systems…………………………………
6) OPERATION OF DM PLANT……………………………………………..
6.1 WAC Exchanger…………………………………………………..
6.2 SAC Exchanger…………………………………………………….
6.3 Degasser system…………………………………………………….
6.4 WBA Exchanger……………………………………………………
6.5 SBA Exchanger…………………………………………………….
6.6 Mixed Bed………………………………………………………...
7) MONITORING OF DM PLANT…………………………………………
8) OPTIMISATION OF DM PLANT OPERATION………………………….
8.1 selection of regeneration system ………………………………………..
8.2 selection of layout & resin types………………………………………….
8.3 atmospheric degasifier……………………………………………….
8.4 outputs based on water quality...............................................................
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1) RAW WATER & IMPURITIES:
Water is one of the basic requirements in raising steam. In nature water is available in
abundance. Its physical and chemical characteristics vary depending upon the source and
strata on which it flows. It picks up mineral salts from the soil, which go in to solution.
Water, therefore contains mineral salts in dissolved condition, in varying proportions,
composition and degree. It gets polluted further with multifarious organic and inorganic
impurities due to disposal of industrial and domestic wastes. Decayed vegetation and micro-
organism also contribute to contamination. Besides dissolved salts water contains coarse
substance in suspended form, constituting of silt and clay matters, generally termed as
turbidity. Silicate matters are present in dissolved as well as in colloidal forms, proportion of
which varies depending mainly on the following conditions:
- Temperature
- Seasonal Conditions
- Chemical characteristics of the particulate
- Velocity of the flow
Table 1 lists the major impurities of raw water, classed in three main groups: first, ionic and
dissolved, second non-ionic and undissolved and third gaseous. The ionic impurities in the
first group are sub divided in to cations and anions. Organic matter and colour appear in both
the first two groups, because there are many types: some dissolved and ionic, such as
hamates, and others colloidal and non-ionic, such as tannins. Also there may be types of
organic matter that are dissolved and non-ionic.
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MAJOR IMPURITIES OF WATER:
CATIONIC ANIONIC NON-IONIC AND UN-DISSOLVED
GASEOUS
Calcium Bicarbonate Turbidity, silt, mud, dirt and other suspended matter.
Carbon dioxide
Magnesium Carbonate Colour Hydrogen SulphideSodium Hydroxide Organic matter AmmoniaPotassium Sulphate Colloidal silica MethaneAmmonium Chloride Micro-organisms OxygenIron Phosphate Bacteria ChlorineManganese Silica Oil
Organic matter colour
Corrosion products (condensate)
HARDNESS
Permanent Hardness: is due to presence of SO4, NO3, Cl of Ca++ & Mg++
Temporary Hardness: is due to presence of HCO3 & CO3 of Ca++ & Mg++ in water
Equivalent Mineral Acidity: (E.M.A.):-
This is the sum of all ions of SO4 + Cl + NO3 in raw water.
Alkalinity is of 3 types.
1. Bicarbonate Alkalinity
2. Carbonate Alkalinity
3. hydroxide or caustic Alkalinity
Total Alkalinity = HCO3 + CO3 + OH
Phenolphthalein Alkalinity (p Alk) is determined by titration with an acid and the colour change (pink to
colorless) takes place at pH of about 8.3.
Methyl Orange Alkalinity (M. Alkalinity):- During titration with an acid, the colour change takes place
at about pH of 4.3 (Orange to Pink)
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2) METHOD OF EXPRESSING DISSOLVED IMPURITIES:
Dissolved impurities may be expressed in terms of ions themselves or in terms of
their equivalent. The preferred method of expression in water treatment field is in
terms of equivalent of Calcium Carbonate abbreviated ‘as CaCo3.’
CaCo3 is a good common denominator because it has a molecular weight of 100,
which facilitates calculations. Moreover in this form of analysis the sum of cations
always equal to sum of anions. This method also aids in predicting the comparative
analysis after various forms of treatment and analysis at consecutive steps in multi
step demineralization.
If analysis expressed in terms of ions, it can be converted to the form of expression in
terms of equivalent CaCo3 (or as CaCo3 ) by dividing figures by equivalent weight of
ions then multiplying by equivalent weight of CaCo3 (i.e. 50).
For example if amount of Calcium in water is 40 ppm as Ca then during expressing it
in terms of CaCo3 it becomes
40 ----- X 50 = 100 ppm as Caco3. (Here 20 is equivalent wt. of Ca) 20
(Equivalent weight of an ion is its molecular weight divided by its valances)
Other units of analysis of water are:
a) one grain/u.s. gallon = 17.1 ppm
b) one grain/imperial gallon = 14.3 ppm
c) one milligram/liter = 1 ppm
Or
one gram/m3 = 1 ppm
[Note 1 U.S. gallon = 8.33 pounds
1 pound = 7000 grams
1 imperial gallon = 10 pounds
1 liter = 1,000,000 mg]
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3) IMPURITIES IN WATER, DIFFICULTIES CAUSED AND MEANS OF TREATMENT:
CONSTITUENT
CHEMICAL FORMULA
DIFFICULTIES CAUSED MEANS OF TREATMENT
1. Turbidity None expressed in analytical units
Imparts unsightly appearance to water. Deposits in water lines and process equipment.
Coagulation, setting and filtration.
2. Colour None expressed in analytical units
May cause foaming in Boilers. Hinders precipation methods such as iron removal and softening.
Coagulation and filtration, chlorination, Adsorption by activated carbon.
3. Hardness Calcium and Magnesium salts expressed as CaCO3
Chief source of scale in heat exchange equipment, boilers, pipelines, etc. forms curds with soap and interferes with dyeing.
Softening, Demineralization, internal boiler water treatment.
4. Alkalinity HCO3, CO3
and OH expressed as CaCO3
Foaming and carryover of solids with steam. Embitterment of boiler steel. Bicarbonate and carbonate produce CO2 in steam a source of corrosion in condensate lines.
Lime & Lime Soda Softening. Acid treatment. Hydrogen Zeolite softening.
5.Free Mineral Acid
H2SO4, HCL, etc. expressed
Corrosion Demineralizing. Dealkalization by H ion exchange. Neutralization with alkalies.
6.Carbon Dioxide
CO2 Corrosion in water lines particularly steam and condensate lines.
Aeration, De-aeration, Neutralization with alkalis.
7. pH Hydrogen-ion concentration defined as PH = log 1/H
PH varies according to acidic or alkaline solids in water. Neutral water have a pH of 6.0 – 8.0
pH can be increased by alkalis and decreased by acids.
8. Sulphate (SO4) Adds to the solids content of water, but in itself, is not usually significant. Combines with Ca++
to form CaSO4 scale.
Demineralization
9. Chloride Cl Adds to the solids content Demineralization
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4) PRE TREATMENT OF RAW WATER:
The purpose of pre treatment is to render raw water fit as influent to a de-mineralizing unit.
Pre treatment is done to make water free from suspended, colloidal and organic impurities.
Since presence of such impurities adversely affect the de-ionisation effect and the final
quality of de-mineralized water, pre-treatment plays a vital role in water treatment. The
different process involved in pre-treatment are:-
a) Settling and Coagulation
b) Filtration
a) SETTLING & COAGULATION:
Used for removal of turbidity and suspended matter. The coarse, heavy particles of
suspended matter gets easily removed by settling the water in a tank, but some suspended
impurities, such as turbidity, micro-organisms and colour are very finely divided or even in
colloidal form, so that they do not settle readily. Settling basins would have to be
excessively large, to remove these fine particles.
Co-agulation is a process of breaking up of a colloidal solution resulting in the recipitation of
the particle of the dispersed phase. It may be spontaneous and brought about by the additions
of an electrolyte which is termed as “coagulant”. Co-agulation, induced by adding chemicals
(Coagulants) to the water, agglomerates the finely divided, suspended solids in to masses that
settle more readily. This occurs in two ways:-
The particles of turbidity and colour have like electric charges on their surfaces,
which keep them apart, because like electric charges repel one another and the co-
agulant ions selected possess charges opposite to those on the suspended particles, so
that they neutralize each other.
The coagulant reacts with the alkalinity of the water to form a gelatinous precipitate,
called “floc” which enmeshes and entraps the finer of the suspended particles.
Alum or Aluminum sulphate is the most commonly used coagulant, because it is the
lowest in cost and least corrosive to handle. The reactions are shown below:-
Al2 (SO4)3 + 3 Ca (HCO3)2 = 2Al (OH) 3 + 3CaSO4 + 6CO2
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The other coagulates are FeSO4 (Ferrous sulphate) and ferric sulphate {Fe2
(SO4)3}
Factors influencing coagulation:
Organic matter, if present in appreciable amounts, inhibits coagulation and
narrows the optimal pH range. For the oxidation of organic matter pre
chlorination and narrows the optimal pH ranges. For the oxidation of organic
matter pre chlorination is desirable, because it broadens the optimal pH range and
there by makes the coagulation easier to control.
If the alkalinity in the water is insufficient to react with the dose of co-agulant, the
pH is below the optimal range, and then the alkalinity must be increased by
adding an alkali. Lime is the cheapest alkali. For Al2 (SO4)3 optimum pH is from
5.5 to 7.5.
b) Filtration:
Filtration is defined as passage of fluid through a porous medium to remove matters held
in suspension. In water purification the matter to be removed includes:-
1. Suspended Silt
2. Clay
3. colloids
4. Micro Organisms including algae, bacteria and virus.
The particles to be removed have approximate size as follows:-
Material Particle Size, Milli-micron
Silt 50,000
Bacteria 5,000
Viruses 50
Colloids 1 – 1,000
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5) DEMINERALISATION SYSTEM:
5.1 DEMINERALIZATION PROCESS & SYSTEMS:
The process of demineralization water by ion exchange comprises of:-
Conversion of salts to their corresponding acids by hydrogen cat-ion exchanger.
Removal of acids by anion exchangers.
The two exchangers are normally in series. Normally cat-ion precedes the anion
exchanger
5.2 ION EXCHANGE MATERIALS:
Major ion exchange materials are synthetic resins made by the polymerization of various
organic compounds.
Most frequently used compounds are:-
1. Styrene
2. Die vinyl – Benzene
The long chained co-polymer formed from these compounds contains a major
proportion of styrene (80-92%) and a minor portion of divinely Benzene (8-
20%). Divinyl Benzene acts as a cross link to hold the polymer chains
together.
To make strong acid cation exchanger polymer is treated with concentrated
sulphuric acid, which attaches – SO3H to the hydro carbon network, to make
most anion exchanger resin the matrix is chloromethylated and animated.
The resin when dry shrinks so that chains come very close together and the
bead cannot be readily penetrated by the ions, but when placed in water, it
takes on water and swells, so that the chains spread apart and permits the
diffusion of the ions.
The degree of swelling depends on the degree of cross linking, i.e. the number
of cross links. The greater the no. of cross links, less the moisture holding
capacity and the swelling.
From the kinetic point of view, for a steady exchange reaction it will be
desirable to have as low as a degree of cross linkage as possible, but this
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would result in a high degree of swelling and an accompanying gelatinous
structure having poor hydraulics properties.
The design of commercial ion exchange resin is therefore involves a choice of
cross linking that represents a compromise between kinetic and hydraulic
performance.
Cation exchanger of the hydrogen type are either strongly acidic or weekly acidic.
Strongly acidic resins contain the sulfonic acidic functional group SO3H where as
weekly acidic resins contain carboxylic acidic group COOH.
Similarly there are weekly basic anion exchanger resins and strongly basic anion
exchanger resins. Strongly basic anion exchangers are again of two types, type I and
type II. Type I resins have less exchange capacity, but more stability than type II.
Type I have quaternary ammonium functional group. Type II has modified
quaternary ammonium functional group where one of the methyl groups is replaced
with an ethanol group.
The week base anion exchangers have polyamine functional groups containing
primary amine – NH2, secondary amine NHR and tertiary amines NR2.
5.3 DIFFERENT DEMINERALISATION SYSTEM:
Various system combinations are available. Selection of particular system depends
on quality of raw water available and the requirement of end product. The various
demineralization systems have been shown below.
Different Demineralization System
Demineralization System
Application Typical Effluent Remarks
1. SA-WB SiO2 + CO2 are no limitation
Conductivity 10-30 micro mhos/cm. SiO2
un-changed
Low equipment and regenerate cost.
2. SA-WB-D No limitation of SiO2
but CO2 removal is required
Conductivity 10-20 micro mhos/cm
Low regenerate cost.
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3. SA-SB Low alkalinity raw water. SiO2 removal required.
Conductivity 5-15 micro mhos/cm. SiO2
= 0.02-0.1
Low equipment cost high chemical cost.
4. SA-D-SB High alkalinity of raw water. SiO2 removal required.
Conductivity 5-15 micro mhos/cm. SiO2
= 0.02 – 0.1
Low chemical cost.
5. SA-WB-D-SB Higher alkalinity, SO4
and Cl in raw water. SiO2 removal required.
Conductivity 5-15 micro mhos/cm. SiO2
= 0.02-0.1
Low chemical with high equipment cost.
6. WA-SA-D-WB-SB High hardness, alkalinity, sulphate and chloride. SiO2
removal required.
Conductivity 5-15 micro mhos/cm. SiO2
= 0.02-0.1
Lowest chemical cost high equipment cost.
7. SA-D-SB-SA-SB High alkalinity, high Na in raw water, high purity treated water required.
Conductivity 1-5 micro mhos/cm. SiO2
= 0.01-0.05
Low chemical cost, high equipment cost.
8. MB Low solids raw water high purity of treated water required.
Conductivity 1.0 micro mhos/cm SiO2
= 0.01 – 0.05
Low equipment cost, high chemical cost
9. SA-D-SB-MB-MB High alkalinity and dissolved solid raw water. High purity treated water.
Conductivity 1.0 micro mhos/cm SiO2
= 0.01 – 0.05
Lower chemical cost, higher equipment cost.
10. SA-D-SB-MB-MB High alkalinity & dissolved solid raw water. Ultra pure water required.
Conductivity 0.5 micro mhos/cm SiO2
= 0.01 – 0.02
Lower chemical cost, higher equipment cost.
KEY
SA - Strong acid Cation Exchanger
SB - Strong Base Anion Exchanger
WB - Weak Base Anion Exchanger
WA - Weak acid Cation Exchanger
D - Degasser
MB - Mixed Bed
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6) OPERATION OF DM PLANT:
A typical DM plant consists of cation, degasser and anion exchangers:
Introducing weak acid cation exchanger and weak base anion along with typical de-
mineralization chain and regenerating strong and weak exchangers by same acid/caustic
increase the efficiency of plant. Also having mixed bed for polishing ex-anion water
improves the quality of D.M. water to great extent.
Degasser usually provided in between cation exchangers and anion exchangers to remove
carbon dioxide that decipates during ion exchange in cation exchangers.
The operating principles of weak acid cation exchanger (WAC), strong acid cation exchanger
(SAC), Degasser, Weak Base Anion Exchanger (NBA), Strong Base Anion Exchanger
(SBA) and mixed bed (MB) are discussed in following paragraphs.
6.1 WEAK ACID CATION EXCHANGERS:
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Weak acid cation exchanger mainly removes Calcium & Magnesium alkalinity from
raw water. For simplicity cation resin is represented by H2R and equation for the service
cycle of WAC can be written as
Ca Ca HCO3 + H2R -R + H2CO3
Mg Mg
H2CO3 H2O + CO2
6.2 STRONG ACID CATION EXCHANGERS:
Strong acid cation exchanger removes sulfates, chloride, nitrates and sodium salts. The
equation for service cycle of strong acid cation can be written as
Ca So4 Ca H2So4
Cl2 + H2R -R + HClMg NO3 Mg HNO3
HCO3
CO3 H2C03
Na SO4 + H2R NaR + H2So4
Cl2 HClNO3 HNO3
H2CO3 H2O + CO2.
Process of exchanging salts in cation exchange continues till resin looses its capacity to
convert salts into corresponding acids. After this the resin to be regenerated by using
hydrochloric acid or sulfuric acid.
During regeneration resin will regain its capacity to exchange salts after which it can again
remove salts from water.
The equation for regeneration cycle can be written as follows:
Ca CaMg R + HCl Mg Cl + RHNa Na
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Ca CaMg R + H2So4 Mg So4 + RHNa Na
Note: Strong Acid Cation Resin can also remove alkaline salts of Calcium & Magnesium.
However as WAC is precedes SAC, there won’t be any load of alkaline salts on SAC.
When Weak Acid Cation and Strong acid cation are in series regeneration is done in through
fare system. Acid after regenerating strong acid cation exchanger passes through weak acid
cation exchanger.
Ion leakage and end points of exhaustion phase:
During re-generation with down flow of acid the top of bed is more completely
converted to the hydrogen form than the bottom, but unless uneconomical amounts of excess
acid are employed, the bottom usually contains a band of sodium at the end of re-generation.
As the next service run starts, the cations in the influent are exchanged for the hydrogen ions
in the top of the bed, releasing the sodium as cation leakage into the effluent. As the run
progresses, this sodium cation leakage decreases, because the sodium band at the bottom is
gradually consumed.
The cations in the water are converted to their corresponding acids. But the conversion is not
complete. The difference between the total mineral acidity (corresponding to the sulphates
and chlorides in the influent) and the free mineral acidity in the effluent is equal to the cation
leakage.
At the end of the exhaustion run, at the break through, the FMA drops, and when the
resulting increased cation leakage reaches the allowable limit, the unit is regenerated.
Normally conductivity is compared during the run. The conductivity ratio is normally
constant during the run. At the exhaustion the conductivity ratio changes indicating the
exhaustion.
Cation leakage is important because it affects the purity of the demineralizer effluent. A
strong base anion exchanger can remove only the acidity, not the sodium. It coverts the
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sodium salts to sodium hydroxide, which creates a high conductivity and pH value in the
effluent. Therefore for a low conductivity of dematerialized water cation leakage (Na) must
be reduced. The several methods adopted area:-
1. Air Mixing of resin Regeneration.
2. Counter flow re-generation.
6.3 DEGASSER SYSTEM
In de-mineralization process carbon dioxide generated by dissociation of carbonic acid at
cation outlet water.
H2CO3 H2O + CO2
The CO2 generated if not removed increases load on SBA resin. So degassers or
Decarborators are placed in cation & anion.
Degassers usually made of acid proof materials (wood or rubber lined steel) as it have to
handle acidic water of cation exchangers. (Redwood or cypresses are usually preferred
woods).
Typical degasser as shown; air blown at the bottom and rises counter current to the
downward trickling water. The spray pipes or trays divide water into droplets or thin films
exposing new surfaces to gas phase. Tray also serves to agitate the water by splashing thus
allow dissolved gases to leave water readily. Agitation overcome tendency of water to retain
gas bubbles through surface tension and viscosity.
Height of the tray stack or Rasching ring proportional to amount of influent CO2.
Decarborators are designed with flow rates that range from 20 to 30 gal/min/sq.foot areas (1
to 1.5 m3/min/m2). The height of Rasching –ring varies from 5 to 15 ft.
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20
Sectional view of degasser tower
6.4 WEAK BASE ANION EXCHANGER:
Weak base anion exchangers can remove only the highly dissociated acids (like H2So4,
HNO3, HCl) from the effluent of cation exchanger. They can not remove weakly dissociated
carbonic acid from alkalinity or silica acid from the silica content in the water.
Exhaustion reaction represented by equation.
H2SO4 SO4
2 HCl + 2ROH (NO3)2 2R + 2H2O 2HNO3 Na
The regenerates of weak base anion resins may be NaOH, Na2CO3 or NH3
Some typical application of WBA exchanger are mirror silvering, processing of ceramics,
deproofing or cutting of alcohol in distilleries, plating, glass manufacture and automobile
pointing.
6.5 STRONG BASE ANION EXCHANGER
This removes weakly dissociated and the strongly dissociated acids.
The reaction of strong base anion exchangers given in following equation:
H2SO4 SO4
2HCl + 2ROH Cl2 2R + 2H2O
2H2CO3 (Co3)2
Regeneration of Anion exchanger usually did by caustic soda. If weak base anion exchanger
& strong base anion exchanger are in series regeneration done in thoroughfare system.
Caustic soda after regenerating strong base anion exchanger passes through weak base anion
exchanger. Following equation represents regeneration equation.
SO4 Na2SO4
Cl2 2NaCl(NO3)2 -2R + 2 NaOH 2 ROH + 2 Na (NO3)2
SiO3 Na2SiO3
CO3 Na2CO3
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6.6 MIXED BED:
OPERATION PRINCIPLE OF MIXED BED DEMINERALISER:
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In mixed bed both strong cation and strong anion exchangers are in same shell, rather in
separate shells. They are mixed together by compressed air after regeneration. Cation and
anion particles being next to each other constitute a series of two bed pairs of beads.
Prior to regeneration the two resins are separated by backwashing. Due to density difference
between two types of beads, the two types of resins separate completely and settle one above
the other (cation in the bottom and anion at the top). A screened interface pipe system,
located between two resins, collects regenerate effluent. Acid usually follows upward and
caustic soda downward.
Cation Regeneration usually proceeds anion, but two may be simultaneously also. The
advantage of sequential regeneration is that the Calcium cations dissipated from the cation
resins before carbonate ions from Anion resins, formation of Calcium Carbonate precipitates
is avoided thus avoiding fouling of interface screening.
A downward blocking flow of water proceeds from top while acid flows upward. Both
blocking water as well as effluent acid escape through interface collector.
The blocking flow avoids expansion of bed and also prevents acid from entering anion bed
above interface.
An upward blocking flow of water or acid proceeds from bottom while caustic soda flows
downward, so that later does not enter cation resin.
Usually mixed bed used for polishing and follows a two bed pair.
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7) MONITORING OF DM PLANT:
S. NO.
DEFECTS CAUSES REMEDIES
1
Decrease in capacity between two regenerations
a. Increase in ionic load Check by analysisb. Flow integrator/indicator defective Checkc. Less amount of regenerant chemical
used for regenerationCheck
d. Resin fouled Give treatment for de-foulinge. Plant being used intermittently Regular running
f. Channeling in resin bedCheck and ensure uniform distribution of bed
g. Resin dirty Give prolonged backwashh. Resin deteriorated Check /replace the resini. Resin quantity become less Check and makeup the level
2
Treated water quality not as specified
a. Cation exhausted Check and regenerateb. Anion exhausted Check and regeneratec. Mixed bed exhausted Check and regenerated. Resin in mixed bed not in mixed
stateAgain remix the resin by air and rinse.
e. Some valves particularly back wash inlet valves passing
Check and rectify.
f. Sodium slip from cation highCheck feed water analysis. Note changes in Na / TC and Silica / TA ratios. Use more chemicals accordingly.g. Silica slip from anion high
h. Unit idle Checki. Unit is not sufficiently rinsed Rinse it to satisfactory quantity.
j. Excessive low flow rateAdjust the unit flow between minimum and maximum flow rate.
k. Channeling of resin bedCheck and ensure uniform distribution in bed.
l. Resin fouled Same as 1 (d ).m. Resin deteriorated
Check the resin and replace.
3Mixed bed quality not good
a. Resin not separated during back wash
Give extended backwash after exhaustion of bed.
b. Air mixing not proper Give extended air mixing.c. Final rinse not proper Give extended final rinse.d. Some valves may be leaking and
contaminating the treated waterCheck and rectify.
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4
High residual CO2 from degasser
a. Due to choked suction filter of degassed air blower
Check and clean filter.
b. Improper air flow to degasserCheck blower discharge valve / damper / speed of blower and it's discharge pressure.
c. Degasser blower not in operationCheck and take blower in line.
.
5Flooding in Degasser
a. Very high air flow rate Reduce air flow rate by adjusting V/V
b. Packed tower choked due to dust or broken packing material
Open, check, clean or replace packing.
6Unit rinse takes long time
a. Flow rate too low Check and increase flow rate.b. Backwash inlet valves not holding Check and rectify.c. Anion resin organically fouled Give alkaline brine treatment.d. Mixed Bed air mix not satisfactory Carryout air mixing again.
e. Acid or alkali pockets formed in unit
Faulty design, Check/rectify. Temporarily give longer backwash and rinse again.
7
Flow rate too low
a. Choked valves or suction strainer of pump
Check and rectify
b. Cavitations in pumpCheck water level in respective tanks.
c. Low inlet pressure Check water pressure. d. Distribution or collector system
chokedCheck and clean
e. Resin trap at outlet unit choked Check and cleanf. Control valves shut due to low off
take. Increase off take.
8
Pressure drop across bed increasing
a. Defective valves Check and rectify / replace.
b. Packing of resin bed due to fines of resin
Give extended backwash with open manhole, scrap fines from top of bed
c. Collecting system choked Check and repeat backwash.d. Pressure gauge defective Check and rectify / replace.
9Resin bed being lost
a. Due to excessive backwash pressure or flow.
Check inlet pressure and flow rate and reduce it if necessary.
b. Faulty collection systemExamine the system for any breakage.
c. Inlet strainer damaged Check and replace.10 Ejector not
working a. Low power water pressure Check and adjust.b. Air lock in the unit Backwash and release air entrapped
in unit. c. Choked or defective valves Examine and rectify
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d. Ejector nozzle may be choked Check and clean.
e. Too much back Pressure from unitCheck for choking of regenerant distribution / collecting system. Passing of inlet and outlet valves.
f. Bulge in rubber linking of pipeline Check and rectify.
11Incorrect reading from rotameters.
a. Choked orifice or impulse line Check and clean
b. Dirty glass or float Check and clean
12
Improper reading from flow recorder integrator
a. Choked impulse line or orifice Check and cleanb. DP transmitter requires re
calibrationRe calibrate
c. Leakage in signal tube between transmitter and panel
Check and repair
d. Low air pressure for D. P. transmitter or recorder
Check instrument air pressure and take remedial measure
13
Level electrodes system for measuring and dilution tank not functioning properly
a. Improper contact between electrodes and control cabling.
Check the contact and rectify.
b. Short circuiting of electrodes due to moisture , dirt etc
Clean and dry contacts of moisture and dirt.
c. Improper working of level controllers
Check and repair.
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Leakage from acid injection or unloading transfer pumps
a. Improper adjustment of mechanical seal
Check and adjust.
15
Corrosion in concentrated acid tanks and lines
a. Low concentration of H2SO4
Check silica gel breather in acid storage tank and replace silica gel charge if necessary.
b. Lining of HCl tank / pipe line damaged.
Rectify.
16
Improper opening closing of pneumatically operated valves
a. Defective solenoid valves. Check and rectify.b. Leaking in air line to solenoid
valve to respective control valve. Check and rectify.
c. Improper contact of micro switches giving false indication on panel.
Check and rectify.
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8) OPTIMISATION OF DM PLANT OPERATION:
DM plant operation can be optimized by
Proper selection of Regeneration system.
Selection of layout & resin type.
Using atmospheric degasser.
Output based on water quality.
8.1 SELECTION OF REGENERATION SYSTEM:
Regeneration system of cation / anion exchanger is normally two types based on regenerate
flow. When the flow of acid / caustic are in the same direction on the service flow the
Regeneration system is known as cocurrent regeneration. And when the flow of acid /
caustic are in opposite direction of service flow it is known as counter current regeneration.
Counter current regeneration have following advantages.
Reduced chemical consumption
Improved water quality and
Less waste volumes
Cation / Anion exchangers which are regenerated by counter-current regeneration system
give more output when compared to the exchangers that are regenerated by Co-current.
This is illustrated in tables below
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STRONG ACID CATION RESIN
Operating capacity verses Regeneration level:
(Na = 40 %, Alkalinity = 50 %)
Regeneration level
(Kg of HCl / M3 of resin
Exchange Capacity
Kg CaCO3 / M3 of Resin
Co – Current Counter Current
50 46 55.2
60 51 59.5
70 55 63.36
80 58.5 66.72
90 61.5 69.12
100 64 71.52
110 66.5 73.44
120 68.5 75.36
STRONG BASE ANION RESIN
Operating capacity verses Regeneration level :
(SO4 = 25 %, CO2 = 20 %, Silica = 25 %)
Regeneration level
(Kg of NaOH / M3 of resin
Exchange Capacity
Kg CaCO3 / M3 of Resin
Co – Current Counter Current
40 26.2 30.2
50 27.6 32.2
60 29.4 34.0
70 31.3 35.48 33.6 36.8
100 36.3 38.2120 38.2 39.1
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Counter-current regeneration systems provide a water quality of better than 2 S/cm and
residual silica of 0.020 to 0.050 mg/l as SiO2. Depending upon water composition and
regeneration conditions, the specific conductivity could be as low as 0.2 S/cm. The normal
counter-current endpoint is 4 S/cm conductivity.
A maximum endpoint value of 0.3 mg/1 SiO2 above the average leakage should not be
exceeded in order to avoid a high contamination of the polishing resin layer and unacceptably
high silica leakage during subsequent cycles. Silica leakage can be minimized by operating
the plant at silica break rather than conductivity end point. This secures the lowest silica
leakage, but at the expense of a 5 – 10 % throughput reduction.
8.2 SELECTION OF LAYOUT & RESIN TYPES:
The plant configuration will depend on the feed water composition, the water quality
required and the economics of operation. The following general guidelines are given to help
in configuration and resin selection.
(A) [SAC] – [WBA]: This combination of strong acid cation [SAC] and weak base
anion [WBA] resins is used to obtain partially deionized water without removal of
CO2 and SiO2.
(B) [SAC] – [SBA]: The combination of strong acid cation and strong base anion [SBA]
is preferred for treating low mineralized water or for small size plants.
(C) [SAC] – [WBA] – [SBA] : This combination of strong acid cation with weak base
and strong base anions is proven to be an excellent choice for larger plants as it
provides an optimum balance between investment and running cost. It is well suited
to treat waters with low alkalinity, when the FMA (Cl + NO3 + SO4) is typically >
60% of the total anions. The normal end-point for a WBA resin corresponds to the
chloride breakthrough, which means that the downstream SBA resin is only
removing the carbon dioxide and silica ions. This situation generally leads to a big
discrepancy between WBA (large) and SBA (low) volumes.
(D) [WAC] – [SAC] – [SBA]: The use of a weak acid cation [WAC] in front of a strong
anion is preferred with feed waters containing a high proportion of temporary
hardness (>60%) and low FMA. The normal end-point for a WAC resin is 10%
29
alkalinity leak. In this condition, the down-stream SAC resin should remove the
permanent hardness and the monovalent cations. This situation generally leads to a
big discrepancy between WAC (large). This is the ideal combination for high
hardness, high alkalinity and high FMA water, as well as large size plants. Again
the cation and anion combinations can be in single or separate vessels.
(E) [WAC] – [SAC] – [WBA] – [SBA]: This is the ideal combination for high hardness,
high alkalinity & high FMA water, as well as large size plants. Again the Cation &
Anion combination can be single or separate vessel.
8.3 ATMOSPHERIC DEGASIFIER:
The decision to install an atmospheric degasifier is based principally on economical
considerations. Removing carbon dioxide before it reaches the anion resins will reduce
NaOH chemical consumption stoichiometrically and this should be balanced against the cost
of the degasifier. Generally the economical balance is not in favor of a degasifier for small
plants (up to about 10 m3/h or 50 gpm). For larger plants, if the total CO2 is greater than 80-
100 mg/1 (ppm), the pay-back time for a degasifier should be short. For very large plants,
the limit can be reduced to 50 mg/l CO2.
8.4 OUTPUT BASED ON WATER QUALITY:
Output of DM plant is depending on water quality, if water quality vary output will also
changed accordingly. So regular monitoring of raw water quality is required and accordingly
output may be calculated.
Some time conductivity of anion remains high since initial stage of service run. This may due
to either CaSO4 precipitation on SAC, Organic fouling & silica deposit on anion resin.
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