Desalination, 73 (1989) 295-312 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

295

A Recarbonation Process for Treatment of Distilled Water

Produced by MSF Plants in Kuwait

H.E. Al-Rqobah, H. and Al-Munayyis, A. Ministry of Electricity and Water, Kuwait.

Abstract

Distilled water produced from MSF plants is a very soft water

that has a low buffer capacity, and as such, is considered quite aggressive to the materials encountered in the water distribution

system. Furthermore, this untreated water is not accepted from a palatability point of view. Different remedial treatment pro-

cesses are therefore being adopted by different MSF plants.

In Kuwait, a recarbonation process has been in operation for

18 months at Shuwaikh Desalination Plant treating 18 MIGD of distilled water as the first step in a major scheme aimed at treating water produced by all MSF plants in Kuwait.

The major steps in the adopted recarbonation process include

extraction of carbon dioxide from the vent gas system of the MSF plant, compression and purification of Cop -air gas stream, acidi-

fication of a pre-calculated distilled water stream in a gas absorption tower, limestone dissolution to augment the water with Caz+ and HCO; necessary for water to be self-inhibiting, degasi- fication of the excess COz, blending with by-passed distilled

water and finally pH adjustment.

This paper presents some engineering design aspects of

the different steps encountered in the recarbonation process as well as an analysis of plant operation experience gained during the last'18 months.

296

Introduction

Kuwait, like most Gulf countries, depends on seawater desalina- tion for its fresh water supply. In 1953, the first submerged tube distillation plant was commissioned in Shuwaikh with a capa-

city of 1.0 MIGD. Today, the Ministry of Electricity and Water (MEW) operates 6 MSF desalination stations with a total installed capacity of 263 MIGD.

The distilled water produced has very low concentrations of

dissolved salts and gases and a total alkalinity of less than 1 mg CaC03/L. The high purity renders the water to be chemically very aggressive towards nearly all components in the water

distribution system, resulting in very severe corrosion problems. One of the by-products of this chemical attack is ferric hydroxide, a red-brown rust, which results in what is called "red water".

To improve the palatability of distilled water, it is usually

blended with brackish water in different blending ratios, depending on the quality of the available brackish water. In

Kuwait, the blending ratio is nearly 1O:l. Negligible amount of underground fresh water is also added. The potable water thus

produced has a total alkalinity of 20-24 mg CaC03/L, which is less than the minimum value of 50 mg CaC03/L necessary to depress the corrosion rate of the different materials involved in the water distribution system. Chemical treatment to increase the

total alkalinity of the potable water is therefore inevitable.

If iron is immersed in water saturated with dissolved oxygen,

rust is developed due to the formation of ferric hydroxide Fe(OH)3 or hydrated ferric oxide Fe203. x H20. The mechanism of this electrochemical reaction can be explained by the following

equations:

Fe + Fe*+ + 2 e- (I)

2 e- t 2 H20 + H + 2 OH- (2) 2

2 e’ + H,O + 1/2 0, + 2 OH-

297

(3)

Equation (1) represents the anodic reaction which. causes ferrous ions and electrons to be released from iron atoms. The electrons released will be consumed according to the cathodic reactions represented by equations (2) and (3). In acidic solu- tions such as water saturated with C02, hydrogen gas is generated and hydroxyl ions are formed according to equation (2). In neutral solutions containing dissolved oxygen, the same amount of hydroxyl ions will be formed according to equation (3), and alkaline conditions will prevail. The ferrous ions are unstable under normal conditions and will be oxidized due to the presence of dissolved oxygen in neutral solutions according to the following reaction:

2 Fe2' + H,O + l/2 0, + 2 Fe3' t 2 OH- (4)

The ferric ions are then hydrolyzed by water precipitating the insoluble ferric hydroxide (iron rust) according to:

Fe3+ + 3 H,O + Fe(OH), + 3 H+ (5)

The overall reaction is the formation of hydrogen ions, which maintain a condition of acidity according to:

2 Fe3' + 5 H20 + l/2 0, + 2 Fe(OH), t 4 H' (6)

Ferric hydroxide is a gelatinous precipitate which partially dehydrates resulting in the red-brown ferric oxide, which is the main constituent of "red water". It has no protective action and

298

as water temperature increases, the PR decreases correspondin! and the corrosion rate will therefore increase. It is therefore important to increase the pH and depress iron dissolution and

thus prevent complete- failure of the pipes in the potable water

distribution system.

The main objective of this work is to describe the recar-

bonation process adopted in Kuwait as the first phase in a comprehensive scheme to solve the problem of "red water". Data

from the first year of operation of this process will also be discussed.

Corrosion Inhibition

There are various methods which can be applied for the corro-

sion inhibition of the pipe network used in the potable water distribution system. During the period 1979-1981, a comprehen- sive study was conducted by the Water Resources Development Centre (WRDC) of MEW with 4 main objectives :

1. To establish a corrosion data base for pipe materials

under prevailing conditions.

2. To evaluate alternative materials of construction.,

3. To assess available metal pacification methods, with

regard to the materials used in different components of the distribution system under the prevailing conditions.

4. To perform a technical and economic study for the most

suitable chemical treatment process for self-inhibition.

One of the main results of the first part of this study was

the recommendation of using ductile iron pipes, lined internally with cement-mortar and seal-coated. Most of the water distribu-

tion system has recently been upgraded to this material of construction.

Pacification of water by increasing its total alkalinity was

investigated, in the second part of the study, using different

299

agents such as calcium and sodium hydroxides, sodium silicate, sodium-zinc phosphates, sodium and calcium bicarbonates. It was concluded that water recarbonation was best performed using bicarbonates, and that the maximum pacification effect was obtained when the total alkalinity, expressed as mg CaC03/L, was increased to the same level as calcium concentration, expressed as mg CaC03/L, in the recarbonated water. It was also recom- mended to use phosphate inhibitors with a higher initial dose of 6-12 mg/L of phosphates followed by a lower 2-6 mg/L dosage.

Water of high alkalinity and calcium content is a stable water, and can produce a thin protective layer of calcium car- bonate by careful increase in the pH. This is the concept of self-inhibition and can be described by the following two reactions:

HCO- t OH- + co2- +HO (7) 3 3 2

CO:- + Ca2+ + CaC03(S) (8)

The hydroxyl ions necessary for the first reaction are produced during the reduction of dissolved oxygen, according to equation (3). There are three conditions which are necessary for self- inhibition of water:

(1) The water must be free of C02. Carbonic acid, even at low concentrations, will neutralize the hydroxyl ions pro- duced according to equation (3). It will also dissolve any protective calcium carbonate layer, and finally it will attack the metal. Free CO2 can be degasified in a stripping tower using air, or by treatment with sodium hydroxide.

(2) The concentration of calcium and carbonate ions must satisfy the solubility product of calcium carbonate, in order to precipitate the inhibition film.

(3) The pH value of water must be carefully adjusted. The Langelier saturation index, LSI, is defined by the following equation:

300

LSI = pti - pHs (9)

where the pH of saturation, pHs, is defined as the pH value at which water containing bicarbonate and calcium is just saturated with CaC03. The pHs can be expressed as

pHs = A + B - log (Ca*+) - log (total alkalinity) (IO)

where A is a constant which depends on the water temperature, and

B is another constant which depends on the water TDS content. Calcium ion concentration and total alkalinity are both expressed as mg CaC03lL.

A positive saturation index is associated with non-corrosive

conditions, and thus indicates a tendency to deposit CaC03. A negative index indicates a tendency to dissolve CaC03 and is therefore associated with corrosive conditions. It can then be deduced that maintaining the pH value of water above its pHs value will result in deposition of the protective CaC03 layer and hence corrosion inhibition can be achieved.

Based on the recommendation of maintaining the same level of

calcium ion concentration and total alkalinity, the following equation can be applied for a recarbonation process, operating at 4O"C, a total alkalinity of 70 mg CaC03/L and calcium con-

centration of 70 mg CaC03/L:

pHs = 11.46 - 2 t log (tota

The Recarbonation Process

1 alkalinity) ( 11)

A recarbonation process was established in 1987 and incorporated with Shuwaikh Power and Water Desalination Plant. The capacity of the recarbonation plant is 18 MIGD and was designed to yield recarbonated water with a total alkalinity of 60-80 mg CaC03/L.

The proc'ess consists of 3 main steps as schematically shown in Fig. 1.

Distilled water from three units of the MSF desalination

plant is acidified in an absorption tower using CO2 $35.. The

CO

? + A

IR

Fig.

1.

Sc

heni

cltic

fl

ow

diag

ram

of

ttl

s re

carb

onat

ion

pr

oces

s.

302

acidified water is then augmented by bicarbonate ions using CaC03 in a number of limestone dissolution filters. .The following

reaction takes place.

CaCOs + H20 + CO2 + Ca*' + Z(HCOa)- (12)

The excess CO2 present in the recarbonated water leaving the limestone dissolution filters is degassified in a stripping tower

using air. Finally, a 15 wt% caustic soda solution is dosed into the water stream for final adjustment of its pH value.

It can be observed that this recarbonation process resembles

the natural carbonation process taking place in the aquatic system.

The carbon dioxide gas, required according to equation (12),

can be produced in many different ways depending on the local conditions. If the water feed to be treated is produced by any thermal desalination process, then extraction of CO2 from the non-condensable gases, released as a result of heating seawater

to high temperatures, would represent the most technically and economically feasible alternative. When seawater is heated to

about 77"C, the calcium bicarbonate will decompose according to the following reaction:

Ca (HCOJ2 +Q b CaCOs + Hz0 + CO2

tem- The amount of CO2 librated depends on the brine top perature from the main heater and the pressure in the first flash chamber of the heat recovery section of the MSF plant.

Dissolved oxygen and nitrogen gases will also be released

together with carbon dioxide. For better heat transfer in the

condenser of the flash chamber, these noncondensable gases are vented and withdrawn, using steam ejectors, to a vent gas con- denser where water vapor is condensed using a by-pass stream of the distilled water to be recarbonated. Almost dry gases are

evacuated from the vent gas condenser using a booster vacuum pump, and fed to a gas compressor. A two-stage reciprocating gas

303

compressor, with intermediate and after coolers, is used to compress the gas to about 7 bars. From the gas receivers, the gas stream containing CO2 and air is fed to a gas purification filter filled with activated carbon to remove any volatile orga- nic components.

According to the block flow diagram shown in Fig. 2, the gas stream is fed to an absorption tower packed with Mella Pack of polypropylene. The extent to which water will be acidified, in the absorption tower, depends on the C02-concentration and flow rate of the gas stream, the flow rate of the water and the operating pressure. The pressure can be kept constant by means of the air accumulated at the top of the tower. If the liquid load is kept constant, the C02-concentration in the exit water stream can then be easily controlled by the flow rate of the inlet gas stream. The height of the absorption tower used in the recarbonation process at Shuwaikh is about 12.0 m with a diameter of 1.3 m.

The acidified water is fed to limestone dissolution filters to produce the required level of total alkalinity in the product. The main factors which affect this step are the size and purity of limestone, bed height, temperature, CO2 concentration in the entering water stream, and the linear velocity of water flowing downwards through the filter bed. The size and purity of limestone was fixed at l-5mm and 90% (as CaC03) respectively. If the water temperature and the bed height are fixed, the reaction represented by equation (12) can then be controlled by the level of C02-concentration and the linear velocity of water. The linear velocity must be optimized since the hold-up time will determine the total bed height, diameter and number of filters. As the linear velocity decreases the diameter or the number of filters must be increased. For high linear velocities, a large bed height is needed which will result in a higher pressure drop. For an optimum linear velocity, the alkalinity can therefore be controlled only by the CO2 concentration in the entering water stream. The linear velocity and filter diameter in the process were fixed at about 18 m/s and 5.0 m, respectively, with a mini- mum limestone bed height of 2.8 m in each filter.

The recarbonated water leaving the limestone dissolution filters contains some excess CO2 and very little dissolved oxy-

VEXT GAS

CO2 + AIR

v

NSF

DESALINATION PLANT

DISTILLED WATER

CO, PURIFICATION

H20 + CO 2

LINESTONE DISSOLUTIOX BACK MASHING

SLURRY

pH ADJUSTNENT

RECAREONATED MATER

Fig. 2. Block flow diagram of the .fecarbonation .process.

305

gen, and is therefore fed to a stripping tower. By direct counter-

current contact with air, more oxygen can be dissolved, and the pH value can be slightly controlled by the flow rate of air. The

stripping tower is packed with the same material as the absorp- tion tower, and is designed with a diameter of 2.6 m and a

height of about 7.7 m.

For final pH adjustment of the water to about 8.0, a dosing

system is used to inject a 15 wt% NaOH solution into the water stream.

As can be observed from Fig. 1 and Fig. 2, the recarbonation process can be designed and operated either in the once-through or by-pass mode. In the once-through mode, all distilled water is augmented by the required amounts of Ca*+ and HC03 ions. In the

by-pass mode, a part of the water is fed to the limestone filters to gain higher values of these ions. The other part is by-passed and blended with the water leaving the stripper tower. As the by-pass ratio, defined as the ratio of by-pass stream to the

total water stream, increases, the number of limestone filters required will decrease. However, more excess CO2 will be needed in the water stream entering the filters to dissolve more limestone, and increase the total alkalinity.

The amount of CO2 which can be extracted from the MSF desali- nation plant is however limited. Each unit, with a capacity of 6 MIGD, can produce about 100Kg vent gas/h.The vent gas contains

60-85% C02. The optimum by-pass ratio depends, therefore on the total capacity of the recarbonation process and the number of units involved in the MSF plant.

Results of Plant Operation

The Shuwaikh Recarbonation Plant was commissioned by Sumitomo Heavy Industries of Japan and its operation started in October 1987. The WRDC of MEW designed a scheme to monitor the quality of the recarbonated water produced, and performed a field survey among the households supplied by this water. Tables 1, 2 and 3 show the quality of drinking water before carbonation, the carbonated

distillate and the carbonated drinking water, respectively. Figures 3-9 display some of the results obtained in this investi-

306

Tab le 1. Typical Characteristics of Kuwait Drink Before Carbonation

ing

Total Dissolved Solids (mg/l) 433.67 PH 7.93 Total Alkalinity (T,Alk.) 18.93 Chloride (mg/l) 91.9 Sulphate (mg/l) 173.14 Calcium (mg/l) 43.64 Saturation Index - 1.0 Ryznar Index 10 Chloride + Sulphate/T. Alk 16.88

Table 2. Monthly Average Characteristics of Kuwait Distillate After Carbonation

Time TDS PI' T.Alk Cl SO4 CA L.1. R.I. Cl.S04/T.Alk

Oct. 87 86.0 8.28 72.68 0.20 5.0 26.05 0.54 7.12 0.069

Nov. 87 77.5 8.21 70.54 0.20 5.0 26.73 0.45 7.30 0.069

Dec. 87 87.8 a.26 69.33 0.20 5.0 25.87 0.48 7.30 0.07

Jan. 88 95.6 7.93 74.84 0.20 5.0 28.60 0.22 7.50 0.065

Feb. 88 9.30 7.85 71.50 0.20 5.0 29.48 0.10 7.65 0.082

Mar. 88 90.6 7.88 74.91 0.20 5.0 28.91 0.18 7.50 0.072

Apr. 88 83.5 8.11 74.82 0.20 5.0 28.53 0.40 7.30 0.058

May 88 87.4 8.09 67.00 0.20 5.0 30.07 0.28 7.50 0.77

Jun. 88 80.3 7.99 72.50 0.20 5.0 26.67 0.25 7.50 0.067

Jul. 88 74.0 7.98 74.33 0.20 5.0 28.80 0.26 7.50 0.066

Aug. 88 32.5 7.90 64.50 0.20 5.0 28.20 0.06 7.80 0.069

Sep. 88 68.0 7.85 65.00 0.20 5.0 24.15 0.02 7.80 0.075

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Table 3. Monthly Average Characteristics of Kuwait Drinking Water After Carbonatian ~

I 1 I’,\’ TDS PH T.Alk Cl SO‘1 CA i.1. R.I. Cl.S04/T.Alk

330.7 7.95 54.72

246.9 8.005 59.94

219.4 7.94 56.66

277.8 7.87 53.1

246.2 7.55 53.41

254.9 7.60 56.30

252.1 7.73 52.90

256.9 7.99 59.38

260.7 7.97 64.45

257.87 7.99 62.88

271.13 7.99 61.67

264.00 7.82 63.82

49.20

37.57

34.41

47.10

40.36

38.11

37.,29

42.77

39.33

37.54

37.07

39.50

95.1)j

74.110

55.25

76.58

62.89

59.81

54.70

60.54

59.17

58.07

68.47

66.15

3il.i -0.034 8.00 3.57

39.18 0.10 7.80 2.236

33.81 -0.01 8.00 1.908

38.58 -0.14 8.20 3.8

36.73 -0.45 8.50 2.296

3'.26 -0.36 8.30 2.064

34.86 -0.28 8.30 1.699

37.65 0.08 7.80 2.02

37.10 0.13 7.70 1.840

38.46 0.13 7.70 1.809

40.58 0.11 7.80 1.883

39.2 -0.03 7.90 1.82

350 ’

0 I 7 r I I I I I I I .I OCT.87 NOV.87 DEC.87 JAN.88 FEB.88 MAR.88 APR.88 MAY 88 JUN.88 JUL.83 AUG.68 SEP.89

TIME CARBONATED DISTILL. + CARBONATED DRINKING

Fig. 3. Total dissolved solids of the carbonated distillate and the final drinking water as a function of time.

308

--

OCT.37 NOV.87 DEC.E7 JM.83 FEB.83 kAZ.83 APR.83 WY 88 JUN.83 JUL.83 AUG.33 SEP.83

TIME CARBONATED DISTILL. f CARBONATED DRINKING

Fig. 4. pH of the carbonated distillate and the drinking water as a function of time.

76 ,

74 -

c 72 -

63 -

66 - \

64

62

60

58

56

54

521 ‘, I I 1 I I I 1 I I OCT.87 NOV.87 DEC.87 JAN.88 FEB.88 MAR.88 APR.83 MAY 88 JUN.88 JUL.83 AUG.38 SEP.33

nh4E CARBONATED DISTILL. f CARBONATED DRINKING

Fig. 5. The total alkallni'ty Of the carbonated distillate and the drinking water as a function of time.

41

40

39

38

37

36

35

34

33

32

31

30

29

28

27 -

26 .

25 -

I I I I I d I I 1 I 1 I OCT.87 NOV.67 DEC.87 JAN.88 FEB.88 MAR.88 APR.88 MAY 88 JUN.88 JUL.88 AUG.88 SEP.88

nME CARBONATED DISTILL + CARBONATED DRINKING

Fig. 6. Calcium ion concentration of the carbonated distillate and drinking water as a function of time.

OX .

0.4

0.3

OCT.87 NOV.67 DEC.87 JAR.88 FE9.88 M.88 APR.88 h’AY 88 JUN.88 ~uL.88 ~uG.66 s~p.86

TIME Cl CARBONATED DISTILL. + CARBONATED DRINKING

Fig. 7. Langelier saturation index (LI) of the carbonated distillate

and drinking water as a function Of time.

310

cl

H

g r-l

8

0

9

8.8 -

8.6 -

8.4 -

OCT.87 NOV.87 DEC.87 JAN.86 FEB.88 MAR.88 Ai'R.88 MAY 89 ~ub4.88 JLJI_.&~ 4~~33 ~~p.88

TIME CARBONATED DISTILL. + CARBONATED DRINKING

Fig. 8. Ryzner Index of the carbonated distillate and drinking water as a function of time.

1.5 -j

/

1

0.5

0

OCT.87 NOV.87 DEC.87 JAN.88 FEB.88 MAR.88 APR.88 MAY 86 JUN.88 JUL.38 AUG.83 SEP.88

TIME CARBONATED DISTILL. + CARBONATED DRINKING

Fi’l. 9. Chloride-sulphate to alkalinity ratio for the carbonated distillate and drinking water as a function of time.

311

gation. The results shown reflect the quality of the recarbonated

‘water before (distillate) and after blending with brackish water (drinking), as a function of time. It can be generally stated that the plant has fulfilled its objectives of treating 18 MIGD to a total alkalinity of 60-80 mg CaCOS/L. The results

shown are monthly average values and apart from a start-up period of 12 days, the plant operation is quite stable. Besides the pH, total alkalinity and calcium content of the recarbonated water, the Langelier CaCOS saturation index (L.I), the Ryznar stability

index (R.1) and the chloride-sulphate to alkalinity ratio are important parameters used to evaluate the water quality. As stated earlier, a zero L.1 value indicates that the treated water is chemically stable while a positive value indicates scale inhi- bition characteristics or the ability of water to form CaC08

scale. On the other hand, a negative value of the L.1 indicates scale dissolving capacity and thus risk of corrosion. Figure 7 shows the variation of the Langelier Saturation Index over the

period October 87 - September 1988. It can be noticed that the L.I. values of the carbonated distilled water are positive except for some short periods, and tend to stabilize towards the end of the investigated period.

The Ryznar Stability Index (R.1) can be calculated using the

equation

R.1 = 2 pHs - pH

Values of R.1 greater than 7.0 indicate tendency for corro-

sion while values less than 7.0 indicate scale formation. Fig. 8

shows R.I. over the same time period. Again the treated water

shows chemical stability towards the end of the period compared with the drinking water quality before carbonation.

The chloride-sulphate to alkalinity ratio is the third para-

meter used to evaluate water quality, and is expressed as

(cl- + SO,*)/Total Alkalinity (mea/l)

Ratios around 0.1 indicate chemical stability in presence of dissolved oxygen and at pH values of 7.0-8.0. However, it is

312

reported by Rabald4 that this ratio can take higher values. Figure 9 shows values of about 0.07 for the carbonated distilled water. Since the brackish water, used for blending, in Kuwait

has high values of chloride and sulphate, ratio values of about 1.0 did not indicate any tendency of water corrositivity.

Conclusion

The operating data, over a period of 12 months, indicated

that the Shuwaikh Recarbonation Plant, built for the treatment of 18 MIGD of distilled water, has fulfilled its objectives. The plant has a smooth and stable operating characteristics. Based on the experience gained and the overall outcome of this process,

MEW has decided to build two recarbonation plants. The first one at Doha with a capacity of 90 MGID and the second one at Az-Zour with a capacity of 40 MIGD. After implementing this scheme, all distilled water produced in Kuwait will be recarbonated in an attempt to overcome the problem of "red water ".

References

1. Abdel-Jawad, M.A. and Al-Turaihi, M.A.

Potable water distribution system corrosivity and corrosion. Part I. Water stability and leaching effect. Ministry of Electricity and Water, Report WRDC/R/48, 1981.

2. Abdel-Jawad, M.A., Al-Saleh, S.A. and Al-Qaisi, M.

Potable water distribution system corrosivity and corrosion. Part II. Metal corrosion and water pacification Ministry of Electricity and Water, Report WRDCJRI48, 1981.

3. Standard Methods for the examination of water and seawater,

1975.

4. Rabald, E.

Corrosion Guide, Elsevier, 1968.