A tailored process for remineralization and potabilization of desalinated water

Desalination. 39 (1981) 509-520 Elsevier Scientific Publishing Company, Amsterdam -

A

TAILORED

PROCESS

SALINATED

Emilio

FOR

503

Priited in The Netherlands

REMINERALIZATION

AND

POTAEILIZATION

OF

DE-

WATER

Gabbrielli

Italconsult

(“)

S.p.A.

- Rome,

Italy

ABSTRACT Economic ed water

analysis

emphasizes

of feasible

remineralization

the desirability

quality

in every

process

is only a fraction

respect

(health,

stability,

of that

and potabilization

of a process

producing

palatability,

required

for

actual

policies

fresh

etc).

water

In fact

desalination

of desalinat-

which

is of high

the cost

of such a

(7.5%

in the reported

case). The larly on

paper

suited

the

discusses

a complete

for desalinated

use

parameters

of

natural

of relatively

(400-500

ppm);

water

remineralization with relatively

carbonate

of the fresh

presence

water

rocks

produced

large quantities

at the same

time

and

and potabilization low residual

CO 2,

and

to be adjusted

of chlorides

purchase

enables

process,

The process

all

to the desired

(100-200

of large

IDS.

the

basic

values,

ppm) and fairly

quantities

of

particuis based stability

even

in the

high final TDS

manufactured

salts

is

avoided. The critical is made

manufactured zation

steps in designing

of the capital salts

such a plant are outlined

and running

such as CaCi2

and potabilization

costs

and a comparative

of the suggested

and NaHC03.

unit is increasingly

assessment

and of another

It is also concluded

becoming

plant, the design of which calls for specific

process

a specialized

using

that the remineralipart of the desalting

know-how.

INTRODUCTION Remineralization particular different defined

light

from

(“)

analyses

of

becoming

the scope

various

i.e. health,

with ITALIMPIANTI

feasible

applying

fresh

produced

to be

looked

being

by

desalination,

in

upon in a somewhat

increasingly

more

clearly

(1). remineralization

a process

palatability,

water

an issue

of desalination

field of application of

that it is worthwhile

all aspects, Now

potabilizaticn is finally

than hitherto,

for each specific

Economic indicate

and

of sea water,

and

which produces

non-corrosiveness,

potabilization

good quality

etc.

In fact,

policies

fresh

water

comparison

of

GABBRIELLJ

SOZ

the cost of implementing a valid distilled water remineralization

process with that for

producing desalinated water shows the former to amount to only a small percentage of rhe when allocating resources for desalting water, it is reasonable to spend

latter. Therefore

just a little extra SO as to provide all the necessary guarantees regarding the quality of the final product. It is in this light that at present the treatment of sea water by reverse osmosis which yields desalinated water with an already high residual salinity seems generany to be a less viable proposition than processes yielding what is practically distilled water, since tF*se permit maximum flexibility and freedom to choose the most appropriate remineralitation technique, whereas reverse osmosis binds the user to what is already a high chlorides content (1). The aim of this paper is to present and discuss, both from the technical and economic point of view, a complete remineralization and potabilization process specifically suitable for water with relatively low residual TDS (say less than 100 ppm), i.e. usually produced by condensation of

vapour ; nevertheless

the process

is applicable,

with appropriate

modifications, to desalinated water of any usuai composition. Though this process is not the only one available, it certainly offers high reliability and flexibility and can ensure a very good quality final product, especially when the latter is intended for domestic use. DESALINATED

WATER REMINERALIZATION

POLICIES

Alternative processes It is possible to imagine quite a few feasible remineralization schemes from the purely chemical point of view. However from the industrial aspect the most interesting processes seem to be those which are fundamentally based on the introduction into the water of the following substances: -

Carbon dioxide or sulphuric acid and calcium carbonate (2, 3, 4, 5).

-

Carbon dioxide or sulphuric acid and lime (2, 6, 7, 20).

-

Calcium chloride and sodium bicarbonate or carbonate (8).

-

Calcium sulphate and sodium bicarbonate (2).

-

Lime and sodium bicarbonate (8). These alternative chemical treatments are basically designed to increase the hardness,

TDS and pH of water to a given exter.t by the solution of two saits or the reaction of a base with an acid that has been injected upstream. Alternatives using hydrochloric acid have been disregarded, as the chloride ion is normally undesirable in product water. Selection of a remineralization policy A specific process for the remineraiization and potabilization of desalinated water has been selected from those indicated above to be developed here. The basic principie involved is the dissolution of

a carbonate

rock (limestone,

dolomite, aragonite, calcite, coral, etc.) by carbon dioxide or by any other acidic agent,

505

GABFUJZLLI with final

adjustment

of pH by means of a basic

treatrrent

in question

ensures

the

purchase

treatment

of

the

calcium,

huge

basic

alkalinity

particular

quantities

also be noted that such a solution salts,

to render

it suitable

produced

for

not only permits

use in the treatment

on site by extraction

stabi!itv

is a relatively

(i.e.

hardness

while

from

this

due

value,

vdue.

to

even

It should

of the use of manufactured

rock on site (grinding,

plant,

with &MEA or DEA

with

to a safe

high TDS

elimination

to process the necessary

it may also be possible

Furthermore,

can be adjusted

and there

The

and does not necessitate

chemicals. water

not temperature)

is present

such as sodium carbonate.

and reliability

manufactured

characterizing

and pH but

when a qur.ntit;r of chloride

flexibility

of

parameters

compound

carbon

sizing,

dioxide

appropriately

could

cleaned

etc)

also be

boiler

flue

gases (2). One final

point is that no solution

procurement carbon

problems

dioxide

with caIcium

only aIternative concerned, however, lime,

this

based

hydroxide

on the

would

on manufactured

the quality

in question, use

due to the potential

solution

based

ensures

oxide,

to the process

is that that

and costs,

or carbonate.

which

of

lime

problems

not be normahy

salts,

of water

with ail the attendant

obtained From

this it follows

may be comparable instead

which

of

may

unless

preferred,

of

that the

as far as results

calcium arise

by the reaction

carbonate.

in handling

there

were

are

It is felt,

and

injecting

particular

iccal

advantages.

GENERAL The

DESCRIPTION complete

schematically

OF THE PROCESS

remineralization

in Fig.

1. The underlying

the hardness

of the distilled

rock,

major

whose

addition excess

and

water

component

chemistry

of

or carbon the

is calcium

dioxide

process

principle,

through

to this, the pH is adjzlsted acidity

potabilization

inc!icated

earlie;,

is that

of a natural

carbonate,

injecting

by the injection

using

developed

the dissolution

if the water

when

process

by

of sodium

is cxcessiveiy

semicalcined

illustrated

of

increasing

or semicalcined

carbon

carbonate

alkaline

dolomite,

is

(for

see

dioxide.

In

in the case details

the

of

on the

paragraph

“An

Example of Application”). The final quality of the water is then adjusted by blending with sea water,

while provision is also made

for

the

injection

of

complementary

chemical

compounds. Potable available,

water

i.e. chlorine

let rays, etc

content

In case

outiet

distilled

the

is likely

to ensure a by-pass

by

the

gas, sodium hyp?thlorite

will be normahy

In order

sterilized

intermediate

from

water

reliability,

be

(3,). As a reference

considered. water

can

here,

storage

is needed,

complete of the CO2

injection

is necessary

for a long time

lost on the natural flexibihty injection

convenient

solution,

method

desalinators to remain

most

very at

chlorine

method

and

dioxide,

ozone,

of sodium hypochlorite mild chlorination

regular

in storage;

intervals

chemical ultraviosolution

is

at the product or

the residual

whenever active

the

chlorine

rock filters. of

the

point

arrangement

and to

ana the dolomite

increase

filters

overall

is provided

in

GABBRIELLI

-

507

GABBFCCELLI

order

to be able

to apply

an emergency

remineralization

procedure

whenever

the

remineralization plant is shut down for maintenance or because of lack of carbon dioxide or carbonate rock. The emergency procedure involves feeding calcium chloride and sodium bicarbonate with the facilities normally used for the optional introduction of additIona compounds, such as sodium fIuoride, and for pH control with sodium carbonate respectively. The feed-units concerned can be designed in such a way as to render them compatible for both services, as far as the materials and flowrate are concerned. Injection of

plant air

in the storage

tanks completes

water

the

potabilization

treatment. This is foreseen as it is generally beneficial for improving the taste and odour of the water (9). Finally facilities

are provided to inject corrosion-preventing

phosphates or silicates (8, 9), whenever the potabilization

agents, such as poly-

system is shut down or water

with safe scaling characteristics cannot be produced for a substantial period. The process can thus be outlined by the following sequence df steps involved (refer

also to Fig. 1): -

Discontinuous or mild chlorination (prevention of long-term bio!ogical fouling).

-

Automatic injection of carbon dioxide (acidification).

-

Flow through contact beds of natural or semical,cined carbonate mck (neutralization).

-

Automatic

pH control through injection of carbon dioxide (correction

of excessive

alkalinity). -

Automatic

blending with sea water p-eviously filtered

through sand and activated

-

Automatic injection of sodium fluoride solution (negligible chemical effect).

-

Automatic injection of sodium hypcchlorite solution (final sterilization).

carbon (TDS adjustment to the pre-set value).

The process has quite a number of positive features,

of which the following

are

definitely worthy of note: The plant is fairly cheap and can be designed to be very easy to run. The composition of potable water produced can be adjusted to ensure a composition similar to many commercial mineral waters. The water produced can be rendered non-corrosive for steel pipes and the scaling characteristics adjusted to the required safety levels. The process is basically self-controlling once exact addition of carbon dioxide has been ensured. Requirements of manufactured chemicals are drastically reduced. If the beds are adequately sized, a whole range of rock can be used. Dangerous chemicals or chemicals not meant for human consumption (e.g. H2S0,,) are avoided, as are salts which do not dissolve easily (e.g. CaSO,)_ The process is reliable, as it is based on the use of proven technolo,y.

AN EXAMPLE OF APPLICATION The

peculiarities

and flexibility

of the process

briefly

outlined

in the General

Description given earlier can perhaps be better explained by an example of an application involving a medium sized pfant. In this case a reference pfant has been chosen having a production capacity of 20,OOcim3fday of distilled water with a normal TDS value of less than 5 ppm. it has to be noted that a reference capacity has been fixed so!ely for the purpose of making the economic analysis reported ahead. An economically

unfavourable

basic assumption has been made, namety that it is

ImpossibIe to ensure a retiable source of suitable rock within the Middle East country where the plant is assumed to be located, so a suitable standard product must he imported from Europe. The product chosen is semicaicined formuIa MgO - CaCC+

dolomite,

considered to have the

Though this product is richer in magnesium than desirable and

lower in carbon dioxide than the original rock, so that more carbon dioxide will have to be used, it has the advantage of being fairly standard, It is not difficult to purchase this type of material, commonly used in acid water neutralization, having the required characteris_ tics as far as purity, composition, particle size distribution, reliability of supplies, etc. are

- concerned. That is why this product, which feads to economically conservative results, was felt to be suitable for the exampie. Carbon dioxide can be obtained in at least three ways: -

Imported from somewhere

-

Produced

-

Produced mixed with air on site by washing flue gases.

within or outside

the country.

pure on site (for instance by absorption with MEA or DEA).

The last alternative

of washing the flue gases, which in most cases would come

directly from the boilers, and then passing the whole stream through the water (2) is

not

considered here, as it tends to cause fesses by evaporation, to require larger equipment, to be less precise as regards control and to be more critical as far as taste and odour of product water is concerned. The other alternative of recovering CO2 from the decarbonator of acid-dosed MSF desalinators (6) will decline in importance probabIy

disappear

due

to

the

corrosion

in the involved

future,

as acid

treatment

wili

most

and the huge quantities needed in

comparison with alternative specialized chemical treatment, The technical solution adopted here is that of producing CO2 inside the plant by absorbing CO2 in the flue gases from a special light fuel oil burner firing a small boiler, which directly supplies steam to the CO2 stripping tower. it is interesting to note that the use of this method permits N2 recovery, which can be used for fang-term dry storage, for instance,

of

YSF

evaporators.

As

the characteristics

of this standard package

for

producing CO2 are well known, they will not be detailed, since the availability of CO2 is all that matters here. What has been accounted for instead are the cost of the package and the quantities of consumables needed to produce CO2 by this method, which cre considered in the paragraph dealing with the economic evaluation.

GASSRIEIJLI

5@9

Chemical features of process using semicalcined doiomite The basic chemicals needed in this case for the process are CO2 and semica!cined doiomite. Commercial MgO

semicalcined

dolomite can be considered to have the formula

CaC03, i.e. about 26% MgO and 74% C&O3

The basic reactions which help to describe what happens in the stream, after the CO2 has been injected,

when it comes

into contact

with

the dolomite

filters,

can be

represented by the following equations C&O_,

Mgo

+ H2C03

* 2H*C03

-

Ca(HCO&

-

Mg(HC09)2

t H20

The doiomite filters also remove all the iron present in the water, thus eliminating one of the major sources of undesirable colouring and turbidity. Moreover, removal of iron, which is precipitated by free chlorine, reduces the quantity of chlorine needed to establish a residual chlorine concentration in the water to ensure sterilization and once again Emits the risk of the presence of staining residuaIs in water. The pH of water hardened to the prefixed extent and blended with sea water wih normally be slightly alkaline already. However, in the case of an acidic product water or

tG attain a sufficiently Though NaHC09

alkaline pH, Na2C03

solution is injected

for pH adjustment.

and Ca(OH)2 or Na(OH) are feasible alternatives,they usually appear less

convenient owing to the quantity involved where the bicarbonate

is concerned and the

handling problems with regard to lime and soda. Adjustment of pH in the case of excessive alkalinity (a situation which could occur on restarting the doiclomitefilters, when the water already contained in the beds may have a pH value higher than 9) is ensured by a fine which can inject the required amount of CO2. In practice, a quantity not exceeding LO mg of semicaicined dolomite will normally be required, including losses for backwashing, to increase the hardness of I litre of water by 1 French degree. To obtain this, about 6 mg CO2 gas will have to have been injected previousIy into the stream of water. With a distribution network made of steel, water with high scaiing properties can be produced and distributed for a certain time after

system start-up,

so a substantial

protective layer of calcium carbonate can form immediately inside the pipes. The product water will stiii be suitabIe for drinking. Product water can then be adjusted to ensure the required quality within the reference

limits adopted, such as the W.H.O. standards for

drinking water. Finally, as there are many indications that th;e incidence of dental caries is high, especially in children, if the water normally drunk does not contain suitable quantities of fluorides (IO), optional sodium fluoride injection facilities are provided as well (these can also be designed so that

they

can

be

used

for adjusting quality by means of other

GABBRIELLI

510

compatible

soluble

economical

and rational

well

come

up against

of fluoride

deeply

standards,

While

solution

On the basis of the above a meaningful

A remineralization desalinated

-

to be 0-S mg/I.

policy and resulting

problems.

tropical

The relative

etc.

may

The quantity

country,

according

consumption

to

of sodium

features

and data,

product

water

the following

procedure

has been adopted

example:

and potabilization

water

The limits

In a given

of pills,

1.8 mg/l.

Scope of work: remineralization

-

in the form

and orgarnsational

around

that the more

it should be observed

fluorides

sociological

all the year

can be assumed

is thus about

to establish

on this subject,

of administering

rooted

ion to be injected

the W-H-0. fluoride

compounds).

to be fed as water

for the various

ranges

plant as in Fig. for domestic

of chemical

I has been specified

use into a steel

additives

injection

for treating

pipe network.

have been established

for the plant on the basis of experience. -

A reference

-

A

reference

according -

composition

Water

operating

to W.H.O.

with

the

for sea water configuration

standards

same

laboratory

information

on such matters

Proceeding

to

characteristics

necessary

and to determine

stable

good

quality

water

has been chosen. has

been

complementary

as pH ard the related

temperatures

ranges

the

to produce

criteria

chemical

obtain

now to the detailed

remineralizdtion

expected

and practice!

resulting

specialized

this at different

has been determined.

rates

in a

chemical

addition

to control

of NaC03

the scaling

analysed

practical

properties

of the product.

description

of the above

procedure,

the following

basic

have been established

for the plan:

perr,l+,g

to ~5s example

produced

of

of

application: -

Maximum dioxide .

hardness

total

(Ca+Mg)

20 French

are attributable -

Sea water blending

-

Sodium

fluoride

potable

water

-

pH adjustment cate

solution

limits,

feed

semicalcined

dolomite

thrrogh

carbon

positive

to render sea

water

rate

under

range

Na2C03

sea water

injection

by laboratory

(10.7 French

degrees

reported

in Table

I, Column

feeding

to ensure

system):

NaHC03

O-2 mg/l NaF

feeding

a pH withm

and a Langelier

miIdly sca!ing blending

water

(emergency

all circumstances

for

the significance

CaCl2

analysis

the water

composition

in product

(emergency

available

cannot

alter

i.e. 200 mg/l as Caid3

to calcium)

through

I.e. 74.5,

degrees,

range: O-l%

to be determined

Clear

by

injection:

system):

W-H-0.

index which

in

feed

desirable

is sufficiently

(ii). has

A,

been considered to hake the reference

disregarding

of the ensuing conclusions.

all

minor

constituents

which

511

GABBRIELLI

TABLE 1 Sea water

reference

anaiysis

limits for drinking water

(Column

(Column

A), World Health Organization

highest desirable

B) COLUMN mg/l

ION

COLUMN mg/l

A

B

Cl-

21,400

200

so;

2,800

200

150

HCO; Br-

100

Ca++

500

Mg++

75

150

1,500

K+

400 11,750

Na+ F-

I

TDS

500

38,600

The value for the Na+ ion has been computed by ?he law of neutraiity of electrical charges for the solution. As a reference

to evaluate the final goal for water quality, the highest

W.H.O. levels referring

to the basic constituents

the F- ion, are reported

on Table

Apart

1, Column

considered

in the sea water

desirable

analysis,

plus

B.

from the difficult and long-drawn out issue of the definition of more precise

generaIIy and internationally recognized standards and criteria for drinking water (9, :i‘, 13, 14, 15, 16, 17) and the refated problem of having a clear definition of the scope of work in desalination (I), the following two practical goals have been fixed: -

water should be mildly scaling water should meet general hygienic requirements. Bearing these points in mind, the following

from

the various

combinations

operating

which are possible

because

configuration

has been

of the flexibility

sekcted

ensured

by the

chemical additions ranges, The chosen condition was considered to be an acceptable theoretical selection, which could also be tentatively

fixed in practice, though only actual

operating experience can indicate the real optimal conditions. In particular a relatively high, but still perfectly stability

of product

acceptable,

percentage of sea water was chosen also to

test

the

water even in thii case.

The above goals and considerations resuit in: A. The decisior to use a tentative CO2 injection rate that gives 15°F of total hardness (Ca”

+ Mg++ ) after the dolomite beds, so that 8°F of hardness would be due to Ca’“.

This injection rate was in fact considered to be the minimum capable guarantee

of ensuring

some

as regards water stability (2), whiie still keeping the concentration of the

GABBRIEZLLI

512 Ca””

big++ions

and

the optimal

range

not the injection laboratory B.

rate would be suitable

level

- i.e. a safer

generally

here as being actually

in this theoretical

recognized

the top value

be reduced

case,

what modifications,

of whether

would be provided

or by a

if any, would be needed.

in the final product (18). This amounts to half the highest

be present

W.H.O.

and ensuring

and at a value in

vaiues

(5). The real proof

in 0.5% of sea water so that just 100 ppm of Cl- ion and 60 ppm

to blend

ion would

desirable

C.

is concerned

test which wou!d also indicate

The decision of Na+

well below the highest desirable W.H.O.

as far as palatability

value

as far

healthier

as risks of corrosion

water

for the desirable

composition

blending

rate.

are concerned

- and was considered

In practice

this rate could

by injecting

enough NaF

(9, 18).

The implementation

of a dental caries

prevention

campaign

to

yieid 0.8 ppm of F- in the final product. The

composition

treatments, As

the

already,

of

starting water

product

so produced

no injection

reference

composition.

TDS

due to pH adjustment_ solution)

No theoretical or corrosive

properties,

Suffice clearly view;

be

reasonably

Therefore,

the influence

was made regarding

the answer

in the following

lowest

Water discussed adding

as will

limits (or even optimal

Complementary

laboratory

being

7 at 20°C

ranges)

of the same 2, Column quantities

by injecting

have

either

of injection

these

three

an acceptable considered for

pH

in this

an increase

of a sterilizing

agent

of (in

pH, alkalinity

(P, Ml or the related

laboratory

analysis,

scaling

the results

of

of Table

2, Column

acceptable

D, with Table

2, Column

B,

from

the potable

does not mean very

are indicated

water

point of

much by itself,

as no

(9, 12, 15, 18).

analysis

practically

the following

is made

left to actual

be appreciated,

(see

Table

to

of

2.

paragraph.

shows that ail the values are perfectly that,

contribution D of Table

aa:rmed

no allowance

it here to note that comparison

a condition

the

none of CO21 has been

(and certainly

Similarly

from

in Column

was also ignored.

forecast

which are reported

resulting

is reported

can

of NaC03

trial

this case NaCiO

water

from pure water,

TDS

D)

and composition

has been

of chemicals

3.6 mg/i NaDH

carefully to distilled


as that produced reproduced water

previously

a starting

by the process

in the

laboratory

adjusted

M Aikailnity

by

to pH =

equal

to 20

mg/i CaCO$. CaC03 H2S04 NaHC03 KHC03

.MgCi2

39.0 mg/,

CaCi*

14.7 mg/!

i-ICI

1.5 mg/l

219.5 mg/i

NaF

1.7 mg/i

KBr

1.0 mg/i

2.0 mg/i

52.2 mg/i

96.2 mg/i

The chemical analysis run on a sample of this water gave the following results: -

pH at 20, 30 and 40°C = 7.8, 7.85 and 8.02 (measured by pHimeter)

513

GABBRIEALI

TABLE Final

2 product of the

tions

water three

Ion

basic

treatments

of tbe

and

“C”

separate

contribu-

applied

to pure

C

D = (ALBiG)

=-G/l

mg/l

mg/l

107.0

107.0

14.0

14.0

0.8

183.8

0.5

0.5

32

2.5

3e_ 5

17

7.5

BrCP

“B”

B

183

3

“A”.

a sum

mg,/‘l

4

HCO-

as

-4

Clso--

“D”

composition

2.0 59.5

193.75

232

TDS

A = 15O Frenchhardness

obtained

plant E = 0. 5%

sea

water

blending

C = 1. 8 mg/l

sodium

fluoride

D = Resuiting

composition

through

I. 8

C02/sem;calcined

427.6

dolomite

water

GABBRIFZLI

514 -

Alkalinity

(P) = 100 mg/l CaCO3

-

Alkalinity

iM) = 312 mg/l CaC03

-

Total

Alkalinity

The following

by volumetric

(measured

titration)

by voiumetric

titration)

= 412 mg/l CaCO3 data,

2 and still valid

(measured

already

obtained

for the sample

for product

of water

water

reported

used for the above

in Column

haboratory

D of Table

testing,

are

also

pertinent: -

Total

=

hardness

blending

18.7 French

to that obtained

ed by laboratory -

Hardness

-

TDS

degrees

(computed

by

through the semicalcined

adding

dolomite

that

due to sea

and substantially

water

confirm-

analysis).

due to calcium = 8.6 French degrees (computed as above).

= 427.6

mg/l

(computed

by adding

the

quantities

in the water and

dissolved

reported in Table 2, Column D). i3y applying temperature -

the

of 20°C,

pH (saturation) From

by

a

of

water

Being positive

that

the chosen

practical

to compare

-

Kilograms

above

data

at a reference

pH of product

alter

water

substantially

that

at 20°C

the

the Langelier

to 8

considered

Index

for

the

= 0.8. is considered

reference

for

suggested

COMPARISON

water

scaling. dealt

subsequent

by field

with here,

it is evident

adjustment

of the final

experience

in the

particular

OF PROCESSES

of processes general

remineralization process

descriptron,

and potabilization with

one using

the overal

quantities

are indicated

manufactured

by means of CaC12 and NaHCO

3

in Table

chemicals,

of chemicals 3.a. In order the data

plus polyphosphate

for

injection

3-b.

For ease of reference, Grams

is a good

of the foregoing

in Table

-

not

it follows

of the product

requirement

AND

remineralization

are reported

does

3a),

than 0.5, the water

and comparison

the suggested

emergency

the

involved.

EVALUATION

for normal

which

Table

of the properties

actual

application

On the basis

(see

composition

for evaluation

needed

with

of the normal

is Ii = pH - pH (saturation)

to the

ECONOMIC Data

NA2C03,

and greater

From the analysis

composition

adjustment

composition

remineralized

(11)

it ensues that:

quantity

water

diagram

= 7.2.

this, and considering

adding

product

Hoover-Langelier

of chemicals

the data included needed to treat

of chemicals

needed

of required

chemicals

in the tables

I m3 of distilled

concern: water.

to run a 20,000 m3/day

plant

at design

rating

for one

day. -

Daily African

cost

country

comparisons; chemicals

like Libya

for

the

are bought

cast

in three

differen t countries

and a Gulf country computation

in Italy.

it

like Saudi Arabia) is assumed

that

(Italy,

a Mediterranean

to allow in all

more general

three

cases

the

515

GABBREELLI

TABLE

3, a

Chemical

Type

consumption

Most

of

ble

chemical

cial

and

for

remineralization

cornmel

General

remarks

form

Quantity

yielding

hardness

Ca++, cluding backwa

Xemical cost -1-S. $/day

Abya

2,400

1

due

already

in-

losses

for

72(

961

50

1 drums

Quantity

hypochloritf

of

16%

to yield

Sodium

50 kg bags

fluoride

1,20

tl

calculated 0. 3 mg/l

Quantity to yield fluoride

(NaF)

Monoethanol

200

amine

drums

(MESA

(QHCH2CH

3. T

78

2(

2’

3

1.8

36

31

31

3r

0.5

10

14

1E

o:

kg

Value

based

plant 100%

2

calculated 0.8 ion

mg/l

on CO

running capacity

at

oi

2

NH,) L

v Sodium

50 kg bags

carbonate

- For sulphur moval from

reflue

gases

@a2C03)

- For

fuel

[L. F. 0.

)

oi

Liquid

de-

livered :ruck

by

Duantity from tion

2

40

10

14

10

200

50

70

50

1, 000

350

70

OH adjust-

ment

TOTAL

Lrabi

chlorine

(NaClO)

Light

iaudi

shing

Sodium

sol.

of

plant

*/m3 S°F

dolomite (MgOeCaCO

and potabilization

Quantity chemical

proba

50 kg bags

Semicalcine

cost

CO2

calculated cons&p-

(L. F. 0,

)

-__ /

1.195

,190

516

GABBRIELL.1

TABLE

3. b

Chemical

consumption

and

cost

for

emergency

remineralization

and

potabilization

plant

rype

Most

of

ble

chemical

cial

Quantity

probacommer-

General

77.

chloride

bags

(CaCl2-

Chemical

remarks

U.S.

cost

$/day

form g/m3

Calcium

of

chemical

8 70,

50 kg

Quantity

calculated

to yield

nH20

ness

kg/day

Italy

Libya

114.1

2.282

525

135.7

2.714

950

753

Saudi Arabiz

981

8OF

due

hard++ to Ca

(l(n<2)

sodium

I 99%

bicarbonate (N~HCO

SO

kg

Stoichiometric quantity

bags

3)

Ca(HC03)2 above CaC12

Sodium

50 1 drums

hypochlorite

16%

solution

(NaClO)

of

with

quantrty l

Quantity to yield

of

nH20

calculated 0. 3 mg/l

50 kg bags

Quantity

3.9

78

20

27

35

1.8

36

31

34

38

of

calculated

fluoride

to yield

0.8

(NaF)

fluorLde

ion

_Anticorro-

50 kg bags

treat-

ment

1,493

chlorine

Sodium

sive

1.221

to yield

Quantity highest

(poly-

mg/l

based

of

on

6

120

228

240

252

envisaged

consumption

phosphate)

rOTAL

_

_-_-_

____--_-_-

__-

-_-

1,754 :

2. 275

2.799,

I

GABBFLIELJJ

517

Where Libya and Saudi Arabia

are concerned,

1981 estimated market prices, are ci.f.;

the costs,

which

are based on Fe5ruary

therefore, they do not include customs duties and

delivery from the harbour to the site. In the case of Italy, prices are ex-works and in&de packing. Therefore in both instances the last delivery step is omitted, since this would be strongly influenced by the specific location of the site within the country. In considering

Tables

3.a and 3.b, which are

self-explanatory

and allow

direct

comment, the following points should be kept in mind: -

To render the two processes reasonably analogous, and therefore

comparable,

the

zmergency chemical additions are such as to produce hardness of 8” F due to Ca++, stoichiometrically

considered as Ca(HC0,).

This means

however

that the product

water obtained with the emergency treatment is of poorer quality than in the other case,

because, among other things, it has much lower total hardness and a substantial

concentration of Cl- ions even without blending with sea water. -

The quantities of basic chemicals needed for on-site production of CC2 have been included in the table; but -when considering these, it should be noted that the quantities of L.F.O.,

sodium

carbonate

for sulphur

removal and monoethanolamine, not directly

injected into the water, are computed by dividing the daily consumption by the amount of water produced daily, thus converting consumption into quantities per cubic meter of water produced. -

All

chemicals

invoived are considered in a commercial

form suitable

for human

consumption. -

Sodium carbonate

consumption for pH adjustment is a mean figure derived from

laboratory tests. Polyphosphate

consumption in the case of emergency

remineralization

checked through operating conditions and the reported

is normally

feed rate is a top figure

estimated from general data available. Quantities do not take into account losses in transportation, storage, handling, etc. The quantity of fuel, though not qecificr’ly

a chemical, is reported in Table 3.a for

completeness of cost analysis, as it is the basic raw material used for the production of co*.

A whole

series of

fuels from methane to Bunker C types are feasible

alternatives. Light fuel oil is also used in the case of Italy for the sake of uniformity, although it is not really economical. Economic considerations In order to provide a point of reference regarding the capital cost of a remineraiization and potabilization plant like that described

in previous paragraphs,

consider the dual purpose pIants assumed to be located

it is possible to

in Libya and discussed in a

previous paper (19). The plants have an overall capacity of 20,000 m3/day of fresh water produced from four MSF evaporators. The average capital cost in the second half of 1979 was about U.S.$ 55 million. The cost of 1 m3 of product water then ranged normally

GdBBRIEJ-.LI

516

between U.S.S

U.S.$

1 and 3, depending

1.70, with

electric

power

to outside

For

uniformity

water

to

production without

feeding

examples

the

solution

in the

plant

described

chemical

injection

5 million.

NaHC03

in

facilities,

of

1 m3 of

fresh

of U.S.$

water

obtained

hypochiorite

value being

supplying would

any

be about

produced

as already

provided,

months would

the provision

including

the

storage

in the paper

injectlon. 3 already made

the

entail

cost

but

a capital for

would be

facilities,

(capital-recovery the cost

for. remineralizing

cost for water

blending

concerned).

of

CO2

operation

as used for computing

(191, the

with

addition

of the equipment

Applying the same formula

for amorrization

blending

The

including

for a few

0.15. This means an overall

was not included

from

(19).

paragraphs,

facilities

in the paper

is U.S.$

apart

and

production

with seawater

A similar

(cost

value

is also

in the case of CaC12 and NaHCO

These

data

percentage

confirm

increase

only acceptable, fresh water

semicalcined therefore,

the

point

in the cost of product

it is virtually

a necessity

by means of desalination

In analysing

the

economic

dolomite

ccsts

with

dolomite,

it emerges

relatively

low and, indeed,

i?voIving

could

it has also as far

of CaC03

Tables

in

the

Introduction,

due to qualitative

once resources

be easily

a high percentage from

water

i.e.

that

remineralization

have been allocated

the

is not

for obtaining

(I).

results

is not optimal

the running

limestone

those

costs

paper

7.5% more than in the case of simple

2.15, namely

of sodium

previous

and polyphosphates,

1 m3 of water

in the

By way of comparison,

method) and the general hypothesis

potabilizing

without

these

facilities

given

storage

about one tenth of the figure just mentioned. factor

a good average

capacity,

1981 level,

no remineralization

U.S.$

CaC12,

At the February

foreseen

obtain

NaClO

conditions, at full

2.00, respectively.

unit and additional

cost of about

operating

of reference,

had been

facili:ies

plant

users.

U.S.$ 65 million and U.S.$

sea

on the operating

the desalination

to be borne

as the

chemical

reduced

by half

can

be used.

in mind that composition

or more, But even

the

use of

is concerned;

especially using

if local

semicalcined

3.a and 3.b that the running costs for consumables

are much lower

pure manufactured

in the case

chemicals,

of the described

process

are

than in

while at the same time water quality

is

better. It should be pointed using carbon 200,000,

respectively,

plant

in designing

rock

shows

that

both types

the higher

on the dissolution

of the latter

type

recovered

by savings

from

to eight

five

and natural

savings

when

in chemical

operating

(i.e.

capital

of a natural based

in running costs

years.

of plants,

Then there

cost

required

rock by carbon

due to chemicals

output

and Saudi Arabia

to which

on the injection

products

at design

400,000 and 500,000 in the case of Italy, Libya

Experience

(i.e. based

dioxide

out that the yearly

Table

with the process are

about

U.S.$

respectively.

3.a and Table

3.b refer

for a plant of the former dioxide)

when compared

of manufactured

in a period

is also the added advantage

chemicals)

which

generally

of the better

type with a

can be varies

quality

of

GABBRIELL.1 the water

519 produced

with

the

Table

3-a plant

(basically

that

in Fig.

when both

I) even

plants ensure the same hardness due to calcium. CONCLUSIONS The paper highlights costs

of

the

solution

of

injection.

errors

hmestone

with

points

water

the

beds

An emergency

bicarbonate, control

supersaturation

procedure

is also

with

beds, and subsequently by pH adjustment fluoride

and

similar

aeration

are

increase

major

point

which

provided

for.

waters.

nature

confirmation

that any properly

sophisticated

than simple blending

for remineralization

designed

fresh

good

quality

potabilization relevant

and

are also coming

processes

are

not

time

if are

between

at all injection

instruments)

or in

of shut-down chloride

of the

and sodium

facilities

(pH

merely

to form

just as highly

not exceeding

the

from

paper

modest

of

any part

the

scheme

more

The extra

cost

and can be

in most cases.

quality,

awareness

of the need

remineralization

of the desalination

and require

is cost

water. is, in fact,

feasible.

that with increasing

a fundamental

specialized

composition

at a very

as shown by the examp!e,

cost for desalination

water

1.8 g sodium

and potabilization

is economically

low,

of

resulting

the

water

dolomite

O-5%, followed

injection

1 m3 of desalinated

emerges

characteris-

1 m 3 distilled

on semicalcined

is obtained

remineralization

is very

with gcod

when

Final

In particular, This result

in the light of these facts

water

if there

feeding

water

is obtained

injection.

with sea water

and potabilization

It should be clear

ancillary

of sea water

which

taken to be no more than 10% of the overall

for

from

is then neutralized

carbonate

mineral

of a general

mixing

of calcium

the

of U.S.$ 0.15 on the basic U.S.$ 2.00 to produce

The

contact

in case

injection

scaling

with a quantity

also

the good

indications

carbonate

and easy to operate

rock and/or chemicals.

are concerned

to 8 with calcium

to some commercial

by sodium

and

is the

of the main plant.

dioxide,

blended

principle

may not be attained

in assigning

by using

shows that mildly

and health

6 g carbon

reliable

or in ensuring

direct

available

injection)

the paper

pH control

remineralization

involving

immediately

tics as far as palatability is treated

(or

basic

with

or faulty

for qualitative

plant,

and sodium fluoride

As an example

rock

is analogous)

range of application,

whose

flexible,

objectives

for the need to feed natural

dioxide/limestone

the criteria, process,

dioxide,

prcject

natural

which

any local

due provision

carbon

of carbon

though

of

and rock,

(to avoid

making

regarding

potabiiization

that the plant is extremely

due attention,

in sizing

acidified

and

by means

It is evident

designed

the main features

remineralization

and

plant;

just as much specific

the

know-

how as do those in the other major parts of the pJant. As

should

example gradually degree

be

evident

from

of application,

where

and partially

attained,

of precision.

the

exposition

a clear

definition

of

the

it is no easy matter

In this regard,

problem

concerning

of a sound scope

the organizations

to define involved

of work

the

reported

has been only

the scope of work with any in setting

standards

and in

~BFtIELIJ

520

establishing criteria will certainly have to concert rheir efforts to produce more precise limits to be used for reference. REFERENCES 1

2 3 4 2 : 9 10 11 I2 13 I4 15 16 I7 18 19 20

E. Gabbrielli, Paper presented at the Round Tabfe organized in connection with the 7th International Symposium on Fresh Water from the Sea, Amsterdam 23-26 September 1980 (in course of publication in Desalination) Degremont, Memento technique de I’eau, Paris (1972) P. Rericha, Vodni Hospod., B. 28 (51, 120-122 (1978) V.M. Sorissov et al., Proceedings from the 7th Internationaf Symposium on Fresh Water from the Sea, I, 371-373 (1980) A.N. Marey, B.N. Belaeva, Hygi&te et Sanitaire, 5, 93-95 (1961) I. Jamjoom,-G. Costes, Desalination, 30, 163-173 (1979) F.C. Wood et al., Desalination, 20, 319-334 (19771 R.E. Margarita, Pure Water, IDEA, 4 (Z&2-7 (1976) A.W.W.A., Water Quality and Treatment, 3rd Edition (1971) W.I-I.O., International Standards for Drinking Water, Geneva (1971) 3. Rodier, L’Analyse de l’eau, 1 (1975) R-3. Weiis, Wat. Poilut. Control, 25-30 f1978) C.H. Tate, R. Rhodes Trussel, Jour. AWWA, 69 <9),486-498 (19771 G-1. Sidorenko, Yn. A. Rakhmanin, Proceedings of the 7th International Symposium on Fresh Water from the Sea, 1, 109-116 (1980) G.M. Berlyne, R. Yagil, Desalination, 13, 217-220 (1973) A.H. Goodman, Proceedings of the Symposium on Maintenance of Water QuaIity, 59-76 (1975) C.I. Sidorenko, Yu.A. Rakhmanin, Environmental Health Perspectives, 30, 133-138 (1979) Yu. A. Rekhmanin et al., Gig. Sanit., 7, 16-22 (1975) E. Gabbrielli, Proceedings of the 7th International Symposium on Fresh Water from the Sea, I, 497-511 (1980) in course of publication in Desalination1 F. A. Drake, Desalination, 18, I-14 (1976)

AKNOWLEDGEMENTS The author is indebted to Mr. E. De Leonibus for technicd Gladwin for assistance with the text.

support and to Mr. LE.