The treatment of industrial effluents containing sodium hydroxide to enable the reuse of chemicals and water

Desalination, 67 (1987) 409-429 Elsevier Science Publishers B.V., Amsterdam

409

-Printed

in The Netherlands

THE TREATRENT OF INDUSTRIAL EFFLUENTS CONTAINING SODIUMHYDROXIDE TO ENABLE THE REUSE OF CHEHICALS AND YATER A.E.

SIMPSON

and C.A. BUCKLEY

Pollution Research Group, Department of Chemical Natal, King George V Avenue, Durban, 4001, Republic

Engineering, University of South Africa

of

SUHMARY An economically viable treatment sequence has been developed and piloted at two textile factories for the recovery and reuse of water, chemicals and heat energy from sodium hydroxide effluent produced during the scouring of cotton fibre. The treatment sequence involves pretreatment of the scour effluent by neutralisation, using an acidic gas, cross-flow microfiltration and charged membrane ultrafiltration (also called nanofiltration). The sodium hydroxide is then recovered in an electrochemical membrane cell with the simultaneous evolution of acidic gas which is recycled within the treatment process. Two possible configurations of the treatment process, where the acidic gas is either chlorine or carbon dioxide, have been discussed. Pilot plant results have been presented for both systems. The carbon dioxide system was the preferred route and'is discussed in detail. The pretreatment sequence neutralised the scour effluent, lowered its chemical oxygen demand by 86% and removed 65% of both the calcium and the organics and 50% of the magnesium. The sodium hydroxide (100 to 200 g/l) and depleted brine solution (total solids 500 mg/l) from the electrochemical membrane cell were of suitable quality for reuse in the factory process. The electrochemical membrane cell produced sodium hydroxide at 62% current efficiency at an electrical power consumption of 4 000 kWh/ton 100% NaOH. The effect of electrolyte, in particular, anolyte flow rate, temperature and concentration on the limiting current density and power consumption has been investigated. Some design data for a full scale treatment plant has been presented. The operation of an acceptable background concentration closed-loop recycle wash system in the scour process was found to reduce the required membrane area by 82%. Minimum dissolution of the precious metal oxide anode coating occurred and long anode life was predicted. Serious electromembrane fouling, with increased resistance, was not apparent.

INTRODUCTIDN The

discharge

problematic receiving

of

industrial

effluents

in that these effluents waters,

thus

reducing

contribute

their

containing

sodium

to the mineral

quality

hydroxide

enrichment

and prejudicing

their

is

of the

potential

for reuse. Sodium

hydroxide

is

The total South African

used market

extensively

in various

NaOH), of which about 6 000 tons (2%) is consumed

OOll-9164/87/$03.50

industrial

applications.

is in the region of 260 000 tons per annum

0 1987 Elsevier Science Publishers

in the textile

B.V.

industry.

(100%

410 The

specific

sodium

factory

is shown

textile four

processes;

hydroxide

for

consumed only

4%

of

associated is

the

severe.

contains

total

ultrafiltration

sequence

is

effluent

to

configurations in Southern

the

The

sodium

the

off and

hydroxide

as it constitutes

environmental

impact

the fibre after scouring

content

pollution

in

sodium

load

of

this

stream,

it

in 10% of the total

Group,

charged

reuse of enable

be

both

neutralisation,

membranes

scour

recovered

of

at the University

of

water

and

the treatment

Natal,

(nanofiltration)

effluents.

the

of

and

sodium

process

to

and

electrochemical of the

hydroxide

the

scour

have been piloted

a

microfiltration,

The net effect

recycled

has developed

cross-flow

from

treatment the

scour

process.

Two

at two textile

mills

Africa.

1

Specific

sodium

hydroxide

consumption

Textile

Percentage

process

total usage

Scouring

17

96

Mercerising

of

Specific

consumption

79

445

g NaOH/kg

Bleaching

1

7

Dyeing

3

21

EFFLUENT

g/l)

fabric

CHARACTERISATION

During

scouring at

the cotton

elevated

The waxes,

pectins

the cotton

during

sodium

the

acceptable

inorganic

demand

dyeing.

of

However,

high

oxygen

consisting

using

the

to

is generally

polyester/cotton

is mainly consumed

is usually washed

of water used in washing

to

woven

and

Discharge

consumption.

Research

sequence

for

typical hydroxide

volume.

Pollution

recovery

a

bleaching

evaporator.

and dyeing

addition

effluent

The

60

an

the disposal

In

treatment

in

Sodium

mercerising,

25% of the chemical

factory

TABLE

in

bleaching

with

1.

over on the fibre after mercerising

reuse

during

usage

in Table

scouring,

carried

recovered

hydroxide

hydroxide

fibre

temperatures

and other washing

solution

is contacted

and,

cotton

with

sometimes,

impurities

also

sodium

at elevated

are saponified

to produce

the

scour

containing

the

inorganic

hydroxide

effluent.

pressures.

and removed

This

and organic

(50 to

effluent

from is a

non cellulosic

411 contaminants

Table

of the cotton.

2 gives

a typical

range

in composition

of

scour effluent.

TABLE 2 Typical

composition

of scour effluent

13.5

PH Conductivity

(S/m)

30

Total carbon

(g/l)

4.0

Inorganic

(g/l)

2.0 0.1 -

(g/l)

1.9 -

3.6

Organic

carbon

carbon

Chemical

oxygen demand

- 90

0.4

(g/l)

4.0 - 14

Sodium

(g/l)

4.0 - 12

Calcium

(mgll)

10

Magnesium

(mg/l)

1.0 - 10

Carbonate

(g/l)

1.0 -

Hydroxide

(g/l)

2.0 - 8

Total

(g/l)

15

("C)

100

solids

Temperature

DESCRIPTION In

development

effluents,

treatment

neutralisation

employed

objectives

sequence

of for

(Fig.

an

1)

acidic

conventional

pH

1)

the sequence

are

neutralisation:

for

the

membranes.

sodium

possible,

treatment

of

sodium

scour

hydroxide

essential

Organics

These

stages.

include

microfiltration

(CFMF),

hydroxide

recovery.

depending

on the type of acidic

or

carbon

dioxide.

the

sodium

hydroxide

The

principle

:-

converts

is

four

cross-flow

chlorine

which

reduction

involves

gas),

of each stage are as follows

This

(ref.

loop recycle of water,

the scour process.

neutralisation,

(i)

a

(NF) and electrochemical

options

salt.

3

- 36

was the closed

sequence (with

nanofiltration

gas

of

the objective

and heat energy within

Two

50

OF TREATMENT SEQUENCE

the

The

-

to

enable

further

into

a

sodium

processing

by

which are soluble at the high pH, flocculate

at the lower pH. (ii) include

cross-flow saponified

(iii) organics,

microfiltration

waxes,

nanofiltration thus ensuring

pectins to

soften

maximum

(ref. 2) to remove suspended

and complexed the

solids,

which

inorganics.

effluent

and

remove

life of the electro-membrane.

trace

colour

and

412 electrochemical

(iv) salt

into

within

sodium

processing

hydroxide

the treatment

and

sequence

in

an

a membrane

acidic

gas.

cell The

to the neutralisation

to

split

acidic

gas

the is

sodium

recycled

stage.

RECOVEREDNaOH;lOto20%

I

MAKE-UPWATER DEPLETEDBRINE pH4to7;0.59/ SODIUMSALT SCOUR WASHRANGE

SCOUR SATURATOR

DIRECTION OF FABRIC

I NON-

1 SCOUREFFLUENT pH14:2Og/lNaOH ORGANICS

7-4

““%!s”~s”

NEUTRALISATION

30to40g/l SODIUMSALT

T

I

REDUCING w SG:::necessary) ORGANICAND POLYVALENT IONCONCENTRATE

ORGANIC CONCENTRATE

Fig. 1. Schematic

Carbon dioxide

equilibrium

depend solution

pH

of the treatment

which (Fig.

wholly

process

system

The viability

varying

of recovery

on

reduces

exists 2).

The

between ionic

the pH value. the

process

pH of the

described

inorganic species

carbon

species

co-existing

Absorption solution

is largely dependent

with

of carbon the

in

solutions

in carbonate dioxide

formation

on the of

solutions

by a hydroxide

of carbonate

ions

413 (CO3') are

which

predominate

increasingly

formed

below 8.6 carbonic acid exists

at high and

0

Below

predominate

acid (H2CO3)

in equilibrium

pH.

is formed

with dissolved

I

I

I

2

4

6

pH 12.6

bicarbonate

pH 11 and 6.5.

between

from the bicarbonate carbon

dioxide

ions (HC03-) At pH values

ions.

Carbonic

gas.

I

8

10

14

12

PH

Fig. 2. Distribution

Careful

pH control the

controlling present

during

stage

with pH

in the

fractions

neutralisation

below 8.6 or carbon (ii)

the

exclusion Monovalent

charge

basis ions and

dioxide

of

treatment

process

monovalent

and

and of

the

membrane It

density.

discriminates low

polyvalent

pH

of

the

effluent

gas will not be absorbed

nanofiltration

negative

divalent

at each

relative

species

is essential divalent

in

anions

:-

(i)

high

of carbonate

charge ions

are

,is an

between

passes

rejected.

through

Hence

the

membrane

organics

ions of different permeate

not be lowered

efficiently.

ultrafiltration

selectively

density

should

on

charges the

with a

(ref.

membrane,

permeate

a

size 6). but

composition

414 during

nanofiltration

of

a

carbonate

At pH values of approximately of

bicarbonate

permeate

ions

will

At

stream.

system

8.6, sodium

pass

through

higher

pH

in solution

is dependent

ions and inorganic the

values,

carbon

membrane

and

divalent

carbonate

be

on

pH.

in the form

in the

recovered

ions

will

be

excluded and the requirements of electroneutrality will cause sodium ions to be rejected, thus lowering the sodium recovery. (iii)

the

reactions occurring

in the

electrochemical

membrane cell

are

detailed in Fig. 3. DEPLETEDBRINE FORREUSEpH7; 0.5 g/lNaHCO,

RECOVEREDNaOH 1

~:0090"~~~'

02ANDC0,

H* -I

7

I

I

I

i ELECTROCHEMICAL MEMYRANE

I

I +

Na+!_

-

H,O+

PRETREATED SCOUREFFLUENT pH8.5;38g/lNaHCO,

electrochemical reactions: anode

02 t 4Ht

2H20 t Ze-, H2 t 20H-

chemical reactions: anolyte

HCOi t H+-t CO2 t H20

Fig. 3. Schematic

The

2H20 - 4e*

cathode

anode

and

cation

permeable

sodium

bicarbonate

of electrochemical

cathode

membrane

compartments

are

ion exchange membrane. is

passed

through

the

cell

separated

by

a

highly

selective

The pretreated effluent containing anolyte

compartment

and

a

dilute

415 sodium

hydroxide

potential anolyte

is

at the cathode

2H20 + 2e

the

where

the catholyte sodium

electrodes,

they combine

by the reduction

with

When a

compartment. ions

migrate

the hydroxide

from

the

ions which are

of water

----j, H2(g) + 20H-

The anodic

2W20 + 4e

reaction

+

and hydrogen

the anolyte release

is the oxidation

of water

:

02(g) + 4H+

The oxygen

the

is passed through

between

to the catholyte

produced

of

solution

applied

causes

of

gases are evolved.

The resulting

a shift in the equilibrium

carbon

dioxide

gas

which

is

increased

of the carbonate

recycled

to

the

acidity

species with

neutralisation

stage.

Chlorine The

system chemistry

dioxide ions

of

at

high

released

the

chlorine

pH

as

values,

as chlorine

system

chlorine

In solution,

system.

is

hypochlorous

gas at pH values

similar

exists acid

below 5.2.

to

that

predominantly at

neutral

of as

pH

the

carbon

hypochlorite

values

and

is

The two systems are compared

in Table 3. The main disadvantages the materials

(i)

highly oxidising

conditions

(ii)

chlorine

(iii)

dissolution

coating

associated

is a hazardous of

on the titanium

(iv)

potential

system are

:-

of the plant need to be resistant

under

and are expensive.

the

low

chemical. chlorine

anode occurs

at the anode at low chloride

mixtures.

with the chlorine

of construction

explosive

overpotential

precious

due to the predominance

metal

oxide

of water oxidation

ion concentrations. hazard

associated

with

chlorine

and

hydrogen

416 TABLE 3 Comparison of the chlorine and carbon dioxide systems

Parm&er

stage

Neutratlsatlon

Carbon

dicxlde

Chlorine

1) acldlc gas

9

C'2

2) product

NW4

N&I.

Ni I

oxldatlon

3)

saws

other effects

N&Cl and

decolcurlsatlon of agan1cs

1) chemical

Nanoflltratlon

reltucingagent

addltlon

Electrcchemlcal

1) anolyte

N&l

recovery

2) cathotyte

NeOH

3)

2C1--2e_,C12(g)

anode reaction

4) chemical

anolyte

reactlo"

-X02(gl+H20

H++HC%

N,I

5) cathode reaction

2H2C+2e-rH2(g)+2C+i-

2H20+2e~H2(g)+2ai-

6) gases released

02. CC2. t$

CI2.

7) materials

PVC,

of cell

constructlo"

PVC, titanium

PolYPropYle~

8) hazards

H2

chlorine toxicity

Nil

explosive.

PROCESS DEMONSTRATION Both piloted

the chlorine at local

effluent

treatment

is discussed The

unit.

specifications

membrane

mills

dioxide

systems

in order

of the treatment

to obtain

The carbon dioxide

design

sequence

were

data for a full scale

system was the preferred

route and

in detail.

electrochemical

The

and carbon

textile

unit

of

the

consisted

pilot of

two

plant cells

are of

detailed a

bipolar

in

Table

stack

with

4.

The

a total

area of 0.1 m*. pilot

treatment

plant

sequence.

trials

were

conducted

batchwise

The trials were aimed at :-

through

each

stage of the

417

(i)

investigating

the performance of the cross-flow microfiltration and

nanoiiltration processes for the pretreatment of the scour effluent. (ii)

determining

the current

determining

the

efficiencies

for the production of sodium

hydroxide. (iii)

specific

power consumption

for the

production of

sodium hydroxide. (iv)

investigating

the

effects

of

operational

applied voltages and electrolyte

temperatures,

current

densities

and

flow and concentration on the

specific power consumption. (v)

determining

closed-loop

recycle

the effect of operating a background salt concentration wash

system on the limiting current density and on the

required electro-membrane area. (vi)

investigating the long term effects of the pretreated scour effluent

on the electrodes and electro-membrane. RESULTS The effluent characteristics after each stage of the treatment sequence of one experiment are summarised in Table 5. The process produced :(i)

two near-neutral

containing

approximately

low volume organic and polyvalent ion concentrates 60 g/l of

total

solids and comprising

10% of the

effluent volume (ii)

hydrogen gas formed at the cathode of the electrochemical cell which

was vented to the atmosphere.

The oxygen and carbon dioxide gases evolved

from the anolyte were fed to the absorption column. recycled

within

the

treatment

sequence

and

the

The carbon dioxide was

oxygen

was

vented

to the

atmosphere. (iii)

a depleted brine solution suitable for recycling as wash water to the

scour process. (iv)

a pure concentrated

scour process.

sodium hydroxide solution for recycling to the

418

TABLE 4 Specifications

of

the

pilot plant using

the carbon

dioxide

system of

treatment sequence

Unit

Comments

Size

Absorption column

Cylindrical perspex column packed with plastic saddles.

Diameter: 140 mm. Height: 1.5 m.

Cross-flow microfilter

Woven polyester tube arranged in a spiral. Inlet pressure: 250 kPa. Pressure drop: 100 kPa. Feed velocity: 1.5 m/set

Diameter: 12 mm. Total membrane area: 0.45 m2.

Nanofil ter

FilmTec NF40 spiral wrap membrane. Operating pressure: 1.6 MPa. Operating temperature: below 45°C.

Total membrane area: 0.56 m2.

Electrochemical cell

Steetley DEM 02 cell (PVC frame). Anode: precious metal oxide coated titanium Cathode: stainless steel Membrane: du Pont Nafion 324 Maximum operating temperature: 55OC. Potential: 4 to 12 V per cell. Current: u to 300 A (6 000 A/m! ). Batch operation from high anolyte concentration (12 g/l Na+) to low anolyte concentration (0.2 g/l Na+). Catholyte concentration: 100 to 200 g/l NaOH.

Capacity

150 1 scour effluent/day. 3 kg 100% NaOHlday as 100 to 200 g/l solution. 135 1 depleted brine. 75 g (840 1) H gas. 600 g (420 1) $2 gas.

2 of 0.05 m2 2 of 0.05 ilfz 2 of 0.05 m2

the

419 TABLE

5

Effect of treatment

sequence

AllalySlS

on a typical

scour effluent

After

Raw sccur effluelrt

After

nartrallsatlon

sample

After

CFW

After

K

electrolysis brine

PH Concklctivlty

13.5

0.6

8.4

9.0

5.2

G/m)

6.4

2.4

2.5

2.3

0.2

-

Total

(g/l)

4.0

1.9

7.6

5.9

0.4

-

carbon

Inorganic Organic

carbon

carbon

Chemical

oxygen demand

(g/t)

0.3

4.3

4.6

5.2

0.0

(g/II

3.1

3.6

3.0

0.7

0.4

-

(g/l)

8.3

9.3

5.3

0.5

0.5

-

Hydroxide

(g/l)

4.1

0.0

0.0

0.0

0.0

(g/t)

2.6

1.9

2.0

3.4

0.0

1.5

Bicarbonate

(g/l)

0.0

lb. 1

16.5

11.5

0.0

0.0 97.0

(g/l)

8.4

8.2

Calcium

(mg/l)

45.0

45.0

Magnesium

(mg/l)

7.0

5.0

6.0

(g/l)

22.0

22.0

20.0

Total

sollds

DISCUSSION

OF PRETREATMENT

Neutralisation and lowered

of

the

The

significant

rejection

chemical

(ref.

7).

Nanofiltration

and

magnesium

significantly

originally

The combined effluent

during

3.0

1.0

-

-

0.5

-

to a

the

-

bicarbonate

was

approximately

magnesium

lowered

solution

and

the

the

in the

in solution

ionic

permeate

feed

surface

pretreatment.

the

neutralised there

was

10%

feed to the unit. Fluxes

dependent

were

on

approximately

salt and 40%

no

in pH. 10%

of the calcium

Flux performance

increased

eight

fold,

feed pH of 9.7 to 8.0.

to occur during the trials. the chemical

of the calcium of

was

which contained

lowered

65%

has been described

species

sodium

pH.

appeared

sequence removed

Approximately

27% of the total

from

by 61% while

(1 MPa, 28°C) with a decreasing

pretreatment by 86%

magnesium.

concentrates

on

removed

system

of

90% of

present

dependent

No fouling of the membrane

scour

0.3 4.0

bicarbonate.

rejection

demand,

from 4 l/m2h to 30 l/m2h

effluent

of

demand

a colourless

oxygen

was

oxygen

37%

of the carbonate The

produced

the chemical

and

of sodium

The nanofiltration

1.2 15.0

PERFORMANCE

hydroxide

microfiltration

calcium

effluent.

detail

the

8.9 8.0

70.0

pH from 13.5 to 8.6.

cross-flow

53%

SEQUENCE

converted

the effluent

On average solids,

the

14.0

Carbonate Sodium

of

NaOH

the

sodium

oxygen demand

and organics salt

was

and

lost

of the 50% of in

the

420 DISCUSSION

OF ELECTROCHEMICAL

Electrolysis solution

of

with

concentrated bicarbonate

a

mimimum

sodium

UNIT PERFORMANCE

nanofiltrate total

hydroxide

concentration

Approximately

the

of the depleted

electrolysis.

Fig. 4 shows

sodium

evolution carbon

of

decrease

0

Electrolysis

nanofiltrate

brine

the

from

carbon

buffered

dioxide (below

ion concentration

2 g/l

solution

relationship

which

in bicarbonate

of

depleted 500

mg/l

lowered 20

in the feed solution

concentration,

concentrations

the hydrogen

colourless

g/l

the to

0.5

indicates the

HC03-)

increased

was dependent between

the

pH of the

extent

g/l.

buffer

sharply

on the degree

the brine of

the anolyte: capacity

pH and the

depletion. at low was

reduced and

concentration.

I

I

I

I

4

6

8

10

1!

Anolyte sodium concentration (g/l)

between the degree electrolysis

of depletion

of pretreated

The

inorganic

at a rate of 1 pH unit per 0.3

I

its pH during

a

sodium

to the electrochemical

2

Fig. 4. Relationship

brine and

hydroxide.

The composition

brine

a

concentration

solution.

in

as sodium

produced

solids

95% of sodium present

cell was recovered

of

the

of the anolyte

scour effluent

and

421

Current efficiency The

current

efficiency

scour effluent current

densities

were allowed

did

maintained

not

and

area,

ions at

thus

hydroxide

back

caused

the membrane

This by

in

turn

led

to

the anode

effective

membrane

ions and hydroxide

increased

and

described

:-

to contact

reducing

ions

the

were

to produce hydrogen

hydrogen

temperatures and catholyte

under

efficiency

flow,

Operational

and

The anolyte

efficiency

of current

from pretreated

62%.

to 1 000 A/m2

40 and 50°C. current

polarisation

membrane

hydroxide

cell averaged

300

electrolyte

surface.

the

current

(ii)

water

membrane

through

the

of the cell

inhibiting

increasing

the

current

between

The main causes

poor operation

(i)

of sodium

membrane

at between

effect

conditions.

periodically,

under

were

to equilibrate

concentrations operation

for the recovery

in the electrochemical

hence

transport

decreased

of

sodium

efficiency.

diffusion

high concentration

of

hydroxide

ions

from

the catholyte

to the anolyte

gradients.

Power consumption The

power

consumption

and voltage.

electrolyte,

(i) effect

of

anolyte

configuration. anolyte flow

and flow

Gas

flow rates.

rates

blinding

is

the product

The operational

had

to

voltages

in

particular,

rate

blinding

through

in the

In either be

of

the applied

were dependent anolyte the

second

stack

above

20

Fig. in

a

was a serious

series or parallel

maintained

flow.

cell cell

conditions

l/min

of current

on :5 shows series

ensure

flow

problem at low

flow configurations, to

the

anolyte

prevention

of gas

and polarisation.

(ii)

electrolyte

electrolyte decreased

temperature.

temperatures by

Fig.

6

illustrates

had on the volt drop across

approximately

30%

as

the

electrolyte

the

effect

the cell stack. temperature

was

that

Voltages increased

from 25 to 60°C. The power consumption scour

effluent,

electrolyte 100% NaOH.

in

the

temperatures

for the production absence of

40

of to

gas

of sodium blinding

60°C. averaged

hydroxide or

from pretreated

polarisation

approximately

4 000

and

for

kWh/ton

8

4

0

1

I

I

I

5

10

15

20

Anolyte flow (Vmin) Fig. 5. Influence of anolyte flow rate on the volt drop across l

first cell

A second cell . cellstack Note: 1) series flow configuration 2) catholyte flow rate

: 15 l/min : 35°C 4) operational current density : 1 000 A/m2 3) electrolyte temperatures

423

8

6 10

I -_

I

I

I

I

2’0

30

40

-

50

60

Electrolyte temperature (OC) Fig. 6. Influence during

Note -*

of electrolyte

anolyte

* 1)

The

current limiting

at which water occurs

density:

electrolysis

at

the

by

specified

electrolyte,

7 shows

out

to

surface.

the

operational

is the maximum current

before

ion starvation

hydroxide

ions

and

It is economically possible

current

hydrogen

advantageous density,

density

and subsequent ions

which

to operate

thus reducing

area.

current

electrolyte (iii)

for a solution,

be carried

unit at the maximum

electrolyte,

Fig.

600 A/m2

decomposition

membrane

The limiting

3 to 7 g/l Nat

density

may

membrane

the electrochemical the required

scour effluent

density current

splitting

on the cell stack volt drop

of pretreated

concentration:

2) current

Limiting

temperature

the electrolysis

density

was dependent

in particular

on

:-

the anolyte,

flow

the anolyte,

concentration.

temperature, in particular relationship conditions

between

the

and

sodium

the

limiting

current

concentration

density of

under

pretreated

424 SCOW

effluent

current

which

density

had

been

decreased

by

spiked 400

with

A/&

sodium

for

bicarbonate.

each

10

g/l

The limiting

decrease

in anolyte

sodium concentration.

Anolyte

Fig. 7. Relationship

between

concentration spiked

Background Since of

the

cell

solution this wash The

in

be

advantage

closed-loop

concentration

stack,

consideration wash

be operated place

recycled

system using

of mains would

brine,

recycle wash

was

(Fig. a

and anolyte scour effluent

the

the

washing

given 8).

the operational a

background

In such depleted

residual

a system sodium

incremental

be recovered

of

required

parameters

concentration the

scour wash

bicarbonate

sodium

picked

in the electrochemical

(background)

be to increase

and hence decrease

to

Only the

section

system

limited

partially

subsequently

containing to

density

of pretreated

greatly

water.

of such a system would

the cell operation

current

(g/l)

bicarbonate

anolyte

stream

depleted

would

with sodium

recycle

would

limiting

for electrolysis

concentration the

closed-loop range

sodium concentration

the

scour

sodium

the limiting

unit.

bicarbonate,

process. current

electro-membrane

brine up by

area.

The

main

density

of

425

MAKE-UP WASH WATER x g/l Na+ as NaHCO,

RECOVERED NaOH lOto2O%

DEPLETED BRINE xg/l Na+ as NaHCO,

SCOUR EFFLUENT pH 14; x+1 0 g/l Na+ as NaOH and NaFO,

t ORGANIC AND POLYVALENT ION CONCENTRATES, H,AND 0,

Fig. 8. Schematic closed

of a background

loop recycle

Fig. g shows the relationship the

background

from spiked the

m2 to 12 3 g/l sodium.

current

The

between

concentration

pretreated

limiting

assumed.

sodium

concentration

wash system

scour effluent density

required

by operating

the required

for the

of

under the conditions

above,

electro-membrane a background

electro-membrane

production

Current area

1 ton/day

used for determining

efficiencies may

closed-loop

of

be decreased recycle

area and 100% NaOH

62%

have

by 82%

concentration

been

from 66 of 30

426

60

I

I

I

I

10

20

30

40

Background recycle loop sodium concentration Fig. 9. Relationship

between

electro-membrane

sodium concentration pretreated

in the recycle

scour effluent

62% current production

area and the background loop for electrolysis

of

at:

efficiency rate of 1 ton NaOH/day

electrolyte

flow rates of 15 l/min

40 to 5o"c

Electrode

life

The lifetime treatment preceded causes

of the anode

sequence. by a gradual

a voltage

interfacial

The

is a major lifetime

dissolution

increase

layer between

due

factor in the economic

of

of the

the

anode

precious

to the build-up

the titanium

is

viability

limited

metal

oxide

by

of the

passivation

coating.

This

oxides

in the

of non-conducting

base metal and the coating.

427 During anode

the

was

electron

pilot-plant

apparent.

trials

no

addition

In

backscattering

techniques

occurred

over

the

Limited

coating

most

Electra-membrane

magnesium,

showed long

where

increase

the

coating

that

negligible

anode

life

the membrane

wear

was

the by had

predicted.

had been in contact

This unusual

abrasion.

across

thickness

effect could

be

of the cell.

of

insoluble

of

formation due

of

is limited

hydroxides

within

Nafion

blisters

specify

ensure

to

by three

between

to the differential

factors

and carbonates

the polymeric

membranes

and 0.1 mg/l Mg in the anolyte

the membrane

a

mechanical

iron and aluminium

manufacturers

(ii)

and

of the electro-membrane

accumulation

(i)

8)

in areas

operation

voltage of

life

The lifetime

The

causing

by improved

(ref.

surface

loss occurred

with the electrodes prevented

of

noticeable monitoring

structure

a maximum

prolonged

of the membrane.

limit of 0.5 mg/l

membrane

the two polymeric

in water

:-

such as calcium,

transport

Ca

life. layers

rates

constituting

between

the two

polymers. (iii)

excessive

Deterioration the

electrical

apparent levels

were

permeate

the

up

the

which of

No

to

trials

35

times

by the presence

formation

temperatures

electro-membrane

resistance.

during

controlled

operational

of

voltage

despite

the

fact

higher

than

the

the hardness

was

reduced

manifest

increase

of low molecular

sequestered blisters

(above gO'C).

would

that

as an

across the

calcium

specified

operation

at

membrane and

limits.

mass organics

in was

magnesium

Fouling

was

in the nanofiltration

ions and inhibited

by

in increase

the

precipitation.

relatively

low

The

current

densities.

OPERATION

OF THE CHLORINE

In summary, at 80% current kWh/ton

efficiency

of the process existed oxidation

organics (ii) effluent (iii) NaCl)

at an average serious

inadequate could due

the organics

predominant

control

the

to

anodic

production

electrical

limitations

sequence

produced

hydroxide

power consumption

of 3 900

concerning

relatively

in the scour effluent

if discharged

over

lead to degradation to

SEQUENCE

the practicalities

:-

which may be hazardous

compared

chlorine

of

OF THE TREATMENT

system of the treatment

However,

100% NaOH.

(i)

SYSTEM

the chlorine

the

low

anolyte

reaction

the

efficiencies

of oxidants

of the nanofiltration

chlor-alkali

was

to the environment.

reduction

conventional

oxidation

to below

produced chlorinated

in the chlorinated

membrane.

concentrations technology of water.

present

(40

g/l

NaCl),

the

This decreased

the

(350

30% and necessitated

g/l

the purchase

of

428 make-up

acidic

anode coating

gas for the neutralisation deterioration

had a high chloride

stage.

and the depleted

content,

making

Water oxidation

caused

rapid

brine produced was very acidic

it unsuitable

and

for reuse as wash water.

CONCLUSION The

treatment

neutralisation, recovery

of

cross-flow

produces two

(i)

(ii)

hydrogen

a depleted

concentrates,

and oxygen brine

is of suitable a

effluent

microfiltration,

soluble organics

(iii)

(iv)

scour

by

the

proposed

nanofiltration

sequence

of

and electrochemical

:-

neutral

other containing

system,

cotton

high

containing

which,

for reuse

sodium

suspended

solids

and

the

inorganics.

gases which are vented

solution

quality

quality

one

and divalent

to the atmosphere.

in the case

of the carbon

dioxide

in the scour process.

hydroxide

stream

for

reuse

in

the

scour

process. Pilot-plant treatment

an electical a

investigations

sequence

process

power

would

significantly effluents

indicated

is economically consumption

allow

reduce

the carbon Sodium

of 4 000 kWh/ton

for chemical, the

that

viable.

water

and

i,mpact associated

dioxide

hydroxide

system

was

100% NaOH.Installation heat energy with

the

of the

produced

recovery discharge

at

of such

and would of

scour

to the environment.

ACKNOULEDGEHENT This

investigation

Commission Scouring

entitled

was i"Water

and Bleaching

The contribution and Nephew

carried

out

Management

under and

a

grant

Effluent

from in

the

the Water Textile

Research Industry:

Effluents".

by the management

and staff of Da Gama Textiles Ltd.,

(Pty) Ltd. and David Whitehead

Smith

and Sons (Pty) Ltd. is appreciated.

REFERENCES C.A. Buckley and A.E. Simpson, Patent 86/4706, Effluent Treatment, June 1986; assigned to the Water Research Commission of the Republic of South Africa. Microfiltration application in the treatment of G.R. Groves, et 2, industrial effEnts, Symposium on Forest Products Research International Achievements and the Future, CSIR, Pretoria, Republic of South Africa, 1985. H.S. Harned and R. Davies, The ionisation constant of carbonic acid in water and the solubility of carbon dioxide in water and aqueous salt solutions from D-5O"C, Journal American Chemical Society, 65 (1943) 2030. H.S. Harned and S.R. Scholes, The ionisation constant of HCD3 from 0-5O"C, Journal American Chemical Society, 63 (1941) 170.

429 5

6 7

8

K.F. Wissburn, D.M. French and A. Patterson, The true ionisation constant of carbonic acid in aqueous solution from O-45"C, J. Phys. Chem. 58 (1954) 693. A charged ultrafiltration M.C. Mickley, membrane process for water softening, IDA Journal l(1) (March 1985) 1-13. Kerr and C.A. Buckley, The effect of pH on the A.E. Simpson, C.A. nanofiltration of the carbonate system in solution, Desalination (in press). J.P. Millington, Electricity Council Research Centre, Capenhurst, U.K., Personal Communication (January 1987).