The nanofiltration and reuse of effluent from the caustic extraction stage of wood pulping

DesaZimtion, 67 (1987) 455-465 Elsevier Science Publishers B.V., Amsterdam-Printed

455 in The Netherlands

THE NANOFILTRATION AND REUSE OF EFFLUENT

FROM THE CAUSTIC EXTRACTION STAGE OF

WOOD PULPING

A. BINDOFFl,

C.J. DAVIESE,

C.A. KERRI and C.A. BUCKLEY1

IPollution Research Group, Department of Chemical Engineering, Natal, King George V Avenue, Durban, Republic of South Africa 2Research

and Development,

SAPPI Limited,

P.O. Enstra,

University

of

1561, Springs.

SUICIARY One of the bleach stages in the treatment of wood for the manufacture of pulp is the extraction of lignin with sodium hydroxide. The resulting wash water is highly coloured at a pH of 7 to 10 and a conductivity of 4 to 7 mS/cm. The high colour precludes its reuse in the bleachino section of the ouloino for the removal 07 colour bodies from'thi process. _ The use of nanofiltratibn effluent while allowing most of the inorganic salts to pass throuqh the In particular, membrane is described. aspects such as -pretreatment and membrane cleaning will be addressed.

INTRODUCTION The

pulp

effluents

and

which

recycling

paper give

industry high

and wastewater

is

a

inorganic

reclamation

large and

and

user

of

organic

reuse

water

and

pollution

are

thus of

discharges

loads.

primary

Water

importance

to the industry.

In the pulp and paper making After

effluents. and/or

mechanical

binding which

pulping.

materials use

(i)

(ii)

of

chemicals process

consisting

pollution

the

pulping

wood

chips

together.

chemical

lignin and other

Various

sulphur,

is the bleach

undergo

is to attack

containing

are

source

processes

oxygen

and

exist

alkali.

:-

of spent cooking

chemicals,

lignin and other

from the wood and

pulp.

The

pulp

fraction This

brightness.

for the degradation normally often

The aim

of

of the pulping

black liquor,

solids extracted

a major

preparation,

that hold the wood fibres

combinations

The products

process,

preliminary

carried

with

constitutes

a

sent

of lignin,

out

for

stage

pollution

bleaching

by further is followed

in several

washing

a major

DDll-9164/87/$03.50

is

is effected

stages in

source

which

lignin

by alkali

in which

between. in many

whiteness

and

Chlorination,

used

extraction.

different It

is

pulp mills.

0 1987 Elsevier Science Publishers

improves

removal.

B.V.

this

Bleaching

chemicals wash

is

are used,

water

which

466 A generalised flow diagram of the pulp and paper making process is given in Fig. 1. The bleach effluent is often highly coloured due to lignin and chlorinated lignin

derivatives

colour

removal,

which

the

are washed out of the cooked

potential

for reuse of

pulp.

this water within

Thus without the mill

is

limited. Methods for colour removal from pulp effluents include :coagulation with metal salts (ref. 1;2).

(i) (ii)

adsorption on polymeric adsorbents (ref. 3).

(iii)

adsorption on activated charcoal (ref. 4;5).

(iv)

electro-chemical methods (ref. 6;7).

This paper examines the possibility of using a nanofiltration membrane for colour removal from bleach effluent. The choice of nanofiltration over other membrane systems was based on the following considerations :the fixed charge on the surface of a nanofiltration membrane (for the

(i)

particular membrane

used the charge is -ve) would enhance the likelyhood of

rejection of the chromophotic organics many of which are negatively charged. This

would

furthermore

membrane.

promote

the

anti-fouling

nanofiltration

Thus

was

characteristics

preferred

over

of

the

conventional

ultrafiltration. (ii) was

Na,

total removal of inorganics, particularly monovalent cations such as not

required,

thus

the

use

of

a

reverse

osmosis

membrane

was

unnecessary. A nanofiltration membrane meets the required criteria and can be considered as

possessing

properties

intermediate

between

those of ultrafiltration

and

reverse osmosis membranes (ref. 8). In addition energy-based

nanofiltration,

being

a low pressure system would offer low

operating costs, and as the membranes are tolerant of a wide pH

range no pH control of the effluent would be required. EXPERIMENTAL HETHOD Preliminary

investigations

using a small scale, flat sheet membrane

test

rig. indicated that nanofiltration was a feasible method of colour removal from the caustic effluent. were

then conducted

allowed

larger

No evidence of irreversible fouling was detected. Tests using a spiral wrap membrane

volumes

of effluent

FilmTec FT40 1812.

to be passed through the membrane

This in a

realistic time period. On

standing,

the

effluent

was

found

to

form

a

colloidal

precipitate,

therefore prior to nanofiltration it was filtered through a 10 micron cartridge

RAW MATERIALS

WASTES

PROCESS

Debarked

Alkaline Sulphate Liquor (Kraft)

Log

Wood

Chips

PULPING

Black liquor

ti Crude Pulp WASHING

THICKENING g

Bleaching and other necessary chemicals

BLEACH EFFLUENT

PREPARATION

PAPER PRODUCTS

Fig.

1. Generalised

flow

sheet

of pulp

and paper making process

458 filter to remove

any fibre particles

Six, 50 1 batches water

recovery)

water

recovery.

to that noted

for

from

two

nanofiltration the

have collected

4th

Samples

and

a colloidal

1

of

Batch

recycle

were

and

were

mode

are

permeate

given

Permeation

This could

that

obtained are given

be due to either

pressure

permeate

fluxes

pressure

effects.

The permeate

through

dilute

a

nitric

the

start

permeation

fact that

the

membrane would

nitric

acid

of

the

cross-flow

acid

solution

precipitate

which

and

finish

of each

rates were monitored

effluent

when

flux versus

permeation

0% water

each

rates

recovery,

run

total

on

total

solids

in

at the start and end

the

effluent

has

little

effluent

seem

the conclusion

corrected

and

not

for

temperature

solution

(ref. 9)

of Batch 4 (75% water

due

to be justified

batch in

to

recovery)

the flux profile

not

that the flux decline

osmotic

pressure

the

for

was 19.5

indicate

by the

(after

but

and was found to be 19.2 l/m2h.

fact

that

pure

effects.

flushing

water

(Fig. 3).

was This

the membrane

permeability

of Batch 4, and again after the conclusion

and even improved

in the

or

were similar would

wash

run

:-

flux was then determined

fluxes

was

effects.

were

fouling

batch

In each case there was a decline

to end of each run.

flux at the completion

A pure water

returned

7 and

and further concentrated

any colloidal

at

batch

of the

when

fouling of the membrane

following

taken

in Table 2.

osmotic

assumption

Batch

with the membrane.

(i)

to

form

recovery.

with

membrane

each

at

(ii)

l/m2h.

water

1, and a plot of

similarity

rates mode

to

the flux rate from the beginning

The

filtered

precipitate,

remove

were

in Table

indicates

to interact

concentration

osmotic

to

Batch 8.

Rates

pertaining

The overall batch

tendency

to

Nanofiltration

Permeation rates

2.

combined

flushed

runs,

of similar appearance

DATA

flux

each

was

(0%

to 75%

the experiment.

EXPERIMENTAL

The

precipitate,

concentrated

on the membrane.

feed

Wanofiltration

with

4

the colloidal

membrane

concentration.

throughout

The

to

batch

was seen in the concentrates.

concentrates

6th

to being

5, 6 and 7, constituted

removing

after

due

prior

effluent,

Batches

composite thus

might

Fig.

hours

solids.

1 to 6) were run on total recycle

(Batch 7) and 95% (Batch 8) overall

The

of

24

and the suspended

(Batches

stage

from Batches

microfilter,

batch

to

At this

Concentrates

to 87%

up

in the untreated

concentrates These

of effluent

test

of Batch 6)

459

N . .?

LL

4

461 TABLE

1

Total Recycle Results

1

2

3

4

5

6

7

8

(new)

(new)

(new)

(new)

(new)

(new)

(1.2.3.4)

(5,6,7)

4.5

5.3

5.3

4.6

4.1

4.0

6.7

11.6

0

25.0

31.9

30.0

20.7

29.6

28.9

32.0

34.0

2

34.4

Batch

Feed TS Time-hr

4

35.8

6

36.1

31.5

a 10 12

29.0

14

25.1

16 18

32.5

20

33.0

22

32.8

34.0 28.3

Total solids

Feed pressure

TABLE

34.5

28.3

31.9

24

TS

34.0 28.0

35.1

(g/l) ; Flux

l/m2h (ccrrected

1.2 M'a ; Membrane

Area 0.3716

to 25%

(ref. 20)

m2

2

Batch Concentration Results

!3atch 1

2

TS

Flux

Stwt

4.5

End

9.5

TS Flux

Total

7

TS

Flux

TS

Flux

TS

Flux

32.8

5.3

31.9

5.3

28.3

4.6

25.1

4.1

31.5

4.0

28

6.7

27.1

12.4

25.5

9.9

21.1

8.2

19.5

0.3

27.4

10.9

16

16.6

solids

75.0

(g/l) ; WR

ANALYTICAL

RESULTS

Analytical

results

Analytical 3.

6

Flux

I/n?h korrected

Table

5

4

TS

75.0

%!m

3

75.0

75.0

75.0

Flux

TS

75

8 Flux

TS

FIUX

35.0

11.6

34.0

29.3

35.8

18.0

87.0

95.0

Water Recovery

to 25-C) ; Feed Pressure

AND COLOUR

results

TS

and

1.2 t.Pa

REMOVAL

the

calculated

membrane

rejections

are

given

in

TABLE 3

11.0 9.8 10.1 10.1 10.6

0

75

75

75

2

3

4

87

95

7

8

10.4

10.3

10.8

10.5

75

75

5

6

1

Feed

Perm

5.9

9.4

10.0

9.9 27.2

15.1

11.4

10.1 10.1

9.2

10.6

12.6

10.0

11.1

9.6

9.3

9.2

10.1

14.0

7.7

5.8

5.1

5.1

5.3

6.1

5.4

2.6

48

49

50

51

41

50

52

46

56

Rej

Perm

data

recovery

Feed

Cond

rejection

%

PH

and membrane

Water

%

Approx.

results

75

Batch

Analytical

9 603

5 812

3 387

2 646

2 570

2 885

3 423

2 841

1 466

Feed

Na

3 822

2 825

1 447

1 134

1 090

1 184

1 270

1 201

547

Perm

m9fl

60

51

57

57

58

59

63

58

62

Rej

%

94

59

93

65

48

46

75

61

21

Feed

Ca

5

2

3

2

2

1

2

2

0

Perm

mg/l

94

97

97

97

96

98

97

97

99

Rej

%

4 750

3 750

2 900

2 000

1 500

2 013

2 500

2 300

800

Feed

655

410

365

207

145

200

275

290

45

Perm

w/l

TC

%

86

89

87

89

90

90

89

87

94

Rej

463 Results are in agreement with the expected performance of the membrane used, with sodium rejections between 51 to 63X, calcium rejections of 94 to 99% and a total soluble carbon rejection of 86% at 95% water recovery. Colour removal Calculation of ADMI Colour values

: In 1970 the American Dye Manufacturers

Institute established an Ecology Committee to study the effect of dyes on the During the course of the investigation it became clear that a

environment.

need existed for a reliable method for the measurement of colour of water. The

method

measurement

devised,

known as ADMI colour

of transmittance

value

(ref.

10) involves the

values of the sample at wavelengths from 400 to

From these values the ADMI colour value is calculated according to a

700 nm.

given procedure. The ADMI colour value provides a measure of colour which is independent of hue, and is now generally accepted as a tentative method of colour measurement (ref. 11). samples. recovery

Table 4 gives the ADMI colour values of the feed and permeate

ADMI colour values of permeate from individual batches at 75% water range

from 45

to 78.

This compares

favourably with ADMI colour

values of tap water which ranged from 39 to 100, varying with time and how long the tap was opened before sampling. Colour rejection at water recoveries up to 95% are uniformly high at 98 to 99%. DISCUSSION While

colour

removal

by

nanofiltration

is

highly

successful

possible

limitations to its use caused by membrane fouling need to be examined. From Fig. 3, colloidal fouling of the membrane was occurring, as shown by the drop in membrane flux, both during a batch concentration batches. ineffective

Clearly in

filtration

removing

all

effluent and, as concentration

through the

a

10

colloidal

micron

run and between

cartridge

precipitate

from

filter

was

the untreated

of the feed solutions proceeded, the residual

suspended solids (probably together with further precipitation) collected on the membrane surface. Attempts to clean the membrane

by flushing with dilute nitric acid after

Batch 4 were successful as the flux profile of Batch 5, is close to that of the original profile (Batch 1). suggesting

that

filtering

However flux performance decline during Batch 6. through

pretreatment for the effluent.

cartridge

filters

is

not

an

effective

464 TABLE 4 ADHI colour and membrane colour rejection values

Batch

%

Feed ADMI

Perm ADMI

%

Water

colour values

colour values

Rejection

recovery 1

0

2 135

2

2

0

2 600

2

99

75

8 550

45

99

3

99

0

1 975

20

99

75

6 050

54

99

0

2 025

28

98

75

6 275

48

99

4 5

75

5 050

43

99

6

75

7 450

78

99

7

+75

6 370

25

99

87

14 500

155

99

+50

6 200

63

99

95

48‘500

745

98

8

Flux profiles for Batches 7 and 8 show marked improvement over the previous six

batches.

Here

these

before nanofiltration.

composite

batches

were cross-flow

microfiltered

Microfiltration therefore appears to offer a feasible

pretreatment method, to ensure acceptable flux rates.

Significantly even at

the 95% water recovery level achieved in Batch 8, there was no reappearance of the colloidal precipitateinthe

concentrate.

Although flux rates declined prior to microfiltration of Batches 7 and 8, it is noteworthy that colour rejection

by the membrane, remained uniformly high

for all runs; indicating the efficiency of the membrane for colour removal. Further

investigations

into

the

level

of

fouling at which colour

removal

becomes impaired is required. For the alkaline extraction effluent studied, nanofiltratfon, preceeded by cross-flow

microfiltration,

offered

an efficient method

for removing colour

from the effluent. The

technology

could

be

incorporated

easily

into

the

existing

bleach

circuit, and would enable a significant reduction in water consumption, without imparing the quality of the pulp.

465

CONCLUSION Nanofiltration colour

removal

using

Nanofiltration colourless highly

FilmTec

from caustic resulted

inorganic

coloured

in

adverse

effects

due

containing

stream

The large volume permeate

membranes

containing

can

to colour

was

found

to

be

suitable

for

effluent.

fractionating

stream

organic

FT40

bleach

the

effluent

monovalent divalent

be returned

ions ions.

to the mill

as the ADMI colour

into a high volume, and

value

a

small

volume

f bleach circuit is close

without

to that of tap

water. Decline acid

in flux rates

flushing

cross-flow Given removal

but can

due

to membrane

be adequately

fouling

controlled

is only partially

by pretreating

improved

by

the effluent

by

microfiltration. adequate

combined

effluent

pretreatment,

with lony-life

expectancy

a highly

efficient

for the membrane

method

for colour

can be expected.

ACKNOWLEDGEMENTS The

research

Treatment

work

was carried

of Pulp Mill

Effluent

out

in terms of a research

grant entitled

the

from SAPPI Ltd.

REFERENCES

8 9

10

11

M. Olthoff and W. Eckenfelder, A laboratory study of colour removal from pulp and paper waste waters, TAPPI, 57(8) August 1974, 55. J. Oswalt and J. Land, Colour removal from kraft pulp mill S. Wright, effluents by massive lime treatment, TAPPI, 57(3), March 1974, 126. Decolourisation of kraft mill Rock, A. Bruner and C. Kennedy, S.L. effluents with polymeric adsorbents, TAPPI, 57(9), September 1974, 87. D.G. MacDonald and T.G. Nguyen, Activated carbon from bark for effluent treatment, Pulp and Paper Magazine of Canada, 75(5), (1974). US Environmental Protection Process Design Manual for Carbon Absorption, Agency, Technology Transfer, October 1973. Electrochemical Decolorization of Kraft Mill Effluents, Journal Water Pollution Control Federation, February 1978. Ion Flotation for Color Removal from Kraft Mill Effluents, Distributed by CPAR Secretariat, Canadian Forestry Service, Ottawa, Ontario, as CPAR Report No. 147-1, 1973. Mickley, for water A charged ultrafiltration membrane process M.C. softening, IDA Journal, l(l), March 1985, 1. C.A. Buckley, K. Treffry-Goatley, M.P.J. Simpson, A.L. Bindoff and G.R. Groves, Pretreatment of fouling and cleaning in the membrane processing of and industrial effluents, Symposium on Reverse Osmosis ACS-I&EC Ultrafiltration, August 1984. Allen, W.B. Prescott, R.E. Derby, C.E. Garland, J.M. Peret, Max W. Saltzman, Determination of Colour of Water and Wastewater by Means of ADMI Proceedings of 28th Industrial Waste Conference, Purdue Colour Values, University, Lafayette, 1972. Standard Methods for the Examination of Water + Wastewater, 16th Edition, 1985.