DESALINATION
ELSEVIER
Desalination 173 (2005) 83-90
www.elsevier.com/locate/desal
Processing of black liquors by UF/NF ceramic membranes
Anton Dafinov*, Josep Font, Ricard Garcia-Valls
Departament d'Enginyeria Quimica, Universitat Rovira i VirgilL
Av. Parsos Catalans 26, 43007 Tarragona, Catalunya, Spain
Tel. +34 (977) 558112; Fax +34 (977) 559667; email: adafinov@etse.urv.es
Received 21 April 2004; accepted 15 July 2004
Abstract
This study deals with the treatment of black liquor from wood pulping by means of membranes. UF/NF
membranes with a skin layer ofTiO 2 and ZrO2 have been applied to recover water and to concentrate the residual
effluent. Three different membranes with molecular weight cut-off of 1, 5 and 15 kDa have been checked. The
membranes have been tested either in single stage operation or in cascade. Total dissolved solids, organic matter,
organic to mineral matter ratio and ash have been determined. In addition, chemical oxygen demand (COD) retention
was calculated by measuring in the feed solution as well as in the permeate and in the concentrated solution. During
the concentration step the steady state was reached after a few minutes running. There was not significant change
in the permeate flow until the volume was reduced at half. Only the 15,000 Da membrane showed continuous
permeate flow declining. Regardless the membrane used, dry matter, organic matter and COD analyses showed
that the retention of organic substances fell in the range of 60-70%, depending on the conditions selected.
Keywords: Ceramic membranes; Ultrafiltration; Nanofiltration; Black liquors; Lignin; Pulping
I. Introduction
In wood pulping, all organic compounds are
dissolved in the digesting liquor except cellulose,
which remains insoluble. After separation of
cellulose fibres, the solution obtained from the
above process, called black liquor, is a waste,
which must be subsequently treated before disposal. The overall annual world production of
black liquor is approximately 500 million tons,
where 10% come from the European Community
*Corresponding author.
[1]. Total dissolved solids (TDS) in the black
liquor usually represents around 15% w/w, which
is composed of lignin derivatives, low molecular
weight organics, and the rest being remaining
chemicals from the digesting liquor. The chemical
oxygen demand (COD) and pH varies from 10,000
to 120,000 m g O / L and from 10 to 13 respectively,
depending on the process applied, usually kraft
or antraquinone [2].
The conventional treatment of the black liquor
is water evaporation until the dry matter reaches
a concentration of 70-80% w/w. Then, the residue
0011-9164/05/$- See front matter © 2005 Elsevier B.V. All rights reserved
doi: 10.1016/j.desal.2004.07.044
84
A. Dafinov et al. / Desalination 173 (2005) 83-90
is incinerated and energy is produced, which can
be used in the pulping process. From the resulting
ash, up to 80% of the digesting chemicals could
be also recovered. A small part of the spent liquor
is further used as a source oflignin that is isolated
either by precipitation or diafiltration. The lignin
is a feasible raw material for many valuable
substances such as activated carbon, vanillin,
vanillic acid, dispersing agents, ion-exchange
agents, polymer fillers, binding agents for the
production of fibre boards, artificial fertilisers and
complexing agents [3-5].
Lignin is avery complex material. Its definition
is ambiguous and often depends on the perspective
of the author. From the chemical point of view,
Meister et al. [6] give an accepted definition stating
that "Lignin is the major non-cellulosic constituent
of wood. It is a complex, amorphous, highly crosslinked polyphenolic". Overall, lignin comprises
17-33% of wood [7] and plays a key role in
cementing the polysaccharide components in the
cell walls, increasing both the mechanical strength
of wood as a composite material and the resistance
toward micro-organisms. All lignins are made of
phenylpropane units, which seem to be randomly
joined by ether and carbon-carbon bonds [8].
Any alternative way for black liquor treatment
must allow recovering of the reagents (which composition depends on the pulping method, e.g.
anthraquinone, sodium hydroxide or sodium sulphide) at low energy consumption, thus substituting
part of the evaporation process and minimising
the polluting load. A promising way for treating
black liquors is the use of membrane processes.
Membranes permit to concentrate the organic
matter in the retentate as inorganic salts dissolved
in the water freely cross the membrane layer. The
studies of P.K. Bhattacharya [9-13] deal with
black liquors from Kraft process plants of small
capacity, total solid content in the liquors was in
the order of 5% and the employment of evaporation units was not justified by the energetic reasons.
The chemical and temperature resistance of the
ceramic membranes compared to polymeric
membranes makes them particularly attractive for
the treatment of black liquor effluents. Cortifias
et al. [14] studied the influence of the transmembrahe pressure and the cross flow velocity on the
elimination of colloidal suspended matter (pitch)
in Kraft black liquors using ceramic microfiltration membranes. They concluded that to get
satisfactory flux high feed velocities are needed.
The influence of the feed temperature and the
transmembrane pressure on the performance of
15 kDa ceramic membrane applied in the treatment of Kraft black liquors is studied by Wallberg
et al. [15]. At the lowest temperature (60°C) the
limiting permeate flux is reached at relatively low
pressure. At temperatures close to that of the black
liquors in the paper mills (75 and 90°C) no limiting
flux is reached in the pressure range studied.
Lui et al. [ 16] treated Kraft spent liquors with
different type of membranes, both polymeric and
inorganic, with cut-off in UF and MF range and
dead-end and crossflow operation modes, proving
the feasibility of membrane treatment.
In this work, multichannel tubular ceramic
membranes were used to treat black liquors. This
kind of membranes offers both physical and
chemical resistance in a wide range of conditions.
The membranes were tested in continuous concentration experiments at different transmembrane
pressures using a soda-anthraquinone black liquor.
The membranes of 5 and 15 kDa were also
checked in discontinuous operation regime.
2. Materials and methods
2.1. Materials
Three channel laboratory scale tubular membranes (CrRAM INSIDE 25 ®) for ultra- or nanofiltration manufactured by TAMI Industries were
used in this study. These membranes are composite elements with a support of t~-alumina
coated with an inner skin layer of either TiO 2 or
ZrO 2. The tubular element was 25 cm long, having
an outside diameter of 10 mm and 3.6 mm
85
A. Dafinov et al. / Desalination 173 (2005) 83-90
hydraulic diameter of each channel. Membranes
with cut-offvalues of 1, 5 or 15 kDa were tested.
The nominal filtration area of each element, provided by the producer was 9.4.10 -3 m 2.
The paper mill CELESA (Tortosa, Spain)
supplied the black liquors. The pulping process
applied by this plant is soda-anthraquinone. Total
dissolved solids (TDS) in the liquor was 8.5% w/w
comprised of organic matter (3.75%) and mineral
salts (4.75%), mostly being reactives used in the
digestion process. The original COD of the black
liquor was 70 g OJL with a pH of 12.
2.2. Experimental set-up
The experimental set-up for cross-flow filtration is schematically presented in Fig. 1. The
apparatus consists of a positive displacement
pump (Hydra-Cell TM) coupled with a pulse
dampener (Hidracar, S.A., 0.65 L). All pipes,
valves as well as the housing of the membrane
module were made of stainless steel. Two valves
are used for activating the dampener, V1, and for
controlling the outlet pressure in the tubular
membrane, V2. Another valve, V3, allows to
v27
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concentrate
~
control the feed flow rate by opening a by-pass.
Valves V4 and V5 give the possibility of applying
a back flow in order to facilitate membrane
cleaning. A 10 L feed tank is coupled with a
thermostat that permits to set the temperature in
the range of 20-90°C with 2°C accuracy.
The feed flow rate was always set at 3.8 L/rain,
which corresponds to a flow velocity inside the
membrane channels of 2.1 m/s corresponding to
a Reynolds number of 8300 calculated for distilled
water at 20°C. Before starting a test, the filtration
system was always rinsed for 30 min with a 0.5 M
NaOH solution at 25°C to wash out any compound
that could remain in the system. Subsequently,
distilled water was passed through the system to
leave it ready for the next run.
The retention characteristics as well as the
permeate flow rates of each membrane were
examined at 3.0, 5.0 and 7.0 bar oftransmembrane
pressure. The experiments were conducted for at
least one hour from starting or until steady state
was established. Previously, the water permeability was determined for each membrane using
distilled water at different transmembrane pressures. The water permeability was also used as a
module
imu|niil.mm,
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amen|aim eiioiiii
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Fig. I. Standard membrane testing equipment for cross flow filtration.
pulse
dampener
V1
UV~
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piston
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86
A. Dafinov et al. / Desalination 173 (2005) 83-90
measure of the cleaning effectiveness. The
cleaning was repeated until the initial water
permeate flow rate was recaptured.
All tests were performed at 30+2°C. It should
be mentioned that at this temperature the effluent
viscosity is quite similar to that of distilled water
at 20°C. First the effect of the transmembrane
pressure on the permeate flux and retention were
studied for different membranes in continuous
experiments, i.e. total recycle of both retentate and
permeate. The membranes with cut-off of 5,000
and 15,000 Da were also tested in discontinuous
operation, i.e., the permeate was collected and
isolated. The process continued until the initial
volume of the liquor was almost halved. In these
cases, the transmembrane pressure was maintained
at 5 bar. The permeate collected in the above
experiments were further filtrated with the membrane of 1 kDa cut-off.
2.3. Analyses
The membrane performance is discussed by
employing several parameters. COD was determined by the standard dichromate method [ 17].
Dry matter was evaluated by heating the sample
to dryness (determined as the steady weight) during
24 h at 110°C. Similarly, organic matter and ash
content were obtained after heating at 500°C and
600°C, respectively for 24 h. The details of the above
analyses have been described elsewhere [18].
3. Results and discussion
3.1. Continuous experiments
Fig. 2 shows the steady state (determined as
that when no further flux decline is observed)
permeate fluxes for all the membranes tested in
function of the transmembrane pressure. For the
membranes of 1 and 5 kDa cut-off, the hydraulic
permeability behaviour was as expected, so the
permeate flow increases with the pressure. In the
case of the 15 kDa membrane, the permeate flux
trough the membrane seems to be independent of
80
1 kDa
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........ ~ J - - - -
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6
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Transmembrane pressure, bar
Fig. 2. Steadystate permeateflux at differenttransmembrane pressuresin continuousexperiments.
the applied transmembrane pressure, showing a
constant value around 52 L m -2h -1. In addition,
the flux for the 15 kDa membrane is surprisingly
lower than that obtained for the 5 kDa membrane
even at transmembrane pressures higher than
5 bar. This result could be due to a pore blocking
mechanism, in addition to the usual gel layer
formation on the membrane surface [ l 9]. The pore
blocking could be attributed to the similar sizes
of the lignin macromoleculesand the pore diameter
of the 15 kDa membrane. On the other hand, TiO 2
mainly forms the skin layer of the membranes of
1 and 5 kDa whereas ZrO2forms that of the 15 kDa
membrane, which could favour some specific
adsorption of lignin molecules inside the pores.
Further investigation is needed in order to highlight the actual interactions between the ceramic
surface and the solutes, which could allow explanation of these different membrane behaviours.
Similar results were obtained by Wallberg et al.
[15] when a 15 kDa ceramic membrane is applied
in the Kraft black liquor treatment in continuous
process. When the experiment is performed at
60°C limiting permeate flux is reached at 175 kPa
of transmembrane pressure. At higher temperatures no limiting flux is observed in the pressure
range studied.
The content of COD, organic matter and dry
matter in the permeate samples is depicted in
87
A. Dafinov et al. / Desalination 173 (2005) 83-90
Figs. 3-5. As it can be seen, the general trends
are similar for each parameter discussed. Nevertheless, it must be noted that, while COD retention
is in the narrow range 40-60 % in the most conditions, there are major differences for the other
two parameters, mainly when the 1 kDa membrane is compared to the other two membranes
tested (Figs. 4 and 5). Organic matter retention
strongly depends on the pore size with a reversing
between the cases of 5 and 15 kDa again attributed
to the pore blocking appearing in the highest cutoff membrane as can be seen in Fig. 4.
The COD and organic matter retentions
obtained for the 1 kDa membrane are quite similar,
around 50%, showing a small dependence on the
applied transmembrane pressure. Examination of
the results for the membranes of 5 and 15 kDa
shows that the COD retention is higher than the
organic matter retention in both cases. A possible
explanation of this fact could be that the gel layer
formed on the membrane surface presents some
selectivity with respect to the less saturated
molecules. The significant dependence o f the
retention on the applied transmembrane pressure
indicates that the f o r m e d gel layer plays a
dominant role in the separation process. Similar
results were obtained by Liu et al. [ 19]. However,
no correlation is observed between the membrane
pore diameter and the permeate flux given when
polymeric and inorganic membranes with a wide
range of cut-off(from 3 kDa to 0.8 gm) are applied
in the treatment o f Kraft black liquor. As pressure
increases the gel layer is progressively compacted
resulting in a higher retention of the organic matter
and COD and a lower permeate flow. Regarding
the dry matter retention (TDS), there is a direct
relation between the pore size and the applied
transmembrane pressure as demonstrated by
Fig. 5.
On the contrary, the ash analyses show no
significant differences among the experiments and
almost no effect o f the applied pressure. The
retention in all cases is low, around 10% in the
case of 5 and 15 kDa membranes and about 13%
60
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...........................
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4
I
5
6
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Transmembrane pressure, bar
Fig. 3. COD retention at different transmembrane pressures in continuous experiments at steady state.
60
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o
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• 15 kDa
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Transmembrane pressure, bar
Fig. 4. Retention of organic matter at steady state in continuous experiments.
40
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Fig. 5. Retention of the dry matter at steady state in continuous experiments.
88
A. Dafinov et al. / Desalination 173 (2005) 83-90
for the 1 kDa membrane, as expected according
to the pore size. The retention of the inorganic
salts is mainly due to the association of the ions
with the lignin macromolecules retained by the
membrane and the separation obtained was expected to be very low.
3.2. Discontinuous experiments
The membranes of 5 and 15 kDa were also
tested in discontinuous operation. The experiments were performed under a constant applied
transmembrane pressure of 5 bar. The permeate
flux and the volume concentration factor (VCF)
are displayed in Figs. 6 and 7. The VCF is defined
as the ratio o f the initial feed volume to the
retentate volume. The initial and final retention
are also presented in Table 1.
The retention in terms of TDS, organic matter
and COD for both membranes depends on the
lignin concentration, which continuously increases
in the feed tank during the operation. On the other
hand, the TDS, organic matter and COD levels in
the permeates are maintained almost constant for
both membranes during the experiments. This fact,
once again, proves that the gel layer formed plays
a dominant role in the separation process and no
reliable relation between the membrane cut-off
and the retention could be established.
Considering the permeate flow, different behaviours were observed for the two membranes
in the discontinuous concentration experiments.
When using the 5 kDa membrane, the permeate
flow reached quite rapidly steady state around
35 L m-2h -1, which remains almost constant
during the 10 h period tested as seen in Fig. 6. In
this case, the initial volume of black liquor was
reduced 1.6 times. On the contrary, when the 15 kDa
membrane was used, the permeation flow rate
decreased continuously throughout the experiment
100
1.7
4, permeate flux
&VCF
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200
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,
300
400
Time, min
,
i
500
1.0
600
1.2
,,L,&
1
(10(1 800 1000 120(1 1400
Time,rain
Fig. 7. Discontinuousoperation for the 15 kDa membrane
at 5 bar.
200
Fig. 6. Discontinuous operation for the 5 kDa membrane
at 5 bar.
400
Table 1
Retention for the 5 and 15 kDa membranes in discontinuous operation
Retention, %
5 kDa membrane
Dry matter (TDS)
Organic matter
COD
initial
36.3
39.0
52.5
15 kDa membrane
final
38.3
39.4
63.4
initial
37.1
40.8
54.6
final
46.2
51.7
63.5
A. Dafinov et al. / Desalination 173 (2005) 83-90
as shown in Fig. 7, reaching a final value of
17 L m -2 h -I, which is significantly lower than that
obtained with the 5 kDa membrane. For the 15 kDa
membrane, a twofold reduction of the initial volume
of black liquor was possible with a final permeate
flux of 17 L m -2 h -I as mentioned. The permeate
flux for that membrane and the same transmembrane pressure in continuous operation was
52 L m-2h-l. In comparison, when working in
continuous operation, the 1 kDa membrane
operating at 5 bar reached a steady state permeate
flux value of 27 L m -2 h -l, which is very close to
the 35 L m -2 h -~ obtained in the concentration test.
The low permeate flux found for the 15 kDa
membrane in discontinuous operation is attributed
to the progressive pore blockage by some of the
compounds present in the treated solution. For
smaller pore sizes, the macromolecules cannot
penetrate into the pores, and they form only a
stable cake layer. It should be mention that for
both experiments very fast flux decline is observed
in the very beginning of the tests. Similar results
were obtained for the continuous experiments, in
the most cases the steady state were reached in
the first minutes. Probably the increasing of the
cross flow velocity will decrease the thickness of
the fouling layer. Further study should be
considered. Due to the low level of the volume
reduction of black liquor it is difficult to notice
the typical permeate flux profile as commented
by Wallberg et al. [ 15].
The permeate collected in the experiments
using 5 and 15 kDa membranes was subsequently
filtrated with 1 kDa membrane at 5 bar of transmembrane pressure. No considerable additional
retention was obtained, only about 10% for the
permeate of the 15 kDa and about 12% for the
permeate of 5 kDa membrane in terms of COD.
This result confirms that the separation is mainly
governed by the gel layer formed on the membrane surface. This gel layer is not significant
when the permeate is fed to be further filtrated,
since the large macromolecules have been mostly
removed in the first membrane pass.
89
In comparison with results present in the literature by Bhattacharya et al. [12], applying a membrane of 5 kDa, a transmembrane pressure of 7.5
bar and 2% of total dissolved solids, in the present
article a TDS concentration three times greater
and a higher permeability was obtained, 72 against
43 L m -2 h-~for an operating pressure of 7 bar.
4. Conclusions
The cross-flow nano- and ultrafiltration techniques can be employed as an alternative way for
the treatment of black liquor from pulping industry.
This filtration process can reduce twofold the
volume of the spent liquor with no significant
changes in the permeate flow across the membranes. The concentrations of TDS, organic matter
and COD in the permeate remained constant with
values of 2.5%, 1% and 30 g O2/L respectively.
Because the ceramic membranes have unique properties such as high physical and chemical resistance, they can withstand severe cleaning procedures (e.g. to strong pH conditions or backwashing),
which results in the rapid permeate flow recovery
without significant damage to the membrane.
In the filtration at the applied conditions, the
formation of a gel layer on the membrane surface
takes place and that may play an important role
in the separation process. Furthermore, in the
filtration experiments performed with the 15 kDa
membrane, pore blocking may occur. This leads
to a continuous decrease in the permeate flux and
retention levels, which however are still higher
than those obtained for the 1 kDa membrane.
Acknowledgements
The authors would like to thank the Spanish
Ministry of Science and Technology (CICYT,
QUI98-0464-C03-01) and the Department of
Chemical Engineering at the Universitat Rovira i
Virgili for its financial support. We also acknowledge the Universitat Rovira i Virgili for providing
a doctoral fellowship.
90
A. Dafinov et aL / Desalination 173 (2005) 83-90
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