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Process intensification in the textile industry: the role
of membrane technology
B. Van der Bruggena,b,*, E. Curciob,1, E. Driolib,1
aLaboratory for Applied Physical Chemistry and Environmental Technology, Department of Chemical Engineering,
K.U. Leuven, W. de Croylaan 46, Heverlee B-3001, BelgiumbResearch Institute on Membrane Technology (ITM-CNR), c/o University of Calabria, via P. Bucci, cubo 17/C, Rende (Cosenza) 87030, Italy
Received 3 June 2003; revised 14 July 2004; accepted 24 July 2004
Abstract
Process intensification is a concept that was recently introduced in the chemical industry for the purpose of reducing environmental
emissions, energy consumption and materials consumption. The principle of process intensification can be used in related industries as well;
textile finishing is an exemplary activity where it may have a significant long-term added value. Membrane technology can be a key factor in
the recycling and reuse of energy, water and chemicals. In this paper, an integral approach for treatment of aqueous process streams in the
textile finishing industry is proposed. The proposed process includes microfiltration pretreatment of used finishing baths, followed by a dual
nanofiltration (NF) unit. These can be operated at elevated temperatures so that no further energy is needed for preheating of recycle streams.
In the proposed treatment scheme, the first of the NF units uses a loose nanofiltration membrane that retains most of the organic fraction but
not the dissolved salts. The second unit uses a tight nanofiltration membrane, which produces a permeate fraction that can be directly reused,
and a concentrated brine that is fed to a membrane crystallizer. In this unit, salts are recovered and recycled for use in new dye baths. The
concentrate stream from the first NF unit is fed to a membrane distillation unit, where the high temperature is advantageously used for further
concentration. The remaining fraction is not reusable, given the fact that most dyes are hydrolyzed after exhaustion of the bath, but has a
significant energetic value, which can be utilized for compensation of energy losses and preheating of suppletion water, by using an
incineration process with energy recovery.
The concept was not tested experimentally, but a simulation for a 500 m3/d production unit shows that it is feasible, although modifications
may be necessary depending on the nature of the finishing baths. Furthermore, the membrane choice in the first NF unit is a critical aspect.
q 2004 Elsevier Ltd. All rights reserved.
Keywords: Process intensification; Textile industry; Finishing baths; Dye baths; Water reuse; Nanofiltration; Microfiltration; Membrane distillation;
Membrane crystallizer
1. Introduction
Due to economical factors, and in view of sustainable
development in industry, there is a growing awareness that
process design requires an integrated approach involving all
processes, including waste water treatment. Therefore,
0301-4797/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jenvman.2004.07.007
* Corresponding author. Address: Laboratory for Applied Physical
Chemistry and Environmental Technology, Department of Chemical
Engineering, K.U. Leuven, W. de Croylaan 46, Heverlee B-3001, Belgium.
Tel.: C32 16 32 23 40; fax: C32 16 32 29 91.
E-mail addresses: [email protected] (B. Van der
Bruggen), [email protected] (E. Drioli).1 Tel.: C39 0984 49 20 14; fax: C39 0984 40 21 03.
environmental engineering should not only consider the
generated waste fractions (i.e. emissions to air, water and
soil), but also develop methods to minimize these fractions,
and eventually realize the ultimate objective of zero
emissions to all environmental compartments. A concept
often used in this context is process intensification. Process
intensification can be defined as the development of
innovative apparatuses, techniques and methodologies that
offer drastic improvements in manufacturing and proces-
sing, substantially decreasing equipment volume, energy
consumption and waste formation, and ultimately lead to
cheaper, safer, sustainable technologies (Stankiewicz and
Moulijn, 2000).
Journal of Environmental Management 73 (2004) 267–274
www.elsevier.com/locate/jenvman
B. Van der Bruggen et al. / Journal of Environmental Management 73 (2004) 267–274268
Process intensification has its origins in the chemical
industry, where large volumes of materials and large
amounts of energy are consumed in unit operations.
Because of limitations of single unit operations, hybrid
processes have been suggested: integrating two unit
operations with different separation principles may have a
synergetic effect, and the resulting separation may be better
than the separation obtained with either unit operation alone
(Stephan et al., 1995). Membrane technology plays an
important role in hybrid processes, mainly because of the
purely physical nature of the separation principle, and the
modular design of membrane processes (Rautenbach and
Mellis, 1995). The advantages of hybrid processes using
membranes were recognized in the beginning of the 1990s,
together with the notion that the flexibility of the systems
can be maximized using membrane technology, because
system operating conditions can be easily adjusted to
accommodate changes in the feed stream or to compensate
for changes that occur in the membrane over time (Ray
et al., 1991). At present the use of hybrid processes has
no significant technical limitations and can be easily
simulated and designed (Brinkmann et al., 2003; Rauten-
bach et al., 1996). A typical hybrid process is the coupling of
pervaporation (or vapor permeation) and distillation, where
a pervaporation unit is combined with a distillation column
to achieve facilitated separation near the azeotropic
composition of the mixture (Fahmy et al., 2001; Lipnizki
et al., 1999). Dehydration of ethanol is a typical application;
a 12% reduction in total annual costs is claimed, which may
even be further optimized (Szitkai et al., 2002). Other
examples of hybrid processes in the chemical industry are
the combination of pervaporation with liquid–liquid extrac-
tors or with chemical reactors; these represent another type
of hybrid process, where the membrane is used to shift the
reaction equilibrium to a higher yield by the selective
removal of the reaction products (David et al., 1991). In
addition to pervaporation, other membrane operations can
be used in hybrid processes, such as coupling of reverse
osmosis (RO) and Multi-Stage Flash (MSF) in seawater
desalination (Van der Bruggen and Vandecasteele, 2002a).
A hybrid process consisting of membrane gas separation
and gas absorption is useful for the removal of acid gases
from natural gases, although the hybrid process is only
advantageous if CO2 and H2S are both present (Bhide et al.,
1998). A possible improvement may involve the replace-
ment of the gas absorption step by a membrane contactor.
The use of hybrid processes has also been suggested for
waste water treatment. Combinations of pressure driven
membrane processes (NF/RO) and adsorbents may yield a
product water with sufficient quality for groundwater
recharge (Ritchie and Bhattacharyya, 2002) or even for
drinking water production when activated carbon is used as
the adsorbent (Konieczny and Klomfas, 2002). Other
possibilities include the combination of pervaporation
with a decanter on the permeate side and an adsorption
unit on the retentate side, which has proven to be a feasible
economic alternative for the treatment of waste waters
contaminated with phenol, with efficient recovery of phenol
(Lipnizki and Field, 2001). Some of these systems are
simple sequences of unit processes in which a membrane
operation plays a central role, in combination with more
traditional methods such as activated sludge treatment or
chemical oxidation (Bohdziewicz et al., 2001). These
systems are still close to the traditional approach of using
separate, consecutive unit processes. Other applications
integrate two or more unit operations, to achieve a
synergetic effect, thus partially approaching the idea of
process intensification. A hybrid process that may become
important in the application of integrated waste water
treatment systems is the coupling of photocatalysis and
nanofiltration, where simultaneous degradation and rejec-
tion can be obtained (Molinari et al., 2002). A further
integration is obtained in a membrane reactor used for the
removal of heavy metals from industrial waste waters,
where fine adsorbent ion exchange particles with high
specific area are retained by a membrane (Flores and
Cabassud, 1999). In some cases, a hybrid process making
use of membranes may even be considered as a single unit
operation, where it is essentially a combination of more than
one operation. Typical examples are membrane contactors,
membrane distillation and membrane bioreactors; all of
these are suitable candidates for use in process intensifica-
tion, given the fact that they combine advantages of two
completely different processes.
Process intensification, however, requires an even more
innovative approach by further optimizing the integration of
different sub-processes in a novel concept; membrane
technology is expected to play a leading role in this field
(Drioli, 2002). Hybrid processes may play a role in this new
concept, but these should fit in a general process scheme
where each unit is optimized taking the requirements of e.g.
flexibility, imposed by the overall system, into account.
In this article, a novel treatment scheme that meets the
requirements of process intensification is proposed for
application in the textile industry, and an estimation of its
efficiency is made. In the textile industry, and in particular
the textile finishing sector, the availability of high-quality
water is a key factor in many processes such as washing,
bleaching, printing and coating of textile products (yarns,
woven fabrics, knitted fabrics, non-woven fabrics, ready-to-
wear articles, etc.). Textile companies often face a shortage
of available water sources, not only because of water
scarcity, but usually as a result of permit systems, which
limit the use of ground water to a predetermined volume.
The permit may be conditional, related to the company’s
efforts to find possibilities for water reuse. In the future,
many of these companies will face the requirement of
reusing a significant part of all incoming fresh water. This
will involve an improvement in the waste water quality to
the standards used for fresh (ground) water. Traditionally
used methods (e.g. biological degradation with activated
sludge) are insufficient for obtaining the required water
Table 1
Typical list of additives used in a strong acid finishing bath, in addition to
dye components
Additive Concentration
Foam reducing agent (g/l) 0.25
Egaliser (g/l) 0.85
Antimoth (g/l) 0.15
Wetting agent (g/l) 7
Salt (g/l) 7.8
Acid (g/l) 2.4
Bleaching agent (g/l) 1.55
Lubricant (g/l) 1.7
B. Van der Bruggen et al. / Journal of Environmental Management 73 (2004) 267–274 269
quality. Membrane technology may offer a realistic
solution: nanofiltration of the effluent provides a permeate
water claimed to have a sufficiently good quality for reuse in
the process (Hao and Zhao, 1994; Sojka-Ledakowicz et al.,
1998; Van der Bruggen et al., 2001). However, the final
destination of the concentrate has not been determined to
date, and methods of coupling the membrane unit to the
other process units are still unclear. The innovative scheme
proposed in this work has the objective of decreasing
drastically the environmental emissions, and ultimately
decreasing the overall costs, with a goal of sustainability.
Acid spender –2. Overview of textile finishing baths
The methodology required for integrated water manage-
ment in a textile finishing company depends largely on
the nature of the finishing operations and, consequently, on
the composition of the finishing baths and waste water. The
term ‘finishing’ covers pretreatment as well as posttreat-
ment of textiles (Centexbel, 2003; Marsh, 1957). Pretreat-
ment includes many possible activities (desizing, boiling
off, bleaching, etc.) on textiles in view of subsequent
treatments. Also textile dyeing and printing are part of the
textile finishing activities. Posttreatment comprises a series
of finishing activities (other than dyeing) to obtain or
improve textile properties. Examples are the softening of
textiles, or treating textiles to become crease-proof, or fire-,
water- and oil-resistant. The dyeing step has the largest risk
for environmental pollution, because this step often requires
high concentrations of organic dyes, additives and salts.
Toxicity of dyes has been proved by several studies (Chung
and Edward, 1992; Meyer, 1981; Preiss et al., 2000).
Dyes can be classified (American Association of Textile
Chemists and Colorists, 1971) on the base of chemical
structure or binding with the textile. Dyes with the same
structural formula can be used to dye different textiles with
different techniques. Different types of dyes are basic
(cationic) dyes, direct dyes, sulphur dyes, azoic dyes and
ingrain dyes, vat dyes, acid dyes, mordant dyes and metal-
complex dyes, disperse dyes, reactive dyes, and pigments
(Abrahart, 1977; Correira et al., 1994; Preston, 1986). The
application technique determines the composition of the dye
bath. Hence, not only the dyes are needed for the dyeing
process; additives such as surfactants and dispersing agents
are also important. Salts are often used as a regulator for the
reaction; depending on the mechanism, high or low salt
concentrations are used in the dye bath. A typical example
of used additives in acid dyeing is given in Table 1. For vat
dyeing, no salts are needed; reactive dyeing uses extremely
high salt concentrations (up to 100 g/l); metal-complex
dyeing and mordant dyeing use salt concentrations
ranging from very low to high. A typical recipe for
disperse dyeing would consist of the following
components: foam reducing agent (on the base of mineral
oil), organic complexing agent (for hardness ions and iron;
example: modified Na-polyacrylate); acetic acid; mixture of
aromatic hydrocarbons as egaliser; dispersing agent;
disperse dyes; base; sodium hydrosulphite. Reactive dyeing
usually requires a pretreatment with a washing agent such as
alkanolethoxylate, organic complexing agent, NaOH,
hydrogen peroxide; the dye bath consists typically of two
or three reactive dyes, dispersing agent, complexing agent
and egaliser. The choice between chloride salts and sulphate
salts is determined by cost and quality: although sulphates
are more expensive, they are preferred because of the better
quality.
As a conclusion, it can be stated that a large diversity can
be found in textile finishing baths. However, a significant
fraction of organic compounds is always present; the
inorganic fraction may range from absent to 10 wt%. All
further considerations concerning treatment methodologies
for an integrated water management system will take this
range of concentrations into account.
3. Process water requirements
Apart from empirical rules of thumb and local standards
(Rozzi et al., 1999b), no guidelines for process water quality
in the textile finishing industry exist. Therefore, require-
ments for process water in the textile industry depend on
individual companies. A few basic conditions are related to
water turbidity, which should be less than the turbidity of
the ground water that is used as fresh water, and the water
hardness, which should be in the normal range for relatively
soft ground water (not more than 50–60 mg/l). Of course, all
colors should be removed before reuse. Furthermore, no
concentration of other components such as heavy metals can
be allowed in the water cycle.
Numerous studies claim that water reclamation is
feasible with pressure driven membrane technology,
because these basic requirements are fulfilled for the
obtained permeates (Akbari et al., 2002; Bes-Pia et al.,
2002; Koyuncu, 2002; Marcucci et al., 2002; Tang
and Chen, 2002; Ciardelli et al., 2001; Rozzi et al.,
1999a,b). In these studies, a simple sequential treatment is
suggested using a combination of traditional processes
B. Van der Bruggen et al. / Journal of Environmental Management 73 (2004) 267–274270
(coagulation/flocculation, adsorption, sand filtration) and
membrane processes (ultrafiltration, nanofiltration, RO). A
fraction of the treated water is lost in each of the
treatment steps, and solid or liquid waste fractions are
produced. Although experimental studies on the practical
consequences of full scale water reuse in textile finishing
are missing, it can be assumed that it is technically
possible to provide reclaimed water leading to a similar
quality of the end product, or even to a quality
improvement. Thus, it will be further assumed that the
final water quality obtained using the processes described
in the literature (ultrafiltration, nanofiltration, RO) is
suitable for reuse.
Fig. 2. Proposed integrated water treatment system using membranes (fullline, water stream; dashed line, energy stream; MD, membrane distillation;
NF, nanofiltration; MC, membrane crystallizer). (1) To be replaced by UF
in the case of sulphur dyeing, disperse dyeing and pigment dyeing. (2) For
reactive dyeing, metal-complex dyeing, mordant dyeing. (3) For vat dyeing
and other techniques where no salts are used. (4) Operated in dead-end.
4. Assessment of an integrated water management
system using membrane technology
A typical sequential treatment system for water reuse in a
textile company making use of nanofiltration is represented
in Fig. 1. The nanofiltration is essentially an extension of the
existing treatment system, opening the possibility of
(partial) reuse of the process water. However, this
methodology generates secondary waste fractions (excess
sludge and the concentrate from the nanofiltration unit), and
energy from hot process streams gets lost by dissipation in
the different treatment steps. Although some attempts have
been made to improve this scenario, the use of membrane
processes in the textile industry is still limited to designs
similar to Fig. 1. An important advance is the simultaneous
reclamation of waste water and energy, by applying
membrane operations at elevated temperatures (Voigt et
al., 2001). The objective of achieving an integrated zero-
discharge system has already been formulated using a
combination of chemical, biological and membrane pro-
cesses (Lee et al., 2001).
The innovative methodology proposed here is rep-
resented in Fig. 2. The waste water fraction considered
might be limited to the exhausted dye baths, or might
include the rinsing baths as well. The latter are less
Fig. 1. Typical current methodology for water reclamation in a textile
company.
concentrated; ideally, rinsing waters can be directly reused
for, e.g. preparation of new dye baths. In practice, however,
the dyes used differ from batch to batch so that direct reuse
is not feasible. The addition of the rinsing baths to the dye
baths before treatment is useful because it dilutes the overall
waste stream so that less problems related to membrane
fouling are expected. On the other hand, the rinsing baths
can be treated more easily; in most cases, it is more
convenient to treat them separately. In this simulation,
separate treatment of rinsing baths is assumed.
The required purification can be divided into two main
steps: removal of the organic fraction (dyes, additives),
and removal of the inorganic fraction (salts). After
pretreatment using a dead-end microfiltration (MF) unit,
the removal of the organic fraction can be done by
nanofiltration (NF-A) using a loose NF membrane with
low salt rejection at a high temperature, corresponding to
the temperature of the dye bath. The permeate fraction
contains a large fraction of inorganics; the organic
fraction is low. (The retentate fraction is a warm,
concentrated organic solution.) The elevated temperature
is necessary for the membrane distillation (MD) unit,
where a temperature difference acts as the driving force.
(In the MD unit, the organic fraction is separated from the
water.) The distillate is recycled to the finishing process;
the remaining organic fraction has an added value by
utilizing its energy content in an incineration process. The
energy yield makes up for the loss of energy through
losses in the different treatment steps.
The NF-A permeate feeds a second nanofiltration unit,
NF-B. In contrast to NF-A, a tight NF membrane is needed
in NF-B, as the objective is to remove the salt fraction. The
removal of salts can be simply omitted for dyeing
techniques where no salts are used, such as vat dyeing.
The modular design of membrane processes allows
flexibility in the process streams, so that combinations of
B. Van der Bruggen et al. / Journal of Environmental Management 73 (2004) 267–274 271
different types of dye baths are possible by using a shortcut
line for waste streams with low inorganic fractions.
The permeate from NF-B is fit for reuse as process water;
small organic fractions remaining from incomplete removal
in NF-A are completely removed. The permeate is recycled
to the textile finishing baths. The retentate from NF-B is a
concentrated salt solution, comparable to the brine from
desalination processes. In a membrane crystallizer (Drioli et
al., in preparation; Curcio et al., 2001), the salts can be
recovered with sufficient purity and reused directly for new
finishing baths. Additionally, further recuperation of
process water is possible.
The overall methodology thus minimizes the waste
fractions and losses of energy and water, as proposed in the
concept of process intensification. The individual treatment
processes will be simulated in a step-by-step approach for a
500 m3/d production unit with composition as indicated in
Table 1, with addition of dyes. The salt is assumed to be
Na2SO4, which is superior in view of the control of the
dyeing process; however, NaCl is also possible. The dyes
are commonly used in mixtures; most regular dyes have a
molecular weight of 900–1400.
4.1. MF
For the MF pretreatment, a dead-end unit using
inorganic membranes is presumably the most robust
solution. Sintered stainless steel membranes are advan-
tageous because they have an extremely high porosity
(between 65 and 85%), which results in permeabilities that
are considerably higher compared to other MF membranes.
Fluxes of 2000–4000 l/m2 h have been reported (Neyens et
al., 2001) at pressures of 0.3–1.5 bar, for pore sizes ranging
from 1 to 10 mm. The membrane surface area needed with
these membranes to treat the feed stream is limited to
10.4 m2. Furthermore, the metallic membranes can be
easily cleaned.
4.2. NF-A
The purpose of the first NF unit is to retain (most of the)
organic fraction, but to allow permeation of salts to the
permeate. The unit should operate at high temperature so
that energy can be recovered.
The membrane for this stage should have a low salt
rejection (around 30% for Na2SO4 and 10% for NaCl), but
should reject a considerable fraction of the organic
compounds and have a high water flux. A nominal
molecular weight cut-off (MWCO) of 1000 is assumed,
which is in the range of the molecular weight of the dye
molecules in the finishing baths. The rejection can be
estimated as a function of molecular weight by using the
following equation (Van der Bruggen and Vandecasteele,
2002b)
sðMW�Þ ¼
ðMW�
0
1
SMW
ffiffiffiffiffiffi2p
p1
MW
!expKðlnðMWÞK lnð ��MWÞ þ 0:56 SMWÞ2
2S2MW
dMW
!
where s is the reflection coefficient, an upper limit for the
rejection at high pressures; ��MW is the MWCO and SMW is a
factor representing the pore size distribution. A rejection
up to 45% for glucose (MW 342) was obtained, so that a
90% rejection for dye compounds with MW 600–900 and
95–99% for dyes with MW above 1000 is estimated.
Ceramic membranes are possible alternatives for the
membrane choice, in view of the specific requirements of
this application (Voigt et al., 2001).
The pressure used is 10 bar; a slightly higher pressure has
to be applied to compensate for the effect of osmotic
pressure. The latter can be estimated using Van’t Hoff’s
equation
Dp Z ncj
RT
M;
where R is the ideal gas constant, T is the absolute
temperature (K), cj is the concentration of the salt (g/l), M is
the molar mass, and n is the number of ions. Only the
rejected ions (ca. 30%) should be taken into account, so that
the osmotic pressure is limited to ca. 1 bar. However, the
effect of osmotic pressure may be more significant for
dyeing methods where more salts are used.
The water flux at elevated temperatures can be estimated
by taking the effect of the temperature on water viscosity
into account (Weast, 1982). This can be described as:
logh
h20
Z1:3272ð20 KTÞK0:001053ðT K20Þ2
T C105; with
J ZDP
hRtot
:
At reference conditions of DPZ40 bar and TZ20 8C, the
water flux J is 250 l/m2 h. With the above equations, a water
flux of 200 l/m2 h is obtained for DPZ10 bar and TZ80 8C.
Taking a 25% flux decline into account due to adsorption of
organic compounds (Van der Bruggen et al., 2002), the
membrane surface area needed is 160 m2. Using 8040 type
spiral wound modules with a 44 mm spacer (22 m2) (Nadir
Filtration, 2003), it can be calculated that six modules are
needed for the 20.8 m3/h process stream. The water
recovery in the spiral wound NF unit is estimated at 75%,
so that the permeate volume is 15.6 m3/h. This fraction
contains 70–85% of the salts in the feed solution
(concentration ca. 6 g/l), due to the low rejection of ions
in the NF-A modules. The organic fraction is limited to
traces. The retentate, which will be fed to the membrane
distillation unit, has a volume of 5.2 m3/h and consists
mainly of organic components in aqueous solution.
B. Van der Bruggen et al. / Journal of Environmental Management 73 (2004) 267–274272
4.3. NF-B
The purpose of the second NF unit is to efficiently
separate salts from the water at elevated temperatures. It is
assumed that mainly sodium sulphate is used in the finishing
baths; however, if sodium chloride is used, NF-B can be
replaced by a RO unit. The membrane is assumed to have a
sulphate rejection of 98% and will further remove traces of
organic compounds because of its low MWCO (150–300).
The water flux at 6.9 bar is 62 l/m2 h; using the same
procedure as above a water flux of 190 l/m2 h is obtained at
80 8C, 10 bar. Evidently, the osmotic pressure will have a
larger influence than in NF-A; an osmotic pressure of 2.5
bar is obtained using the Van’t Hoff equation. Using a
standard 32.5 m2 surface area for a 8040 module, 82 m2 is
needed, corresponding to 2.5 (3) modules.
The recovery rate is fixed at 85%, somewhat higher
because fouling should not have a significant effect, given
the absence of organic compounds. The retentate fraction
(2.3 m3/h) is a brine with approximately 40 g/l of sulphates
(if complete rejection of Na2SO4 can be assumed); it will be
fed to a membrane crystallizer. The permeate contains ca.
100 mg/l of dissolved salts, taking a permeation fraction of
2% into account, and can be used for the preparation of new
dye baths.
4.4. MC
Crystallization from solution is a common unit operation
due to its ability to provide large amounts of a high-purity
product, generally in one processing step and with lower
energy requirement than other conventional separation
processes. Coupling of membrane processes and crystal-
lization operations has been proposed for selected appli-
cations: desalting scaling mine water (Juby et al., 1996),
softening of drinking water by CaCO3 removal (Sluys et al.,
1996), and production of BaSO4 nanoparticles (Zhiqian and
Zhongzhou, 2002).
The membrane-crystallization unit aims to induce super-
saturation in solution by removing solvent in the vapour
phase, thus originating a metastable state allowing nuclea-
tion and growth of crystals. The solubility of Na2SO4 is on
the order of 29.8 g/100 ml water, so that a sufficient
oversaturation can be obtained by using a recovery rate of
90% (i.e. 2100 l/h) in the membrane distillation prior to the
precipitation.
Microporous hydrophobic membranes separate the salted
solution; a typical module that can be used is MD150CP-2N
(Microdyn Inc.), containing 178 polypropylene hollow
fibres packed in a shell of 150 mm diameter and 1 m length.
The nominal pore size of the membranes is 0.20 mm; the
total membrane area in the module is 10 m2. The solvent
evaporates at the membrane interface on the warm
(retentate) side, diffuses through the pores and condenses
on the opposite (distillate) side (Curcio et al., 2001). In this
application, the high temperature of the retentate from NF-B
can be advantageously used to obtain a relatively high flux
in the distillation unit. A constant flux of 0.4 l/m2 h was
reported for a temperature of 35 8C at the feed side and
15 8C at the permeate side. Taking the flux-driving force
proportionality into account
Jdist wDpvap;
the operational flux can be calculated. In order to obtain a
maximal temperature effect, the feed can be preheated to
95 8C; with a permeate temperature of 60 8C, a vapour
pressure difference of 486 mmHg is obtained, correspond-
ing to a flux of 6.7 l/m2 h. Thus, a membrane surface area of
315 m2 is needed to treat 2100 l/h, corresponding to 32
modules.
The Na2SO4 crystals can be directly reused for new dye
baths; according to recent research, they have a superior
quality as compared to crystals produced with traditional
methods (Drioli et al., in preparation).
4.5. MD
The membrane distillation unit can be operated similar to
the unit used in the membrane crystallizer and is fed with the
NF-A retentate (5200 l/h). From the average composition of
the dye baths, the concentration of organics is estimated at
40 g/l. Again, the feed solution is preheated to 95 8C so that
a maximal flux is obtained. If the permeate is held at 60 8C, a
flux of 6.7 l/m2 h is obtained; a total surface area of 665 m2
is needed, corresponding to 67 MD150CP-2N modules
(Microdyn Inc.).
4.6. Incineration
An exergy analysis (Molinari et al., 1995) already
showed that membrane operations result in large substi-
tution coefficients, defined as the ratio of primary energy
saved and the electrical energy consumed in the process.
In this section, the analysis is extended by estimating the
energy balance resulting from the application of an
incineration process on the organic sludge from the MD
unit. The organic fraction, mainly consisting of hydro-
lyzed dyes, is not fit for reuse, so that further separations
are not useful. However, the caloric value of the organic
components can be efficiently used in the overall process.
The final incineration of the organic sludge obtained from
the MD unit may comprise several sub steps such as
drying, pyrolysis and combustion. From measured values
for combustion of similar energy sources (combustible
waste from agriculture, sludge from mechanical and
biological waste water treatment), the energetic value of
the residue is estimated at 4–5 kWh/kg (Energy Saving
Now, 2003). The organic fraction in the dye bath may
range from 5 to 15 g/l, decreasing to 3–10 g/l after
expiration. If a concentration of 8 g/l is assumed, a total
volume of approximately 170 kg/h of organic sludge is
B. Van der Bruggen et al. / Journal of Environmental Management 73 (2004) 267–274 273
obtained, with an energetic value estimated at 680–
850 kW. A temperature drop of 20 8C in the NF-B
permeate is assumed. Reheating the permeate stream
(13,300 l/h) to the required process conditions (i.e. 90 8C)
requires a heat supply of ca. 210 kW. The recycle water
from the MC and MD is obtained at 60 8C. Taking a
supplemental 5 8C temperature loss into account, a heat
supply of 265 kW is needed for reheating. A total volume
of 1000 l/h of fresh water is needed as suppletion water;
heating this water volume from 15 to 90 8C requires
90 kW. The total heat supply needed is 660 kW. Thus, the
energy supply from the incinerator should be sufficient to
compensate for energy losses throughout the treatment
system, even if conservative predictions are used.
5. Economic aspects
Economic aspects have not been considered in detail in
this study. Process intensification essentially creates a
possibility to save or reuse considerable amounts of energy,
materials and water. The weight of these factors in the
economic evaluation is expected to increase dramatically
during the next decades, which will make systems allowing
the reuse of energy, materials and water more profitable.
Innovative process designs might not be competitive at
present, but rapidly changing economic situations will
undoubtedly prove that process intensification is the most
sustainable option, combining economic and environmental
aspects.
6. Conclusions
The simulations show that the proposed concept for an
integrated membrane system in the textile finishing industry
is feasible. Recycling of water (from NF units, MD and
MC), of energy (intrinsic energy content of recycled water
and from combustion) and of materials (salts from MC) can
be realized; furthermore, although the dyes are not reusable
as dye compounds, their caloric value is efficiently reused.
The only waste streams generated by the system are the
ashes from the incineration unit, and small cleaning streams
for the membrane units.
The concept needs to be further confirmed using lab-
scale and pilot scale experiments for the respective
membrane operations suggested. The integration of the
processes is expected to be achievable with maintenance of
the flexibility of the system, given the intrinsic compatibility
of membrane operations. Critical aspects that can be
identified on the basis of the conceptual study include the
choice of the membrane in the first NF unit, and efficient
methods to control organic fouling in this unit. Possible
future developments that may reduce the latter problem are
the use of tubular ceramic nanofiltration membranes or
capillary polymeric membranes; the former aspect will be
facilitated by the development of new membrane materials
operable at high temperatures, combining a low surface
charge with a low MWCO.
Acknowledgements
B. Van der Bruggen gratefully acknowledges FWO-
Vlaanderen for a post-doctoral grant and for financial
support.
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