# Fouling of Ultrafiltration Membranes during Isolation of Hemicelluloses in the Forest Industry

Authors: Tobias Persson; Ann-Sofi Jönsson
Journal: Scholarly Research Exchange
 (2009)

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## Abstract

The process streams in the forest industry contain a large amount of hemicelluloses that today ends up in the wastewater. This is an unfortunate loss of a renewable raw material. The hemicelluloses can be isolated from the process stream by using membrane filtration in a process that produces purified water as a by-product, thereby facilitating increased recirculation. However, process streams from the forest industry contain both aromatic compounds and inorganic ions that are known to cause fouling of the membranes. Thus, the most suitable membrane and pretreatment from a cost-efficient point of view must be applied to avoid fouling and life-time shortening of the membranes during operation. In the present investigation, fouling during ultrafiltration of a process stream from the production of Masonite was studied. The fouling of a hydrophilic membrane made of regenerated cellulose was less severe than the fouling of hydrophobic membranes made of polyether sulphone and a composite fluoropolymer. Pretreatment of the wastewater with activated carbon resulted in higher flux and less fouling of both hydrophilic and hydrophobic membranes. Lowering the pH of the wastewater led to less severe fouling of the regenerated cellulose membrane, but lower flux of the composite fluoropolymer membrane.


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## Body

## 1. Introduction

There is a
universal aim in the forest industry to reduce the consumption of fresh water
by recycling process streams as far as possible. This makes it necessary to
introduce artificial kidneys in order to prevent the accumulation of matter in
the internal circulation systems. Membrane processes have been used as kidneys
in the forest industry since the late 1960s, mainly for the treatment of white
water [1], and bleach plant effluent [2, 3] in pulp and paper mills. Recently,
an additional benefit when treating process streams in the forest industry has
attracted attention. Process streams in the forest industry are often rich in
hemicelluloses that can be used as hydrogels [4], barrier films [5, 6] and
paper additives [7]. The main interest in this area has been focused on the
isolation of hemicelluloses in mechanical and thermomechanical pulp mills
[8–11]. When studying the potential of isolation of hemicelluloses in process
streams from the Masonite process, severe fouling was observed. This does not
come as a surprise as aromatic compounds, such as lignin and extractives, act as
natural glue in Masonite board and have been identified as foulants during
membrane filtration of pulp and paper effluent [12–14].The aim of this
work was to study different measures to reduce fouling when treating wastewater
from the Masonite process. Fouling of six ultrafiltration (UF) membranes was
investigated. The membranes were made of different materials including
regenerated cellulose (hydrophilic), polyether sulphone (hydrophobic), and a
composite fluoropolymer (hydrophobic material modified to be hydrophilic). The
influence of pH on fouling and the removal of foulants by adsorption on active
carbon before ultrafiltration was studied.


## 2. Materials
and Methods

### 2.1. Masonite Wastewater

Experiments were
performed with wastewater from Masonite AB (Rundvik, Sweden). Masonite is a type of hardboard
formed by pressing a board of long cellulose fibres at elevated temperature.
The main raw material used at Masonite AB is softwood
chips and cutter dust. The chips are defibrated by steam explosion and ground
in a refiner together with the cutter dust, producing a wet fibre-rich pulp.
The pH of the pulp is adjusted by formic acid. A hydrophobic dispersion of
paraffin wax, glue (phenolic resin), retention aids, and catalysts (aluminium
and ferric sulphate) is added. The pulp is dewatered and cut into lengths.
These are then pressed and heated to form Masonite boards. All process streams
are collected and treated by flotation at Masonite AB. The sludge is recycled
to the fibre system, and 80% of the liquid is reused in the process. The
remaining 20% of the liquid is discharged. The wastewater studied in this work
was withdrawn after the flotation basin and had a temperature of 60°C. It
contained 1-2 g/L,  of hemicelluloses, various inorganic compounds in low
concentration (1–6 mg/L of Al, Fe, Si, Mn, 10 mg/L, Mg and 50 mg/L, Ca) and the
UV light absorption at 280 nm, which is a measure of the content of lignin, was
around 10 absorption units. Wastewater was withdrawn at various occasions,
which means that the properties of the wastewater vary slightly between the
four experiments performed.


### 2.2. Concentration of Hemicelluloses

The turbidity of
the wastewater was rather high, 700 Nephelometric Turbidity Units (NTUs). The
wastewater was therefore prefiltered in order to
remove insoluble material which could otherwise block the feed channels in the
ultrafiltration module. Prefiltration was performed in a Vibratory Shear-Enhanced Processing
(VSEP) unit (series L/P, New Logic,
Emeryville, Calif, USA) using a microfiltration membrane with a pore diameter of
10μm. The total membrane area was 1.57 m2. Microfiltration was
carried out at room temperature and with a vibration frequency of 50 Hz, which
corresponds to an amplitude of 19 mm. The transmembrane pressure must be above
200 kPa at all times to avoid damage of the membranes caused by the vibrations.
To ensure this, the microfiltration was performed at 250 kPa in this study. A
higher pressure was not applied since it increases the transport of particles
to the membrane surface and thus the probability of the formation of a filter
cake. The turbidity after microfiltration was 18 NTU. The total solids (TS)
content was decreased from 0.6% to 0.5%.Ultrafiltration
was carried out in a DDS 20 plate-and-frame module (Alfa Laval A/S, Nakskov,
Denmark), equipped with eight double-sided membrane discs with a total membrane
area of 0.28 m2. The ETNA10 membrane
(Alfa Laval A/S) is a composite fluoropolymer membrane with a nominal molecular mass cut-off of 10 000 Da.The membranes were
cleaned before the experiment, at a volume reduction of 0.7 and after the
experiment. The volume reduction (VR) is defined as the ratio between the
volume of permeate withdrawn and the initial feed volume. The membranes were
cleaned with an alkaline cleaning agent, 0.5 wt% Ultrasil 10 (Henkel Chemicals
Ltd, Düsseldorf, UK) at 50°C for about 45 minutes. The system was thoroughly rinsed
with deionized (DI) water after cleaning. The pure water flux (PWF) was
measured at 50°C and 1.0 MPa.During
concentration of the wastewater, the transmembrane
pressure was 1.0 MPa, the temperature was 50°C, and the circulation
velocity was 0.5 m/s. The same operating conditions were previously
successfully used to isolate hemicelluloses from pulp mill process water [8]. The
retentate was recycled to the feed tank, and the permeate was continuously
withdrawn. The permeate flow rate was measured gravimetrically with a balance
(PL 6001-S, Mettler Toledo Inc., Columbus, Ohio, USA). Pressure, temperature, and
permeate flow rate were recorded by a PC equipped with LabView 6.0 software
(National Instruments Co, Austin, Tex, USA). Data were recorded every 5 seconds.


### 2.3. Membrane Screening

The
wastewater was prefiltered in the VSEP unit as described above. Ultrafiltration was performed in an equipment with four cross-flow modules connected in parallel. Each module was equipped with a circular flat membrane with an
area of 0.002 m2. Different membranes were used in each module. Fouling of six membranes was studied. The characteristics of the
membranes are given in Table 1. Both retentate and permeate were recirculated
to the feed tank during ultrafiltration. New
membranes were used in all experiments. The transmembrane pressure was 1.0 MPa.
The membranes were cleaned before the
experiment. Pressure, temperature, and permeate flow rate were recorded every 60
seconds. The temperature was 50°C and the
permeate flow of the UP005, UC005, and PLCC membranes was measured
gravimetrically with a balance (PL 6001-S, Mettler Toledo Inc.), and the
permeate flow of the ETNA01, ETNA10, and NP030 membranes was measured with a
PhaseSep flow meter (Phase Separations Ltd, Queensferry, UK).Table 1
Properties of the ultrafiltration membranes used in the
investigation. The pure water flux (PWF) was measured at 1.0 MPa and 50°C.
ManufacturerMembraneMaterialMax. operating
temperature (°C)Cut-off
(Da)PWF (L/m2h)Alfa
LavalETNA01Composite
fluoropolymer601000290Alfa
LavalETNA10Composite
fluoropolymer6010 000890Microdyn-NadirNP030Polyether
sulphone951000110Microdyn-NadirUP005Polyether
sulphone955000590Microdyn-NadirUC005Regenerated
cellulose555000180MilliporePLCCRegenerated
cellulose505000200Octanoic
acid was used to study the hydrophobicity of the ETNA10, UP005, and UC005
membranes. Octanoic acid is a saturated fatty acid with similar properties as
some of the extractives in wood. A solution with 0.5 g/L, of octanoic acid was
used as feed at a temperature of 20°C and a
pressure of 0.2 MPa. After stabilizing the flux for 20 minutes, the flux was
measured and compared to the pure water flux at the same conditions. The
hydrophobicity of the membranes was reflected by the magnitude of the flux
decrease.


### 2.4. Adsorption by Activated Carbon

The wastewater was prefiltered in the VSEP unit as described above. Ultrafiltration was performed in the same way and with the
same equipment as used in the membrane screening but with the same type of membrane
in all four modules. The pressure was 0.7 MPa in order to decrease the flux and
thus reduce the transport of foulants to the membrane surface. The
membranes were cleaned before and after the experiment. Adsorption with
activated carbon was performed in a stirred tank
for 30 minutes at room temperature. Activated carbon with a size of 3–5 mm
(KEBO AB, Stockholm, Sweden) was added at a ratio of 1 : 24 (g/g wastewater). The
same ratio has been used for the removal of phenolic compounds from rice straw
hemicellulosic hydrolysate [15]. The activated carbon was removed by filtration
through a filter paper (Quality 3, Munktell Filter AB, Falun, Sweden) before
ultrafiltration. The retentate was recycled to the
feed tank, and the permeate was continuously withdrawn during
ultrafiltration.


### 2.5. Adjustment of pH

The
wastewater was prefiltered in the VSEP unit as described above. Ultrafiltration was performed in a cross-flow module with a circular
flat membrane with an area of 0.002 m2. The permeate flow was measured with a PhaseSep flow
meter (Phase Separations Ltd). The retentate and the
permeate were recycled to the feed tankThe transmembrane
pressure corresponding to a pure water flux of 100 l/m2h at 50°C was measured at the pH to be investigated before each
experiment. The pH of the pure water (deionized water) and the wastewater
was adjusted with sodium hydroxide (50 wt%, Merck, Darmstadt, Germany) and
hydrochloric acid (32 wt%, Merck) in an external tank at room temperature
before ultrafiltration. The transmembrane pressure
corresponding to a pure water flux of 100 l/m2h at the investigated
pH and a temperature of 50°C was then used during ultrafiltration of the pH
adjusted waste water. Thus, all membranes were subjected to similar conditions
during the experiments. New membranes were used in each experiment. They were
cleaned before the experiment.


### 2.6. Analysis

The content of
total solids was determined according to the standardized method of the
National Renewable Energy Laboratory (NREL, U.S. Department of Energy, USA)
[16], and the turbidity was measured with a turbidimeter (HACH 2100P, Hach
Lange, Düsseldorf, Germany).The
concentration of hemicelluloses was analyzed by acid hydrolysis according to the standardized method of the NREL [17]. Monomeric sugars were
analyzed before and after acid hydrolysis, and the hemicellulose content was
calculated from the difference in monosaccharide concentration before and after
hydrolysis. Anhydro corrections of 0.9 and 0.88 were used for hexoses and
pentoses, respectively. High-performance anion-exchange chromatography coupled
with pulsed amperometric detection (HPAEC-PAD) using an ED40 electrochemical
detector (Dionex, Sunnyvale, Calif, USA), was used to analyze the monomeric
sugars. The chromatograph was equipped with a gradient pump (GP40, Dionex), an
autosampler (AS50, Dionex) with a Carbo Pac PA1 guard, and a PA10 analytical
column (Dionex). Millipore water with 2 mM NaOH was used as eluent at a flow
rate of 1 mL/min, and the injection volume was 10 μL. D-mannose, D-glucose, D-galactose, D-xylose,
and L-arabinose (Fluka Chemie AG, Buchs, Switzerland) were used as standards.The content of
lignin residues was measured with Shimadzu UV-160 spectrophotometer (Kyoto,
Japan) at a wavelength of 280 nm. Before measurement, the samples were diluted
with deionized water. The concentration of inorganic cations was determined by
inductively coupled plasma atomic emission spectroscopy (ICP AES) with a Perkin
Elmer (Waltham, Mass, USA) Optima 3000DV ICP AES instrument.


## 2.1. Masonite Wastewater

Experiments were
performed with wastewater from Masonite AB (Rundvik, Sweden). Masonite is a type of hardboard
formed by pressing a board of long cellulose fibres at elevated temperature.
The main raw material used at Masonite AB is softwood
chips and cutter dust. The chips are defibrated by steam explosion and ground
in a refiner together with the cutter dust, producing a wet fibre-rich pulp.
The pH of the pulp is adjusted by formic acid. A hydrophobic dispersion of
paraffin wax, glue (phenolic resin), retention aids, and catalysts (aluminium
and ferric sulphate) is added. The pulp is dewatered and cut into lengths.
These are then pressed and heated to form Masonite boards. All process streams
are collected and treated by flotation at Masonite AB. The sludge is recycled
to the fibre system, and 80% of the liquid is reused in the process. The
remaining 20% of the liquid is discharged. The wastewater studied in this work
was withdrawn after the flotation basin and had a temperature of 60°C. It
contained 1-2 g/L,  of hemicelluloses, various inorganic compounds in low
concentration (1–6 mg/L of Al, Fe, Si, Mn, 10 mg/L, Mg and 50 mg/L, Ca) and the
UV light absorption at 280 nm, which is a measure of the content of lignin, was
around 10 absorption units. Wastewater was withdrawn at various occasions,
which means that the properties of the wastewater vary slightly between the
four experiments performed.


## 2.2. Concentration of Hemicelluloses

The turbidity of
the wastewater was rather high, 700 Nephelometric Turbidity Units (NTUs). The
wastewater was therefore prefiltered in order to
remove insoluble material which could otherwise block the feed channels in the
ultrafiltration module. Prefiltration was performed in a Vibratory Shear-Enhanced Processing
(VSEP) unit (series L/P, New Logic,
Emeryville, Calif, USA) using a microfiltration membrane with a pore diameter of
10μm. The total membrane area was 1.57 m2. Microfiltration was
carried out at room temperature and with a vibration frequency of 50 Hz, which
corresponds to an amplitude of 19 mm. The transmembrane pressure must be above
200 kPa at all times to avoid damage of the membranes caused by the vibrations.
To ensure this, the microfiltration was performed at 250 kPa in this study. A
higher pressure was not applied since it increases the transport of particles
to the membrane surface and thus the probability of the formation of a filter
cake. The turbidity after microfiltration was 18 NTU. The total solids (TS)
content was decreased from 0.6% to 0.5%.Ultrafiltration
was carried out in a DDS 20 plate-and-frame module (Alfa Laval A/S, Nakskov,
Denmark), equipped with eight double-sided membrane discs with a total membrane
area of 0.28 m2. The ETNA10 membrane
(Alfa Laval A/S) is a composite fluoropolymer membrane with a nominal molecular mass cut-off of 10 000 Da.The membranes were
cleaned before the experiment, at a volume reduction of 0.7 and after the
experiment. The volume reduction (VR) is defined as the ratio between the
volume of permeate withdrawn and the initial feed volume. The membranes were
cleaned with an alkaline cleaning agent, 0.5 wt% Ultrasil 10 (Henkel Chemicals
Ltd, Düsseldorf, UK) at 50°C for about 45 minutes. The system was thoroughly rinsed
with deionized (DI) water after cleaning. The pure water flux (PWF) was
measured at 50°C and 1.0 MPa.During
concentration of the wastewater, the transmembrane
pressure was 1.0 MPa, the temperature was 50°C, and the circulation
velocity was 0.5 m/s. The same operating conditions were previously
successfully used to isolate hemicelluloses from pulp mill process water [8]. The
retentate was recycled to the feed tank, and the permeate was continuously
withdrawn. The permeate flow rate was measured gravimetrically with a balance
(PL 6001-S, Mettler Toledo Inc., Columbus, Ohio, USA). Pressure, temperature, and
permeate flow rate were recorded by a PC equipped with LabView 6.0 software
(National Instruments Co, Austin, Tex, USA). Data were recorded every 5 seconds.


## 2.3. Membrane Screening

The
wastewater was prefiltered in the VSEP unit as described above. Ultrafiltration was performed in an equipment with four cross-flow modules connected in parallel. Each module was equipped with a circular flat membrane with an
area of 0.002 m2. Different membranes were used in each module. Fouling of six membranes was studied. The characteristics of the
membranes are given in Table 1. Both retentate and permeate were recirculated
to the feed tank during ultrafiltration. New
membranes were used in all experiments. The transmembrane pressure was 1.0 MPa.
The membranes were cleaned before the
experiment. Pressure, temperature, and permeate flow rate were recorded every 60
seconds. The temperature was 50°C and the
permeate flow of the UP005, UC005, and PLCC membranes was measured
gravimetrically with a balance (PL 6001-S, Mettler Toledo Inc.), and the
permeate flow of the ETNA01, ETNA10, and NP030 membranes was measured with a
PhaseSep flow meter (Phase Separations Ltd, Queensferry, UK).Table 1
Properties of the ultrafiltration membranes used in the
investigation. The pure water flux (PWF) was measured at 1.0 MPa and 50°C.
ManufacturerMembraneMaterialMax. operating
temperature (°C)Cut-off
(Da)PWF (L/m2h)Alfa
LavalETNA01Composite
fluoropolymer601000290Alfa
LavalETNA10Composite
fluoropolymer6010 000890Microdyn-NadirNP030Polyether
sulphone951000110Microdyn-NadirUP005Polyether
sulphone955000590Microdyn-NadirUC005Regenerated
cellulose555000180MilliporePLCCRegenerated
cellulose505000200Octanoic
acid was used to study the hydrophobicity of the ETNA10, UP005, and UC005
membranes. Octanoic acid is a saturated fatty acid with similar properties as
some of the extractives in wood. A solution with 0.5 g/L, of octanoic acid was
used as feed at a temperature of 20°C and a
pressure of 0.2 MPa. After stabilizing the flux for 20 minutes, the flux was
measured and compared to the pure water flux at the same conditions. The
hydrophobicity of the membranes was reflected by the magnitude of the flux
decrease.


## 2.4. Adsorption by Activated Carbon

The wastewater was prefiltered in the VSEP unit as described above. Ultrafiltration was performed in the same way and with the
same equipment as used in the membrane screening but with the same type of membrane
in all four modules. The pressure was 0.7 MPa in order to decrease the flux and
thus reduce the transport of foulants to the membrane surface. The
membranes were cleaned before and after the experiment. Adsorption with
activated carbon was performed in a stirred tank
for 30 minutes at room temperature. Activated carbon with a size of 3–5 mm
(KEBO AB, Stockholm, Sweden) was added at a ratio of 1 : 24 (g/g wastewater). The
same ratio has been used for the removal of phenolic compounds from rice straw
hemicellulosic hydrolysate [15]. The activated carbon was removed by filtration
through a filter paper (Quality 3, Munktell Filter AB, Falun, Sweden) before
ultrafiltration. The retentate was recycled to the
feed tank, and the permeate was continuously withdrawn during
ultrafiltration.


## 2.5. Adjustment of pH

The
wastewater was prefiltered in the VSEP unit as described above. Ultrafiltration was performed in a cross-flow module with a circular
flat membrane with an area of 0.002 m2. The permeate flow was measured with a PhaseSep flow
meter (Phase Separations Ltd). The retentate and the
permeate were recycled to the feed tankThe transmembrane
pressure corresponding to a pure water flux of 100 l/m2h at 50°C was measured at the pH to be investigated before each
experiment. The pH of the pure water (deionized water) and the wastewater
was adjusted with sodium hydroxide (50 wt%, Merck, Darmstadt, Germany) and
hydrochloric acid (32 wt%, Merck) in an external tank at room temperature
before ultrafiltration. The transmembrane pressure
corresponding to a pure water flux of 100 l/m2h at the investigated
pH and a temperature of 50°C was then used during ultrafiltration of the pH
adjusted waste water. Thus, all membranes were subjected to similar conditions
during the experiments. New membranes were used in each experiment. They were
cleaned before the experiment.


## 2.6. Analysis

The content of
total solids was determined according to the standardized method of the
National Renewable Energy Laboratory (NREL, U.S. Department of Energy, USA)
[16], and the turbidity was measured with a turbidimeter (HACH 2100P, Hach
Lange, Düsseldorf, Germany).The
concentration of hemicelluloses was analyzed by acid hydrolysis according to the standardized method of the NREL [17]. Monomeric sugars were
analyzed before and after acid hydrolysis, and the hemicellulose content was
calculated from the difference in monosaccharide concentration before and after
hydrolysis. Anhydro corrections of 0.9 and 0.88 were used for hexoses and
pentoses, respectively. High-performance anion-exchange chromatography coupled
with pulsed amperometric detection (HPAEC-PAD) using an ED40 electrochemical
detector (Dionex, Sunnyvale, Calif, USA), was used to analyze the monomeric
sugars. The chromatograph was equipped with a gradient pump (GP40, Dionex), an
autosampler (AS50, Dionex) with a Carbo Pac PA1 guard, and a PA10 analytical
column (Dionex). Millipore water with 2 mM NaOH was used as eluent at a flow
rate of 1 mL/min, and the injection volume was 10 μL. D-mannose, D-glucose, D-galactose, D-xylose,
and L-arabinose (Fluka Chemie AG, Buchs, Switzerland) were used as standards.The content of
lignin residues was measured with Shimadzu UV-160 spectrophotometer (Kyoto,
Japan) at a wavelength of 280 nm. Before measurement, the samples were diluted
with deionized water. The concentration of inorganic cations was determined by
inductively coupled plasma atomic emission spectroscopy (ICP AES) with a Perkin
Elmer (Waltham, Mass, USA) Optima 3000DV ICP AES instrument.


## 3. Results

### 3.1. Concentration of Hemicelluloses

The concentration
of hemicelluloses in the prefiltered wastewater was 1.4 g/L. Mannose, galactose,
and glucose residues constituted about 80% of the hemicellulose content. The
ratio between these sugar residues in the wastewater equals that in the hemicellulose
galactoglucomannan [18]. The remaining 20% of the hemicellulose content
consisted of arabinose and xylose residues.The wastewater was
concentrated using the ETNA10 membrane. This membrane had been found to be
successful when treating a process stream from a thermomechanical pulp mill in
a previous study [8]. However, the flux when concentrating the Masonite
wastewater declined rapidly, as shown in Figure 1. The membrane was therefore
cleaned at a VR of 0.7. The flux was partially recovered after cleaning, as can
be seen in the figure.Figure 1
Flux during ultrafiltration of prefiltered
Masonite wastewater. There was no sampling of flux overnight between VR 0.21
and 0.36.Concentration was
interrupted at VR 0.99 (corresponding to a volume reduction factor VRF of
100). The content of total solids was 0.5% initially, 1.1% at VR 0.7, and 12.4%
in the final retentate. The increase in the concentration of hemicelluloses in
the retentate is shown in Figure2. The concentration increases steeply at VR
0.95, as can be seen in the figure. The concentration of hemicelluloses at VR
0.7 was 4.5 g/L, and the final concentration was 65 g/L.Figure 2
Concentration of hemicelluloses in the
retentate during the experiment illustrated in Figure1.The pure water flux was 260 L/m2h before the experiment,
235 L/m2h after cleaning at VR 0.7, and 25 L/m2h after
cleaning at the end of the experiment. It was not possible to restore the pure
water flux by repeated cleaning with the alkaline cleaning agent Ultrasil 10.


### 3.2. Membrane Screening

A screening study
was carried out to find a membrane less sensitive to fouling than the ETNA10
membrane. Hydrophilic
membranes are less susceptible to fouling than membranes with a more
hydrophobic character in most applications [12, 13, 19, 20], but the chemical
stability of hydrophobic membranes is generally superior [20]. In this
investigation, hydrophilic membranes made of regenerated cellulose (UC005 and
PLCC), hydrophobic membranes of polyether sulphone (NP030 and UP005), and
membranes made of a hydrophobic material modified to be
hydrophilic (ETNA 01 and ETNA10) were included. The influence of the pore size
was studied using membranes with cut-offs between 1000 and 10 000 Da.The
TS content of the prefiltered wastewater used in the experiments was 0.4%, and the concentration of hemicelluloses was 1.0 g/L. A significant
difference in flux decline between the hydrophilic and the hydrophobic
membranes was observed. The flux decline of the hydrophilic membranes was minor
(see Figure3(a)), whereas the flux of all other membranes was less than 5% of the
original pure water flux after 24 hours (see Figure 3(b)). The normalized flux used
in the figure is the ratio between the wastewater flux and the pure water flux
before the experiment. The cut-off had no significant influence on the flux
decline.Flux decline during
ultrafiltration of wastewater. Flux of (a) hydrophilic and (b) hydrophobic and
modified membranes. The sampling of flux from the UC005 membrane was
interrupted after 15 hours because of instrument failure. Note the difference
in the scale of they-axes.
(a)(b)The
hydrophobictiy of the ETNA10, UP005, and UC005 membranes were analyzed by using
a solution containing 0.5 g/L octanoic acid and measuring the magnitude of the
flux decrease caused by the fatty acid. No flux decrease was observed for the
hydrophilic UC005 membrane. A minor flux decline was noticed for the ETNA10
membrane (flux being 90% of the pure water flux). The hydrophobic UP005
membrane was severely fouled. The flux was less than 70% of the pure water flux.
These results agree with the results from the screening study shown in Figure3,
which implies that the flux decline of the hydrophobic membranes was due to fouling
by organic molecules.


### 3.3. Adsorption by Activated Carbon

Adsorption by activated carbon was employed prior to ultrafiltration
in order to reduce fouling of the ETNA10 and the UC005 membrane. The TS content decreased from 0.5% to 0.4%, and the
concentration of hemicelluloses decreased from 1.6 g/L to 1.4 g/L after
adsorption with activated carbon. Pretreatment with activated carbon
increased the flux of both the UC005 membrane (see Figure4(a)) and the ETNA10
membrane (see Figure 4(b)). Both membranes had an average hemicellulose retention
of 85%, which shows that both can be used to concentrate the hemicelluloses.Flux during ultrafiltration of wastewater
with and without pretreatment with activated carbon. Flux of (a) the UC005
membrane, and  (b) the ETNA10 membrane. The curve
“without carbon” in (b) originates from the “concentration of hemicelluloses”
study. The hemicellulose concentration in the beginning and in the end of the
experiments is shown in the figure.
(a)(b)The positive
effect of the pretreament with activated carbon was most marked for the ETNA10
membrane. Fouling was not only reduced, the flux of the ETNA10 membrane was
even higher than the flux of the UC005 membrane. When using activated carbon,
the pure water flux was fully recovered after the experiment with the ETNA10 membrane
and to 90%, for the UC005 membrane.


### 3.4. Influence of pH

To
change the properties of the foulants, the initial pH of the wastewater (4.5)
was adjusted to pH 3 and pH 10 by adding hydrochloric acid and sodium
hydroxide. The TS content of the prefiltered wastewater used in the experiments
was 0.3%, and the concentration of hemicelluloses was
1.0 g/L. The influence of pH on the flux of the hydrophilic UC005 membrane was
significant, as can be seen in Figure5(a). The flux of the UC005 membrane was
stable for a longer time at pH 3 than at pH 4.5. At pH 10, the flux decreased
rapidly. On the other hand, the flux of the ETNA10 membrane at pH 3 was
considerably lower; whereas the flux at pH 4.5 and 10 was quite similar, as
shown in Figure 5(b).Influence of pH on flux during
ultrafiltration of wastewater with, (a) the UC005 membrane, and (b) the ETNA10
membrane.
(a)(b)The pressure
needed to reach a pure water flux of 100 L/m2h varied with pH for
the ETNA10 membrane, but not for the UC005 membrane. A pressure of about 0.28 MPa was needed for the UC005 membrane at both pH 3 and pH 10. The pressure
needed to be increased from 0.02 MPa at pH 10 to 0.07 MPa at pH 3 for the
ETNA10 membrane. These results suggest that the surface charge of the UC005
membrane is independent of pH in the investigated pH interval and that the
charge increases with increased pH for the ETNA membrane. That might explain the
rapid decline of the flux at pH 3 in Figure 5(b) for the ETNA membrane since a
decreased surface charge will increase the hydrophobicity and thus the fouling.


## 3.1. Concentration of Hemicelluloses

The concentration
of hemicelluloses in the prefiltered wastewater was 1.4 g/L. Mannose, galactose,
and glucose residues constituted about 80% of the hemicellulose content. The
ratio between these sugar residues in the wastewater equals that in the hemicellulose
galactoglucomannan [18]. The remaining 20% of the hemicellulose content
consisted of arabinose and xylose residues.The wastewater was
concentrated using the ETNA10 membrane. This membrane had been found to be
successful when treating a process stream from a thermomechanical pulp mill in
a previous study [8]. However, the flux when concentrating the Masonite
wastewater declined rapidly, as shown in Figure 1. The membrane was therefore
cleaned at a VR of 0.7. The flux was partially recovered after cleaning, as can
be seen in the figure.Figure 1
Flux during ultrafiltration of prefiltered
Masonite wastewater. There was no sampling of flux overnight between VR 0.21
and 0.36.Concentration was
interrupted at VR 0.99 (corresponding to a volume reduction factor VRF of
100). The content of total solids was 0.5% initially, 1.1% at VR 0.7, and 12.4%
in the final retentate. The increase in the concentration of hemicelluloses in
the retentate is shown in Figure2. The concentration increases steeply at VR
0.95, as can be seen in the figure. The concentration of hemicelluloses at VR
0.7 was 4.5 g/L, and the final concentration was 65 g/L.Figure 2
Concentration of hemicelluloses in the
retentate during the experiment illustrated in Figure1.The pure water flux was 260 L/m2h before the experiment,
235 L/m2h after cleaning at VR 0.7, and 25 L/m2h after
cleaning at the end of the experiment. It was not possible to restore the pure
water flux by repeated cleaning with the alkaline cleaning agent Ultrasil 10.


## 3.2. Membrane Screening

A screening study
was carried out to find a membrane less sensitive to fouling than the ETNA10
membrane. Hydrophilic
membranes are less susceptible to fouling than membranes with a more
hydrophobic character in most applications [12, 13, 19, 20], but the chemical
stability of hydrophobic membranes is generally superior [20]. In this
investigation, hydrophilic membranes made of regenerated cellulose (UC005 and
PLCC), hydrophobic membranes of polyether sulphone (NP030 and UP005), and
membranes made of a hydrophobic material modified to be
hydrophilic (ETNA 01 and ETNA10) were included. The influence of the pore size
was studied using membranes with cut-offs between 1000 and 10 000 Da.The
TS content of the prefiltered wastewater used in the experiments was 0.4%, and the concentration of hemicelluloses was 1.0 g/L. A significant
difference in flux decline between the hydrophilic and the hydrophobic
membranes was observed. The flux decline of the hydrophilic membranes was minor
(see Figure3(a)), whereas the flux of all other membranes was less than 5% of the
original pure water flux after 24 hours (see Figure 3(b)). The normalized flux used
in the figure is the ratio between the wastewater flux and the pure water flux
before the experiment. The cut-off had no significant influence on the flux
decline.Flux decline during
ultrafiltration of wastewater. Flux of (a) hydrophilic and (b) hydrophobic and
modified membranes. The sampling of flux from the UC005 membrane was
interrupted after 15 hours because of instrument failure. Note the difference
in the scale of they-axes.
(a)(b)The
hydrophobictiy of the ETNA10, UP005, and UC005 membranes were analyzed by using
a solution containing 0.5 g/L octanoic acid and measuring the magnitude of the
flux decrease caused by the fatty acid. No flux decrease was observed for the
hydrophilic UC005 membrane. A minor flux decline was noticed for the ETNA10
membrane (flux being 90% of the pure water flux). The hydrophobic UP005
membrane was severely fouled. The flux was less than 70% of the pure water flux.
These results agree with the results from the screening study shown in Figure3,
which implies that the flux decline of the hydrophobic membranes was due to fouling
by organic molecules.


## 3.3. Adsorption by Activated Carbon

Adsorption by activated carbon was employed prior to ultrafiltration
in order to reduce fouling of the ETNA10 and the UC005 membrane. The TS content decreased from 0.5% to 0.4%, and the
concentration of hemicelluloses decreased from 1.6 g/L to 1.4 g/L after
adsorption with activated carbon. Pretreatment with activated carbon
increased the flux of both the UC005 membrane (see Figure4(a)) and the ETNA10
membrane (see Figure 4(b)). Both membranes had an average hemicellulose retention
of 85%, which shows that both can be used to concentrate the hemicelluloses.Flux during ultrafiltration of wastewater
with and without pretreatment with activated carbon. Flux of (a) the UC005
membrane, and  (b) the ETNA10 membrane. The curve
“without carbon” in (b) originates from the “concentration of hemicelluloses”
study. The hemicellulose concentration in the beginning and in the end of the
experiments is shown in the figure.
(a)(b)The positive
effect of the pretreament with activated carbon was most marked for the ETNA10
membrane. Fouling was not only reduced, the flux of the ETNA10 membrane was
even higher than the flux of the UC005 membrane. When using activated carbon,
the pure water flux was fully recovered after the experiment with the ETNA10 membrane
and to 90%, for the UC005 membrane.


## 3.4. Influence of pH

To
change the properties of the foulants, the initial pH of the wastewater (4.5)
was adjusted to pH 3 and pH 10 by adding hydrochloric acid and sodium
hydroxide. The TS content of the prefiltered wastewater used in the experiments
was 0.3%, and the concentration of hemicelluloses was
1.0 g/L. The influence of pH on the flux of the hydrophilic UC005 membrane was
significant, as can be seen in Figure5(a). The flux of the UC005 membrane was
stable for a longer time at pH 3 than at pH 4.5. At pH 10, the flux decreased
rapidly. On the other hand, the flux of the ETNA10 membrane at pH 3 was
considerably lower; whereas the flux at pH 4.5 and 10 was quite similar, as
shown in Figure 5(b).Influence of pH on flux during
ultrafiltration of wastewater with, (a) the UC005 membrane, and (b) the ETNA10
membrane.
(a)(b)The pressure
needed to reach a pure water flux of 100 L/m2h varied with pH for
the ETNA10 membrane, but not for the UC005 membrane. A pressure of about 0.28 MPa was needed for the UC005 membrane at both pH 3 and pH 10. The pressure
needed to be increased from 0.02 MPa at pH 10 to 0.07 MPa at pH 3 for the
ETNA10 membrane. These results suggest that the surface charge of the UC005
membrane is independent of pH in the investigated pH interval and that the
charge increases with increased pH for the ETNA membrane. That might explain the
rapid decline of the flux at pH 3 in Figure 5(b) for the ETNA membrane since a
decreased surface charge will increase the hydrophobicity and thus the fouling.


## 4. Discussion

Fouling of the
UC005 membrane was much less than fouling of the ETNA10 membrane, as shown in
Figure3. In addition, the flux of the UC005 membrane could be increased by
lowering the pH from 4.5 to pH 3 (see Figure 5(a)), and by pretreatment of the
wastewater with activated carbon (see Figure 4(a)). The increase in flux at pH 3
might be due to the increased solubility of inorganic compounds at this pH
[20]. Activated carbon is known to adsorb both organic molecules, such as
phenolic compounds [15], and various inorganic ions, such as aluminium [21].
Hence, the removal of inorganic ions from the wastewater could explain both
results.The influence of
pretreatment on the flux of the ETNA10 membrane was somewhat different. The
flux increased in a similar way after pretreatment with activated carbon (see
Figure4(b)). However, the flux decreased more rapidly at pH 3 than at pH 4.5 and
10 (see Figure 5(b)). To investigate if the ETNA membrane was fouled by organic molecules, new
samples of ETNA10 and UC005 membranes were cleaned with 0.5 wt% Ultrasil 10 and
placed in a beaker with waste water for 20 hours. The pressure needed to reach
a pure water flux of 100 L/m2h after rinsing with deionized water was
measured before and after the experiment. The pressure had to be increased by 18%
for the ETNA membrane to reach the same flux after the experiment, while no
significant difference could be seen for the UC005 membrane. This suggests that
adsorption by organic foulants is significant for the ETNA membrane but not for
UC005 membrane.The fouling was successfully inhibited by using
activated carbon, but it would be very costly to use this method in larger
scale due to the high price of activated carbon (2000€/metric ton). With
the investigated method, it corresponds to an additional cost of 80 €/m3 waste water containing approximately 1 kg of hemicelluloses. This can be
compared with the total cost of a similar isolation process developed for
isolating hemicelluloses from pulp mill process streams which was 0.7 €/kg of
hemicelluloses [8]. The dosage of activated carbon must be decreased
significantly if used in this application.To decrease fouling by lowering pH is a less
costly method than using activated carbon. The buffer capacity of the waste
water is very low; hence, only a small dosage of acid (34% HCl) is needed to
lower the pH from 4.5 to pH 3 (0.5 g/kg waste water). The price of hydrochloric
acid is around 100–150€/metric ton which
corresponds to a treatment cost of less than 0.1 €/m3 waste water, which is several times less expansive than using activated carbon.
However, this method was only successful for the UC005 membrane.


## 5. Conclusions

The results of
this study could be of great importance in the treatment of process streams
from forest industries by ultrafiltration. Several important results that
support this belief have been achieved in this work, for example,(i)
it is possible to reach 99% volume
reduction during ultrafiltration,(ii)
the hemicelluloses can be
concentrated to at least 60 g/L,(iii)
fouling can be avoided by using a
suitable combination of pretreatment methods and ultrafiltration membrane.In this
application, both the ETNA10 and the UC005 membranes could be used. The ETNA10
can withstand harsher operating conditions, which is beneficial since the
temperature of the wastewater is 60°C and must be cooled to 55°C if the more
foulant-resistant UC005 membrane is to be used in a future application.
However, if the ETNA10 membrane is to be used, a cost-efficient
fouling-reduction pretreatment step will be necessary.



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*Source: 624012-2009-04-05.xml*