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Citation: Al Namazi, M.; Li, S.;
Ghaffour, N.; Leiknes, T.; Amy, G. A
Fouling Comparison Study of Algal,
Bacterial and Humic Organic Matters
in Seawater Desalination
Pretreatment Using Ceramic UF
Membranes. Membranes 2023,13, 234.
https://doi.org/10.3390/
membranes13020234
Academic Editor: Zhaoxiang Zhong
Received: 13 December 2022
Revised: 23 January 2023
Accepted: 27 January 2023
Published: 15 February 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
membranes
Article
A Fouling Comparison Study of Algal, Bacterial and Humic
Organic Matters in Seawater Desalination Pretreatment Using
Ceramic UF Membranes
Mohammed Al Namazi 1, 2, *, Sheng Li 2, Noreddine Ghaffour 1, TorOve Leiknes 1and Gary Amy 1
1Water Desalination and Reuse Center (WDRC), Biological and Environmental Science and Engineering
Division (BESE), King Abdullah University of Science and Technology (KAUST),
Thuwal 23955-6900, Saudi Arabia
2Desalination Technology Research Institute (DTRI), Saline Water Conversion Corporation (SWCC),
Al Jubail 31951, Saudi Arabia
*Correspondence: mnamazi@swcc.gov.sa
Abstract:
This study investigates three types of organic matter, namely algal organic matter (AOM),
bacterial organic matter (BOM), and humic organic matter (HOM). These organics are different in
properties and chemical composition. AOM, BOM and HOM were compared in terms of organic
content, fouling behavior, and removal efficiency in ceramic UF filtration. UF experiments were
conducted at a constant flux mode using 5 kDa and 50 kDa ceramic membranes. Results showed
that 5 kDa membrane removed more transparent exopolymer particles (TEP)/organics than 50 kDa
membranes, but less fouling formation for all the three types of organic matters tested. Membranes
exhibited the lowest trans-membrane pressure (TMP) during the filtration of HOM, most probably
due to the high porosity of the HOM cake layer, contributed by big HOM aggregates under Ca
bridging effect. AOM shows the highest MFI-UF (modified fouling index-ultrafiltration) and TMP
(transmembrane pressure) values among the three organics and during all filtration cycles for both
membranes. The AOM fouling layer is well known for having high fouling potential due to its
compressibility and compactness which increase the TMP and eventually the MFI values. AOM
and BOM organics exhibited a similar fouling behavior and mechanism. Furthermore, the divalent
cations such as calcium showed a significant impact on membrane fouling. That is probably because
calcium ions made the membranes and organic matter less negatively charged and easier to deposit
on membranes, thus, enhancing the membrane fouling significantly.
Keywords: fouling; ceramic UF membrane; seawater desalination; pretreatment
1. Introduction
A study in the intake bay of Al-Jubail area at the site of the SWRO plant intake location
revealed excessive growth of filamentous algae, as revealed by biofouling monitoring
coupons [
1
]. The intake point is at the dead end of the bay, where the water turnover is low,
and the water column is somewhat stagnant. Although the algae are disinfected by chlorine
dosage, they reach the plant structure and create severe filtration problems [
1
]. The conse-
quence of this is severe reduction in product water output and frequent membrane cleaning
due to fouling. Nevertheless, many studies nowadays have revealed that membrane and
permeate flux are affected by fouling in general and bio-fouling in particular. In RO system,
a positive relationship has been found between Transparent Exo-polymer Particles (TEP)
levels in feed water and biofouling rate on the membrane [
2
–
5
]. Furthermore, autopsy
analysis of fouled RO membranes showed clearly the existence of alcian blue substances
representing TEP on the membrane surfaces. Likewise, other studies (Villacorte et al. 2010
and Bar-Zeev et al. 2012) outlined the occurrence of biofouling associated with TEP in
seawater membrane filtration [
6
,
7
]. Moreover, the fouling mechanism is strongly linked to
Membranes 2023,13, 234. https://doi.org/10.3390/membranes13020234 https://www.mdpi.com/journal/membranes
Membranes 2023,13, 234 2 of 12
TEP source. TEP particles can be derived either from Algae (AOM) or Bacteria (BOM). In
both cases, AOM and BOM can be purified or separated from their source. However, TEP
cannot purify or separate from AOM and BOM. Therefore, this point is highly considered
when a time comes to fouling study. HOM is totally different in terms of molecular structure
than AOM and BOM.
For this reason, HOM has been added to study the differences between them within
the following hypothesis. Many studies have been carried out to explain the role of TEP in
membrane biofouling.
Once the role of TEP in membrane fouling has been asserted, many studies have been
carried out to remove TEP from water treatment systems. About two decades ago, UF was
discovered to be one of the promising technologies in water industry. Ten years later, UF
has proved by itself a wide spectrum of contaminants removal such as viruses, Giardia,
and bacteria [
8
]. Earlier studies showed that particulate TEP (
≥
0.4
µ
m) can be easily
removed by UF with in-line coagulation pretreatment. Kennedy et al. (2009) concluded
that 100% removal of TEP (
≥
0.4
µ
m) was achieved after UF [
9
]. However, this study was
only concerned about particulate TEP removal, indicating that colloidal TEP has not been
taken into account. Another supportive study by Villacorte et al. (2009) outlined that
particulate TEP was readily removed from integrated membrane system (IMS) by typical
pretreatments such as low-pressure membranes [
10
]. They also stated that colloidal TEP,
less than 0.1
µ
m, could not be completely removed from source water by microfiltration
(MF) and UF. Furthermore, TEP levels were also reduced by only 30% after a combination of
coagulation, sand filtration, and MF [
11
]. In summary, most previous studies of particulate
TEP removal were clearly successful whereas removal of colloidal TEP from various areas
of integrated membrane systems remains a challenge. For this reason, it is very likely that
these TEP precursors (
≤
0.4
µ
) can reach RO membranes and foul them. These days, ceramic
membranes have been gained a high reputation among water production society due to
their mechanical, chemical, and thermal stability [
12
]. These inorganic membranes have
also long working life, high flux, and fouling potential compared to organic membranes
such as polymeric membranes [
13
]. Seawater temperature in the Arabian Gulf ranges
from 16
◦
C in winter and 33
◦
C in summer [
14
]. Hence, ceramic membranes make it
possible because of their resistance to high temperature values and eventually, UF ceramic
membranes represent a successful combination for organic fouling pretreatment.
Although they are quite similar in organic fracture, AOM and BOM could behave
differently in terms of characterization and fouling mechanism compared to HOM. HOM
might exhibit a different fouling behavior and mechanism compared to AOM and BOM
due to its small molecular weight cut off or (MWCO) and its ability to interact with divalent
cations such as calcium. Smaller LMWCO UF ceramic membranes remove more efficiently
TEP/organics than larger ones, will have lower flux but more stable than membranes with
larger pore sizes. However, it was not clear yet what size of ceramic membrane is more
efficient on removing different organics. Moreover, the fouling mechanism of AOM, BOM,
and HOM on ceramic UF membranes is not well understood, and the potential impact
of divalent cations on the fouling formation of different organic matter is also not yet
systematically investigated.
Therefore, this study was conducted to illustrate the potential effectiveness of ceramic
UF membranes on removing different organics in seawater, and its possible influencing
factor on fouling formation.
2. Materials and Methods
2.1. AOM, BOM, and HOM Extraction
To control algal growth, water samples (100 mL) were collected from various cul-
tures every three days. In addition to TOC, DOC, pH, chlorophyll–a, and total cells were
monitored using flow cytometry. The AOM extracted at the stationary-decline phase
(10–18 days)
using the protocol previously reported in literature [
15
]. The algal cultivation
was conducted in an environmental chamber. This chamber is fully equipped with incuba-
Membranes 2023,13, 234 3 of 12
tion as well as aeration system to maintain the best environmental conditions for biological
culturing. The method developed by Myklestad (1995) was applied to cultivate and extract
AOM in the lab using a new strain (Cheatocers affines, CA) imported from Culture Collection
of Algae and Protozoa (CCAP) company, Oban, Scotland [
16
]. The strain was preserved in
the environmental chamber at a constant growth temperature (20
◦
C) and a light intensity
of approximately (50
µ
mol/m
2·
s) To mimic real day and night intervals, an artificial light
control set for 12 h-on/12 h-off was used. The culturing protocol was started with a
2 mL
inoculum of new strain in a 50 mL sterile culture tube enriched with marine nutrients
based on Guillard F/2 medium, and then incubated in the environmental chamber. The
Guillard’s Marine Water Enrichment Solution, from Sigma Aldrich, is enriched in major
nutrients required for diatoms cultivations as described by Guillard [
17
]. A week later,
another inoculation in a 1000 flask was carried out by spiking 5–7 mL of 50 mL culture
tube into 200 mL of autoclaved and enriched raw seawater (RSW). The 200 mL was then
transferred into 10 L autoclaved glass bottles containing 5 L of culture medium after a week
of incubation. Finally, the 5 L solution was kept in the environmental chamber at the set
conditions mentioned above for 20 days. Extraction of BOM was conducted after 10 days
of culturing and extracted according to the protocol reported by Li et al. [
15
]. A commercial
seawater humic substance (SWAN RIVER) was used as HOM model. The HOM is a popular
model compounds used in membrane fouling mechanism study, which is isolated from the
Suwannee river, and its characteristics has been previously reported [18].
2.2. Filtration Experiment Protocol
A bench scale dead-end UF ceramic membrane setup used in this study. The membrane
holder is provided by TAMI Company and has a diameter of 45 mm (Section 2.3). The
UF ceramic membranes were also supplied by the same company and they have the same
diameter for membrane specifications. The operational conditions set for experiments
were 18 filtration cycles for 30 min each with synthetic seawater (Section 2.4) and 1-min
backwashing using Milli-Q water (Section 2.5). The setup is mainly consisting of 2 gear
pumps, for feed and backwash respectively. The pumps connected to feed water tank
(2 L)
and backwash tank (2 L) containing MQ water. A digital balance from Mettler Toledo
Company was used to measure the flux. The setup is automated using a data logger that
directly connected to LabVIEW computer software. TMP as a function of flow rate versus
time was monitored every 30 s through the computer program. The flux was constant at
(241 L/m
2
/h) with constant flow rate at 7 mL/m. At the beginning of filtration run, the
initial permeability was tested using MQ water to ensure the standard permeability by
membrane manufacture. Furthermore, due to the SEM and TEP visualization destructive
analysis, each filtration cycle has used new ceramic membrane.
After experiments, the corresponding water samples (feed, permeate) and used mem-
branes were analyzed using the analysis techniques described in Section 2.6.
2.3. Ceramic UF Membranes
The ceramic UF membrane utilized in this study was obtained from TAMI company.
The characteristics of the ceramic membranes are shown in Table 1, and clean water
permeances of 50 kDa and 5 kDa membranes are 92.6 and 76.9 L/(m2·h·bar), respectively.
Table 1. Water quality of synthetic seawater used in this study.
Manufacturer TAMI
Pore size, or MWCO 50 kDa, 5 kDa
Materials Support layer: TiO2
Active layer ZrO2+ TiO2
Surface Area (cm2)17.4
Membranes 2023,13, 234 4 of 12
2.4. Feed Water
The feed water was prepared using synthetic seawater quality as presented in (Table 2).
The feed water solution was diluted with AOM, BOM, and HOM with total DOC 0.7 mg/L.
Table 2. Water quality of synthetic seawater used in this study.
Parameters Concentration mg/L
Chloride (Cl) 19,290 mg/L
Sodium 10,780 mg/L
Boron 5.6 mg/L
Sulfate 2660 mg/L
Potassium 420 mg/L
Calcium 400 mg/L
Magnesium (Mg)
DOC mg/L
1320 mg/L
0.7
2.5. Backwash Water
The previous studies revealed Milli-Q water was strongly able to remove NOM
fouling from UF membranes [
19
–
23
]. While the previous studies were mostly carried out
on polymeric membranes and different NOM characteristics, this study used MQ water for
determine the role of divalent cations such as calcium on fouling mechanism on ceramic
membrane as MQ water is free from any cations.
2.6. Membrane and Water Sample Analysis
2.6.1. Scanning Electron Microscope (SEM)
To help studying the fouling mechanism on ceramic membranes, SEM equipped with
cryo-stage and cryo preparation chamber has been used for membrane morphology. For
cross sectional image, Focused Ion Beam (FIB) was used in combination with SEM. Energy
Dispersive X-ray or EDAX was used for elemental analysis. This analysis was carried out
at KAUST Core Lab.
2.6.2. AOM/BOM/HOM/TEP Visualization
All the three organic samples have been visualized using alcian blue staining. After
each filtration cycle, membrane samples were dried in room temperature and a couple of
alcian blue drops were added on the membrane surface and, 10 min later, these samples
were visualized using a microscope [15].
2.6.3. AOM and BOM Cultivation
In this experiment, the most dominant algae species present in the Gulf seawater such
as Chaetoceros affins (CA) was acquired from the Culture Collection of Algae and Protozoa
(CCAP; Oban, Scotland). CA represents the main producer of micro algae biopolymers [
15
].
All these species belong to the diatom group that represents one of the most common types
of phytoplankton dominated in seawater. The cultivation was taken place at the WDRC
lab. The cultivation period for AOM extraction to be ready was 20 days. Dominant marine
bacteria in the Red Sea water, mainly Pseudidiomarina, atlantica (P. atlantica) was cultured
at WDRC lab to extract BOM [4,15,24].
2.6.4. TEP Analysis
TEP concentrations in AOM and BOM were measured using a method developed
by Li (2015) [
15
]. Particulate TEP (P-TEP) having higher sizes than 0.4
µ
m was measured
by 0.4
µ
m pore size polycarbonate membrane (PC) filter (Whatman Nuclepore) whereas
colloidal TEP (C-TEP) was measured using 0.1 µm PC.
Membranes 2023,13, 234 5 of 12
2.6.5. MFI-UF
This tool is basically developed to measure the membrane fouling potential of the
feed water of membrane filtration system. MFI developed by Schippers and Verdouw
(1980) [
25
], then improved by Boerlage et al. (2004) and Salinas Rodriguez et al., (2012) as
MFI-UF inspector to quantify membrane fouling [26,27].
MFI = (ηI)/(2∆P A2)
where
∆
P is the reference pressure value of 2 bar,
η
is the reference viscosity at 20
◦
C and A
is the reference filtration area of 13.8 ×10−4m2.
MFI-UF is a further developed method, using ultrafiltration membranes at constant
flux. ‘I’ is the slope of the linear region in a plot of
∆
P versus time and is a characteristic of
the cake/gel filtration mechanism.
2.6.6. Particle Size Distribution and Calcium Binding Experiment
The Malvern Nano-sizer equipment has been used for measuring the particle size and
the ability of these three organics for binding the calcium molecules. All feed samples;
AOM, BOM, and HOM were tested before and after the addition of calcium in order to
examine the ability of binding. All samples were tested in duplicate.
2.6.7. LC-OCD Characterization
To characterize the NOM, LC-OCD was developed to identify quantitative information
and qualitative results regarding organic compounds in natural water. LC-OCD model
8 system has UV detector (UVD), online organic carbon detector (OCD), and organic
nitrogen detector (OND). In this study, the characteristics of different constituents of NOM
typically identified are; biopolymer, humics, building blocks, LMW acids, and LMW
neutrals. Seawater samples were collected in 20 mL glass vials and filtered with a Whatman
filter (pore size = 0.45
µ
m). Then, compounds were separated using two-column size
(250 nm ×20 mm, Toyo pearl TSK HW-50S).
3. Results and Discussion
3.1. Characteristics of Organics
LC-OCD results (Figure 1) showed the AOM growth during cultivation days. The
results show the chromatograms of AOM culture medium. The biopolymer growth started
from the exponential phase (day 2 to day 8) to stationary-death phase (day 10–day 18). The
biopolymer peaks are clearly detected between 26–38 min retention times. This retention
time is within the range described by Huber et al., (2011) [
15
,
18
]. The highest biopolymer
peak was observed on day 14 when the algal cells counts reached the maximum. Another
major peak of the chromatogram can be observed between 45–53 min and represents
the low molecular weight (LMW) acids. This LMW acid peak could be attributed to the
F/2 culture medium where the EDTA agent is one of its components [
2
]. Small peaks
for building blocks and LMW neutrals were also detected as shown in (Figure 1). The
chromatograms of culture medium correlated positively with AOM culture. They show
one major peak for LMW acids which appeared in the same retention time range of LMW
acids for AOM. Apart from this, no peaks were observed for culture medium during all
AOM growth phases. Furthermore, minor peaks of biopolymer, building blocks, humics,
and LMW acids and neutrals appeared in the RSW solution. However, the amount of
these fractures is very low when they compared to the final AOM concentration (Figure 1).
Thus, the RSW fractions have little impact or effect on the final AOM composition. From
the perspective of the LC-OCD results, AOM mainly consists of a biopolymer fraction
(i.e., polysaccharides and proteins)
and some minor concentrations of building blocks and
LMW neutrals. A similar biopolymer increase during marine bacteria growth was observed
in a previous study as well [4,15].
Membranes 2023,13, 234 6 of 12
Membranes 2023, 13, x FOR PEER REVIEW 6 of 15
Apart from this, no peaks were observed for culture medium during all AOM growth
phases. Furthermore, minor peaks of biopolymer, building blocks, humics, and LMW ac-
ids and neutrals appeared in the RSW solution. However, the amount of these fractures is
very low when they compared to the final AOM concentration (Figure 1). Thus, the RSW
fractions have little impact or effect on the final AOM composition. From the perspective
of the LC-OCD results, AOM mainly consists of a biopolymer fraction (i.e., polysaccha-
rides and proteins) and some minor concentrations of building blocks and LMW neutrals.
A similar biopolymer increase during marine bacteria growth was observed in a previous
study as well [4,15].
Figure 1. LC-OCD chromatograms for AOM during cultivation (a), algae cultivation medium (b),
and background seawater (c).
3.2. Filtration Performance of Ceramic UF Membranes
Flux profile presented in (Figure 2) reveals the fouling scenario during UF ceramic
filtration experiment as a function of TMP versus time for all organic matters. During the
initial period of UF filtration using 50 kDa membranes (A), the TMP increased smoothly
and gradually, and then stabled after 5–10 min of filtration cycle. Generally, 50 kDa mem-
branes was more exposed to pore blockage followed by cake layer fouling compared to 5
kDa membranes. This is attributed to the large pore sizes of the 50 kDa membrane that
allow the small particles of the three organics particularly HOM to penetrate the pores. In
addition, it is probably due to the deformability TEP derived from AOM and BOM parti-
cles in the feed water, both leading to pore blockage fouling mechanism. The highest TMP
was observed at 5 kDa filtration operation (4.5 bars) whereas the 50 kDa UF ceramic mem-
branes had the highest TMP at 3.5 bars, due to the lower MWCO of the 5 kDa membranes
and their corresponding higher membrane resistance. AOM and BOM showed thinner
cake layer during all filtration cycles using 5 kDa membranes. As mentioned earlier, TEP
particles derived from AOM and BOM have evolved this process by making the cake layer
more compact and compressible. Furthermore, some previous studies support this fact
which in turns enhance the fouling resistance and reduce the porosity of the cake layer
[5,7,28]. On the other hand, HOM cake layer was thicker than those for AOM and BOM.
20 30 40 50 60 70 80 90 100
0
2
4
6
8
10
12
14
16
18
20
22
LMW neutrals
LMW acids
HS BB
BP
(b)
(c)
(a)
OCD
UVD
UVD
DAY 2
DAY 4
DAY 8
DAY 10
DAY 14
OCD
UVD
Signal response
Retention time (min)
Biopolymer
Building blocks
LMW acids and HS
LMW neutrals
Figure 1.
LC-OCD chromatograms for AOM during cultivation (
a
), algae cultivation medium (
b
),
and background seawater (c).
3.2. Filtration Performance of Ceramic UF Membranes
Flux profile presented in (Figure 2) reveals the fouling scenario during UF ceramic
filtration experiment as a function of TMP versus time for all organic matters. During the
initial period of UF filtration using 50 kDa membranes (A), the TMP increased smoothly and
gradually, and then stabled after 5–10 min of filtration cycle. Generally, 50 kDa membranes
was more exposed to pore blockage followed by cake layer fouling compared to 5 kDa
membranes. This is attributed to the large pore sizes of the 50 kDa membrane that allow the
small particles of the three organics particularly HOM to penetrate the pores. In addition, it
is probably due to the deformability TEP derived from AOM and BOM particles in the feed
water, both leading to pore blockage fouling mechanism. The highest TMP was observed
at 5 kDa filtration operation (4.5 bars) whereas the 50 kDa UF ceramic membranes had
the highest TMP at 3.5 bars, due to the lower MWCO of the 5 kDa membranes and their
corresponding higher membrane resistance. AOM and BOM showed thinner cake layer
during all filtration cycles using 5 kDa membranes. As mentioned earlier, TEP particles
derived from AOM and BOM have evolved this process by making the cake layer more
compact and compressible. Furthermore, some previous studies support this fact which
in turns enhance the fouling resistance and reduce the porosity of the cake layer [
5
,
7
,
28
].
On the other hand, HOM cake layer was thicker than those for AOM and BOM. However,
this cake layer was found to be more porous and less compact, and this was more likely
to occur when HOM particles bound with calcium offering big aggregates and leading to
high porosity of the HOM cake layer on the membrane surface, and thus less impact on
TMP and MFI values as shown in (Figures 2and 3).
As shown in (Figure 3), the fouling potential was higher with the 50 kDa membrane
compared to the 5 kDa one. The AOM represented the highest MFI-UF value (3200 s/L
2
)
whereas HOM gave the lowest value (2000 s/L
2
). This observation suggests that the pore
blockage mechanism is more dominant for the 50 kDa UF membranes and cake layer of the
5 kDa membrane is most probably dominating the fouling mechanism. In fact, these results
are consistent with other studies (Li et al., 2011) [
29
]. The possibility of the three organic
particles to be trapped into the larger membrane pores was higher during UF filtration
using 50 kDa membranes, which explains the higher values of MFI-UF. AOM shows the
Membranes 2023,13, 234 7 of 12
highest MFI-UF values among the three organics and during all filtration cycles for both
membranes. The AOM fouling layer is well known for having high fouling potential due to
its compressibility and compactness, which increase the TMP and eventually the MFI-UF
values [28].
Membranes 2023, 13, x FOR PEER REVIEW 7 of 15
However, this cake layer was found to be more porous and less compact, and this was
more likely to occur when HOM particles bound with calcium offering big aggregates and
leading to high porosity of the HOM cake layer on the membrane surface, and thus less
impact on TMP and MFI values as shown in (Figures 2 and 3).
Figure 2. TMP comparison for 50 kDa (A) and 5 kDa (B). Each filtration experiment was duplicated,
and the variation of two experimental data sets are within 10%.
Figure 2.
TMP comparison for 50 kDa (
A
) and 5 kDa (
B
). Each filtration experiment was duplicated,
and the variation of two experimental data sets are within 10%.
Membranes 2023, 13, x FOR PEER REVIEW 8 of 15
Figure 3. MFI-UF for 50 kDa and 5 kDa ceramic membranes. The MFI-UF were calculated twice for
each experiment, and the deviation between two times calculations were within 5%.
As shown in (Figure 3), the fouling potential was higher with the 50 kDa membrane
compared to the 5 kDa one. The AOM represented the highest MFI-UF value (3200 s/L2)
whereas HOM gave the lowest value (2000 s/L2). This observation suggests that the pore
blockage mechanism is more dominant for the 50 kDa UF membranes and cake layer of
the 5 kDa membrane is most probably dominating the fouling mechanism. In fact, these
results are consistent with other studies (Li et al., 2011) [29]. The possibility of the three
organic particles to be trapped into the larger membrane pores was higher during UF fil-
tration using 50 kDa membranes, which explains the higher values of MFI-UF. AOM
shows the highest MFI-UF values among the three organics and during all filtration cycles
for both membranes. The AOM fouling layer is well known for having high fouling po-
tential due to its compressibility and compactness, which increase the TMP and eventu-
ally the MFI-UF values [28].
As shown in Figure 4, AOM and BOM showed the highest TEP concentrations in ce-
ramic membrane filtration experiments for both 5 and 50 kDa membranes, which are in
agreement with literature as AOM and BOM contain more polysaccharides and eventually
more TEP. However, TEP on AOM was higher than BOM because of higher biopolymer
fraction in AOM compared to BOM. Regarding TEP removal, this study clearly revealed
that UF 5 kDa membranes can remove more TEP/organics (66–80%) compared to the 50 kDa
membrane (53–57%) due to their lower MWCO (Figure 4). The results from the ceramic UF
membranes were better than those reported before with existing pretreatment DMF and
previous reported polymeric UF, which were about 10% and 40%, respectively [3,5].
Figure 3.
MFI-UF for 50 kDa and 5 kDa ceramic membranes. The MFI-UF were calculated twice for
each experiment, and the deviation between two times calculations were within 5%.
Membranes 2023,13, 234 8 of 12
As shown in Figure 4, AOM and BOM showed the highest TEP concentrations in
ceramic membrane filtration experiments for both 5 and 50 kDa membranes, which are in
agreement with literature as AOM and BOM contain more polysaccharides and eventually
more TEP. However, TEP on AOM was higher than BOM because of higher biopolymer
fraction in AOM compared to BOM. Regarding TEP removal, this study clearly revealed that
UF 5 kDa membranes can remove more TEP/organics (66–80%) compared to the
50 kDa
membrane (53–57%) due to their lower MWCO (Figure 4). The results from the ceramic UF
membranes were better than those reported before with existing pretreatment DMF and
previous reported polymeric UF, which were about 10% and 40%, respectively [3,5].
Membranes 2023, 13, x FOR PEER REVIEW 9 of 15
Figure 4. TEP removal as a function of absorbance of (a) 50 kDa and (b) 5 kDa for all organic matters.
The particle size distribution results presented in (Figure 5) showed that the smallest
particle sizes were HOM, then BOM, and AOM, respectively. However, HOM shows the
smallest size before addition of calcium and the largest size after binding with calcium
molecules. The HOM particles were in the range of 8–13 nm, suggesting that they can
penetrate easily the 50 kDa membrane pores. The study also shows pore blockage with
AOM and BOM samples that are larger in terms of particles sizes than the 50 kDa mem-
brane pores. The reason behind this is most likely that the AOM and BOM contain mainly
deformable TEP particles that can penetrate the pores structures [30]. In addition, the par-
ticle size distributions presented in (Figure 5) show only the average values (mean size),
therefore the smaller particles that not in the range presented with particle size distribu-
tion from AOM and BOM could pass the 50 kDa UF membranes and cause pore blocking.
For 5 kDa membrane, most of organic particles have been rejected probably because of
the lower MWCO or this membrane. For 50 kDa membranes, AOM and BOM showed the
highest TEP concentrations, which are in agreement with literature as AOM and BOM
contain more polysaccharides and eventually more TEP. However, TEP on AOM was
higher than BOM because of higher biopolymer fraction in AOM compared to BOM. Re-
garding TEP removal, the study clearly revealed that UF 5 kDa membranes can remove
more TEP/organics compared to the 50 kDa membrane due to their lower MWCO.
Figure 4.
TEP removal as a function of absorbance of (
a
) 50 kDa and (
b
) 5 kDa for all organic matters.
The particle size distribution results presented in (Figure 5) showed that the smallest
particle sizes were HOM, then BOM, and AOM, respectively. However, HOM shows the
smallest size before addition of calcium and the largest size after binding with calcium
molecules. The HOM particles were in the range of 8–13 nm, suggesting that they can
penetrate easily the 50 kDa membrane pores. The study also shows pore blockage with
AOM and BOM samples that are larger in terms of particles sizes than the 50 kDa membrane
pores. The reason behind this is most likely that the AOM and BOM contain mainly
deformable TEP particles that can penetrate the pores structures [
30
]. In addition, the
particle size distributions presented in (Figure 5) show only the average values (mean
size), therefore the smaller particles that not in the range presented with particle size
distribution from AOM and BOM could pass the 50 kDa UF membranes and cause pore
blocking. For
5 kDa
membrane, most of organic particles have been rejected probably
because of the lower MWCO or this membrane. For 50 kDa membranes, AOM and BOM
Membranes 2023,13, 234 9 of 12
showed the highest TEP concentrations, which are in agreement with literature as AOM
and BOM contain more polysaccharides and eventually more TEP. However, TEP on AOM
was higher than BOM because of higher biopolymer fraction in AOM compared to BOM.
Regarding TEP removal, the study clearly revealed that UF 5 kDa membranes can remove
more TEP/organics compared to the 50 kDa membrane due to their lower MWCO.
Membranes 2023, 13, x FOR PEER REVIEW 10 of 15
Figure 5. Size distributions of feed water before and after addition of Ca for all types of organic
matters. HOM shows the smallest size before addition of calcium and the largest size after binding
with calcium molecules.
The AOM fouling layer came at last with very high organic content/TEP and less
calcium concentration. It is well known that CA species can produce more organic matter
rather than BOM and HOM and this is most probably the reason behind this phenomenon.
Further, the AOM fouling layer was gel-like layer and sticky due to its content of polysac-
charides. All these fouling layers of AOM, BOM, and HOM were porous. However, the
HOM cake layer was found to be the highest porous layer compared to AOM and BOM
layers because of the role of calcium binding that enhances the size of HOM particles,
allowing them to create larger particles, porous and less compact cake layer as clearly
indicated in TMP and MFI-UF results.
3.3. Impact of Calcium on fouling
Comparative results of calcium concentration deposited on the surface of UF ceramic
membranes shown in EDAX elemental analysis presented in (Figure 6). By comparing the
two membranes used, the calcium concentration was found to be higher on 50 kDa than
5 kDa membrane surface. Furthermore, the fouling layer of HOM on 50 kDa membrane
was found to be the highest compared to AOM and BOM, resulting in a high binding
Figure 5.
Size distributions of feed water before and after addition of Ca for all types of organic
matters. HOM shows the smallest size before addition of calcium and the largest size after binding
with calcium molecules.
The AOM fouling layer came at last with very high organic content/TEP and less
calcium concentration. It is well known that CA species can produce more organic matter
rather than BOM and HOM and this is most probably the reason behind this phenomenon.
Further, the AOM fouling layer was gel-like layer and sticky due to its content of polysac-
charides. All these fouling layers of AOM, BOM, and HOM were porous. However, the
HOM cake layer was found to be the highest porous layer compared to AOM and BOM
layers because of the role of calcium binding that enhances the size of HOM particles,
allowing them to create larger particles, porous and less compact cake layer as clearly
indicated in TMP and MFI-UF results.
3.3. Impact of Calcium on fouling
Comparative results of calcium concentration deposited on the surface of UF ceramic
membranes shown in EDAX elemental analysis presented in (Figure 6). By comparing the
Membranes 2023,13, 234 10 of 12
two membranes used, the calcium concentration was found to be higher on 50 kDa than
5 kDa
membrane surface. Furthermore, the fouling layer of HOM on 50 kDa membrane was
found to be the highest compared to AOM and BOM, resulting in a high binding between
humic substances and calcium molecules. These findings confirm that 5 kDa membranes
were able to remove more organics than 50 kDa membrane. Thus, the concentrations of
the three organics particularly humics were observed low on 5 kDa membrane surface,
resulting in decreased adsorption between HOM and calcium molecules. However, the
adsorption of HOM and the calcium molecules for the 5 kDa membrane was also higher
than AOM and BOM due to high calcium concentration deposited on the membrane surface
(Figure 6).
Membranes 2023, 13, x FOR PEER REVIEW 12 of 15
(a)
(b)
(c)
Figure 6. EDAX elemental analysis for (a) HOM, (b) AOM, and (c) BOM in 5 kDa UF membrane
filtration.
Figure 6.
EDAX elemental analysis for (
a
) HOM, (
b
) AOM, and (
c
) BOM in 5 kDa UF
membrane filtration.
Membranes 2023,13, 234 11 of 12
4. Conclusions
The following conclusions can be drawn from this study.
•
AOM gave the highest MFI-UF and TMP values among the three organics and during
all filtration cycles for both membranes. The AOM fouling layer is well known for hav-
ing high fouling potential due to its compressibility and compactness which increase
the TMP and eventually the MFI-UF values. AOM and BOM organics presented a
similar fouling behavior and mechanism. However, AOM was significantly higher
compared to BOM in terms of TEP concentrations and gel-like formation. This is
probably attributed to the high polysaccharide concentration in AOM.
•UF 5 kDa membranes can remove more TEP/organics compared to the 50 kDa mem-
branes due to their lower MWCO. A cake layer fouling while the 50 kDa membrane
showed a blockage fouling mechanism followed by a cake layer formation. For 5 kDa
membranes, AOM and BOM showed thinner cake layer during all filtration cycles
as TEP particles derived from AOM and BOM have evolved this process by making
the cake layer more compact and compressible which in turns enhance the fouling
resistance and reduce the porosity of the cake layer. HOM cake layer was thicker
than those for AOM and BOM. This cake layer was found to be more porous and less
compact, and this occurred more likely when HOM particles bind with Ca molecules
offering big aggregates and leading to high porosity of the HOM cake layer on the
membrane surface.
•
The divalent cations such as calcium revealed a strong influence on membrane fouling.
In this experiment, the HOM particles were most likely influenced by this phenomenon
which bridge/adsorb more organic molecules when interacting with calcium ions
making the membrane less negatively charged and enhancing the membrane fouling.
However, this fouling was less severe compared to AOM and BOM fouling.
Author Contributions:
Conceptualization, M.A.N. and N.G.; methodology, M.A.N. and S.L.; formal
analysis, M.A.N.; investigation, M.A.N.; data curation, M.A.N.; writing—original draft preparation,
M.A.N. and S.L.; writing—review and editing, M.A.N. and S.L.; supervision, N.G., G.A. and T.L.;
project administration, N.G., G.A. and T.L. All authors have read and agreed to the published version
of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Data is unavailable due to company restriction.
Conflicts of Interest: The authors declare no conflict of interest.
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