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Citation: Akkoyunlu, B.; Daly, S.;
Cerrone, F.; Casey, E. Investigating
Mass Transfer and Reaction
Engineering Characteristics in a
Membrane Biofilm Using Cupriavidus
necator H16. Membranes 2023,13, 908.
https://doi.org/10.3390/
membranes13120908
Academic Editor: Vladislav A.
Sadykov
Received: 23 October 2023
Revised: 23 November 2023
Accepted: 11 December 2023
Published: 14 December 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
Investigating Mass Transfer and Reaction Engineering
Characteristics in a Membrane Biofilm Using
Cupriavidus necator H16
Burcu Akkoyunlu 1,2 , Sorcha Daly 1,2,3, Federico Cerrone 2,4,5 and Eoin Casey 1, 2, *
1School of Chemical and Bioprocess Engineering, University College Dublin, D04 V1W8 Dublin, Ireland;
burcu.akkoyunlu@ucdconnect.ie (B.A.); s.b.daly@greenwich.ac.uk (S.D.)
2BiOrbic Bioeconomy SFI Research Centre, University College Dublin, D04 V1W8 Dublin, Ireland;
federico.cerrone@dcu.ie
3School of Engineering, Faculty of Engineering and Science, University of Greenwich, Medway Campus,
Chatham ME4 4AG, UK
4UCD Earth Institute, School of Biomolecular and Biomedical Sciences, University College Dublin,
D04 V1W8 Dublin, Ireland
5School of Biotechnology, Dublin City University, Glasnevin Campus, D09 N920 Dublin, Ireland
*Correspondence: eoin.casey@ucd.ie
Abstract:
Membrane biofilm reactors are a growing trend in wastewater treatment whereby gas-
transfer membranes provide efficient bubbleless aeration. Recently, there has been a growing interest
in using these bioreactors for industrial biotechnology using microorganisms that can metabolise
gaseous substrates. Since gas fermentation is limited by the low solubilities of gaseous substrates
in liquid media, it is critical to characterise mass transfer rates of gaseous substrates to enable the
design of membrane biofilm reactors. The objective of this study is to measure and analyse mass
transfer rates and reaction engineering characteristics for a single tube membrane biofilm reactor
using Cupriavidus necator H16. At elevated Reynolds numbers, the dominant resistance for gas
diffusion shifts from the liquid boundary layer to the membrane. The biofilm growth rate was
observed to decrease after 260
µ
m at 96 h. After 144 h, some sloughing of the biofilm occurred.
Oxygen uptake rate and substrate utilisation rate for the biofilm developed showed that the biofilm
changes from a single-substrate limited regime to a dual-substrate-limited regime after 72 h which
alters the localisation of the microbial activity within the biofilm. This study shows that this platform
technology has potential applications for industrial biotechnology.
Keywords:
membrane biofilm reactor; Cupriavidus necator; oxygen transfer; mass transfer model;
biofilm
1. Introduction
The membrane biofilm reactor, sometimes called a membrane aerated biofilm reactor,
is an emerging reactor system for wastewater treatment [
1
]. In such systems, biofilms form
on gas-permeable membranes where gaseous substrates such as oxygen diffuse through
the membrane into the biofilm. Conventionally, biofilms grow on static surfaces where
nutrients are delivered from the biofilm-liquid interface (see Figure 1a). In membrane
biofilm reactors, substrates are delivered from both sides, resulting in distinct nutrient
profiles. Depending on the concentration of nutrients and oxygen, the location of the
biocatalytic activity within the biofilms can be controlled [
2
]. This influences the overall
performance of the membrane biofilm reactors. Recently, these bioreactor systems have
been investigated to produce various chemicals such as volatile fatty acids and ethanol
using mixed-culture systems [
3
]. However, there is limited information on using pure
cultures in membrane biofilm reactors.
Membranes 2023,13, 908. https://doi.org/10.3390/membranes13120908 https://www.mdpi.com/journal/membranes
Membranes 2023,13, 908 2 of 13
Membranes 2023, 13, x FOR PEER REVIEW 2 of 13
For industrial biotechnology applications, a defined microbial species is used within
the membrane bioreactor [3]. Although high product purities can be obtained compared
to mixed cultures, using pure culture faces challenges such as contamination and sensi-
tivity to gas composition supplied through the membrane [4]. In the scientific literature,
methane-oxidising bacteria (methanotrophs), nitrate-reducing bacteria and acetogenic
bacteria have been studied extensively in membrane biofilm reactors [3,5,6]. Although
there is another group of microorganisms called chemoautotrophs that can utilise various
gas substrates, there are no reports on their use in these bioreactors.
Chemoautotrophs can oxidise inorganic chemical substances such as hydrogen (H
2
)
as their energy source and use carbon dioxide (CO
2
) as the main source of carbon [7].
Although this microorganism offers an opportunity to valorise waste CO
2
, the overall re-
action faces challenges such as low solubility of gas in liquid media and usage of explosive
gas mixtures. Membranes have the potential to help achieve high gas transfer efficiencies
at low gas supply rates due to the high specific surface area available for transfer. In the
case of chemoautotrophs, oxygen and the carbon source (CO
2
) can be supplied using
membranes which would lead to unique biofilm structures. Possible nutrient profiles for
the traditional biofilms and chemoautotrophs are shown schematically in Figure 1. Fur-
thermore, using dense membranes for gas transfer through diffusion can prevent explo-
sive environments that may occur with bubbling. Thus, membrane biofilm reactors are
suggested as a promising reactor system for gas fermentation using chemoautotrophic
microorganisms.
Figure 1. A schematic of different nutrient profiles for biofilms formed on (a) a solid surface, (b) a
membrane used for aeration, and (c) a membrane used for supplying gaseous substrate.
In this study, biofilm characteristics of a chemoautotrophic bacteria, Cupriavidus ne-
cator H16, was investigated using a single tube membrane biofilm reactor. The mass trans-
fer coefficient for O
2
was measured, and a dimensionless model was developed to predict
the effect of operational conditions on mass transfer characteristics. The effect of biofilm
thickness on substrate diffusion rates and oxygen uptake rate was also investigated using
the same membrane setup. This is the first study to investigate biofilm characteristics of a
chemoautotrophic bacteria on a tubular membrane and it aims to be the basis of future
studies with membranes as gas transfer devices for cultivating Cupriavidus necator H16.
2. Materials and Methods
2.1. Membrane Biofilm Reactor Design
A tubular non-porous polydimethylsiloxane (PDMS) membrane (Fisher Scientific
Ltd., Loughborough, UK) was used in this study to construct the membrane biofilm reac-
tor (see Figure 2). The tubular membrane was poed in a 6 mm nylon tubing using trans-
lucent epoxy resin (MG Chemicals, Santa Clara, CA, USA). Membrane biofilm reactor was
constructed using a glass column with a 10 mm outer diameter and 8 mm inner diameter
Figure 1.
A schematic of different nutrient profiles for biofilms formed on (
a
) a solid surface, (
b
) a
membrane used for aeration, and (c) a membrane used for supplying gaseous substrate.
For industrial biotechnology applications, a defined microbial species is used within
the membrane bioreactor [
3
]. Although high product purities can be obtained compared to
mixed cultures, using pure culture faces challenges such as contamination and sensitivity
to gas composition supplied through the membrane [
4
]. In the scientific literature, methane-
oxidising bacteria (methanotrophs), nitrate-reducing bacteria and acetogenic bacteria have
been studied extensively in membrane biofilm reactors [
3
,
5
,
6
]. Although there is another
group of microorganisms called chemoautotrophs that can utilise various gas substrates,
there are no reports on their use in these bioreactors.
Chemoautotrophs can oxidise inorganic chemical substances such as hydrogen (H
2
)
as their energy source and use carbon dioxide (CO
2
) as the main source of carbon [
7
]. Al-
though this microorganism offers an opportunity to valorise waste CO
2
, the overall reaction
faces challenges such as low solubility of gas in liquid media and usage of explosive gas
mixtures. Membranes have the potential to help achieve high gas transfer efficiencies at low
gas supply rates due to the high specific surface area available for transfer. In the case of
chemoautotrophs, oxygen and the carbon source (CO
2
) can be supplied using membranes
which would lead to unique biofilm structures. Possible nutrient profiles for the tradi-
tional biofilms and chemoautotrophs are shown schematically in Figure 1. Furthermore,
using dense membranes for gas transfer through diffusion can prevent explosive environ-
ments that may occur with bubbling. Thus, membrane biofilm reactors are suggested as a
promising reactor system for gas fermentation using chemoautotrophic microorganisms.
In this study, biofilm characteristics of a chemoautotrophic bacteria, Cupriavidus necator
H16, was investigated using a single tube membrane biofilm reactor. The mass transfer
coefficient for O
2
was measured, and a dimensionless model was developed to predict
the effect of operational conditions on mass transfer characteristics. The effect of biofilm
thickness on substrate diffusion rates and oxygen uptake rate was also investigated using
the same membrane setup. This is the first study to investigate biofilm characteristics of
a chemoautotrophic bacteria on a tubular membrane and it aims to be the basis of future
studies with membranes as gas transfer devices for cultivating Cupriavidus necator H16.
2. Materials and Methods
2.1. Membrane Biofilm Reactor Design
A tubular non-porous polydimethylsiloxane (PDMS) membrane (Fisher Scientific
Ltd., Loughborough, UK) was used in this study to construct the membrane biofilm
reactor (see Figure 2). The tubular membrane was potted in a 6 mm nylon tubing using
translucent epoxy resin (MG Chemicals, Santa Clara, CA, USA). Membrane biofilm reactor
was constructed using a glass column with a 10 mm outer diameter and 8 mm inner
diameter with two T-shaped push-in fittings (RS) at the ends. The membrane module with
the single PDMS tube was fitted into the glass column by connecting it to the T-shaped
push-in fitting with a reducer to fit a 6 mm nylon tubing end. The perpendicular ends of
Membranes 2023,13, 908 3 of 13
the T-shape push-in fittings were used to connect the liquid inlet and outlet using 6 mm
outer diameter Tygon tubing was to minimise possible gas transfer through the tubing. Gas
lines were connected to the membrane module using 6 mm nylon tubing. Details of the
membrane biofilm reactor are summarised in Table 1.
Membranes 2023, 13, x FOR PEER REVIEW 3 of 13
with two T-shaped push-in fiings (RS) at the ends. The membrane module with the sin-
gle PDMS tube was fied into the glass column by connecting it to the T-shaped push-in
fiing with a reducer to fit a 6 mm nylon tubing end. The perpendicular ends of the T-
shape push-in fiings were used to connect the liquid inlet and outlet using 6 mm outer
diameter Tygon tubing was to minimise possible gas transfer through the tubing. Gas
lines were connected to the membrane module using 6 mm nylon tubing. Details of the
membrane biofilm reactor are summarised in Table 1.
Figure 2. Membrane bioreactor module used in the study.
Table 1. Details of the membrane biofilm reactor design.
Membrane Material PDMS
Inner diameter of the membrane fibre (mm) 1.0
Outer diameter of membrane fibre (mm) 2.0
Wall thickness of membrane fibre (mm) 0.5
Effective fibre length (cm) 33
Surface area of membrane fibre (m
2
) 2.07 × 10
−3
Glass column effective length (cm) 30
Inner diameter of glass column (cm) 0.8
2.2. Oxygen Transfer Rate Measurement
Oxygen transfer rate (OTR) was measured using two methods with an experimental
setup shown in Figure 3. In the first method (the dynamic method), a dissolved oxygen
(DO) probe (CellOx
®
325, WTW, Los Angeles, CA, USA) was used to measure the oxygen
concentration in the liquid. For abiotic experiments, deionised water was pumped
through the shell side of the membrane bioreactor using a peristaltic pump (Watson Mar-
low 323, RS, Manchester, UK). The deionised water was deoxygenated by sparging with
pure nitrogen until the DO reading reached 0–1 mg/L. Subsequently, pure oxygen was
introduced to the lumen of the membrane fibre at a pressure of 90 mbar g and a minimum
flowrate of 300 mL/min to maintain the pressure within the fibre during the experiment.
The DO probe was fied into a separate tube and connected to the membrane biofilm
reactor outlet to measure the dissolved oxygen concentration over time. Oxygen concen-
tration readings were collected using a Pico Logger and ploed as a function of time until
the DO concentration reached saturation point. The rate of oxygen transferred into the
liquid was calculated using the following linearised mass balance equation [8]:
ln (C*− C0
C*− C1
) = kLa t (1)
where C* is the DO saturation concentration at 20 °C, and k
L
a is the volumetric mass trans-
fer coefficient.
Figure 2. Membrane bioreactor module used in the study.
Table 1. Details of the membrane biofilm reactor design.
Membrane Material PDMS
Inner diameter of the membrane fibre (mm) 1.0
Outer diameter of membrane fibre (mm) 2.0
Wall thickness of membrane fibre (mm) 0.5
Effective fibre length (cm) 33
Surface area of membrane fibre (m2) 2.07 ×10−3
Glass column effective length (cm) 30
Inner diameter of glass column (cm) 0.8
2.2. Oxygen Transfer Rate Measurement
Oxygen transfer rate (OTR) was measured using two methods with an experimental
setup shown in Figure 3. In the first method (the dynamic method), a dissolved oxygen
(DO) probe (CellOx
®
325, WTW, Los Angeles, CA, USA) was used to measure the oxygen
concentration in the liquid. For abiotic experiments, deionised water was pumped through
the shell side of the membrane bioreactor using a peristaltic pump (Watson Marlow 323, RS,
Manchester, UK). The deionised water was deoxygenated by sparging with pure nitrogen
until the DO reading reached 0–1 mg/L. Subsequently, pure oxygen was introduced to
the lumen of the membrane fibre at a pressure of 90 mbar g and a minimum flowrate
of 300 mL/min to maintain the pressure within the fibre during the experiment. The
DO probe was fitted into a separate tube and connected to the membrane biofilm reactor
outlet to measure the dissolved oxygen concentration over time. Oxygen concentration
readings were collected using a Pico Logger and plotted as a function of time until the DO
concentration reached saturation point. The rate of oxygen transferred into the liquid was
calculated using the following linearised mass balance equation [8]:
ln(C∗−C0
C∗−C1
) = kLa×t (1)
where C* is the DO saturation concentration at 20
◦
C, and k
L
a is the volumetric mass
transfer coefficient.
The second method for measuring OTR is referred as the pressure drop method [
9
]
whereby two valves are connected at either end of the membrane module. Oxygen gas was
introduced to the lumen of the membrane at a minimum flowrate of 200 mL/min, then the
valves were closed to maintain the oxygen pressure in the membrane and allow oxygen to
leave the lumen only through the membrane. High sensitivity pressure measurements were
achieved by using a pressure transducer connected to a Pico Logger to record the pressure
drop. For a given intramembrane pressure, the difference between the initial and final
pressures indicates the change in mass of oxygen inside the membrane tube [
10
]. The OTR
was calculated by using the pressure drop and converting it to moles by using the ideal gas
Membranes 2023,13, 908 4 of 13
law. The gas volume was calculated by combining the volume inside the membrane fibre
with the tubing between the valves and it remained constant for all experiments. OTR was
measured using this method when the biofilm was present on the membrane.
Membranes 2023, 13, x FOR PEER REVIEW 4 of 13
Figure 3. Experimental setup schematic diagram for OTR measurements.
The second method for measuring OTR is referred as the pressure drop method [9]
whereby two valves are connected at either end of the membrane module. Oxygen gas
was introduced to the lumen of the membrane at a minimum flowrate of 200 mL/min,
then the valves were closed to maintain the oxygen pressure in the membrane and allow
oxygen to leave the lumen only through the membrane. High sensitivity pressure meas-
urements were achieved by using a pressure transducer connected to a Pico Logger to
record the pressure drop. For a given intramembrane pressure, the difference between the
initial and final pressures indicates the change in mass of oxygen inside the membrane
tube [10]. The OTR was calculated by using the pressure drop and converting it to moles
by using the ideal gas law. The gas volume was calculated by combining the volume in-
side the membrane fibre with the tubing between the valves and it remained constant for
all experiments. OTR was measured using this method when the biofilm was present on
the membrane.
For both methods, each measurement was taken at least in triplicates and measure-
ments were performed in random order. Results for OTRs for deionised water (abiotic
experiments) at different liquid recirculation rates were compared in Figure 4. The statis-
tical error in the OTR measurements were found to be less than 5%.
Figure 3. Experimental setup schematic diagram for OTR measurements.
For both methods, each measurement was taken at least in triplicates and measure-
ments were performed in random order. Results for OTRs for deionised water (abiotic
experiments) at different liquid recirculation rates were compared in Figure 4. The statistical
error in the OTR measurements were found to be less than 5%.
Membranes 2023, 13, x FOR PEER REVIEW 5 of 13
Figure 4. Pressure drop method and the dynamic method oxygen transfer rate comparison at the
same operational conditions.
2.3. Mass Transfer Model Development
Mass transfer in gas–liquid membrane systems occurs across three parts: the gas
layer, membrane layer and liquid layer. The overall mass transfer coefficient can be de-
scribed based on the general resistance in series model [11]:
1
K = 1
kmH + 1
kl
+ 1
kgHcc (2)
where K is the overall mass transfer coefficient (m/s), k
m
is the mass transfer coefficient
through the membrane (mol m
−2
Pa
−1
s
−1
), H is the Henry’s solubility constant (Pa m
3
mol
−1
),
k
l
is the mass transfer coefficient at the liquid side (m/s), k
g
is the mass transfer coefficient
through the gas side (m/s) and H
cc
is the dimensionless Henry’s constant. The literature
suggests that gas mass transfer resistance is negligible when compared to membrane and
liquid resistance [11,12]. The membrane mass transfer coefficient for a nonporous mem-
brane is defined as [13]:
km = P
δ (3)
where P is the permeability of the gas through the membrane (mol m
−1
Pa
−1
s
−1
) and δ is
the membrane thickness (m).
The mass transfer model developed follows the following assumptions:
1. Membrane temperature and flow conditions are at steady state;
2. Ideal gas law is used for all gas streams;
3. Gas is physically absorbed/desorbed in the liquid; chemical reactions are negligible;
4. Axial concentration gradients in the gas and liquid streams are negligible;
5. No diffusional resistance in the gas side of the membrane.
Using these assumptions, the overall mass transfer coefficient was calculated using
the nondimensional Sherwood (Sh) number correlation [14].
Sh = A Rex Scy (4)
A is a constant and Sherwood, Schmidt (Sc) and Reynolds (Re) numbers are calcu-
lated as follows [14]:
Sh =K deff
D (5)
Figure 4.
Pressure drop method and the dynamic method oxygen transfer rate comparison at the
same operational conditions.
Membranes 2023,13, 908 5 of 13
2.3. Mass Transfer Model Development
Mass transfer in gas–liquid membrane systems occurs across three parts: the gas layer,
membrane layer and liquid layer. The overall mass transfer coefficient can be described
based on the general resistance in series model [11]:
1
K=1
kmH+1
kl
+1
kgHcc (2)
where K is the overall mass transfer coefficient (m/s), k
m
is the mass transfer coeffi-
cient through the membrane (mol m
−2
Pa
−1
s
−1
), H is the Henry’s solubility constant
(
Pa m3mol−1
), k
l
is the mass transfer coefficient at the liquid side (m/s), k
g
is the mass
transfer coefficient through the gas side (m/s) and H
cc
is the dimensionless Henry’s con-
stant. The literature suggests that gas mass transfer resistance is negligible when compared
to membrane and liquid resistance [
11
,
12
]. The membrane mass transfer coefficient for a
nonporous membrane is defined as [13]:
km=P
δ(3)
where P is the permeability of the gas through the membrane (mol m
−1
Pa
−1
s
−1
) and
δ
is
the membrane thickness (m).
The mass transfer model developed follows the following assumptions:
1. Membrane temperature and flow conditions are at steady state;
2. Ideal gas law is used for all gas streams;
3. Gas is physically absorbed/desorbed in the liquid; chemical reactions are negligible;
4. Axial concentration gradients in the gas and liquid streams are negligible;
5. No diffusional resistance in the gas side of the membrane.
Using these assumptions, the overall mass transfer coefficient was calculated using
the nondimensional Sherwood (Sh) number correlation [14].
Sh =A RexScy(4)
A is a constant and Sherwood, Schmidt (Sc) and Reynolds (Re) numbers are calculated
as follows [14]:
Sh =K deff
D(5)
Sc =µ
Dρ(6)
Re =ρudeff
µ(7)
where
µ
is the viscosity (kg m
−1
s
−1
),
ρ
is the fluid density (kg/m
3
), u is the velocity of the
fluid (m/s), D is the diffusion coefficient (m
2
/s) and d
eff
is the module effective diameter
calculated as [14]:
deff =dcolumn2−ndfiber2
dcolumn+ndfiber
(8)
To calculate an effective Reynolds number, the effective diameter was used when cal-
culating the average velocity of the fluid. Schmidt number is independent of the membrane
configuration, and it is the same for all operating conditions. The exponent y in
Equation (2)
is generally 0.33 in the literature and the same is assumed in this study [
14
]. For the mem-
brane biofilm reactor constructed, the exponent x and A is calculated using the data for O
2
via the dynamic method and minimising the error between Equations (4) and (5).
Membranes 2023,13, 908 6 of 13
2.4. Biofilm Development
A chemoautotrophic microorganism Cupriavidus necator H16 was used in this study.
Overnight cultures were prepared using 50 mL nutrient-rich LB media in 250 mL Erlen-
meyer flasks at 30
◦
C. For biofilm experiments, minimal salts media (MSM) was used
and prepared using (per litre) 9 g Na
3
PO
4·
12H
2
O, 1.5 g KH
2
PO
4
, 1 g NH
4
Cl, 200 mg
MgSO
4·
7H
2
O and 1 mL of trace elements that include 4 g ZnSO
4·
7H
2
O, 1 g MnCl
·
4H
2
O,
0.2 g Na
2
B
4
O
7·
10H
2
O, 0.3 g NiCl
2·
6H
2
O, 1 g Na
2
MoO
4·
2H
2
O, 1 g CuCl
·
2H
2
O and 7.6 g
FeSO
4·
7H
2
O per litre. After autoclaving at 121
◦
C for 15 min, the media was supplemented
with fructose as the carbon source to reach an initial concentration of 15 g/L.
The single-tube membrane biofilm reactor was operated in batch mode for approx-
imately 30 h, until the optical density (OD) value reached 2.5–3.0. The medium was
continuously recirculated at a flowrate of 50 mL/min using a peristaltic pump (Watson
Marlow 323, RS) and air was fed into the membrane at 300 mL/min. After the initial
bacterial attachment on the membrane, the setup was switched to continuous operation
mode at very low liquid flowrates of 2 mL/min to avoid cell washing. The reactor was
operated for 6 days in continuous mode to grow biofilms. Experiments were repeated four
times. The initial biofilm formation was observed after 24 h for each experiment.
2.5. Biofilm Thickness Measurement
Biofilm thickness was measured using ImageJ to analyse photos of the biofilm every
24 h
for 6 days. Biofilm thickness was measured from multiple sections for each photo that
was taken every 24 h, and the average biofilm thickness was calculated for each time point.
Since the biofilm obtained is fragile, photos were taken in situ as a non-disruptive technique.
Due to the magnifying effect of the glass, each biofilm measurement was corrected by
measuring the silicone tube without any biofilm attached using the same method.
2.6. Substrate Uptake Measurement
OTR was measured using the pressure drop method every 24 h at different intramem-
brane pressures and average oxygen uptake rate (OUR) was calculated at different biofilm
thicknesses. Residual fructose was determined using a high-performance liquid chromatog-
raphy (HPLC) system equipped with RID-10A refractive index detector (Shimadzu, Kyoto,
Japan) at 50
◦
C. A total of 20
µ
L of the sample was injected into the column after the culture
supernatant was filtered through Mini-UniPrep syringeless filter devices (Agilent, Santa
Clara, CA, USA). The fructose was analysed by Aminex-87H column (Bio-Rad, Watford,
UK) at 40
◦
C. The samples in the column were eluted with 0.014 N H
2
SO
4
at a flow rate
of 0.55 mL/min and pressure of 4.3 MPa. A standard curve for fructose was used for
quantification. Using the change in residual fructose concentration, fructose utilisation rate
was calculated.
3. Results
3.1. Effect of Intramembrane Pressure on Oxygen Transfer
Figure 5shows the effect of intramembrane pressure on OTR using the pressure
drop method. In the set of experiments reported here, the OTR is proportional to the
intramembrane pressure, and it is higher at elevated intramembrane pressures. As the
intramembrane pressure increases, the amount of oxygen at the lumen side of the membrane
increases which creates a higher driving force for gas transfer. Thus, the OTR becomes
higher at higher intramembrane pressures. Figure 5further shows that the setup used for
OTR calculations using the pressure drop method is robust since the linear correlation is
observed with the experimental data.
Membranes 2023,13, 908 7 of 13
Membranes 2023, 13, x FOR PEER REVIEW 7 of 13
(Agilent, Santa Clara, CA, USA). The fructose was analysed by Aminex-87H column (Bio-
Rad, Watford, UK) at 40 °C. The samples in the column were eluted with 0.014 N H
2
SO
4
at a flow rate of 0.55 mL/min and pressure of 4.3 MPa. A standard curve for fructose was
used for quantification. Using the change in residual fructose concentration, fructose uti-
lisation rate was calculated.
3. Results
3.1. Effect of Intramembrane Pressure on Oxygen Transfer
Figure 5 shows the effect of intramembrane pressure on OTR using the pressure drop
method. In the set of experiments reported here, the OTR is proportional to the intramem-
brane pressure, and it is higher at elevated intramembrane pressures. As the intramem-
brane pressure increases, the amount of oxygen at the lumen side of the membrane in-
creases which creates a higher driving force for gas transfer. Thus, the OTR becomes
higher at higher intramembrane pressures. Figure 5 further shows that the setup used for
OTR calculations using the pressure drop method is robust since the linear correlation is
observed with the experimental data.
Figure 5. Effect of intramembrane pressure on the oxygen transfer rate calculated using pressure
drop method.
3.2. Mass Transfer Model
The experimental Sherwood number was calculated using Equation (4) and the OTR
data obtained above. Since the Sc number is constant, Sh/Sc
0.33
was ploed against Re num-
ber to calculate component x in Equation (4). The constant A was then calculated from the
intercept of the graphs. As the component x and A were determined, following correlation
was proposed for this system:
Sh = 0.38 Re0.36 Sc0.33 (9)
Experimental K values for O
2
data as a function of Re was ploed in Figure 6 with
the proposed model.
Figure 5.
Effect of intramembrane pressure on the oxygen transfer rate calculated using pressure
drop method.
3.2. Mass Transfer Model
The experimental Sherwood number was calculated using Equation (4) and the OTR
data obtained above. Since the Sc number is constant, Sh/Sc
0.33
was plotted against Re
number to calculate component x in Equation (4). The constant A was then calculated
from the intercept of the graphs. As the component x and A were determined, following
correlation was proposed for this system:
Sh =0.38 Re0.36 Sc0.33 (9)
Experimental K values for O
2
data as a function of Re was plotted in Figure 6with the
proposed model.
Membranes 2023, 13, x FOR PEER REVIEW 8 of 13
Figure 6. Mass transfer coefficient for O
2
as a function of the Re. Circles represent the experimental
values using the pressure drop method and the solid line represents the predicted model.
The maximum mass transfer coefficient was observed at the highest liquid flowrate.
As the Re increases, the mass transfer coefficient increases but the slope of the graph be-
gins to decrease. This can be explained by the liquid boundary layer where K becomes
independent of the Re at high liquid flowrates. Similarly, as the Re decreases, K tends
towards zero which indicates that the liquid boundary layer is the dominant resistance.
3.3. Biofilm Development
Figure 7 shows the development of average biofilm thickness with respect to time.
Sloughing was observed approximately 120 h for each experiment where a small section
of biomass along the single tube fibre detached uniformly, shown in Figure 8a. For each
experiment, the length of biofilm that sloughed off was between 10 and 20% of the total
membrane length. However, regrowth of biofilm in the region of the sloughed areas were
rapid. Figure 8b shows the regrowth of biofilm after 24 h. Nevertheless, average thickness
measurements at the 144 h point were performed on the parts where no sloughing oc-
curred.
Figure 7. Average biofilm thickness development over time.
Figure 6.
Mass transfer coefficient for O
2
as a function of the Re. Circles represent the experimental
values using the pressure drop method and the solid line represents the predicted model.
The maximum mass transfer coefficient was observed at the highest liquid flowrate.
As the Re increases, the mass transfer coefficient increases but the slope of the graph
begins to decrease. This can be explained by the liquid boundary layer where K becomes
Membranes 2023,13, 908 8 of 13
independent of the Re at high liquid flowrates. Similarly, as the Re decreases, K tends
towards zero which indicates that the liquid boundary layer is the dominant resistance.
3.3. Biofilm Development
Figure 7shows the development of average biofilm thickness with respect to time.
Sloughing was observed approximately 120 h for each experiment where a small section
of biomass along the single tube fibre detached uniformly, shown in Figure 8a. For each
experiment, the length of biofilm that sloughed off was between 10 and 20% of the total
membrane length. However, regrowth of biofilm in the region of the sloughed areas were
rapid. Figure 8b shows the regrowth of biofilm after 24 h. Nevertheless, average thickness
measurements at the 144 h point were performed on the parts where no sloughing occurred.
Membranes 2023, 13, x FOR PEER REVIEW 8 of 13
Figure 6. Mass transfer coefficient for O
2
as a function of the Re. Circles represent the experimental
values using the pressure drop method and the solid line represents the predicted model.
The maximum mass transfer coefficient was observed at the highest liquid flowrate.
As the Re increases, the mass transfer coefficient increases but the slope of the graph be-
gins to decrease. This can be explained by the liquid boundary layer where K becomes
independent of the Re at high liquid flowrates. Similarly, as the Re decreases, K tends
towards zero which indicates that the liquid boundary layer is the dominant resistance.
3.3. Biofilm Development
Figure 7 shows the development of average biofilm thickness with respect to time.
Sloughing was observed approximately 120 h for each experiment where a small section
of biomass along the single tube fibre detached uniformly, shown in Figure 8a. For each
experiment, the length of biofilm that sloughed off was between 10 and 20% of the total
membrane length. However, regrowth of biofilm in the region of the sloughed areas were
rapid. Figure 8b shows the regrowth of biofilm after 24 h. Nevertheless, average thickness
measurements at the 144 h point were performed on the parts where no sloughing oc-
curred.
Figure 7. Average biofilm thickness development over time.
Figure 7. Average biofilm thickness development over time.
Membranes 2023, 13, x FOR PEER REVIEW 9 of 13
Figure 8. (a) Sloughing of biofilm on single tube after 120 h; (b) regrowth of biofilm on the sloughed
areas in the same region after a further 24 h.
At 24 h intervals, the OTR was measured at both the normal (steady state) intramem-
brane pressure (200 mbar) and also at a range of elevated intramembrane pressures (rang-
ing from 120 to 450 mbar). Figure 9 shows the effect of intramembrane pressure on OTR
corresponding to a range of different biofilm thicknesses as the experiment progressed.
OTR increases linearly as intramembrane pressure increases, which is not unexpected as
the driving force for oxygen transfer increases. This suggests that the system is not oxygen
limited for the range of oxygen pressures and biofilm thicknesses tested.
Figure 9. Oxygen transfer rate for biofilm thickness with different intra-membrane pressures.
Figure 10 shows the average oxygen utilisation rate (OUR) results for measured bio-
film thickness. OUR increases as the biofilm thickness increases, levels and then falls. The
trend in OUR with respect to biofilm thickness can be explained as follows. In alignment
with previously published results for membrane-aached biofilms [2], the trend shown in
Figure 10 is a result of the concept of “optimal biofilm thickness”, a unique feature of
Figure 8.
(
a
) Sloughing of biofilm on single tube after 120 h; (
b
) regrowth of biofilm on the sloughed
areas in the same region after a further 24 h.
At 24 h intervals, the OTR was measured at both the normal (steady state) intramem-
brane pressure (200 mbar) and also at a range of elevated intramembrane pressures (ranging
from 120 to 450 mbar). Figure 9shows the effect of intramembrane pressure on OTR corre-
sponding to a range of different biofilm thicknesses as the experiment progressed. OTR
increases linearly as intramembrane pressure increases, which is not unexpected as the
driving force for oxygen transfer increases. This suggests that the system is not oxygen
limited for the range of oxygen pressures and biofilm thicknesses tested.
Membranes 2023,13, 908 9 of 13
Membranes 2023, 13, x FOR PEER REVIEW 9 of 13
Figure 8. (a) Sloughing of biofilm on single tube after 120 h; (b) regrowth of biofilm on the sloughed
areas in the same region after a further 24 h.
At 24 h intervals, the OTR was measured at both the normal (steady state) intramem-
brane pressure (200 mbar) and also at a range of elevated intramembrane pressures (rang-
ing from 120 to 450 mbar). Figure 9 shows the effect of intramembrane pressure on OTR
corresponding to a range of different biofilm thicknesses as the experiment progressed.
OTR increases linearly as intramembrane pressure increases, which is not unexpected as
the driving force for oxygen transfer increases. This suggests that the system is not oxygen
limited for the range of oxygen pressures and biofilm thicknesses tested.
Figure 9. Oxygen transfer rate for biofilm thickness with different intra-membrane pressures.
Figure 10 shows the average oxygen utilisation rate (OUR) results for measured bio-
film thickness. OUR increases as the biofilm thickness increases, levels and then falls. The
trend in OUR with respect to biofilm thickness can be explained as follows. In alignment
with previously published results for membrane-aached biofilms [2], the trend shown in
Figure 10 is a result of the concept of “optimal biofilm thickness”, a unique feature of
Figure 9. Oxygen transfer rate for biofilm thickness with different intra-membrane pressures.
Figure 10 shows the average oxygen utilisation rate (OUR) results for measured biofilm
thickness. OUR increases as the biofilm thickness increases, levels and then falls. The trend
in OUR with respect to biofilm thickness can be explained as follows. In alignment with
previously published results for membrane-attached biofilms [
2
], the trend shown in
Figure 10 is a result of the concept of “optimal biofilm thickness”, a unique feature of
biofilms where co-diffusion of limiting substrate occurs. Under constant loading conditions,
as the biofilm thickness increases, the biofilm changes from a single-substrate limited regime
to a dual-substrate-limited regime. Unlike conventional biofilms, the layer of maximum
microbial activity within thinner biofilms may not necessarily be located adjacent to the
biofilm-liquid interface and as a result, a diffusion barrier exists that reduces the effective
concentration of the carbon substrate.
Membranes 2023, 13, x FOR PEER REVIEW 10 of 13
biofilms where co-diffusion of limiting substrate occurs. Under constant loading condi-
tions, as the biofilm thickness increases, the biofilm changes from a single-substrate lim-
ited regime to a dual-substrate-limited regime. Unlike conventional biofilms, the layer of
maximum microbial activity within thinner biofilms may not necessarily be located adja-
cent to the biofilm-liquid interface and as a result, a diffusion barrier exists that reduces
the effective concentration of the carbon substrate.
Figure 10. Average oxygen uptake rate measurements at the specified biofilm thicknesses.
Figure 11 shows the steady state fructose utilisation rate as the experiment progresses
with increasing biofilm thickness. The reactor is operated as a batch initially and does not
become steady state continuous until 72 h, which corresponds to a biofilm thickness of
200 µm. It can be seen that the fructose utilisation rate starts to decrease between 125 µm
and 200 µm and continues to fall significantly as the biofilm thickness progresses. This
provides further evidence of the dual-limitation condition that is inherent in membrane
aached biofilms.
Figure 11. Fructose utilisation rate at corresponding biofilm thickness.
4. Discussion
This study aims to show the initial aachment and biofilm growth of a chemoauto-
trophic organism Cupriavidus necator H16 for proof-of-concept and investigates the gas–
mass transfer characteristics of the designed membrane biofilm reactor. PDMS was used
as the membrane material for gas transfer due to the high permeability of this material to
Figure 10. Average oxygen uptake rate measurements at the specified biofilm thicknesses.
Figure 11 shows the steady state fructose utilisation rate as the experiment progresses
with increasing biofilm thickness. The reactor is operated as a batch initially and does not
become steady state continuous until 72 h, which corresponds to a biofilm thickness of
200 µm
. It can be seen that the fructose utilisation rate starts to decrease between 125
µ
m
and 200
µ
m and continues to fall significantly as the biofilm thickness progresses. This
Membranes 2023,13, 908 10 of 13
provides further evidence of the dual-limitation condition that is inherent in membrane
attached biofilms.
Membranes 2023, 13, x FOR PEER REVIEW 10 of 13
biofilms where co-diffusion of limiting substrate occurs. Under constant loading condi-
tions, as the biofilm thickness increases, the biofilm changes from a single-substrate lim-
ited regime to a dual-substrate-limited regime. Unlike conventional biofilms, the layer of
maximum microbial activity within thinner biofilms may not necessarily be located adja-
cent to the biofilm-liquid interface and as a result, a diffusion barrier exists that reduces
the effective concentration of the carbon substrate.
Figure 10. Average oxygen uptake rate measurements at the specified biofilm thicknesses.
Figure 11 shows the steady state fructose utilisation rate as the experiment progresses
with increasing biofilm thickness. The reactor is operated as a batch initially and does not
become steady state continuous until 72 h, which corresponds to a biofilm thickness of
200 µm. It can be seen that the fructose utilisation rate starts to decrease between 125 µm
and 200 µm and continues to fall significantly as the biofilm thickness progresses. This
provides further evidence of the dual-limitation condition that is inherent in membrane
aached biofilms.
Figure 11. Fructose utilisation rate at corresponding biofilm thickness.
4. Discussion
This study aims to show the initial aachment and biofilm growth of a chemoauto-
trophic organism Cupriavidus necator H16 for proof-of-concept and investigates the gas–
mass transfer characteristics of the designed membrane biofilm reactor. PDMS was used
as the membrane material for gas transfer due to the high permeability of this material to
Figure 11. Fructose utilisation rate at corresponding biofilm thickness.
4. Discussion
This study aims to show the initial attachment and biofilm growth of a chemoau-
totrophic organism Cupriavidus necator H16 for proof-of-concept and investigates the gas–
mass transfer characteristics of the designed membrane biofilm reactor. PDMS was used
as the membrane material for gas transfer due to the high permeability of this material
to gas components, especially oxygen [
15
]. Using dense membranes makes it possible to
supply oxygen at high pressures which would overcome the oxygen diffusional limita-
tions in conventional aerated bioreactors and oxygen-transfer efficiencies would approach
100%. Also, dense membranes do not exhibit intrapore fouling and wetting as is the case
with microporous membranes [
16
]. Thus, a PDMS membrane was used in this study for
gas transfer.
The Reynolds numbers in this study are in the turbulent range, higher than many
studies that investigate the shell side mass transfer correlations. This is due to the small
diameter of the membrane bioreactor column used in this study, which increases the liquid
velocity within the bioreactor and causes the Reynolds number to be in the turbulent
range. Using high Re numbers makes the liquid boundary layer resistance lower than the
resistance caused by the membrane layer. Therefore, at elevated liquid velocities, the mass
transfer coefficient (K) becomes independent of the Re. In the literature, the power of the
Re number was reported between 0.3 and 0.93, which shows that the suggested correlation
is comparable with the previously reported studies [
14
]. Furthermore, the proposed model
showed the best fit to the experimental data of the study.
Single tube (or single fibre) membrane systems are useful when designing membrane
biofilm reactors. Using a single fibre makes it easier to analyse the performance of the
biofilm formed on the membrane. Depending on the results, the system can be scaled
up by simply adding more fibres or having multiple single fibre membrane systems in
parallel. Furthermore, imaging the biofilm in a single tube system is easier compared to
using multiple membranes in a bundle. Biofilm characteristics of the chemoautotrophic
bacteria used in this study were investigated by supplying pure oxygen through the single
fibre membrane. In the case of C. necator, carbon and oxygen can be delivered from the
membrane side since this microorganism can utilise CO
2
as its carbon source. However,
the membrane was used to supply only pure oxygen and fructose was used as the carbon
source which was present in the liquid media. Thus, the substrate concentration profiles of
the biofilm formed should resemble Figure 1b where the highest oxygen concentration is at
the biofilm–membrane interface (substratum) and the highest fructose concentration is at
the biofilm–water interface.
Membranes 2023,13, 908 11 of 13
As the biofilm continues to grow, the dissolved oxygen concentration eventually falls
to zero within the biofilm. Since the microorganism used in this study is aerobic, this could
potentially lead to oxygen limitation. However, OTR results at various intramembrane
pressures and biofilm thicknesses indicate that oxygen limitation is not a concern within
the range of testing. Thus, it can be concluded that the resulting biofilm is entirely aerobic,
and there were no instances of oxygen limitation throughout the experiments.
Formation of the biofilm on the membrane surface creates an extra mass transfer
resistance for oxygen. However, Casey et al. reported that for biofilm thicknesses up to
600
µ
m, the OTRs are higher compared to abiotic measurements [
2
]. In this study, when
the OTRs are compared in the presence and absence of a biofilm, it is seen that the OTRs
are higher until the biofilm thickness reaches 124
µ
m after 24 h. Since microorganisms are
utilising the oxygen provided, the concentration of oxygen within the biofilm decreases.
Due to the concentration difference between the biofilm and the membrane lumen, the
oxygen flux through the membrane increased.
Sloughing is a common occurrences at later stages of biofilm development and it can
be caused by a combination of shear stress and biofilm growth rate [
17
]. As the biofilm
grows, shear stress (or velocity) in the glass column increases as the diameter decreases.
In larger bioreactors, the effect of biofilm thickness on velocity can be insignificant since
larger vessels are used. However, in the biofilm reactor used in this study, the diameter
of the column is relatively small. Thus, biofilm growth can significantly change the shear
stress on the biofilm which might have played a role in the sloughing. The maximum
thickness reached before sloughing in the present study was approximately 300
µ
m, which
is considered a relatively thin biofilm [18].
In membrane-attached biofilms, biofilm thickness is a major influence on determining
the process performance. There is an optimum biofilm thickness for each specific applica-
tion and condition. An excessively thin biofilm might not provide enough activity where
an excessive biofilm proliferation might hinder the reaction by increasing the resistance
to diffusion by substrate transfer. For nitrogen removal, Terada et al. showed that the
performance of a membrane-aerated biofilm reactor (MABR) is optimal when the biofilm
thickness stabilises at approximately 1600
µ
m [
19
]. Another study by Matsumoto et al.
showed that the nitrogen removal efficiency of an MABR can exceed 70% when the biofilm
thickness is between 600 and 1200
µ
m [
20
]. Sanchez-Huerta et al. also concluded that
biofilm thickness over 580
µ
m enhances the removal of organic micropollutants in an
MABR [
21
]. These studies show that the biofilm thickness has an important effect on the
performance of the membrane biofilm reactors. Therefore, it is important to identify biofilm
growth characteristics including biofilm growth and stripping.
In this study, product formation was not considered since the main objective of the
study was to investigate the biofilm characteristics and its effect on substrate diffusion
rates. The model organism used in this study can produce a polymer that could be used
to create bioplastics. However, it is possible to use a genetically modified version of this
strain to produce a wide range of products.
This study provides a basis for future studies with biofilm characteristics of Cupri-
avidus necator H16 on gas transferring membranes. Here, the substrates are introduced to
the biofilm from opposing sides and the effect of biofilm thickness on oxygen and fructose
utilisation rates are discussed. Due to the chemoautotrophic metabolism of this microor-
ganism, more complex bioreaction scenarios can be studied in the future, such as supplying
CO2as the carbon source through the membrane.
5. Conclusions
The goal of this study is to assess the mass transfer coefficients for O
2
in a single tube
membrane biofilm reactor and investigate the biofilm characteristics of Cupriavidus necator
H16. Membranes provide better mass transfer rates for low soluble gaseous substrates for
gas fermentation. Also, they provide a safe delivery system when explosive gas mixtures
are used. The dimensionless model used for predicting O
2
mass transfer coefficients was
Membranes 2023,13, 908 12 of 13
validated through experimental measurements. The highest liquid flowrate yielded the
highest mass transfer coefficient. After a biofilm of 200
µ
m biofilm thickness had developed,
a decline in both the oxygen uptake rate and fructose utilisation rates was observed,
indicating that dual-substrate limitation came into effect after three days of biofilm growth.
A decline in biofilm growth rate was observed after 96 h, which corresponds to 260
µ
m
biofilm thickness.
Author Contributions:
Conceptualisation, B.A. and E.C.; methodology, B.A. and F.C.; validation,
B.A. and E.C.; writing—original draft preparation, B.A.; writing—review and editing, S.D., F.C. and
E.C.; visualisation, B.A.; supervision, E.C.; project administration, E.C.; funding acquisition, E.C. All
authors have read and agreed to the published version of the manuscript.
Funding:
This research was funded by Science Foundation Ireland (SFI) under Grant Number
16/RC/3889 for BiOrbic Bioeconomy SFI Research Centre, which is co-funded under the European
Regional Development Fund and BiOrbic industry partners.
Institutional Review Board Statement: Not applicable.
Data Availability Statement:
The data presented in this study are available on request from the
corresponding author.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
Nerenberg, R. The membrane-biofilm reactor (MBfR) as a counter-diffusional biofilm process. Curr. Opin. Biotechnol.
2016
,38,
131–136. [CrossRef] [PubMed]
2.
Casey, E.; Glennon, B.; Hamer, G. Biofilm development in a membrane-aerated biofilm reactor: Effect of flow velocity on
performance. Biotechnol. Bioeng. 2000,67, 476–486. [CrossRef]
3.
Akkoyunlu, B.; Daly, S.; Casey, E. Membrane bioreactors for the production of value-added products: Recent developments,
challenges and perspectives. Bioresour. Technol. 2021,341, 125793. [CrossRef] [PubMed]
4.
Esquivel-Elizondo, S.; Delgado, A.G.; Rittmann, B.E.; Krajmalnik-Brown, R. The effects of CO
2
and H
2
on CO metabolism by
pure and mixed microbial cultures. Biotechnol. Biofuels 2017,10, 220. [CrossRef] [PubMed]
5.
Jiang, M.; Zheng, J.; Perez-Calleja, P.; Picioreanu, C.; Lin, H.; Zhang, X.; Zhang, Y.; Li, H.; Nerenberg, R. New insight into
CO
2
-mediated denitrification process in H
2
-based membrane biofilm reactor: An experimental and modeling study. Water Res.
2020,184, 116177. [CrossRef] [PubMed]
6.
Zhang, Y.; Jiang, M.; Ma, J.; Wang, Y.; Zhang, X.; Wei, Q.; Wang, X.; Zhang, X.; Zheng, J. Development of a novel periodic venting-
controlled membrane biofilm reactor for hydrogenotrophic denitrification: Process performance and microbial mechanism. Chem.
Eng. J. 2023,463, 142529. [CrossRef]
7. Mara, D.; Horan, N. Handbook of Water and Wastewater Microbiology; Academic Press: Cambridge, MA, USA, 2003.
8.
Elisiário, M.P.; De Wever, H.; Van Hecke, W.; Noorman, H.; Straathof, A.J.J. Membrane bioreactors for syngas permeation and
fermentation. Crit. Rev. Biotechnol. 2021,42, 856–872. [CrossRef] [PubMed]
9.
Casey, E.; Glennon, B.; Hamer, G. Oxygen mass transfer characteristics in a membrane-aerated biofilm reactor. Biotechnol. Bioeng.
1999,62, 183–192. [CrossRef]
10. Rishell, S.; Casey, E.; Glennon, B.; Hamer, G. Characteristics of a methanotrophic culture in a membrane-aerated biofilm reactor.
Biotechnol. Prog. 2004,20, 1082–1090. [CrossRef] [PubMed]
11.
Matula, E.E.; Nabity, J.A. Characterization of simultaneous heat, oxygen, and carbon dioxide transfer across a nonporous
polydimethylsiloxane (PDMS) hollow fiber membrane. Chem. Eng. J. Adv. 2021,6, 100106. [CrossRef]
12.
Rishell, S.; Casey, E.; Glennon, B.; Hamer, G. Mass transfer analysis of a membrane aerated reactor. Biochem. Eng. J.
2004
,18,
159–167. [CrossRef]
13.
Heile, S.; Rosenberger, S.; Parker, A.; Jefferson, B.; McAdam, E.J. Establishing the suit- ability of symmetric ultrathin wall
polydimethylsiloxane hollow-fibre membrane contactors for enhanced CO
2
separation during biogas upgrading. J. Memb. Sci.
2014,452, 37–45. [CrossRef]
14.
Kavousi, F.; Syron, E.; Semmens, M.; Casey, E. Hydrodynamics and gas transfer performance of confined hollow fibre membrane
modules with the aid of computational fluid dynamics. J. Membr. Sci. 2016,513, 117–128. [CrossRef]
15.
Zhang, H. The Permeability Characteristics of Silicone Rubber. In Proceedings of the SAMPE Fall Technical Conference, Global
Advances in Materials and Process Engineering, Dallas, TX, USA, 6–9 November 2006.
16.
Montoya, J.P. Membrane Gas Exchange: Using Hollow Fiber Membranes to Separate Gases from Liquid and Gaseous Streams; MedArray#
Inc.: Ann Arbor, MI, USA, 2010.
17.
Heffernan, B.; Murphy, C.D.; Syron, E.; Casey, E. Treatment of Fluoroacetate by a Pseudomonas fluorescens Biofilm Grown in
Membrane Aerated Biofilm Reactor. Environ. Sci. Technol. 2009,43, 6776–6785. [CrossRef] [PubMed]
Membranes 2023,13, 908 13 of 13
18.
Li, T.; Liu, J. Factors affecting performance and functional stratification of membrane-aerated biofilms with a counter-diffusion
configuration. RSC Adv. 2019,9, 29337–29346. [CrossRef] [PubMed]
19.
Terada, A.; Hibiya, K.; Nagai, J.; Tsuneda, S.; Hirata, A. Nitrogen removal characteristics and biofilm analysis of a membrane-
aerated biofilm reactor applicable to high-strength nitrogenous wastewater treatment. J. Biosci. Bioeng.
2003
,95, 170–178.
[CrossRef] [PubMed]
20.
Matsumoto, S.; Terada, A.; Tsuneda, S. Modeling of membrane-aerated biofilm: Effects of C/N ratio, biofilm thickness and surface
loading of oxygen on feasibility of simultaneous nitrification and denitrification. Biochem. Eng. J. 2007,37, 98–107. [CrossRef]
21.
Sanchez-Huerta, C.; Medina, J.S.; Wang, C.; Fortunato, L.; Hong, P.-Y. Understanding the role of sorption and biodegradation in
the removal of organic micropollutants by membrane aerated biofilm reactor (MABR) with different biofilm thickness. Water Res.
2023,236, 119935. [CrossRef] [PubMed]
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