& IWA Publishing 2011 Water Science & Technology 9 63.8 9 2011
1701
The effect of mixed liquor suspended solids (MLSS) on
biofouling in a hybrid membrane bioreactor for the
treatment of high concentration organic wastewater
A. Damayanti, Z. Ujang, M. R. Salim and G. Olsson
ABSTRACT
Biofouling is a crucial factor in membrane bioreactor (MBR) applications, particularly for high
organic loading operations. This paper reports a study on biofouling in an MBR to establish a
relationship between critical flux, Jc, mixed liquor suspended solids (MLSS) (ranging from
5 to 20 g L1) and volumetric loading rate (6.3 kg COD m3 h1) of palm oil mill effluent (POME).
A lab-scale 100 L hybrid MBR consisting of anaerobic, anoxic, and aerobic reactors was used with
flat sheet microfiltration (MF) submerged in the aerobic compartment. The food-to-microorganism
(F/M) ratio was maintained at 0.18 kg COD kg1 MLSSd1. The biofouling tendency of the
membrane was obtained based on the flux against the transmembrane pressure (TMP)
behaviour. The critical flux is sensitive to the MLSS. At the MLSS 20 g L1 the critical flux
is about four times lower than that for the MLSS concentration of 5 g L1. The results showed
A. Damayanti
Z. Ujang (corresponding author)
M. R. Salim
Institute of Environmental & Water
Resource Management (IPASA),
Universiti Teknologi Malaysia (UTM),
81310 Skudai,
Johor Bahru,
Malaysia
E-mail: zaini@utm.my
G. Olsson
Industrial Electrical Engineering and
Automation (IEA),
Lund University,
Box 118, SE-221 00 Lund,
Sweden
high removal efficiency of denitrification and nitrification up to 97% at the MLSS concentration
20 g L1. The results show that the operation has to compromise between a high and a low MLSS
concentration. The former will favour a higher removal rate, while the latter will favour a higher
critical flux.
Key words 9 biofouling, flux, membrane bioreactor, MLSS, TMP
INTRODUCTION
Palm oil mill has significantly contributed to the generation of
wastewater in Malaysia. As the world’s largest producer and
exporter of palm oil, Malaysia has more than three million
hectares of oil palm cultivated area. More than 500 palm oil
mills have been built and operated for the past 40 years
(Damayanti et al. 2010). Palm oil processing is carried out
in mills where oil is extracted from the oil palm fruit bunch
(Vijayaraghavan et al. 2007). Extraction of crude palm oil
needs large quantities of water, contributing to about 50%
of the POME. POME is characterized as a thick brownish
liquid containing high amounts of total solids (E40 g L1),
oil and grease (E8 g L1), COD (E43 g L1), BOD
(E25 g L1), ammonia nitrogen (E0.1 g L1), and pH of
3.570.1 (Ahmad et al. 2008).
doi: 10.2166/wst.2011.338
In recent years, membrane technology has been used
basically for low strength wastewater like domestic wastewater in aerobic conditions similar to the conventional
activated sludge process (Hamdzah et al. 2005; Ujang et al.
2007). The membrane bioreactor (MBR) has been successfully
used for treatment of municipal and industrial wastewaters
(Alaboud 2009). The research reported in this paper has been
conducted under different operating conditions using a
hybrid MBR (HMBR) with a combination of anaerobic,
anoxic, and aerobic reactors to treat POME. A sustainable
process performance in the last aerobic tank was obtained at
different levels of MLSS concentration 5 to 20 g L1. The
membrane replaces the function of the settling tanks that is
conveying the supernatant to the water body as well as
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A. Damayanti et al. 9 The effect of mixed liquor suspended solids (MLSS) on biofouling in a hybrid membrane bioreactor
returning the solids back to the reactor (Qin et al. 2007).
Internal biofouling due to the adsorption of dissolved matter
into the membrane pores and pore blocking is considered
irreversible and is generally only removed by chemical cleaning. There are principally three strategies for limiting biofouling in submerged MBR process systems: maintaining the
permeate flux below the critical flux, Jc, increasing the membrane aeration, and employing physical or chemical cleaning
(Razak et al. 2007). Under constant-TMP or constant-flux
conditions, the fouling mechanism will increase hydraulic
resistance and the trans membrane pressure will be increased
as permeate flux will be declined, as further studied by Qin
et al. (2007).
Many studies about biofouling at MBR in different MLSS,
for several example Chang & Kim (2005) using synthetic
wastewater, showed which cake resistance (Rc) decreased
as MLSS concentration (between 0.09–3.7 g L1) decreasing.
Another study compared attached growth (MLSS between
0.1–2 g L1) and suspended growth (MLSS 3 g L1) microorganisms in MBR using municipal wastewater showed the
rate of attached growth system 7 times higher affected the
biofouling rate compare with suspended growth (Lee et al.
2001). Therefore this study was carried out to investigate
performance and biofouling tendency at four different
MLSS concentrations between 5 to 20 g L1 in a hybrid
MBR treating high strength wastewater, like POME. Thus
the study has two aims: (1) to observe the biofouling through
the critical flux, Jc, which was determined using the flux step
method and (2) to investigate the performance of the organic
and nitrogen removal in the system at different MLSS levels.
Water Science & Technology 9 63.8 9 2011
Table 1 9 Characteristics of POME compared to municipal sewage
Symbol
POME Valuesa
Sewageb,c
pH
5.6
7
Tot COD
45
0.60
TS (Total Solid)
43
0.80
TDS (Total Dissolved Solid)
35
0.23
SS (Suspended Solid)
8.2
0.56
NVSS (Non Volatile
4.0
0.25
NO
3
0.10
0.01
NO
0.30
0.06
þ
NH4
0.20
0.03
Total Nitrogen
0.50
0.01
Suspended Solid)
2
*Note:
a This study,
b Nor-Anuar et al. (2007),
c Ujang et al. (2008)
All parameter’s units in gL1 except pH.
from the experiment. The values of F/M range from 0.7 to
1 kg BOD5 kg MLSS1 day1 (Metcalf and Eddy 2004).
F
QS0
¼
M
VX
ð1Þ
MATERIALS AND METHODS
where F/M ¼ food to microorganism ratio
S0 ¼ influent concentration (ML3)
Q ¼ flowrate (L3T1)
V ¼ volume (L3)
X ¼ biomass concentration (ML3).
POME samples and pretreatment process
Laboratory scale trials
The characteristics of raw POME collected from Felda Bukit
Besar Palm Mill, Johor, Malaysia are shown in Table 1. Upon
sampling the temperature of POME was measured at
approximately 801C751C. POME samples used in this
study were stored at 41C. The operating temperature was
251C (room temperature in a tropical country). The POME
was mixed in the reactor using a stirrer according to Hofvendahl et al. (1999) for anaerobic compartment and Ramalho
et al. (2004) for anoxic compartment, for agitation speed
120 rpm or Newton number 4 to maximize the process.
Finally the POME was collected in the feed tank to study
the performance of different levels of MLSS. The F/M was
calculated using Equation (1) based on measurements taken
The schematic process flow diagram for the lab scale HMBR
is shown in Figure 1. A specification of the HMBR system is
given in Table 2. For acclimatization the HMBR system was
seeded with biological sludge taken from the wastewater
treatment plant of the Felda Bukit Besar Palm Mill, Johor
Bahru. The experiments were carried out at room temperature 251C. The system was operated continuously (24-h)
during this study, with acclimatization taking placed within
two months and a total study duration of six months. Trials
for different MLSS levels were conducted to obtain biofouling
conditions. The HRT of the anaerobic, anoxic, and aerobic
reactors, as shown in Table 2, were 11, 7, and 8 hours
respectively. The flat sheet membrane (15cm 10cm) used
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A. Damayanti et al. 9 The effect of mixed liquor suspended solids (MLSS) on biofouling in a hybrid membrane bioreactor
Water Science & Technology 9 63.8 9 2011
Figure 1 9 Schematic process flow diagram.
in this study had been kindly donated by Kubota, Inc. and was
vertically submerged in the bioreactor. The size of the membrane bioreactor is 24cm 10cm 90cm. The height between
the membrane module and the base of the reactor is 30 cm.
The TMP was measured using an online system by installing a
pressure transducer. For the aeration, the compressed air
supplied individually and the flowrate has been used is 12
LMH. Recirculation flow rate in this study was 1:3.
The system is operated for acclimatization to the new
media was about one a half months before starting experiments. The sludge divided into 4, for MLSS study 5, 10, 15
and 20 g L1. After that, the sludge placed in cool room.
Then the system operated using lowest MLSS first, 5 g L1,
acclimatized again around one to two weeks until steady state
phase, and finished it in one month. After that, the sludge was
replaced from reactor, using new sludge (from cool room)
which have acclimatized before, but it is still need acclimatized again around one to two weeks until steady state phase,
using MLSS 10 g L1, until finished in one month. The same
step as MLSS 15 and 20 g L1. We excess sludge withdrawal
and stationary MLSS for same SRT, 70 days. All step study
finished in four months excluded acclimatization in two
Table 2 9 The Specifications of the HMBR system
Item
Specifications
Membrane type
Flat sheet
Membrane material
Chlorinated Polyethylene
Membrane pore size
0.4 mm
Membrane area per module
0.1 m2
Number of modules
3
Effective volume of anaerobic reactor
50 L
Effective volume of anoxic tank
30 L
Effective volume of aeration tank
20 L
months. Flux 11 Lm2 h1 applied for operating condition
in HMBR.
During the studies, the fouled membranes were removed
from the aeration reactor and cleaned with a sponge followed
by water flush cleaning. The membranes were then soaked in
a 0.5% ppm NaOCl and left overnight (Ujang et al. 2005).
After this, the cleaned membranes were ready to be used
again in the process (Ujang et al. 2005).
Critical flux evaluation
The flux step method is used to determine the critical flux
(Le-Clech et al. 2003). The critical flux measured at the start
for each period. This carried out by increasing values of
permeate flux and recording the relative TMP variations.
For each flux step two TMP values were considered: the
initial TMP, corresponding to the initial sudden increase of
the filtration resistance, and the final TMP, i.e. the TMP at the
end of the step (Le-Clech et al. 2003). From these two TMP
values two parameters connected to fouling can be evaluated,
the average TMP and the rate of change of the TMP (dTMP/
dt). Any flux with dTMP/dtZ0.5 is defined as the critical flux
(Bottino et al. 2009). By using resistance series model as in
Equation (2), Rtot or total filtration resistance then could be
calculated (Chang & Kim 2005):
Rtot ¼ Rm þ Rc þ Rf
ð2Þ
where Rtot is the total filtration resistance, Rm is the intrinsic
membrane resistance (obtained from pure water permeability
measurements), Rc is the cake resistance, and Rf is the fouling
resistance due to irreversible adsorption and pore plugging
(Bottino et al. 2009). Rm and Rtot were calculated according to
Darcy’s law below:
R ¼ TMPm1 J 1
ð3Þ
40
35
30
25
MLSS 5 gL–1
20
MLSS 10 gL–1
15
MLSS 15
10
dTMP/dt (mbar/sec)
45
J (Lm–2h–1)
Water Science & Technology 9 63.8 9 2011
A. Damayanti et al. 9 The effect of mixed liquor suspended solids (MLSS) on biofouling in a hybrid membrane bioreactor
1704
18
16
14
12
10
8
6
4
2
0
MLSS 5 gL–1
MLSS 10 gL–1
MLSS 15 gL–1
MLSS 20 gL–1
0
5
gL–1
MLSS 20 gL–1
5
10
15
Jc (Lm–2h–1)
20
25
Figure 3 9 Trend of dTMP/dt (mbar/sec) as a function of the flux at MLSS concentrations 5–
20 gL1 with Y-error bar 5%.
0
0
100
200
300
TMP (mbar)
400
500
Figure 2 9 TMP and J behavior during ascending and descending runs at different MLSS
concentrations with Y error bar 5%.
where m is the fluid viscosity and J is the flux. Permeate flux (J)
applied from 0 to 40 Lm2 h1 for critical flux test, 15 minute
for one value constant. The critical flux measured at the start
each test period pf MLSS. Each MLSS concentration used
new membrane.
Samples analysis
Laboratory experiments were carried out in the Environmental Engineering Laboratory, UTM. All experimental analysis
was conducted according to Standard Methods (APHA 1998)
within three hours after sampling. MLSS concentrations were
regularly measured. Chemical Oxygen Demand, NO3-N,
NO2-N, NH4-N, and PO4-P were analysed using a spectrophotometer (HACH/DR 4000). The measurements were
carried out in duplicate according to HACH testing procedures. The average values were taken for different MLSS
concentrations between 5 and 20 g L1. Dissolved oxygen
concentration, temperature and pH were monitored using an
on-line measurement system, and the biotransformation was
observed using on-line respirometry. The DO probe used in
this work was the WTW Oxil oxygen meter. It was calibrated
with pure water before being used in the membrane bioreactor.
RESULTS AND DISCUSSION
Behaviour of membrane critical flux in MBR operation
Figures 2 and 3, and Table 3 show the influence of different
MLSS concentrations on the TMP and biofouling behaviour.
The values in Table 3 are calculated using Equation (3). The
results clearly demonstrate that the membrane biofouling rate
increases with an increase in the MLSS concentration. The
results obtained indicate that the MLSS concentration has a
significant influence on the trans membrane pressure (TMP).
Figure 2 shows the flux vs. TMP behaviour, during the
ascending and descending run of membrane with POME at
different MLSS concentrations. The differences between two
runs were significant.
During the ascending run, the flux first proportionally
increased with an increase in the TMP and leveled off, more
quickly at higher MLSS concentrations, due to membrane
biofouling. The detrimental effect of fouling on the membrane
flux is also apparent by observing the behaviour of the
membrane flux as a function of the applied TMP during the
descending run.
Figure 3 shows the trend of dTMP/dt as a function of the
flux determined from the ascen-ding runs of various hysteresis loops of Figure 2. From the reported trend the critical
flux, i.e. the flux corresponding to dTMP/dt ¼ 0.5 can
be easily calculated. The critical flux values are reported in
Table 3, along with the values of various resistances. Figure 3
also shows that by increasing the MLSS concentration from 5
to 20 g L1 the critical flux becomes four times lower.
Although the Rm value of the membrane was very low,
Table 3 9 Effect of MLSS concentration on critical flux and resistances of POME
No MLSS (g/L)
Jc (Lm2 h1) Rtot (1012 m1)
Rm (1012 m1) Rc þ Rf (1012 m1)
1 5
9.2
1.73
0.19
1.54
2 10
7.1
2.04
0.19
1.85
3 15
4.7
2.83
0.19
2.64
4 20
2.5
3.90
0.19
3.71
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A. Damayanti et al. 9 The effect of mixed liquor suspended solids (MLSS) on biofouling in a hybrid membrane bioreactor
cake formation and fouling occurred during the treatment
process. Table 3 demonstrates that higher MLSS will lead to
higher Rtot. The Rm value led to a significant increase of the
total resistance, since biofouling and cake formation take
place, especially at high MLSS concentration (Bottino et al.
2009). Unlike Bottino et al. (2009), Chang & Kim (2005)
showed that the Rm value not always led to a significant
increase of the total resistance, especially at low MLSS
concentration. This is in line also with Chang & Kim (2005)
that found that Rc, cake resistance, will decrease as MLSS
between 3.7 to 0.09 g L1 decrease.
Chang & Kim (2005) also showed that higher MLSS
concentrations lead to higher higher Rtot. The rate of TMP
increase was found to generally increase with increasing
MLSS concentration at each flux tested because of higher
suspended solids in the reactor (Bottino et al. 2009). Furthermore, at each MLSS concentration, dTMP/dt increased with
increasing flux-step. In addition, Rosenberger & Kraume
(2002) who found that higher SS concentration affected
lower F/M ratio and also lower EPS concentration, led to
better filterability.
þ
COD, NO 3 , NO 2 , and NH 4 removal efficiency at
different flux and MLSS
Removal efficiency (%)
þ
COD, NO
3 , NO2 , and NH4 removal efficiencies at different
MLSS are shown in Figure 4. Towards the end of the
experimental period the feed COD concentration had an
average of 40.5 g L1, while the effluent concentration was
kept in the range 200 to 350 mg L1 depending on the POME
source quality and MLSS used in the MBR. These mean
values here is continuous with acclimatized sludge from lab
scale hybrid MBR. This effluent quality need to be treated
further to meet effluent quality standard (Environmental
Quality Regulation 2009).
Figure 4 also demonstrates a COD removal efficiency of
up to 99%. The NO3-N effluent concentration depends on the
Water Science & Technology 9 63.8 9 2011
POME source quality and the average removal rate is up to
97%. This is comparable with other studies, where NO
3
removal using aerobic treatment was up to 95% (Kumbasar
2009) and 95% NH4þ removal (Sarioglu et al. 2009). In Figure
4 it is shown that the rate of removal efficiency generally
increases with increasing MLSS concentration at each flux
tested because of higher biomass activity working in the
reactor (Alaboud 2009) Furthermore, at each MLSS concentration, dTMP/dt increased with increasing flux-step. This is
why balancing the biofouling tendency and effluent quality is
needed to reduce operation and maintenance costs in a
wastewater treatment plant (Ujang et al. 2008). It is not a
big difference but it is important for the effluent quality at
high MLSS concentrations. However it is also important to
consider the biofouling effect at high MLSS concentrations.
CONCLUSIONS
This study has indicated that the membrane biofouling rate
increases with an increasing MLSS concentration, as
observed through the critical flux analysis and extensive
experimental results for POME. During the experiments of
increasing MLSS the flux first increased proportionally with
the increase in TMP and levelled off, more quickly at higher
MLSS concentration, due to membrane biofouling. The detrimental effect of fouling on the membrane flux is also apparent
by observing the behaviour of the membrane flux as a function of the applied TMP during the descending run. The rate
of change in the TMP was found to generally increase with
increasing MLSS concentration at each flux tested. The study
also showed that the removal increased with increasing
MLSS concentrations. In general, the results from this
paper indicate the need to balance the biofouling tendency
and the effluent quality to reduce operation and maintenance
costs in wastewater treatment plants, for high strength wastewater such as POME.
ACKNOWLEDGEMENTS
100
99.5
99
98.5
98
97.5
97
96.5
COD
NO3–
NO2–
NH4+
5
10
15
MLSS (gL–1)
20
þ
Figure 4 9 COD, NO
3 , NO2 , and NH4 removal efficiency at different MLSS concentrations of
the POME with Y error bar 0.2%, respectively.
This study was made possible by the financial support from
the Ministry of Science of Technology (MOSTI) Malaysia
through eScience funding (Vot 79006) and Islamic Development Bank for providing scholarship. Thanks to Mr. Zulkifli
Ahmad for special assistance and discussions, and thanks to
Institute Technology of Sepuluh Nopember for supporting the
first author. Membrane modules were kindly donated by
Kubota Inc.
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A. Damayanti et al. 9 The effect of mixed liquor suspended solids (MLSS) on biofouling in a hybrid membrane bioreactor
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