Process Limits of Municipal Wastewater Treatment
with the Submerged Membrane Bioreactor
Manuscript number: EE/2003/023623
By
R. Shane Trussell, P.E.1
Samer Adham, Ph.D2
R. Rhodes Trussell, Ph.D, P.E.3
1
Doctoral Candidate, University of California, Berkeley, 377A 60th Street, Oakland, CA 94618
2
Vice President, MWH, 300 North Lake Avenue, Pasadena, CA 91101
3
President, Trussell Technologies, Inc., 939 East Walnut Street, Pasadena, CA 91106
1
Subject Headings
Membrane Processes, Activated Sludge, Suspended Solids, Fouling, Residence Time,
Wastewater, Organic Loads, Chemical Oxygen Demand
Abstract
The submerged membrane bioreactor (SMBR) is a promising technology for wastewater
treatment and water reclamation. This paper presents results from two pilot scale SMBR systems
operating in parallel on municipal wastewater in San Diego, CA. The SMBRs were operated to
address the limitations and advantages of the SMBR process compared to conventional activated
sludge processes. Minimal membrane fouling was observed through out the year of testing with
the exception of the process limitations. Both pilot units provided consistently high quality
effluents throughout the study, even when operating at hydraulic retention times (HRT) as low as
1.5 h. Two sets of experiments were conducted to identify different fouling conditions. The first
experiments were conducted to explore operation at high suspended solids concentrations. The
SMBR process experienced adverse performance at mixed liquor suspended solids (MLSS)
concentrations greater than approximately 20 g/L. The second experiments explored operation at
low mean cell residence time (MCRT). At an MCRT of <2 d, membrane fouling was rapid.
Chemical cleaning with sodium hypochlorite solution provided full recovery of the membrane
permeability.
1
Keywords
Ultrafiltration, Submerged Membrane Bioreactor, External Membrane Bioreactor, Nitrification,
Membrane Flux, Membrane Permeability, Hydraulic Retention Time, Mean Cell Residence
Time, Mixed Liquor Suspended Solids, and Mixed Liquor Volatile Suspended Solids
Introduction
Membrane bioreactors are an innovative wastewater treatment process that eliminates one of the
principle limitations of contemporary activated sludge, namely, the requirement for gravity
separation. The membrane bioreactor (MBR) process uses a microfiltration (MF) or
ultrafiltration (UF) membrane to perform the solid-liquid separation and combines activated
sludge, clarification, filtration and MF/UF into one unit operation (see Figure 1). The MBR
process offers 4 key advantages over conventional activated sludge processes:
1) Compact footprint – the combination of several unit operations into one and the ability to
operate at elevated solids concentration significantly reduces the footprint of the MBR
process compared to conventional processes (vanDijk and Roncken 1997).
2) Perfect solid barrier – the study of the sedimentation properties of floc has been the
concern of every wastewater professional since the development of biological treatment.
Using membranes for solids separation, poor sludge settleability is no longer a concern
and operation with high solids concentrations in the mixed liquor is possible (Adham et
al. 2001; Cote et al. 1997; Trussell et al. 2000).
3) Removal of difficult to degrade organics – the MBR process enables a wide-range of
operating conditions that makes the removal of organic compounds, previously too slow
2
to degrade, possible. MBR technology can treat high molecular weight compounds,
leachate wastewater and even oil contaminated wastewater (Cicek et al. 1998; Scholzy
and Fuchs 2000; Seo et al. 1997; vanDijk and Roncken 1997; Winnen et al. 1996;
Wintgens et al. 2002). In addition, the high solids concentrations in the MBR make this
process robust in the treatment of concentrated industrial wastewaters (Sutton et al.
2002).
4) Expanded range of operation – the gravity clarification step in conventional activated
sludge processes limits the range of MCRTs and HRTs that are possible for treating
wastewater. The MBR process provides perfect solids separation at high solids
concentrations and expands the current operating realm of the activated sludge process.
The MBR process is commercially available in two configurations: external membrane
bioreactor (EMBR) and submerged membrane bioreactor (SMBR). The EMBR is also referred to
as an “In-Series”, “Side-Stream”, “Conventional”, or “Cross-Flow” MBR, but simplification of
the terminology has lead to a more accurate description of the process as an EMBR. The EMBR
process pumps a large recycle flow of mixed liquor from the reactor to an external membrane
module for solid-liquid separation (Figure 2). The EMBR process was the original MBR
configuration and early applications of the MBR process treated industrial wastewaters and
landfill leachate (vanDijk and Roncken 1997). Using a pump to provide cross-flow, the EMBR
process is capable of operating at relatively high membrane fluxes with typical values of 50 –
150 l/m2.h (vanDijk and Roncken 1997). However, in order to maintain adequate shear at the
membrane surface, recirculation flows for the EMBR process are often 20-30 times the product
water flow. The high recirculation flows associated with this process increase energy
consumption, making EMBR uneconomic for large municipal wastewater applications (Gander
3
et al. 2000). The innovation of the SMBR configuration in 1989 significantly reduced the
operational costs of the MBR process, making the SMBR the configuration of choice for
municipal wastewater treatment (Yamamoto et al. 1989). Using coarse bubble aeration to
provide the cross-flow, the inefficiencies of pumping large recycle flows have been eliminated
and significantly reduced power requirements from 7 to 0.3 kWh/m3, for EMBR to SMBR,
respectively(Cote et al. 1997; vanDijk and Roncken 1997).
There are approximately 500 MBR installations worldwide with a majority of large-scale MBR
applications focused in Europe and the United States. Italy has a 10 MGD SMBR facility
treating municipal wastewater using Zenon Environmental’s membrane technology. Kubota,
another large SMBR manufacturer, has a number of installations in the UK with the largest plant
capacity of 3.5 MGD in Dorset, UK. Although the number of MBR installations is growing
logarithmically, the limitations of the SMBR process for the treatment of municipal wastewater
are not well understood. The focus of this project was to investigate possible operating regimes
to aide design engineers.
Materials and Methods
Membrane Flux and Vacuum Pressure
In membrane systems, the flux, or flow rate of water through a given area of membrane, is a
decisive parameter to accurately determine the process design. The membrane flux was
calculated as follows:
J=
Q Permeate
AMembrane
Equation 1
4
!
where,
J = Membrane Flux (L/m2 .h or m/s)
QPermeate = Membrane Permeate Flow (L/h or m3/s)
AMembrane = Outside Membrane Surface Area (m2)
To assess membrane performance, it is important to describe the driving force required
(transmembrane pressure) to pass water through the membrane at a constant temperature.
Equation 2 is the commonly employed Darcy’s law for filtration, illustrating the influence of
temperature on membrane flux. Increased viscosity, as a result of decreasing water temperature,
will increase the pressure required to maintain a constant membrane flux for a given hydraulic
resistance.
J=
"P
#W $ R T
Equation 2
where,
ΔP = Transmembrane Pressure (Pa)
ηW = Absolute Viscosity of Water (kg/m.s)
RT = Total Hydraulic Resistance (1/m)
The resistance term, RT, is a characteristic of the membrane and is not a function of temperature.
However, the viscosity of water is a function of temperature and was normalized to 20oC using
equation 3. Equation 3 is accurate within 5% error for a temperature range of 5 to 40oC.
"WT
= e(-0.0239(T -20))
20 o C
"W
where,
T = Temperature of Water (oC) !
"WT = Absolute Viscosity of Water at T (kg/m.s)
5
!
Equation 3
"W20
o
C
= Absolute Viscosity of Water at 20oC (kg/m.s)
Membrane permeability, LP, is the ratio of membrane flux divided by the transmembrane
!
pressure required and has been normalized to 20oC combining equation 2 and equation 3.
20 o C
P
L
=
1
o
"W20 CR T
=
J # e(
-0.0239( T -20))
$P
Equation 4
where,
o
!
L20P C = Membrane Permeability at 20oC (L/m2 .h.bar or m/s.Pa)
Test Site
The test site was the Aqua 2000 Research Facility at the North City Water Reclamation Plant
(NCWRP) in San Diego, California. The NCWRP is a state of the art, conventional wastewater
reclamation plant with headworks, primary sedimentation, activated sludge, and tertiary
filtration, located in the heart of an affluent commercial area. The NCWRP operates the
activated sludge process at an MCRT of 7 d with an HRT of 6-8 h, resulting in typical MLSS
concentrations of 2 g/L. The filters are 7 feet deep with anthracite filter media and the plant
produces reclaimed water that meets the Title 22 criteria of the State of California. The NCWRP
began operation in January of 1997 with a design capacity of 30 MGD and data collected from
the NCWRP during this study were used to provide a process comparison between the SMBR
and conventional activated sludge with filtration. The two pilot-scale MBR units were located at
a site that had direct access to the primary effluent of the full-scale plant.
6
Feed Water Characteristics
The SMBR pilots treated primary effluent from the NCWRP with no additional pre-screening.
The NCWRP receives mainly domestic sewage, however, some industrial sewage is also present
due to the large industrial community in the surrounding area. Table 1 summarizes the influent
wastewater quality characterized by the composite primary effluent sample.
Process Monitoring
The transmembrane pressure was monitored using a vacuum gauge installed in the permeate line
and the temperature of each reactor was measured in-situ using a portable thermometer. Data
was recorded daily on all flows and pressures. Table 2 presents the lab analyses that were
performed and are presented in this paper along with the method used for each. The mixed
liquor suspended solids (MLSS) and mixed liquor volatile suspended solids (MLVSS) were
performed three times per week. The nitrate, nitrite, ammonia, pH and turbidity for both SMBR
units were monitored five times per week. Data presented from the full-scale plant is courtesy of
the NCWRP staff and is routinely collected for process performance.
Description of MBR Pilot Units
Two MBR pilot units were custom designed and constructed to meet the needs of the research
team. In each process tank, four ZeeWeed®-10 modules were submerged in the reactor and
operated under vacuum pressure at a constant flux. The ZeeWeed®-10 modules were polymeric
ultrafiltration membranes with a nominal pore size of 0.035 µm and a total surface area of 0.93
m2 (10 ft2). Complete membrane specifications are presented in Table 3.
7
The membranes were operated with a production cycle of 15 min and a backpulse (backwash)
duration of 15 s. The vacuum pump applied suction to the membranes for 15 min, producing
permeate via the direct ultrafiltration of the mixed liquor. At the end of each 15 min cycle, the
valves changed position to reverse the flow back through the membranes, pushing permeate from
the clean in place (CIP) tank through the membranes and into the process tank for 15 s. By
reversing the flow through the membranes, the backpulse process helped reduce the effects of
long term fouling. Figure 3 is a simplified illustration of the pilots that were constructed. In
order to maintain a constant flux of 33.9 L/m2.h (20 gfd or 9.43x10-6 m/s), a design was required
which allows the hydraulic retention time (HRT) to be independent of the membrane flux. By
pumping permeate from the membranes into an overflow tank, the HRT of the system can be
changed without altering the membrane flux. The feed flow to the system was controlled to
maintain the bioreactor at a constant level. Since the waste rate (see QWaste, Figure 3) is
negligible compared to the effluent flow from the system (see QEffluent, Figure 3), the feed rate
(see QFeed, Figure 3) is equal to the flow pumped out of the overflow tank, thus determining the
HRT. Any excess permeate returns to the bioreactor to maintain the tank level and the HRT.
The flux is independently controlled by the flow rate of the vacuum pump, QPermeate.
Each reactor had an active volume of 70 gallons with a depth of 1 meter. To control the short
term fouling due to cake build up and compression, each membrane module was scoured with
0.94 L/s (2.0 ft3/min) of coarse aeration. In addition to controlling membrane fouling, the coarse
aeration ensured the bioreactor was well mixed. Due to the poor oxygen transfer rate of the
coarse aeration, fine air diffusers were also installed to maintain the dissolve oxygen
8
concentration above 2 mg/L. Pure oxygen was supplemented via the fine air diffusers when
required.
Membrane Cleaning
Maintenance cleans were performed on a weekly basis to further reduce the effects of deep pore
and long term fouling. The maintenance cleans were performed in-situ by pumping 1L/min of
200 mg/L NaOCl solution back through the four ZeeWeed®-10 modules for 1 min. Pumping was
stopped and the NaOCl solution was then left inside the membrane lumen for 9 min. The 200
mg/L NaOCl solution was refreshed (1 min at 1L/min) 6 times for a total of 1 h. Between major
experiments, both membranes were cleaned by soaking the membranes for 4-6 h in a 1000 mg/L
NaOCl solution. Fouling by metal precipitates was not observed with this wastewater. As a
result, cleanings with citric acid were not needed to restore membrane permeability.
Results and Discussion
Although the SMBR process has proven to be a reliable wastewater treatment process at long
sludge ages, it is not clear under what spectrum of operating conditions the SMBR process can
be implemented. A clear advantage of the SMBR process is the elevated solids concentrations
achievable under normal conditions. A goal of this work was to understand the extent to which
this advantage could be exploited to minimize 1) sludge production and 2) the hydraulic
retention time. Another focus of this study was the potential operation of the SMBR process
without nitrification. Wastewater treatment plants are often designed for young sludge ages to
prevent nitrification, substantially reducing the oxygen demand.
9
Before the experiments were begun, both pilot units were operated in parallel, under the same
operating conditions, to achieve steady state and demonstrate comparable results. Two sets of
experiments were conducted, each with two reactors operating in parallel (SMBR #1 and SMBR
#2). The first set of experiments addressed operation with high solids concentrations. In this set,
SMBR #1 was operated without wasting until the process failed due to high solids, while the
solids concentration in SMBR #2 was increased by steadily reducing the HRT, holding the SRT
constant, until this process also failed due to high solids. The second set of experiments
addressed operation at low MCRTs. In this set, the MCRT of SMBR #1 was reduced until the
process failed. During this experiment SMBR #2 operated as a control.
Control Test
Before the experiments, both SMBR pilots were operated for 3,000 h at mean cell residence
times (MCRTs) of 35 d with hydraulic retention times (HRTs) of 4 h. SMBR # 1 continued to
operate under these conditions for another 1,000 h. Figure 4 and Figure 5 present the solids
concentrations in each SMBR throughout the testing period. Under these operating conditions,
the MLSS stabilized around 10 g/L and no membrane fouling was observed. Figures 4 and
Figure 5 also present the membrane flux and permeability. These tests confirmed that both units,
when operated under the same conditions and on the same influent, produce comparable results
and stable operation was obtained at these conditions.
High Solids Experiments
These experiments began after both SMBRs had been through the stabilization period just
described. The first experiment was conducted by halting the wasting of mixed liquor from
10
SMBR #1 to observe the effects of high solids concentrations on membrane performance.
Membrane fouling occurred when the MLSS exceeded 20 g/L and approached 25 g/L (compare
MLSS and permeability in Figure 4).
In a second experiment, conducted in SMBR #2, the HRT was reduced in steps from 4 h to 1.5 h
over a 1,700-hour period. The SRT was held constant at 35 d and the dissolved oxygen was
maintained at 2 mg/L or greater throughout the test. Once the HRT dropped below 2 h, pure
oxygen was required but membrane fouling did not occur. Membrane fouling did occur when
the HRT dropped to 1.5 h and the solids concentration reached 20 g/L, approaching 25 g/L
(Figures 5).
The hydraulics of the pilot membrane system, the shear force at the membrane surface (coarse air
flow rate and header design), the characteristics of the activated sludge, and the membrane flux
influence membrane fouling under these high solids concentrations. Under different growth
conditions, SMBR #1 approaching an SRT of 100 d and SMBR #2 remaining constant at 35 d,
membrane fouling occurred at the same solids concentration, approximately 20 g/L. A
concentration of 20-25 g/L as a solids limit agrees with literature available on the hydraulics of
Zenon Environmental’s SMBR process (Mourato et al. 1999). With the SMBR configuration,
cross-flow is provided by coarse air introduced to the header beneath the membrane module. In
this study, a coarse airflow rate of 0.94 L/s (2.0 ft3/min) was introduced into each of the four
membrane modules. Ueda et. al. has shown that increasing the air flow or air intensity (air
flow/area) can increase permeability under high solids conditions (Ueda et al. 1997). In these
11
experiments the SMBRs were operated at the manufacturer’s recommended operating conditions
and the effect of higher air rates was not explored.
Low Solids Experiments
Following the high solids experiments, the sludge was removed and the membranes in both
SMBRs were chemically cleaned and then recharged with return activated sludge from the
NCWRP. SMBR #2 was then operated as a control with an MCRT of 10 d and an HRT of 4 h
during the duration of these low solids experiments. SMBR #1 was first operated at an MCRT of
5 d with an HRT of 4 h. Figure 6 presents the solids concentrations observed during this phase,
which stabilized around 4 g/L. Upon achieving steady state (3 MCRTs), the MCRT of SMBR #1
was reduced to 4 d and no membrane fouling had occurred up to this point (Figure 6). The
MCRT was then reduced to 3 d and examination of ammonia and nitrite data began to indicate
that the process was close to losing nitrification. The MCRT was subsequently reduced to 2 d
and the membrane performance remained unaffected. During the operation at longer MCRTs,
wasting had been accomplished on a batch basis. Beginning with an MCRT of 2 d a new
wasting system was installed that provided wasting at a constant flow from the aeration tank.
This new wasting system improved operational stability at these lower
MCRTs. With a new wasting system installed, the MCRT was further reduced to 1.5 d and
membrane fouling began (see permeability in Figure 6).
Fouling under low MCRT conditions is likely due to the effects of partially degraded organics
combined with the effects of a reduced filtration cake. Previous work has demonstrated that
increased effluent COD values and increased fouling rates are obtained in the MBR process as
the MCRT is lowered (Cicek et al. 2001; Lee et al. 2001; Lu et al. 2001). Cicek et al. 2001
12
found that the effluent COD increased dramatically from 3.54 mg/L to 22.9 mg/L for an MCRT
decrease from 5 to 2 d, respectively. For the work presented here, the effluent COD value was
only slightly affected by the decreasing MCRT (Figure 7), but it is well known that the effluent
COD from the activated sludge process increases as the MCRT approaches the limiting MCRT.
Under the MCRT conditions tested, organic fouling is likely the mechanism. In addition the
MLSS concentration in the low solids experiments was only 1 g/L and this low concentration
reduced the effectiveness of the filtration cake, allowing the soluble organics to reach the
membrane surface more quickly, aggravating the situation.
Water Quality
The SMBR pilot units provided a consistent, high-quality effluent through out the entire duration
of the study (Figure 8 and Figure 9). The SMBR process did not experience the large
fluctuations in effluent turbidity and BOD5 that the conventional plant experienced while
continuously treating the same wastewater. The perfect barrier that the membrane provides for
the solid-liquid separation produces an effluent that is capable of meeting the most stringent
discharge requirements.
Conclusions
Two limiting conditions of the SMBR process were determined through this work. The first is a
rapid loss of membrane permeability at high solids concentrations and this occurred above
approximately 20 g/L under the conditions tested. The high solids limit was reached in two
different reactors, operated under different conditions to increase the solids level. The second
limit was determined for low MCRT operation where rapid membrane fouling occurred at an
13
MCRT < 2 d. The SMBR process experienced minimal fouling during the year of testing except
at the process limitations. The SMBR process provided complete organic degradation and
nitrification at an HRT of 1.5 h. In addition, the SMBR process produced a consistent effluent
quality, equivalent or superior to the tertiary wastewater effluent from the full-scale wastewater
reclamation plant.
Acknowledgements
The authors would like to acknowledge the Water Environment Research Foundation who
providing funding for this work, Project #98-CTS-5. The authors would also like to
acknowledge the City of San Diego for operational and lab support provided through out the
project. In addition, the authors would like to express their gratitude to Dr. Jenkins for his
contributions to this work.
14
Table 1. Water Quality of Primary Effluent from the Full-Scale
Composite Sample
Analyte
Ammonia
Biological Oxygen
Demand
o-Phosphate
pH
Total Suspended
Solids
Turbidity
Volatile Suspended
Solids
Units
mg/L N
# of Samples
41
Median
28.6
Range
21.2 - 32.0
mg/L
315
115.0
52.2 - 261
mg/L P
-
49
319
2.87
7.71
0.41 - 3.66
7.52 - 7.92
mg/L
320
74.0
22.0 - 168
NTU
319
60.0
24.0 - 117
mg/L
320
64.0
22.0 - 112
15
Table 2. Summary of Analyses Performed and Method Used
Analyte or Parameter
Ammonia
Method Number and Type Detection Limit
Units
EPA350.1
0.015
mg N/L
SM5210B
2
mg/L
EPA300A
0.2
mg/L
Nitrite
SM4500B
0.005
mg/L
o-Phosphate
EPA300A
0.2
mg/L
Total Phosphorus
EPA365.1TP
0.07
mg/L
Total Kjeldahl Nitrogen
EPA351.2
0.08
mg N/L
Chemical Oxygen
Demand
SM5220D
5
mg/L
Total and Volatile
Suspended Solids
SM2540D&E
1.6
mg/L
Dissolved Organic
Carbon
SM5310B
0.25
mg/L
Biological Oxygen
Demand
Nitrate
16
Table 3. Specifications for ZeeWeed -10 Membrane Module
Module Specification
Total membrane surface area
Value
0.93 m2 (10 ft2)
Membrane surface chemistry
Neutral and hydrophilic
0.035 µm
Nominal Pore Size
0.62 bar @ 40°C
Maximum transmembrane pressure
Typical operating transmembrane pressure
0.07-0.48 bar @ 40°C
40°C
Maximum operating temperature
Operating pH range
5-9
Cleaning pH range
2-10.5
Maximum Hypochlorite Exposure
1,000 ppm
Maximum TMP backwash pressure
0.62 bar
17
Figure 1. Conventional process and SMBR with comparable product
water quality
18
Figure 2. Fundamental Illustration of EMBR
19
Figure 3. Process Schematic of SMBR Pilot Units (pumps not shown)
20
Figure 4. SMBR #1 solids concentration and membrane operation
during high solids experiment
21
Figure 5. SMBR #2 solids concentration and membrane operation
during high solids experiment
22
Figure 6. SMBR #1 solids concentration and membrane operation
during low solids experiment
23
Figure 7. COD data for both phases of testing
24
Figure 8. Turbidity data for both phases of testing
25
Figure 9. BOD5 data for both phases of testing
26
Figure 1. Conventional process and SMBR with comparable product
water quality
Figure 2. Fundamental Illustration of EMBR
Figure 3. Process Schematic of SMBR Pilot Units (pumps not shown)
Figure 4. SMBR #1 solids concentration and membrane operation
during high solids experiment
Figure 5. SMBR #2 solids concentration and membrane operation
during high solids experiment
Figure 6. SMBR #1 solids concentration and membrane operation
during low solids experiment
Figure 7. COD data for both phases of testing
Figure 8. Turbidity data for both phases of testing
Figure 9. BOD5 data for both phases of testing
27
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