1

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 60

th 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.

2

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

3

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

4

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 =QPermeate

AMembrane

Equation 1

5

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.

!

"W

T

"W

20oC

= e(-0.0239(T -20)) Equation 3

where,

T = Temperature of Water (oC)

!

"W

T = Absolute Viscosity of Water at T (kg/m.s)

6

!

"W

20oC = 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.

!

LP

20oC =

1

"W

20oCRT

=J # e

-0.0239 T -20( )( )

$P Equation 4

where,

!

LP

20oC

= 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.

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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.

8

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

9

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.

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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

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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

12

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

13

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

14

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.

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Table 1. Water Quality of Primary Effluent from the Full-Scale Composite Sample

Analyte Units # of Samples Median

Ammonia mg/L N 41 28.6 21.2 - 32.0

Biological Oxygen

Demandmg/L 315 115.0 52.2 - 261

o-Phosphate mg/L P 49 2.87 0.41 - 3.66

pH - 319 7.71 7.52 - 7.92

Total Suspended

Solidsmg/L 320 74.0 22.0 - 168

Turbidity NTU 319 60.0 24.0 - 117

Volatile Suspended

Solidsmg/L 320 64.0 22.0 - 112

Range

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Table 2. Summary of Analyses Performed and Method Used

Analyte or Parameter Method Number and Type Detection Limit Units

Ammonia EPA350.1 0.015 mg N/L

Biological Oxygen

DemandSM5210B 2 mg/L

Nitrate 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

DemandSM5220D 5 mg/L

Total and Volatile

Suspended SolidsSM2540D&E 1.6 mg/L

Dissolved Organic

CarbonSM5310B 0.25 mg/L

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Table 3. Specifications for ZeeWeed -10 Membrane Module

Module Specification Value

Total membrane surface area 0.93 m2 (10 ft

2)

Membrane surface chemistry Neutral and hydrophilic

Nominal Pore Size 0.035 µm

Maximum transmembrane pressure 0.62 bar @ 40°C

Typical operating transmembrane pressure 0.07-0.48 bar @ 40°C

Maximum operating temperature 40°C

Operating pH range 5-9

Cleaning pH range 2-10.5

Maximum Hypochlorite Exposure 1,000 ppm

Maximum TMP backwash pressure 0.62 bar

18

Figure 1. Conventional process and SMBR with comparable product water quality

19

Figure 2. Fundamental Illustration of EMBR

20

Figure 3. Process Schematic of SMBR Pilot Units (pumps not shown)

21

Figure 4. SMBR #1 solids concentration and membrane operation during high solids experiment

22

Figure 5. SMBR #2 solids concentration and membrane operation during high solids experiment

23

Figure 6. SMBR #1 solids concentration and membrane operation during low solids experiment

24

Figure 7. COD data for both phases of testing

25

Figure 8. Turbidity data for both phases of testing

26

Figure 9. BOD5 data for both phases of testing

27

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

28

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