Technical
Paper
Membrane Bioreactor
Performance Compared to
Conventional Wastewater
Treatment
Authors: Thomas C. Schwartz and Brent R. Herring,
Woodard and Curran Incorporated
Ricardo Bernal and Janet Persechino, GE
Introduction
Governor Dummer Academy (GDA) is the oldest
private day and boarding school in the United
States. Founded in 1763, it is located on a 350-acre
campus in Byfield, Massachusetts. The wastewater
treatment plant (WWTP) that served the Governor
Dummer Academy campus was originally constructed in the 1960s, with upgrade work performed in the 1990s. Despite the upgrade efforts,
the WWTP was unable to consistently meet the
effluent quality required by the permit under which
it operates. This school was faced with the major
dilemma of having to increase the throughput
capacity of their current conventional wastewater
treatment system in order to consistently meet the
required discharge quality. At the same time, the
school was faced with building constraints due to
the wetlands that surround the WWTP, and,
therefore, could not increase the physical size of
the plant.
After a thorough review of the options available
to the school including prefabricated package
treatment units, sequencing batch reactors and
moving bed bioreactors, it was decided that the
submerged membrane bioreactor (MBR) system
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was the best alternative for this application. In
August of 2000, the installation of a 380 m3/day
(100,000 gpd) MBR system used to treat domestic
wastewater was completed. The new MBR-based
facility was renamed the David A. Gaouette
Wastewater Treatment Facility. Steady state operations were achieved during the month of September 2000, and the facility has been successfully
operating since.
The MBR approach to upgrading this facility
allowed the academy to actually slightly decrease
the WWTP footprint by maintaining the use of the
existing treatment tank and removing the former
filtration system building and tankage.
The MBR system has successfully increased the
WWTP treatment capacity, enabling the academy
to meet tough permit discharge limitations for BOD,
TSS, (<10 mg/L) and Total and Fecal Coliform
(<10 FCU/100 ml sample).
The David A. Gaouette Wastewater Treatment
Facility is now serving not only as the GDA treatment plant, but also as a showcase facility and a
research site for MBR process optimization. Influent
and effluent parameters are being continually
examined to refine the optimum operating conditions, and permeate water samples are monitored
every week to ensure that all environmental discharge limits are met. The objective of this paper is
to report the first year of operation of the plant and
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to compare the MBR performance with the
conventional plant that was previously operated at
the school.
submerged MBR system at Governor Dummer
using a 0.4 micron pore size polyethylene hollow
fiber membrane.
The Conventional Process
The MBR replaces the secondary clarification. The
MBR separates treated effluent from the mixed
liquor solids utilizing a hollow fiber microfiltration
membrane with a 0.4-micron pore size. The submerged membranes are typically placed directly
into the existing aeration tank. The membranes
allow the purified water to pass through the pores
(permeate), while creating a complete barrier to the
passage of any solid greater than 0.4-microns,
which includes almost all bacteria (mixed liquor solids). The permeate is drawn through the membranes using a suction lift pump leaving the
suspended biomass material in the aeration tank.
Biomass (mixed liquor) is removed using a sludge
pump on an as-required basis. Figure 1 gives a
basic flow sheet of a typical MBR chamber.
A typical treatment train for municipal and domestic
wastewater treatment is generally broken into primary, secondary, and tertiary treatment levels.
1. Primary treatment is the removal of floating
and settleable solids through processes including screening and sedimentation.
2. Secondary treatment is typically the aerobic
biological treatment process by which bacteria
oxidize the organic matter in the wastewater,
producing cell mass (sludge) and carbon dioxide. In suspended growth systems, the bacteria
are maintained in an aeration basin and
referred to as mixed liquor. Blowers supply air
to the mixed liquor to supply the necessary
oxygen. The bacteria are usually separated
from the purified wastewater in a clarifier. The
purified water is discharged to the next step
and the sludge is returned to the aeration basin for reuse and a small portion is removed for
disposal (waste).
3. Tertiary, or advanced treatment, includes processes beyond secondary treatment, most often
to remove specific constituents or improve the
quality of the final effluent. It is most often a
form of filtration.
The MBR Process
There are two popular types of MBR processes. A
submerged system consists of a microfiltration (MF)
or ultrafiltration (UF) membrane with pore sizes
ranging from 0.1 – 0.4 microns. These membranes
are submerged in the reaction tanks, with the
permeate being drawn into the membranes using
a vacuum capable pump. Tubular systems are also
available. These systems will treat a side stream of
the mixture in the aeration tank. This type of system requires a high amount of pumping power to
keep the velocities high to prevent membrane fouling, and high pressure to force the water through
the membrane. In addition, tubular systems have a
larger footprint than submerged systems due to
the external location of the membranes. For these
reasons, the decision was made to install a
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Figure 1: Typical MBR Process
In a conventional wastewater treatment plant, the
secondary clarifier limits the solids concentration in
the aeration tank. Typical mixed liquor suspended
solids (MLSS) concentrations are 1,500 mg/l to
5,000 mg/l. The larger the clarifier relative to hydraulic and solids loading of the facility, the higher
the possible concentration in the aeration tank.
Membranes create a solids barrier, and, therefore,
are not subject to gravity settling solids limitations,
as in conventional clarifiers. MBRs are limited
instead by the fluid dynamics of high solids mixed
liquor, the effects on the ability to get permeate
through the membrane, premature fouling of the
membranes, and the effect on oxygen transfer.
Typical MLSS concentrations in MBR systems are
10,000 mg/l to 15,000 mg/l and have been
reported to be as high as 20,000 mg/l in certain
instances.
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Hydraulic retention times (HRT), the amount of time
the wastewater spends in the system, for MBRs are
typically 4-20 hours. On most domestic wastes, this
is enough time to allow for the oxidation of organic
material and ammonia (nitrification). The average
sludge age, the time the biomass spends in the
aeration tank, or sludge retention times (SRT) is
15-45 days. The older sludge ages and the higher
MLSS concentrations in the MBR process compared
to conventional systems enable the MBR to
produce significantly less sludge for disposal than
conventional treatment systems.
tain operation, or any time the TMP across the
membrane increases more than 30 kPa (4.4 psi)
above the start-up pressure.
As mentioned previously, the limitations on MLSS
concentration and HRT are based on the solids
content of the mixed liquor and the effects on permeate flow, fouling, and oxygen transfer. In high
concentrations of MLSS, the permeate flow can be
limited by the physical presence of solids at or
near the individual pores which restricts flow.
In addition, higher MLSS concentrations have a
higher propensity to foul the membranes and restrict flow of water and air at the surface of the
membrane rendering the air scouring ineffective at
cleaning the membranes.
MBRs were developed in the late 1980s and are
now being used on wastes ranging from livestock
waste with influent BODs of 18,000 ppm, industrial
wastes with BODs from 5,000 - 10,000 ppm, and
domestic wastes with BOD ranges of 200-600 ppm.
This technology has served customers who need
integration of water and wastewater management
for total water management systems. These systems must be consistent in producing water with
low BOD and TSS, need to minimize space usage,
and need to minimize sludge disposal amounts.
When the solids concentration becomes too high,
the flow of permeate will become restricted and
require a greater vacuum to initiate flow. Because
of this, the pressure differential across the membranes, or the trans-membrane pressure (TMP),
is continuously monitored and used to gage the
degree of fouling in a system that is operating
within acceptable solids concentrations. Under
normal operation, the rate of fouling of the MBR
membranes is reduced by injecting the blower air
directly under the membranes. The continual agitation caused by the flow of air and water over the
membrane surface serves as a surface scour. In
addition, the permeate pump runs for eight minutes and then is turned off for two minutes. This
allows the membranes to “relax,” and with no vacuum on the membranes, the air scour has a
greater impact. Despite this continuous cleaning,
a gradual accumulation of organic substances such
as glycoprotein will occur at the membrane surface. For this reason, in-situ chemical cleaning of
the membranes is recommended. Cleanings consist of in situ reverse flow of a dilute (3,000 ppm,
0.3% solution) sodium hypochlorite solution through
the membranes for two hours. The cleanings
should occur roughly every three months to mainTP1036EN 0601
In addition, out-of-tank cleanings will be occasionally performed by removing the membranes from
the aeration tank and soaking the membranes in a
dilute (5,000 ppm, 0.5% solution) sodium hypochlorite and 4% (40,000 ppm) sodium hydroxide solution for 12-15 hours. This should be done when the
in-situ chemical cleanings do not show significant
reduction in the TMP.
Description of the Conventional
Governor Dummer Academy
Wastewater Treatment Plant
The treatment plant as it was prior to the MBR
upgrade was a small conventional suspended
growth type treatment plant built in the early
1960s and is illustrated in Figure 2. As depicted in
Figure 3, the plant consisted of the following operations and tankage:
• Grinder (bypassed)
• Influent flow measurement manhole–flume insert
• Flow equalization tank
• Common-wall secondary tankage
– Aeration basin
– Secondary clarifier
– Chlorine contact tank (chlorine no longer used)
• Filtration building
– Filter wetwell with two submersible lift pumps
– Two UV disinfection modules
• Effluent flow measurement manhole—flume insert
• Sand drying beds
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The layout of this tankage and building is depicted
in Figure 4. The site was built-out to the fullest extent possible, due to the presence of surrounding
wetlands on three sides and a steep hill on the influent side of the site. The lack of space at the site
made sighting conventional treatment system extremely difficult and expensive. It also would have
been challenging to maintain the existing treatment
system or any other temporary system during
construction.
Figure 2: Original Conventional Plant
The plant was drastically undersized and provided
poor performance and poor effluent quality. Frequent pump-outs of the clarifier and the aeration
tank by tanker truck were necessary to keep the
system functioning at even a minimal level. As part
of the initial investigation into upgrade alternatives
for this facility, a capacity evaluation was performed on the existing treatment units. It was determined the aeration tank was capable of treating
roughly 11,000 gallons per day (gpd) (42 m3/day),
while meeting the ammonia limit of 1.0 mg/l. The
clarifier was estimated to be capable of roughly
30,000 gpd (114 m3/day) if improvements were
made. The flow data from the preceding years
demonstrated an overall yearly average flow of
roughly 30,000 gpd (114 m3/day) with several
months between 40,000 gpd (151 m3/day) and
60,000 gpd (227 m3/day), while school was in session. In addition several continuous days had influent flows between 80,000 and 100,000 gallons per
day (303 to 495 m3/day) with peaks during those
days in excess of 140,000 gpd (530 m3/day). The
existing system was clearly not up to the task at hand.
Figure 4: Layout of Pre-Existing GDA WWTP
It was determined that the best alternative for this
facility was to retrofit it with a submerged MBR system. This allowed for the continued usage of all
tankage in a reconfigured flow pattern. The reuse
of tankage greatly simplified construction and the
downtime necessary.
Description of the Upgraded MBR
System
Figure 3: Governor Dummer Academy Conventional
WWTP PFD
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The MBR approach required no major new infrastructure or tankage. With the MBR, the clarifier
was no longer needed and the new rating for the
aeration tank was estimated to be approximately
100,000 gpd (379 m3/day). Further, the system was
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promised to produce an effluent superior to that
from a tertiary sand filter. This allowed for the discontinuation of tertiary filtration at this facility and
the removal of the associated building. Figure 5
illustrates the modifications to the aeration basin
for the upgrade.
headworks, which consists of a grinder with a
3.0-millimeter auger screen. The grinder breaks up
any large solids. Any of the ground solids that do
not pass through the fine screen are removed by
the associated auger. Fine screening is important
for the submerged MBR to any small fibers that
could wrap around the membrane fibers and also
removes any particulates that could adhere to the
membranes. The chopper will automatically
reverse direction if it gets jammed, and the screen
is cleaned with a screw auger which deposits the
solids in a catch bag.
Figure 5: Upgraded Aeration Basin with MBR Retrofit
The MBR units formed the core modifications
needed for the upgrade and established the envelope of work for the overall upgrade, which
included the following activities:
•
New headworks including efficient grit removal,
solids grinding and screening, and proper
influent flow metering
•
Piping and general process modifications for the
new configuration
•
An additional UV disinfection module and a positive
flow splitting control structure to evenly divide flow
to all three units operating in parallel
•
New operations building to provide an office, laboratory, bathroom, storage room, and equipment
room for the blowers, pumps, flow meter, flow
splitter UV units, and other ancillary equipment
New discharge flow metering
Filter building removal
•
•
•
Electrical—new equipment, instrumentation, and
controls, relocation of electrical feed and needed
upgrades of existing services
•
Improvements to site drainage, potable
water supply, landscaping, and pavement
A process flow diagram of the MBR system at Governor Dummer is shown in Figure 6. The incoming
raw water first enters the system at the new
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Figure 6: Governor Dummer Academy WWTP MBR
Upgrade PFD
Also in the new headworks, a Parshall flume measures the influent flowrate. From the flume, the
wastewater is directed into the former secondary
clarifier basin, now used in a reverse flow configuration as the aerated grit chamber. Heavy solids
settle out of the wastewater while aeration from
the blower system provides adequate turbulence
to keep the more buoyant-neutral organic solids in
suspension. The grit tank serves as a settling tank
for the removal of grit and as an equalization tank.
The tank is aerated to prevent the settling of suspended organic materials. The water flows into the
aeration tank/MBR chamber via a weir between the
grit tank and the aeration basin.
Two racks of membrane modules were placed into
the existing 97 m3 (25,650 gallon) tank. The modules are on a dual header arrangement. Each
membrane is isolated one at a time for in-situ
chemical cleaning by the use of ball valves.
A walkway across the top of the aeration tank was
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constructed for easy access to all connections on
the MBR modules and piping. Figure 7 shows a
picture of the walkway and connections on the
MBR system.
Figure 7: Connections and Walkway
Aeration for the MBR is provided by two blowers,
with one used for stand-by. Each membrane module has an individual connection to the aeration
header located conveniently along the walkway.
The air provides the necessary oxygen for the biological digestion process, and also scours the
membranes to keep them from fouling. The air
enters the basin from the bottom portion of the
membrane cassette. There is a dissolved oxygen
(DO) meter in the aeration basin to warn of low DO
levels, and pressure gauges on the blower discharge lines help to indicate if the diffusers are
becoming clogged. Separate lines divert a portion
of the system air to the grit tank and sludge
holding tank.
Each module also has its own connection to the
permeate line and water recycle line. There are
two permeate pumps included with the system,
one is used for stand-by purposes only. Each
pump is equipped with a variable frequency drive
(VFD) to adjust the flow based on the liquid level in
the MBR tank. The flow setpoint is automatically
adjusted to match the number of MBR modules in
service. Each permeate pump has local inlet and
discharge pressure gauges. Check valves are
installed on the permeate lines. When the permeate pump shuts off, the check valves close and
prevent the permeate pipes from draining, which
aides in maintaining vacuum pump prime. The flow
of permeate is measured using a magnetic flow
meter and transmitter placed on the final discharge
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piping. Figure 8 gives the layout of the entire
WWTP at the site.
A sludge pump was installed at the center of the
aeration tank to remove sludge when the MLSS of
the tank accumulates over the recommended
10,000 ppm for optimal operation. The sludge is
pumped to a holding tank. Due to the high sludge
age, the excess sludge produced by the MBR does
not settle readily. In order to thicken the sludge, a
polymer is added to the sludge holding tank which
causes the sludge to rise. A submersible pump
placed on the bottom of the sludge tank then
pumps the water that accumulates at the bottom of
the sludge tank back to the aeration basin. Once a
month the sludge is trucked off site.
The pumps, blowers, controls, and power panels
have been installed inside an operations building.
This building also contains the UV disinfection,
which is the last step in the process before the
water is discharged.
The new electrical system includes a motor control
center (MCC) that provides power to the control
system (contained inside the control cabinet),
pumps, blowers, and the wiring to convey power to
pumps, blowers, instruments, the control system,
and two VFDs. The control cabinet is located inside
the electrical room in the operations building, and it
contains a programmable logic controller (PLC),
relays to buffer high voltage output devices, and an
operator interface panel, as well as switches and
indicators necessary for system operation. The
system is monitored by a SCADA software package
where data is logged twenty-four hours a day. It is
set up to run on automatic control and requires
very low labor hours and maintenance. Normal
operator interaction, other than daily checks, is
administered through the operator interface on the
SCADA system.
Data Comparison Between the
Conventional and MBR System
Table 1 summarizes the plant operational data
from ten continuous months of operations prior to
the MBR upgrade and the eight months of continuous operations (up to June of this year) after the
MBR upgrade and startup.
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Table 1:
Plant Performance Conventional vs. MBR Upgrade
cal size of the system, and still be able to meet
tough discharge regulations, they opted to install
an MBR system. The MBR technology replaces the
sedimentation process that was used in their conventional wastewater treatment plant.
The facility includes a chopper, 3.0 mm screen, grit
tank, aeration tank where the membranes are
located, blowers, permeate pumps, ultraviolet disinfection lamps, data logging system and instrumentation, PLC controller, sludge pump and holding
tank, and a tank to mix the clean-in-place (CIP) solution. The product water is consistently below the
discharge limits containing less than 2 mg/l of suspended solids, BOD and COD, also fecal coliform
units less than 10 colonies/100 ml and ammonia
less than 1 mg/l. The MBR plant has produced 22%
less sludge than the conventional treatment plant.
After being sent through ultraviolet disinfection, the
academy can safely and confidently discharge the
water to a local river. Since plant start up in August
2000, one hypochlorite CIP has been performed,
and the optimum mixed liquor suspended solids
concentration, flux rate, air diffuser cleanings, dissolved oxygen levels, and other minor operating
parameters have been established. Data is logged
24 hours a day to a SCADA system. Influent and
effluent parameters are being examined and permeate water samples are monitored every week.
As illustrated in Table 1, the plant performance
since the upgrade is greatly improved, and is consistently well within discharge permit limitations.
The conventional plant operations produced
roughly 12.3 dry kilograms (27 lbs.) per day of
sludge and discharged 3.6 (8 lbs.) dry kilograms per
day of solids in the effluent for a total sludge production of 15.9 (35 lbs.) dry kilograms per day. This
is in contrast to the MBR system which produces
roughly 11.8 (26 lbs.) dry kilograms per day of
sludge and discharges a mere 0.27 (0.6 lbs.) dry
kilograms per day of solids in the effluent for a total
sludge production of 12.2 (27 lbs.) dry kilograms
per day or a 22% reduction in solids production.
Conclusion
When the oldest boarding school in the United
States was faced with the problem of having to
increase the throughput capacity of their wastewater treatment system without increasing the physiTP1036EN 0601
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