Treating the Cause, Not the Symptom

Since the early 1990s, submerged
membrane bioreactor (MBR)
technologies have been reshaping
wastewater treatment industries
around the world and creating new
reuse opportunities through decentralization
and plant upgrades. In
fact, in the U.S. alone, there are now
hundreds of operating MBR plants
and more than 4,000 installations
worldwide. With the development of
MBR technology over the last
decade, it has become apparent that
an overall system approach is necessary
to design and operate a successful
MBR plant.

Assuming adequate pretreatment,
this type of approach accounts for the
interdependency between control
strategies, plant hydraulics and biological
processes. This interdependency
is referred to as biohydraulics. To
understand the principles of biohydraulics,
it is important to note that
all submerged membranes have a
biofilm that must be properly managed
using a variety of design and
operational techniques.

A biofilm creates a dense dynamic
membrane that allows for enhanced
nutrient removal and degradation of
refractory organics, and more importantly,
prevents reversible and irreversible
fouling. Enviroquip, Inc. has
developed proactive design and
operational strategies to prevent both
types of fouling by managing biofilm
properties. This approach deals with
the causes of biofouling, rather than
the effects, and gives operators better
tools to run MBR systems.

Proactive biofilm management

Until recently, MBR suppliers
focused on the effects of biofilm
formation and not the causes. In
other words, the question was, How
can operators react to changing
biofilm conditions and eliminate
them through physical/chemical
methods such as backpulsing and
chemical cleaning?

Instead, Enviroquip has focused on
changing the question to, How do I
manage fouling conditions to prevent
irreversible fouling in the first place?

Aside from temperature,
EPS/SMP, colloidal particle size distribution
and particle diversity can all
be addressed through design and
operational choices.

It is widely believed that extracellular
polymeric substances (EPS) and soluble
microbial products (SMP) are the
main culprits that cause reversible and
irreversible biofouling. At a short
solids retention time (SRT), polysaccharides,
which are secreted by
microbes in an effort to stabilize
their environment and to aid in
flocculation, can combine to form
colloidal material that subsequently
blocks biofilm pores and increases
filtration resistance.

In fact, using hollow fiber membranes,
researchers at the University
of Berkeley observed that flux rates
declined roughly 800% faster as F:M
ratios were increased from roughly 0.5
day-1 to greater than 1.5 day-1. These
results are consistent with a 2004
UNESCO survey evaluating the cause
of MBR system problems in North
America for hollow fiber systems,
which pointed out that an SRT of <20
days may have accelerated biofouling.
At the other end of the spectrum,
microbes begin to lyse if the SRT is
too high (50+ days) and generate
SMP or protienaceous EPS, which
are also known foulants.

Although SMP concentrations
increase at long SRT, there is evidence
that average particle size also
increases at higher mixed liquor suspended
solids (MLSS) and at long
SRT. Particle size is important because
it determines the rate at which particles
migrate away from biofilm due
to lift forces induced by air scouring.
In other words, bigger (heavier) particles
move faster back into bulk solution
(mixed liquor) at a constant
cross-flow velocity induced by air
scour. Considering EPS/SMP data and
similar information regarding particle
size distribution as a function of SRT,
it appears that the optimum SRT
range is 12 to 50+ days.

SRT control through sludge management
is the best way to keep EPS
concentrations down and to maximize
air-scouring efficiency. Adding coagulants,
however, can bind up free EPS
and agglomerate colloidal material to
increase average particle size. During a
recent pilot study using a hollow-fiber
MBR system, stable TMP values maintained
for weeks with a 25-mg/L dose
of aluminum sulfate (alum) increased
within 48 hours after alum dosing was
terminated. Within 12 days, the average
TMP increased by 61% (in spite
of backpulsing and daily chemicalcleans) and did not fully recover after
resumption of alum dosing. It was
speculated that colloidal polysaccharides,
which formed after alum dosing
was terminated, may have caused irreversible
fouling.

Enviroquip has been working with
the Nalco Corp. to develop and integrate
a specially formulated polymer,
known as a flux enhancer, to sustain
high fluxes at cold temperatures.
MPE50 is a long chain cationic polymer
that bridges between negatively
charged floc particles to reduce free
EPS concentrations and decrease the
number of small particles, e.g. colloids,
that can clog biofilm and/or
membrane pores. The product is integrated
into Enviroquip MBR systems
to increase maximum monthly flows
30 to 50%, and in certain cases,
decrease air scour requirements.

Hydraulics

Topics seemingly unrelated to
biohydraulics, such as flow splitting,
gate types and hydraulic profiles,
often determine the success of an
MBR plant, irrespective of the
membrane technology selected.

Proper flow splitting is essential to
the successful operation of any MBR
system and is dependent on the
plant layout and flow control methods.
Without proper flow splitting,
there is a potential for preferential
flow to one or more basins that can
cause a significant imbalance in
MLSS. Even with the proper plant
layout, the type of gate used in
between basins can be an issue.

For example, using sluice gates
instead of weir gates can trap nuisance
organisms such as Microthrix
parvicella in isolated areas and create
foaming conditions at long SRT conditions
common to MBR operation.
Also, submerging return points can
mask flow-splitting problems and
uneven flow distribution.

Conventional wisdom says that
internal recycle should be pumped
back from MBRs to the head of the
plant to reduce pumping costs.
However, this strategy saves only a
small amount of energy (<1%) at the
expense of increased plant maintenance
and operational complexity.

Enviroquip standard MBR design
practice reverses the typical hydraulic
profile in order to gain precise control
of the recycle rate and partially
equalize flows upstream of the MBRs.
Constant water level in the MBR generally
improves overall performance.

Operational controls

To successfully run any MBR system,
it is very important to monitor
and control biological conditions
while sustaining high permeability,
defined as flux over TMP (gpd/psi).
Generally, this is done by maintaining
a target MLSS that corresponds to an
approximate SRT, and in some cases
(Enviroquip UNR), using online biomonitoring.
In order to optimize SRT,
systems must be designed with the
flexibility to operate over a range of
MLSS concentrations.

For example, Enviroquip allows for
MLSS concentrations up to 18,000
mg/L without derating hydraulic
capacity. Even with such flexibility,
physical/chemical methods to
address both reversible and irreversible
fouling are necessary.

The relaxation and aeration
strategies employed at a given
plant can have a tremendous
impact on operation. For example,
Enviroquip has developed unique
strategies, such as proportional
aeration and enhanced relaxation,
which can reduce energy costs by
up to 30% while increasing sustainable
fluxes. Whatever the strategy,
it is evident that the industry trend
is to move away from complicated
and potentially counterproductive
backpulsing, and toward variations
on relaxation.

Whatever the means of fouling
control, it is generally a good idea
to keep TMP as low as possible to
avoid collapsing biofilm and minimize
the rate of irreversible fouling.
Enviroquip generally employs
permeability control automation
and TMP interlocks to avoid high
TMP operation and to reduce
CIP requirements.

Understanding biohydraulics is
the key to successfully designing and
operating an MBR system. Moreover,
taking a proactive approach to
biofilm management, using the concepts
grounded in biohydraulics, can
significantly reduce MBR system costs
and maintenance requirements.

Dennis Livingston is MBR product manager at
Enviroquip, Inc. He can be reached at 512/834-
6019 or by e-mail at dennis.livingston@enviroquip.
com. Hiren Trivedi is technical manager,
biological processes at Enviroquip, Inc. He can
be reached at 512/834-6015 or by e-mail at
hiren.trivedi@enviroquip.com.

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