A membrane bioreactor (MBR) is a biological
wastewater treatment process that uses a
microfiltration (MF) or ultrafiltration (UF)
membrane to provide the solid-liquid separation.
The MBR combines three separate unit operations
of a conventional activated sludge and tertiary filtration plant into
one process (Figure 1). Using a membrane for solid-liquid separation,
the MBR process is not subject to deterioration in effluent water quality,
commonly observed in gravity separated activated sludge plants. Although
there is a long list of advantages with the MBR process, all these advantages
can be described in three principal categories:
1) Consistent, excellent, effluent water quality
2) Compact footprint
3) High mixed liquor suspended solids (MLSS)
concentrations
There are two fundamentally different MBR configurations
being manufactured: an external MBR (EMBR) (Figure 2) and a submerged
MBR (SMBR) (Figure 3). Due to the high pumping costs associated with
the EMBR, the SMBR configuration is predominant in the global municipal
wastewater market. EMBR was the original MBR configuration and EMBRs
are still being widely applied for industrial wastewater treatment.
Yamomato et al. (1989) originated the SMBR configuration and only 16
years later there were more than 1000 SMBR installations worldwide.
The first full-scale installation in the United States was located in
Arapahoe, Colorado and began operation in the summer of 1998.
Irrespective of the membrane manufacturer, all SMBR designs and operating
MBR plants are impacted by:
1) Pretreatment
2) Biological conditions in the reactor
3) Membrane flux and backtransport efficiency
For an SMBR facility, it is important that a design engineer understand
the significance of the above listed criteria
and the interplay that exists between all three elements. The biological
conditions are impacted by the solids retention time, hydraulic retention
time, the reactor hydraulics, the reactor shear environment, influent
wastewater characteristics, location of solids wasting with drawl point,
and electron acceptor conditions. MBRs have the advantage of the perfect
solid-liquid separation provided by the membrane barrier, but MBRs also
have the disadvantage that all treated wastewater flow must be filtered
through the membranes before being discharged. As a result, MBRs treating
high peak and wet weather flows need to be carefully designed and expertise
on membrane flux rates, membrane operating conditions, mixed liquor
properties, and controlling these parameters to maximize the hydraulic
throughput requires diligent, knowledgeable and conservative engineering.
Trussell Technologies, Inc. is a leader in applying fundamental science
to the design of the MBR technology and is capable of providing an optimized
system for a specific application. Dr. Shane Trussell has completed
two Water Environment Research Foundation (WERF) projects that have
brought forth significant new understanding of the MBR process with
great impact on the municipal wastewater industry (Project #98-CTS-5
and #01-CTS-19-UR). Dr. Shane Trussell demonstrated that two fundamentally
different process limits exist for the SMBR process:
1) High organic loading rates, or high food to microorganism (F/M)
ratios.
At high organic loading rates, steady-state
membrane fouling rates increase due to rapid organic fouling. The organic
fouling resulted from increased concentrations of carbohydrate and protein
soluble microbial products (SMP) present in the mixed liquor. Dr. Shane
Trussell further proved that indeed protein and carbohydrate concentrations
increased on the membrane surface and that the molecular weight of these
organics increased with increasing loading rates. Finally, although
the majority of fouling resulted from SMP, the cake properties of materials
rejected at the membrane surface increased in adhesion, also known as
a “sticky cake.” The total concentration of extracellular
polymeric substances (EPS) increased with organic loading as did the
molecular weight of the EPS. This work was performed on real wastewater
with two different MBR manufacturers, one a MF membrane and the other
UF.
2) High MLSS concentrations
At high MLSS concentrations, membrane fouling
increases due to the effects of increasing
mixed liquor viscosity on the coarse
bubble aeration backtransport efficacy.
As the MLSS concentration increases,
the mixed liquor viscosity increases and alters the flow regime of
the coarse bubble swarms. This increase in viscosity, coupled with
increased MLSS loading on the membrane, results in a “blinding” of
the membrane surface with rejected materials
not adequately resuspended to the bulk
solution. Loss of membrane permeability due to excessive MLSS concentrations
can often be restored by reducing the
MLSS concentration and aerating the membrane without producing permeate
for a period of time. Reducing the membrane flux, or increasing the
coarse bubble aeration rate (or intensity), allows MBR operators to
sustain stable operation at high MLSS concentrations (>20 g/L)
assuming the aeration system is adequate
to maintain the appropriate dissolved
oxygen concentrations. Membranes are
now commonly being considered for sludge thickening (SMBR configuration)
by operating at low membrane flux rates.
MBRs are a promising wastewater treatment technology with great potential
for producing high quality reclaimed water in a compact footprint. As
the cost of membranes decrease and discharge regulations become more
strigent, MBRs will become a predominant force in the wastewater treatment
industry, reaching beyond its roots in water reuse. Understanding the
science and biology of the MBR process allows utilities to understand
operational and economic decisions that are being made at their facility
and ensures that goals are achieved and needs are exceeded.
Links to key SMBR manufacturers in the USA:
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