Evaluation of Conventional
Activated Sludge Compared to
Membrane Bioreactors
Short Course on Membrane Bioreactors
3/22/06
R. Shane Trussell, Ph.D., P.E.
shane@trusselltech.com
Outline
• Introduction
• Process Design
• Effluent Water Quality
• Peak Flows
• Mixed Liquor Properties
• Conclusions
Outline
• Introduction
• Process Design
• Effluent Water Quality
• Peak Flows
• Mixed Liquor Properties
• Conclusions
Introduction
• Biological processes have become the
preferred municipal wastewater
treatment process
• Activated Sludge Process (ASP) has
developed into a mature process over
the past century
• Membrane Bioreactor (MBR) process is
relatively new to wastewater treatment
with the concept of direct sludge
filtration emerging four decades ago
Introduction
• Membrane
Bioreactor (MBR)
– Modified activated
sludge process
– UF/MF membrane
• Two configurations
– External (EMBR)
– Submerged (SMBR)
Flow Schemes for the MBR and Conventional
Activated Sludge Process
Conventional
Microfiltration
Tertiary
Secondary
Treated
Clarifier
Wastewater
Aeration Basin
Backwash
Water
WASTE
Primary Treated
Wastewater
Flow Schemes for the MBR and Conventional
Activated Sludge Process
Conventional
Microfiltration
Tertiary
Secondary
Treated
Clarifier
Wastewater
Aeration Basin
Backwash
Water
WASTE
Aeration Basin
MBR
Primary Treated
Wastewater
(Equivalent to
a 1-3 mm screen)
Tertiary
Quality
Wastewater
WASTE
Submerged MBR (SMBR)
Q
Aeration Basin
Effluent
WASTE
Primary Treated
Wastewater
QR = 3-5xQ
Solids Recycle
Waste Activated Sludge
External MBR (EMBR)
Effluent
Aeration Basin
Q
Primary Treated
Wastewater
Solids Recycle
QR = 20-30xQ
Waste Activated Sludge
Outline
• Introduction
• Process Design
• Effluent Water Quality
• Peak Flows
• Mixed Liquor Properties
• Conclusions
Process Design
• MBRs combine activated sludge
technology with membrane filtration to
expand the normal operating region
• MBRs can be designed at higher MLSS
concentrations because they are not
affected by the limitations of gravity
sedimentation for solid-liquid
separation
• SMBRs are typically designed for
MLSS concentrations 8-12 g/L
Advantages of High MLSS
• SMBRs operate at 2 to >6 times ASP
MLSS concentrations
• Higher MLSS concentrations translate
into:
–Longer SRT
same HRT, or
–Shorter HRT
same SRT
Longer SRT Designs
• For a given HRT, the SMBR process can
operate at 2 to >6 times the SRT of ASP
• “Traditional” SMBR design has been to
operate at “conventional” HRTs with long
SRTs (i.e. > 20 days)
• Long SRTs have the following advantages:
– Complete nitrification can occur even in cold
climates
– Reduced biological sludge production
– Complete oxidation of influent organics
– Possibility that slow growing microorganisms
can degrade persistent organics
Shorter HRT Designs
• In general, for a given SRT, the SMBR process can
treat wastewater in 1/2 to 1/4 the HRT of ASP
• Short HRTs have the following advantages:
– Reduce overall plant footprint
– Capital cost savings from reduced land and tank volume
• Concept of shorter HRTs brings about one of the
principle limitations of SMBRs compared to ASP
– Minimum SRT
• There is a minimum SRT where membrane fouling
becomes rapid
– A general design guideline is target the minimum SRT
for nitrification plus an additional safety factor
– Some manufacturers have established their own lower
limit at 12 days
Effect of SRT on Steady-State Fouling Rate
MCRT, d
10
5
4
3
2
4.0
HRT = 1 h
3.5
3.0
y = 1.661x2.1977
R2 = 0.9517
2.5
2.0
HRT = 4 h
1.5
1.0
0.5
0.0
0.0
0.2
0.4
0.6
0.8
F/M, g COD/g VSS.d
1.0
1.2
1.4
1.6
Process Design
• Key difference is in solid-liquid separation
– ASP is not sensitive to low SRTs and can
successfully operate in a “conventional” mode
– SMBRs are sensitive to low SRTs and compact
designs can result in increased membrane
fouling rates
• This difference in solid-liquid separation
also makes pretreatment imperative
– Fine screening is an absolute must in SMBRs
– The MBR community has been and is still
learning how important reliable screening
equipment is
Process Design
• Higher MLSS concentrations influence the
oxygen transfer efficiency
• Oxygen transfer from coarse bubble
aeration required for membrane agitation
needs to be considered
Krampe
and
Kauth,
2002
Outline
• Introduction
• Process Design
• Effluent Water Quality
• Peak Flows
• Mixed Liquor Properties
• Conclusions
Effluent Water Quality
• Biological process
applied to oxidize
organics and remove
nutrients
• Principle difference is
solid-liquid separation
mechanism
• Membrane provides a
more consistent, higher
quality effluent
Effluent Water Quality
Membrane provides an absolute barrier and effluent
quality is no longer a concern.
ASP Effluent
MBR Effluent
<30
ND (<2)
Turbidity, NTU
2 to >10
<0.2
Total Coliform,
#/100 mL
10,000 to
100,000
ND to 100
BOD5, mg/L
<2 to 30
ND (<2)
TSS, mg/L
Effluent Water Quality
• MBR eliminates the need
for monitoring sludge
settleability as an
operational parameter
» Effluent quality is not
dependent on operations
» Not necessary to determine
TSS/VSS concentrations to
maintain desired SRT
» Can use fixed waste rate
SRT=V/QWAS
Effluent Water Quality
• Public Health Benefit
– membranes increase the distance
between reclamation and the risk of
microbial disease
» pathogens are removed by size
exclusion
•
not a highly selective chemical or photochemical
reaction
» pathogens can be rejected, not just
reduced
– Results from operating MBR plants:
Indigenous Coliphage
Primary Effluent
Reactor #2
Reactor #2 (Non-Detect)
Reactor #1
Reactor #1 (Non-Detect)
Tertiary
1.E+06
Start -up
Period
New Membranes
Reactor # 2
1.E+05
1.E+04
Repaired
Integrity
1.E+03
1.E+02
1.E+01
1.E+00
1.E-01
0
1000
2000
3000
4000
Hours of Operation
5000
6000
7000
8000
Effluent Water Quality
• MBR Effluent Allows Modern Objectives
to be Realized
– Ideal for UV disinfection
» All particulate matter and suspended solids that
can interfere with UV have been rejected at
membrane barrier
» High percent transmissivity (>70%)
» Dose of 80 mJ/cm2 adequate for MBR effluent,
while 100 mJ/cm2 required for granular filtered
wastewater
– Ideal pretreatment process for reducing TDS
» Suitable for direct feed to RO
» Chloramine residual is required
Aqua 2000 Bureau II Study
[Filmtec BW 30-4040, low pressure TFC RO membranes]
Net Operating Pressure
Temperature
200
Feed TDS = 1200 mg/L
35
150
30
125
25
100
20
75
15
50
10
11 weeks
25
0
0
250
500
750
1000
1250
Tim e of Operation, h
1500
1750
5
0
2000
Temperature, °C
175
Net Operating Pressure, psi
40
Plant shutdow n
Outline
• Introduction
• Process Design
• Effluent Water Quality
• Peak Flows
• Mixed Liquor Properties
• Conclusions
Peak Flows
• Peak flows are well addressed in ASP, but
can be troublesome for MBRs
• Membranes are designed for a certain
throughput (design flux)
• MBRs are typically limited to a peaking
factor of 1.5Q
– Dependent on design flux (aggressive or conservative),
temperature, and mixed liquor conditions
• ASP is capable of sustaining larger peak
flows (>2.5Q) for longer periods of time
– Possible deterioration in effluent quality
Peak Flows
• MBR designs for large peak flows consider
the following solutions:
– Additional membrane area for peak flow service
– Flow equalization tanks (frequently primary eff)
– “Flux enhancing” polymers or coagulant
addition
• Currently, the most conservative and cost
effective solution is typically some kind of
flow equalization
• Flux enhancing polymers and coagulant
addition are showing great promise, but
research on long-term effects is needed
Peak Flows
Peak flows will become less of a disadvantage
for MBRs as membrane costs continue to
decline and our understanding of conditions
affecting membrane fouling increase
Outline
• Introduction
• Process Design
• Effluent Water Quality
• Peak Flows
• Mixed Liquor Properties
• Conclusions
Mixed Liquor Properties
• Mixed liquor properties are important
because they determine how easily a sludge
can be filtered through a membrane, settled
or dewatered
• Differences in solid-liquid separation apply
different selective pressures
– ASP requires a biology that flocculates and
settles well to remain in the system
– MBRs retain all biomass, even single cells in the
mixed liquor
Mixed Liquor Properties
• Merlo et al. (2004) revealed some key
findings comparing SMBR and ASP under
steady state conditions for 2, 3, 4, 5, 10-d
SRTs
– SMBR has higher colloidal content
– SMBR has higher filament concentrations
– Both SMBR/ASP particle size distribution
(excluding colloidal - i.e. >2 µm) was controlled
exclusively by mixing intensity, G
Particle Size Distribution
ASP Hi vs. SMBR
0.6
CMAS
ASP Hi
Frequency
0.5
0.4
0.3
0.2
0.1
0.0
2-4
4-6
6-8
8-10
10-20
20-40
40-100
0.6
100-2000
SMBR
0.5
0.4
0.3
0.2
0.1
0.0
2-4
4-6
6-8
8-10
10-20
20-40
Characteristic Length, µm
40-100
100-2000
Outline
• Introduction
• Process Design
• Effluent Water Quality
• Peak Flows
• Mixed Liquor Properties
• Conclusions
Conclusions
• SMBRs have advantages compared to ASP
(compact, high quality effluent, high MLSS
concentrations)
• SMBRs have disadvantages compared to ASP (low
SRT limit, peak flow issues)
• Mixed liquor properties are different in SMBRs
compared to the ASP because of the reactor
conditions
• Engineers have been studying mixed liquor
properties to improve the settleability of ASP
• Future of the SMBR process will be studying
mixed liquor properties that improve filterability
Questions?