Aerated Anoxic Principles
Arkansas Water Environment Association’s
2012 “Water’s Worth It” Specialty Conference
Page 1
Water Technologies
Getting Started
Define: Aerated - Anoxic
Page 2
Water Technologies
The terms can be very confusing – and a
considerable number of definitions arrived at
Aerated-anoxic …
Anoxic-aeration …
Anaerobic-aeration …
Anammox bacteria
…what in the world am I talking
about!
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Water Technologies
Aerated-Anoxic is not limited to Siemens
aeration systems – but can be included in
any activated sludge aeration processes,
such as:
 Multichannel Oxidation Ditch
 Aeration tanks in series
 Multi-pass plug flow systems
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Water Technologies
Conventional plant designs use dedicated
anoxic and aerobic biological reactors
Influent
Anoxic
Reactor
Aerobic
Reactor
Secondary
Clarifier
Effluent
4Q
Internal Recycle
RAS
WAS
 No air added to the anoxic reactor
 Nitrification achieved in the aerobic reactor
 Nitrates brought back through internal recycle at 4Q
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Water Technologies
Aerated anoxic processes operate at constant
oxygen deficit in the first part of the process
Influent
Aerated
Anoxic
Reactor
Aerated
Anoxic
Reactor
Aerobic
Reactor
Secondary
Clarifier
Effluent
Internal Recycle
RAS
WAS
 O2 supplied to aerated anoxic reactors is less than O2 demand
 No internal recycle required for total nitrogen removal
Page 6
Water Technologies
A conventional oxidation ditch with aerobic and anoxic
zones is not the same as aerated anoxic reactors in series
Conventional Oxidation Ditch


O2 supply must exceed demand for complete treatment
It is difficult to control aerobic and anoxic zones in a single reactor
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Water Technologies
Aerated Anoxic Reactors
Oxygen Deficit
Q: What does Oxygen
Oxygen Deficit Condition
Deficit mean?
Deficit Condition:
O2 Supplied < 75% of O2 Demand
Result = ‘0’ DO
100
90
 For single reactor system It means “You’re in
Trouble”!
80
70
60
50
40
 For systems with reactors in
a series, it pertains to the
conditions desired in the
first reactor.
Page 8
30
20
10
0
O2 Demand
O2 Supplied
Water Technologies
Aeration Tanks
Complete Mix vs. Plug Flow
Complete Mix
Influent
Effluent
Plug Flow
Influent
Effluent
Aeration Tanks
Typical Oxidation Ditch
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Water Technologies
Aeration Tanks
Converting Plug flow to Complete Mix
Option 1: Add some pipes
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Water Technologies
Orris Albertson
“Control of Sludge Bulking”
Discussed the “history of activated
sludge innovations” used over 70
years ago
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Water Technologies
Early Innovators
Wellington Donaldson … 70 years ago
decided to “compartmentalize” plug flow
tanks
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Water Technologies
Orris Albertson
“Control of Sludge Bulking”
Phoenix 91st Avenue WWTP:
Aerated-anoxic system
 Established that anoxic tank could be “mixed” with
course bubble air instead of conventional mixers
 Anoxic tank equaled 25% of total volume
“Air did not hurt denitrification.”
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Water Technologies
Aerated Anoxic
Nitrification
 Adequate oxygen
Advantages of Nitrification in
Aerated Anoxic Reactor:
 Alkalinity
“Air did not hurt denitrification.”
Nitrification Requirements:
 Adequate sludge age
 Adequate ammonia
 Immediate source of nitrates for
denitrification
 Simultaneous nitrificationdenitrification takes place
 A short-cut nitrification/
denitrification pathway is
available
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Water Technologies
I’ve got a problem with what
you’re saying …
…that’s not the way I learned it!
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Water Technologies
Where does the nitrification occur?
Q: Does all the nitrification
occur in the high DO zone?
Y or N
A: Not in aerated anoxic systems
… the low DO reactors are
the BEST environment for the
nitrifiers
0 0.5
2
The nitrification occurs where
the majority of O2 is delivered.
Orbal DO profile in mg/l
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Water Technologies
Aerated anoxic saves power
 Majority of the oxygen is
supplied at zero DO
Oxygen transfer is more efficient
when supplied at zero DO
α(Cs – C)
 20% - 30% power savings
Page 18
Water Technologies
Carey, OH WWTP Case Study
Overview
Challenge: Five rectangular tanks
operating in parallel with nitrification
“problems”. Ammonia level was not
meeting requirements
Solution: Switch operation from parallel
to series.
Results:
 Before (Two in parallel): eff. Ammonia
1.7 mg/l
 After (Three in Series): eff. Ammonia
0.03 mg/l
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Water Technologies
Reactors in series improve nitrification
Carey, OH WWTP case study
Challenge: 5 tanks in parallel
with nitrification “problems”
Parallel
Series
 Ammonia not meeting limits
Solution: Switch parallel to series
Explanation:
Results:
 Before
Switch to
series eliminated
(parallel):
shortNH3
circuiting
eff.
1.7 mg/l
Reactors
in series created
 After
(series):
aerated anoxic conditions that
eff. NH3 0.03 mg/l
promote simultaneous N-D
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Water Technologies
Unusual Titles for Papers
“Lower Oxygen Delivery Can Lead to
Superior Nitrification”
“Increasing Oxygen Delivery in
Anoxic Tanks to Improve
Denitrification”
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Water Technologies
INCREASING oxygen delivery in anoxic tanks to
IMPROVE Denitrification
Sounds backwards, huh?
Wouldn’t denitrification be better with no oxygen
delivery in anoxic tanks?
Case Study: Hammonton, NJ WWTP
Increasing O2 delivery in aerated anoxic tanks improves denitrification
By increasing O2 delivery in outer channel NO3-N dropped from 2.8 mg/l to 1.6 mg/l!
175
O2 Supply Before
O2 Supply After
150
O2 Demand, lbs/hr
O2 Demand
125
100
75
50
25
0
Outer
Middle
Channel
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Inner
Water Technologies
Hammonton, NJ WWTP
Nitrogen Balance
Q: With 0.3 mg/l nitrate, how much
denitrification is due to recycle?
 Influent N to be nitrified @ 200 lbs/day
 N denitrified @ 197 lbs/day
 N in recycle @ 13 lbs/day
 N denitrified due to simultaneous N-D
@ 184 lbs/day
A: More than 93% is denitrified due to
simultaneous N-D!
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Water Technologies
University of Wisconsin F.I.S.H. Study
Compared nitrifiers from:
1. Orbal plants with aerated anoxic
reactors
2. Nitrification plants with conventional
upfront non-aerated anoxic tanks
Found different types of nitrifiers for two different types of plant:

NITROSOMONAS – typical ammonia oxidizer for most nitrification
plants, including those with non-aerated anoxic reactors

NITROSPIREA – dominant ammonia oxidizer for plants with aeratedanoxic reactors

NITROBACTER – dominant nitrite oxidizer
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Water Technologies
University of Wisconsin F.I.S.H. Study
Dormant Nitrifiers
NITROSPIREA:
 The dominant ammonia
oxidizer in systems where
there is “simultaneous
nitrifcation-denitrification”
 A nitrifier that is very good
at going dormant
 There are a lot of
“dormant” nitrifiers in our
BNR designs
Staging – with aerated anoxic reactors in
front – is a good way to increase (even
double) the nitrifier population.
Nitrifier population does not exclusively
depend upon the amount of ammonia
entering the plant.
Under typical conditions:
 10,000 units
 50% active – 50% inactive
With increased dormant population:
 25,000 units
 20% active – 80% inactive
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Water Technologies
Nine Springs WWTP: Madison, WI
Conventional FB plant with upfront anaerobic tank
Anaerobic
Aerobic (fine bubble)
Operates at a 10 day SRT … side by side test with one side being
aerated anoxic
Loading MLSS eTN eP
CFM/lb of BOD
Conventional:
14
2800 15
0.4
1100
Aerated Anoxic:
26
4100
11
0.2
673
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Water Technologies
Denitrification
Shortcut Pathway
 5-step pathway:
Ammonia – nitrite – nitrate –
nitrite – nitrogen gas
 3-step Shortcut pathway (in
aerated anoxic tanks):
“Explains lack of nitrite oxidizers in
Orris Albertson’s Phoenix study”
Ammonia – nitrite – nitrogen gas
Requires 33% less carbon
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Water Technologies
Aerated Anaerobic
It sounds too odd …
But works in designs with LARGE
aerated anoxic tanks and limited
oxygen
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Water Technologies
McMinnville, Oregon WWTP
 Two 3-channel Orbal Basins
 1st channel operates as
“aerated anaerobic channel”
 2nd channel operates as
“aerated anoxic channel”
 1.52 m.gal. Aeration Volume per
basin
 Four 50 hp Drives
 0.07 mg/l eff. P required
 Designed for 8 day sludge age
Only 18% of total basin volume is
“aerobic”
 0.5 mg/l eff. ammonia required
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Water Technologies
McMinnville, Oregon WWTP
P-removal Performance
Typical Municipal Plant
 Operates first channel with discs
at 29 rpm – less than 10% of the
total oxygen (aerated anaerobic)
 Second channel operates with
70% of the air – and a zero DO
(aerated anoxic)
Effluent ammonia at 0.2 mg/l (with 8 day sludge age)
Sol. P from Orbal/FC is 0.03 mg/l
Total P from Orbal/FC is 0.1 mg/l
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Water Technologies
Comparison of Two Identical Aeration Systems
System A

SAE @ 3 lbs/hp-hr

DO levels 0 - 2 - 4

alpha @ 0.9

power draw @ 544 hp
System B

SAE @ 3 lbs/hp-hr

DO levels 0 - 2 - 4

alpha @ 0.9

power draw @ 444 hp
Page 32
Water Technologies
Case Study – @ Little Lower Miami, OH
Adding more O2 at Zero DO saves power DO
Aerated Anoxic Optimization Recommendations
800
Oxygen Delivered, lbs/d
700
Add more O2 where absorption rate
is greatest
Add less O2 where absorption rate
is less
Add less O2 where absorption rate
is less
O2 absorbed
from aerators
(AOR)
600
Total O2 Supplied (lb/hr)
500
Actual
Optimized
Difference
400
For AOR
1050
1000
300
For FCF
583
322
200
Total
1633
1322
22.5% less
544 HP
444 HP
22.5% less
Unabsorbed
O2 from
aerators
(FCF)
100
0
Power Use
Actual
Optimized
Aerated Anoxic Zone
0 mg/l DO
0.9 FCF
Actual
Optimized
Aerobic Zone 1
2mg/l DO
0.7 FCF
Actual
Optimized
Aerobic Zone 2
4mg/l DO
0.5 FCF
Operating Conditions: DO levels and alpha/FCF same
Page 33
Water Technologies
Anammox Bacteria
“Nitrogen Removal Efficiency at Centralized Domestic
Wastewater Treatment Plant in Bangkok, Thailand”
VLR @ Nongkhaem:
I
E
BOD
32.8
3.8
N
11.8
6.7
Of the several plants (8) in the study, Anammox type
bacteria was only present at the Nongkhaem plant
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Water Technologies
Anammox Bacteria
2.5 mgd Orbal plant with primaries
Ratio of BOD:N@ 3.5:1 (105 mg/l BOD – 30 mg/l TKN)
Effluent ammonia @ 0.1 mg/l
Effluent nitrate @ 2 mg/l
Anammox Pathway:
50% of the ammonia oxidized to NO2-N
50% of the ammonia uses NO2-N for oxygen supplier
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Water Technologies
Where do we go from here?
Blending Aerobic …
With Anaerobic
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Water Technologies
50,000 gpd Pilot Study in Singapore
Page 37
Roof Installation
Roof with Canvas
Gas Holder Installation
Digester with Gas Holder
Water Technologies
With DAFT and Anaerobic Digestion
Page 38
Water Technologies
BOD Extraction and Digester Recycle
Plant main gate
Construction
Temporary
Access
Page 39
Water Technologies
Prime Float – Normal Loading Conditions
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Water Technologies
Prime Float – High Solids Loading Conditions
(Simulate Captivator)
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Water Technologies
Test Plans and Summary for High Solids Loading Conditions
Influent Flow
Q Inf = 30 gpm,
BOD = 400 mg/L
TSS = 375 mg/L
BOD = 320 mg/L
TSS = 210 mg/L
FeCl3,
0 - 10 ppm
DAF Influent
QDAF = 18 gpm,
Primary
Clarifier
Biosolids Flow, TSS Source
DAF Float Sludge, 3.5%
DAF
System
Contact
Tank
DAF Effluent, to Aeration Tanks
BOD = 50 mg/L, TSS = 35 mg/L
Directly To Aeration Tanks
Q R = 6 gpm, TSS = 6000 mg/L
High Solids Loading
Conditions
Base Case
Base Case +10 ppm FeCl3
Base Case + 5 ppm FeCl3
Q = 18 gpm
Biosolids TSS Loading
/Influent BOD Loading
Targeted
2.7
2.7
2.7
Actual
3.5
3.1
3.1
DAF performance,
Removal Efficiencies, %
TSS
98
99
97
BOD
93
95
97
COD
96
96
96
Half of raw influent was treated by combined primary clarifier and contact tank (CT) + DAF:
DAF effluent BOD = 50 mg/L with total removal of 88%: 20% removed by clarifier and 68% removed by CT + DAF
DAF effluent TSS = 35 mg/L with total removal of 91%: 44% removed by clarifier and 47% removed by CT + DAF
Estimated reduction in aeration required = 0.5*Q*(320-50)/(Q*320) = 42% based on BOD balance
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Water Technologies
Sand Island WWTP, Hawaii
Currently no secondary treatment
Upgrade cost estimate: $1.2 Billion
Power Cost: $0.21/kW
Limited land area for upgrade
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Water Technologies
Aeration Energy Reduction/Biogas Production
Conventional Mode
2700 kW
1800 kW
BE/DR
Energy used
Energy recovered
1300 kW
4800 kW
Total revenue gain of $8M/year
Total surplus of $6M/year
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Water Technologies
Remembering Wellington Donaldson
He had great “ideas”.
But for the most part they were:
ignored
neglected
forgotten
not used
Maybe now we are finally back on the right
course…
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Water Technologies