Transcript 09 SND nutrient removal

Waste Water Treatment Technology
Oxygen supply Major investment (1 M$/y per treatment plant)
Fine bubble diffusers
Nitrogen Removal How? :
Aerobic Nitrification NH3 + O2  NO3
Anaerobic Denitrification NO3 + organics  N2
Problems
Nitrifiers grow slow and are sensitive and need oxygen
Denitrifiers need organics but no oxygen
Nitrification can be either sequential or simultaneous:
List Pollutants to be removed
• Suspended material (inorganic, bacteria, organic)
• Dissolved organics (COD,BOD)
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COD = chemical oxygen demand (mg/L of O2)
dichromate as the oxidant
BOD5 = biochemical oxygen demand(mg/Lof O2 in 5 days
microbial O2 consumption over 5 days
N
P
pathogens
odor, colour
ultimate aim: recycle of water for re-use
Why organic pollutant removal?
Organic pollutants represent an oxygen demand (COD or
BOD)
Bacteria in the environment will degrade the pollutants and
use oxygen.
If oxgygen uptake > oxygen transfer
 oxygen depletion .
 Collapse of ecosystem
3
Why nutrient removal?
Simplified Sequence of events of
eutrophication
Pristine aquatic ecosystems are typically limited by
nutrients.
Supply of nutrients (N or P)
 photosynthetic biomass (primary and secondary).
 More oxygen production and consumption
 Sedimentation and decay of dead biomass
 Depletion of oxygen in sediment/water column
 Collapse of ecosystem
4
Why nutrient removal?
comprehensive Sequence of events of
eutrophication (needs understanding of
anaerobic
respirations)
Pristine aquatic ecosystems are typically limited by nutrients.
Supply of nutrients (N or P)
 photosynthetic biomass (primary and secondary).
 More oxygen production and consumption
 Sedimentation and decay of dead biomass
 Depletion of oxygen in sediment/water column
 Oversupply of e- donors
 Use of other electron acceptors (anaerobic respirations)
 Ferric iron reduction to ferrous iron (Fe3+ --> Fe2+)
 Sulfate reduction to sulfide (H2S) (poison, oxygen scavenger
Solubilisation of iron and phosphate (ferric phosphate poorly soluble)
Further supply of nutrients  cycle back to beginning
O2 depletion, sulfide and ammonia buildup
Upwards shift of chemocline --> Killing of aerobic organisms
Further sedimentation
5
Collapse of ecosystem
Simplified Principle of of Activated Sludge
COD,
NH4+, phosphate
to ocean
Activated Sludge
(O2 + X)
Clarifyer
100:1
Excess sludge
Biomass Recycle (Return Activated Sludge)
•After primary treatment (gravity separation of insoluble solids)
•Secondary treatment: Oxidation of organic pollutants, (COD and
BOD removal, partial N removal
•Needed: NH4+ conversion to N2 ? How?
6
What is Nitrification?
Microbial oxidation of reduced nitrogen compounds
(generally NH4+).
Autotrophic ammonium oxidising bacteria (AOB)
(Nitrosomonas, Nitrosospira etc.):
NH4+ + 1.5 O2

NO2- + H2O + 2 H+
Autotrophic nitrite oxidisers (Nitrobacter, Nitrospira etc.)
NO2- + 0.5 O2
 NO3-
Aerobic conversion of NH4+ to NO3
+ removes some of the oxygen demand (COD)
+ removes NH4+ toxicity ot fish and odor from wastewater
- does not accomplish nutrient removal
What is denitrification?
•Microbial reduction of oxidised nitrogen compounds (generally NO3-).
•Anoxic process using nitrate as an alternative electron acceptor to
oxygen (anaerobic respiration)
•Catalysed by non- specialised factultative aerobic heterotrophic
bacteria.
•A series of reduction steps leading to potential accumulation of
intermediates
•Electron donor: organic substances (BOD, COD)
NO3- + 2 H+ + 2 e-  NO2- + H2O (nitrate reductase)
NO2- + 2 H+ + e-
 NO + H2O
2 NO + 2 H+ + 2 e-  N2O + H2O
(nitrite reductase)
(nitric oxide reductase)
N2O + 2 H+ + 2 e-  N2 + H2O (nitrous oxide reductase)
Review of Terms
•Metabolic processes can be differentiated between:
•Processes that make use of exergonic redox reactions, conserve the
energy of the reaction as ATP
Catabolism or Dissimilation or Respiration
typically oxidative process (degradation or organics to CO2)
•Processes that drive endergonic reactions by using the ATP generated
from Dissimilation
Anabolism or Assimilation or Biomass Synthesis
typically reductive processes (synthesis of complex organics from
small building blocks
If the building block is CO2  autotrophic
E n e rg y s o u rc e
P h o to
Chemo
E le c tro n
donor
O rg a n o
L ith o
C -s o u rc e
H e te ro
A u to
The Nitrogen cycle
Ox
State
-3
-2
-1
0
+1
+2
+3
+4
+5
CNH2
NH4+
N2
NO
NO2NO3-
The Nitrogen cycle
Ox
State
-3
-2
-1
0
+1
+2
+3
+4
+5
CNH2
NH4+
Dotted lines are assimiliative paths N2
NO
NO2NO3-
The Nitrogen cycle
Ox
State
-3
-2
-1
0
+1
+2
+3
+4
+5
CNH2
NH4+
Nitrogen fixation:
Atmospheric N2 reduction to
ammonium and amino acids.
N2
Syntrophic Rhizobia types, free living
bacteria and cyanobacteria.
NO2Reactions serves assimilation.
NO3-
NO
The Nitrogen cycle
Ox
State
-3
-2
-1
0
+1
+2
+3
+4
+5
CNH2
NH4+
N2
NO
NO2NO3-
Ox
State
The Nitrogen cycle
-3
CNH2
-2
-1
0Nitrification step 1
Nitritification:
+1
+2
Ammonium as the electron
donor for aerobic respiration.
+3
+4
Chemo-litho-autrophic.
+5
Nitrosomonas type species.
NH4+
N2
NO
NO2NO3-
Ox
State
The Nitrogen cycle
-3
CNH2
-2
-1
0
Nitrification
step 2
Nitratification:
+1
+2 as electron donor for
Nitrite
aerobic
+3 oxidation to nitrate
+4
Chemo-litho-autrophic
+5
Nitrobacter type species.
NH4+
N2
NO
NO2NO3-
Ox
State
-3
-2
-1
0
+1
+2
+3
+4
+5
The Nitrogen cycle
Denitrification
using either nitrate
CNH2
(NO3-) or nitrite (NO2-)
as the electron
eacceptor for
anaerobic respiration.
NH4+
N2
Most COD can serve
as electron donor.
Non-specific bacteria
replacing O2 with
Nitrate as e- acceptor
when oxygen is
depleted.
NO
NO2NO3-
How to accomplish overall Nremoval?
Nitrification typically occurs during the aerobic treatment of wastewater:
COD + O2
Ammonium + O2
 CO2
 Nitrate
In addition to the aerobic activated sludge treatment an anaerobic
treatment step is included aiming at N-removal (tertiary treatment)
Insufficient N removal is typically achieved. why?
Clarifier
Aerobic
Treatment
Anaerobic
Treatment
Recycled sludge
Effluent
How to accomplish overall Nremoval?
•N removal by the anaerobic step requires an electron donor to reduce
NO3- to N2.
•This electron donor is organic material.
•Solution A: Add organic material to the anaerobic treatment step.
•Example: Methanol
•Problems: costs, contamination
•Alternative solutions?
NH4+
COD
N2
CO2 Clarifier
NO3CO2
Aerobic
Treatment
Anaerobic
Treatment
Recycled biomass
(sludge)
Effluent
How to accomplish overall Nremoval?
•The obvious solution to successful N removal:
•Use the COD as electron donor for nitrification and denitrification
•How to allow anaerobic denitrification to occur in the presence of
oxygen?
NH4+
COD
N2
CO2 Clarifier
NO3CO2
Aerobic
Treatment
Anaerobic
Treatment
Recycled biomass
(sludge)
Effluent
How to accomplish overall Nremoval?
•Observations in the laboratory have shown that aerobic nitrification and
anerobic denitrification can sometimes occur at the same time.
•This simultaneous nitrification and denitrification (SND) has been the
focus of many R&D projects for improved N-removal.
NH4+
COD
N2
CO2 Clarifier
NO3CO2
Aerobic
Treatment
Anaerobic
Treatment
Recycled biomass
(sludge)
Effluent
Idea for SND
• Q: How to allow anaerobic denitrification at the same
time as aerobic nitrification?
• A: Intelligent oxygen control, not straightforward:
• Aerobic:
COD + O2
•
Ammonium + O2
• Anaerobic:
COD + Nitrate
 CO2
 Nitrate
N2 + CO2
• COD should be e-donor for nitrate reduction, not
oxygen reduction.
• Oxygen supply will burn COD faster than ammonium
• No COD  No denitrification  NO3- pollution
• Goal for improved N removal: Slow down aerobic
COD oxidation, to leave electron donor for denitrif.
Ideas for SND
• 1: Alternating aeration
• 2: Limiting aeration
• 3: SBR technology: Slowing down COD oxidation by
conversion to PHB
• Intelligent aeration control
Plug flow allows alternating aerobic /
anaerobic conditions without time schedule
Clarifier
Influent
Effluent
Waste Sludge
Return Activated Sludge
Air Line
Biomass Retention in WWTP
Alternating Aeration in Batch Systems
• Aerobic: COD + NH4+ + O2  NO3- + residual COD
• Anoxic: Residual COD + NO3-  N2
• There is always substantial COD + O2  CO2
wastage. Effective N removal is limited
Which phase is anaerorobic, which lines are COD, NO3- and NH4+ ?
Alternating Aeration in Batch Systems
• Aerobic: COD + NH4+ + O2  NO3- + residual COD
• Anoxic: Residual COD + NO3-  N2
• There is always substantial COD + O2  CO2
wastage. Effective N removal is limited
COD
anoxic
NH3
NO3-
aerobic
Alternating Aeration in Batch Systems
• Aerobic: COD + NH4+ + O2  NO3- + residual COD
• Anoxic: Residual COD + NO3-  N2
• There is always substantial COD + O2  CO2
wastage. Effective N removal is limited
COD
NH3
NO3-
anoxic
COD
oxidation
with NO3-
aerobic
COD
and NH3
oxidation
What is SND (Simultaneous
Nitrification and Denitrification) ?
•Compromise with DO to go so low that ammonium
oxidation is still working and denitrification is enabled.
•Basically: Run nitrification and denitrification at same
speed  sophisticated control needed.
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Oxygen dependency of Nitrification
Rate
Nitrification is not only limited
by the substrate concentration
(nitrate) but also by the oxygen
concentration
double limitation
\
Nitrif.
DO (mg/L)
SURNH 3

S NH 3
 max SURNH 3 
 S NH  k NH
3
3

 SO 


 X NH 3


 SO  kO 

Oxygen dependency of Denitrification
Rate
Oxygen inhibition constant (ki)
can be measured and used
for modeling
Similar to half saturation
constant
Denitri.
half inhibition constant
DO (mg/L)
SURNH 3
 S NO3
 max SURNO3 
 S NO  k NO
3
3

 kiO 

 X
 SO  kiO  NO3

Oxygen dependency of SND
Underoxidation
Overoxidation
Underoxidation: NH3 build- up
Over-oxidation: NO3- build-up
Rate
To match Nitrif. and Denitri.:
Nitrif.
Denitri.
SND
DO (mg/L)
Flux of reducing power (NH3,
COD) should match flux of
oxidation power. But how?
What is the magical DO level that
enables max SND?
How does the SND curve change
with different loading rates,
biomass levels and N:C levels?
Why Simulaltaneous nitrification and
denitrification(SND) ?
•Minimise aeration costs by running at low DO
•Avoid external COD addition to
(a) lower costs
(b) encourage (AOB) rather than heterotrophs 
 adapt high N-removal performance sludge
•Avoid pH fluctuations (costs, performance loss)
•Save further O2 and COD by SND via nitrite
•Simplified operation
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Why Simulaltaneous nitrification and
denitrification(SND) ?
•Minimise aeration costs by running at low DO
•Avoid external COD addition to
(a) lower costs
(b) encourage (AOB) rather than heterotrophs 
 adapt high N-removal performance sludge
•Avoid costs for pH corrections (nitrification uses acid
while denitrification produces acid (can you show this with stoichiometric
equations?)
•Save further O2 and COD by SND via nitrite
•Simplified operation
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SND pathway
NH3
NH2OH
O2
N 2O
NO2NO3-
NO2-
N2
If nitrification and
denitrification can occur
simultaneously there is a
possibility of by-passing
nitrate formation and nitrate
reduction 
SND via nitrite.
COD
Has the advantage of
oxygen savings and COD
savings.
DO Effect on Nitrification and Denitrification
Rate
SND via NO2- can operate more
easily than SND via NO3- as
oxygen has a stronger inhibition
effect on nitrate reduction than
nitrite reduction
Nitrification
NO3-
NO2- reduction
DO (mg/L)
If SND proceeds via nitrite,
then: how much savings are
generated?
Underoxidation
Overoxidation
Nitrif.
Rate
[N] in outflow
Nitrif.
NH3
Denitri.
DO (mg/L)
Denitri.
DO (mg/L)
Conclusion: For best N-removal in the outflow of
the treatment process, a low DO should be chosen
Laboratory Sequencing Batch Reactor
Tenix / Murdoch
University
SND SBR
pilot plant
(Woodman Pt.
03-12-24)
Labview control
Bioselector,
Online OUR
monitoring,
N2O emission,
O2 minimisation
Return activated sludge ready to be contacted with
incoming feed to allow “feast time” and enhance floc
Why Storage Driven Denitrification?
Idea: Making use of bacteria’s behaviour of taking up organic
substances for storage as PHB.
Denitrification needs organic reducing power:
• Either sufficient COD or PHB storage
• Problem with COD: degrades quicker than NH3
•  no COD left for denitrification
Advantages of bacterial Storage of COD as PHB as PHB:
1. Oxidises slower  lasts longer  important for SBR
2. Reducing power inside the floc rather than outside
3. Reducing power can be settled and build up.
41
PHB
Why Storage Driven Denitrification?
Denitrification needs organic reducing power:
• Either sufficient COD or PHB storage
• Problem with COD: degrades quicker than NH3
•  no COD left for denitrification
Advantages of bacterial Storage of COD as PHB as
PHB:
1. Oxidises slower  lasts longer  important for SBR
2. Reducing power inside the floc rather than outside
3. Reducing power can be settled and build up.
42
PHB
BOD storage as PHB
needs ATP
2 Acetate
2 CoA
4 ATP
2 Acetyl-CoA
(16 e-)
Mechanisms for ATP generation:
•O2 respiration
•Nitrate respiration
•Glycogen fermentation
•Poly-P hydrolysis
PHB
(18 e-)
1 NADH
Bio(2 e-)
mass
2 CoA
TCA
cycle
8 NADH
(16 e-)
Glycogen, P complicated
NO3- too low.
Aerobic bioselector?
H2O
ETC
2 CO2
24
ATP
Our results:
Storage under some
O2 supply
O2
PHB
Expected Benefit of Storing Reducing
Power Inside the Floc
N2
COD
O2
NO2- PHB
anoxic
NH3
• PHB physically
separated from O2
• Selective availability
of O2 to AOB.
• PHB may be more
readily oxidised by
nitrate or nitrite being
formed by the aerobic
reaction
aerobic
CO2
PHB
A
B
C
D
Increasing PHB (dark) buildup in
bacterial biomass (red) during early
phase of SBR
PHB
4
Aerobic
Carb. comp. (CmM)
Nitrog-comp. (mM)
10
Anoxic
8
3
6
PHB
2
4
NO3-
1
2
SOUR (mgO2/g/h)
0
0
0
50
40
30
20
10
0
0
50
100
Time (min) 250
300
350
Three phases could be
observed
•1st : COD PHB
•2nd : PHB driven SND
(60%)
•OUR indicates NH3
depletion
•3rd : wastage of
reducing power
•69 % N-removal, no reducing power left
NH3
OUR
50
150 200
Time (min)
•Needed: Automatic stopping of aeration
when ammonia is oxidised to prevent PHB
oxidation with oxygen
•Could be detected from OUR monitoring
Effect of auto-aeration cut-off on
PHB levels and N-removal
Aerobic
Anoxic
8
3
6
PHB
2
4
NO3-
1
2
Nitrog. comp. (mM)
0
Carb. comp. (CmM)
4
010
4
Anoxic
Aerobic
Settle
8
3
6
2
4
1
2
0
0
0
50
100
150
200 250
Time (mins)
300
350
Carb. comp.(mM)
Nitrog-comp. (mM)
10
Aim: Avoid wastage of
reducing power
by: auto-aeration cutoff
Outcomes:
•More PHB preserved
•N-rem 6986%
•Less air
•Shorter treatment
Special features of PHB hydrolysis kinetics
PHB degradation kinetics is ~ first order:
dependent on PHB, but independent of biomass
However, ammonium oxidation is proportional to biomass:
higher sludge concentrations should favour autotrophic
over heterotrophic activity  helps SND.
4
Anoxic
Aerobic
Settle
8
3
6
2
4
1
2
0
0
0
50
100
150
200 250
Time (mins)
300
350
Carb. comp.(mM)
Nitrog. comp. (mM)
10
Use of negative derivative of OUR to
detect ammonium depletion
Ammonium
depletion
-d(SOUR)/dt (mg/g/h2 )
2
1
Effect of aeration cut-off
on next cycle?
0
50
100
150
Time (mins)
200
Longer term effects of PHB buildup (not
examinable)
70
PHB analysis and SPOUR monitoring show:
60
PHB can be build up over several cycles
SOUR (mg/L)
50
Cycle
12
improved SND
explains biomass “adaptation”
40
30
8
5
 no need for emptying cells
Cycle 1
one over-aerated cycle can
20
loose all “savings” from prev. cycl.
NH3 –
OUR
10
review end of aeration DO high?
0
0
50 Time (min) 150
PHB build-up over 12 cycles
PHB analysis and
SPOUR monitoring
show:
5
PHB (mM)
4
PHB can be build up
over several cycles
3
2
 enabling more
reducing power and
better denitrification
1
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13
cycle
PHB driven SND performance
after 12 cycles of controlled PHB build-up
1.6
With close to complete N-removal:
no point for front denitrification phase
 DO required for COD storage
1.4
Conc (mM)
1.2
1
0.8
NH4+
0.6
0.4
NO3NO2-
0.2
0
0
50
100
150
200 Time (min)
Below this point for 2007 only
• Nitrogen removal by separating nitrifiers
from denitrifiers
Biological nutrient removal
•
As the main influent N species of wastewater
is ammonia, nitrification must precede
denitrification
•
BUT if oxygen and organic carbon are
present, heterotrophic organisms will
consume the carbon
•
This is a waste of both oxygen ($$) and
carbon ($$) causing the cost of operation to
increase
•
If the influent COD can instead be stored
internally by the heterotrophs for later use in
denitrification, this would save on both
oxygen and carbon
BIO301 - Leonie Hughes
Multiple sludge approach to WWT
Stage 1 - storage of influent COD
Influent
wastewater
BIOFILM
Heterotrophic
denitrifiers
Acetate and
Ammonia
Effluent
wastewater
Ammonia
Acetate
BIO301 - Leonie Hughes
PHB
Multiple sludge approach to WWT
Stage 2 - oxidation of ammonia
BIOFILM
Heterotrophic
denitrifiers
Influent
wastewater
BIOFILM or
SBR
Autotrophic
nitrifiers
Ammonia
Nitrate
Ammonia
BIO301 - Leonie Hughes
Effluent
wastewater
Nitrate
Multiple sludge approach to WWT
Stage 3 - reduction of nitrate
Effluent
wastewater
BIOFILM
Heterotrophic
denitrifiers
Influent
wastewater
Nitrate
Nitrogen gas PHB + Nitrate
BIO301 - Leonie Hughes
BIOFILM or
SBR
Autotrophic
nitrifiers
Commercialisation of PHB
•
Enhanced bacterial food source for use in
aquaculture
•
Biopol - biological alternative to
petrochemical plastics
BIO301 - Leonie Hughes
The need for biodegradable
plastics
•
6 billion plastic bags are used every year in
Australia
•
All plastic products make up 4% of all waste
going to landfill
•
Reduction in plastic going to landfill will make
landfill lifespans longer
BIO301 - Leonie Hughes
History of Biopol
•
ICI/Zenica published the first patents in the 1980s for a
complete production pathway of PHB with minimal
cost extraction
•
Biological fermentation method
•
Shampoo bottle for Wella was highest profile
product
•
In 1996 Monsanto purchased the patents and shifted
the focus to PHB production in genetically modified
crops
•
Continued public perception affecting
commercialisation of GM crops contributed to the
selling of the PHB patents to Metabolix
•
Metabolix now have exclusive rights to manufacture,
sell and use PHA related products regardless of origin
BIO301 - Leonie Hughes
Wastewater - free source of PHB?
•
One of the limitations of PHB production is the
high cost compared to petrochemical based
thermoplastics
•
If we know that
•
Activated sludge can make it and
•
Wastewater can be used as the substrate
•
Surely this may change the economics?
•
Much research is focused on pursuing this
BIO301 - Leonie Hughes
Wastewater - free source of PHB?
Question:
•
Consider that wastewater is a waste product
that people are currently paid to remove
•
If it becomes a resource, what would stop
governments charging those who want it
•
What if this counteracts the previous
economic statement?
BIO301 - Leonie Hughes
Phosphorous Removal
• Called “phosphorous accumulating organisms” (PAO’s)
• Require fluctuating conditions of aerobic and anaerobic
conditions à SBR can provide perfect environment.
• The PAO’s have a pool of poly-inorganic phosphate
(poly-Pi) inside the cell.
Phosphorous Removal
Anaerobic conditions
• hydrolyse a phosphate bond to produce energy in order
to import substrate (typically acetate) into the cell.
• Hydrolysed Pi released into the medium and PHA is
produced
• Called the “P release phase”.
Aerobic conditions
• the bacteria take up phosphorous to regenerate poly-Pi
pool
• PHA as the energy source
• Called the “P uptake phase”
Overall net reduction of phosphorus in the wastewater.
Nitrous Oxide (N2O) Production During SND
The Environmental Impact of N2O
• Nitrous oxide is a greenhouse gas
• global warming potential 250 times greater than CO2
• Estimated N2O responsible for 6% of global warming
• involved in the destruction of the ozone layer
• leading to an increase in the incidence of skin cancer
and related health problems
Nitrous Oxide (N2O) Production During SND
• N2O is an intermediate of denitrification
• Produced from the reduction of NO2- (nitrite reductase)
• N2O is reduced to N2 (nitrous oxide reductase)
• Nitrous oxide reductase is highly oxygen sensitive
• Oxygen, even at very low levels (0.02 mg O2/L), will stop
the enzyme working and cause N2O to be emitted
Nitrous Oxide (N2O) Production During SND
• N2O also produced by Autotrophic ammonium oxidising
(nitrifying) bacteria, if the oxygen concentration is very
low.
• In an SBR operated for SND both nitrifiers and
denitrifiers in the flocs will be exposed to low dissolved
oxygen concentrations
Result:
• SBR's operated for SND have a greater tendency to
emit N2O than traditionally wastewater treatment plants
• could be of environmental concern.
In a nutshell
• Nutrient rich wastewater released into waterways can
lead to eutrophication.
• During nutrient removal of wastewater, aerobic and
anaerobic processes need not be separated as
traditionally thought.
• Under oxygen limitation, simultaneous nitrification
(aerobic) and denitrification (anaerobic) can be
achieved, due to anoxic zones inside the floc.
• Effective denitrification requires a carbon source.
• Control of aeration to DO < 1 can help conserve carbon
for heterotrophic denitrification, improving denitrification.
• SND via nitrite provides savings in reduced oxygen and
BOD consumption.
Bulking sludge due to Filamentous Bacteria (S. natans)
Anaerobic Ammonium Oxidation (Anammox)
ation of ammonium to dinitrogen gas (N2) with nitrite as the electron acceptor by autotrophic
Discovered at the Kluyver Laboratory, Delft, The Netherlands in 1995.
vered to be oxidised in the absence of oxygen by a rare species of bacteria Planctomycetes
NH4+ + NO2-  N2 + 2 H2O (Go’ = -357 kJ mol-1)
rectly to dinitrogen gas, without the need for the multi-step process of aerobic nitrification an
Foaming sludge due to Nocardia