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Low Energy Process Control
January 23rd, 2013
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WEF WebcastLow Energy Process Control
Ammonia‐based aeration control
Leiv RIEGER, inCTRL Solutions Inc., Canada
• Peter L. Dold, EnviroSim
• Richard M. Jones, EnviroSim
• Charles B. Bott, HRSD
• William J. Balzer, HRSD
Acknowledgement
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Overview
ContextAeration controlNitrification fundamentals
Aeration control strategies
Control fundamentals
Case studiesConclusions
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Aeration costsContext
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Rieger et al., WER 2012
BenefitsContext
WWTP Morgental35,000 PE3.5 mgd
WWTP Thunersee130,000 PE
10 mgd
WWTP Werdhoelzli600,000 PE
50 mgd
simulation full-scale simulation full-scale simulation
Energy -30% -20% -30% -16.5% -25%
TN removal +48% +40% +60% +40% +32%
Annual net savings
$ 53’000 $ 360’000 $ 1’200’000
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Ammonia-based aeration control
Case study HRSD’s Nansemond WWTP5-stage Bardenpho, 60,000 m3/d (16 mgd), 250,000 PE
Simulation study
BenefitsContext
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Influent variability
WWTPs are highly dynamic systems ...
Olsson, 2008
Context
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• High variability of incoming load
• Fixed reactor volumes
• WWTP design based on peak load
Unused capacities
Context
Nitrification is the rate limiting step and therefore the primary target of BNR aeration control strategies
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Sufficient provision of dissolved oxygen
Ammonia as substrate (+ essential nutrients)
Sufficiently long aerobic sludge retention time
Sufficient mass of nitrifiers
Nitrification requirementsNitrification fundamentals
Auto-troph
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At 2 mg DO/L: ca. 80% of max. rate
DO constraints – nitrification kineticsNitrification fundamentals
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Ammonia as substrateNitrification fundamentals
Typical ammonia profile from fully aerated plant
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Typical effluent ammonia variation from fully aerated plant
SRTaerob = 8 days Average ≈ 0.5 mgN/L
Reasons for peaks?
Ammonia effluent variationsNitrification fundamentals
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• The mass of nitrifiers changes slowly
• The total mass depends on average ammonia load and SRT
• The influent ammonia load may vary substantially over a day
Ammonia break-through often:
due to limited mass of nitrifiers
not a problem of insufficient oxygen(or other limiting components)
Nitrifier massNitrification fundamentals
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PST Denitrification Nitrification FST
Control handle: Aeration
DO versus ammonia controlAeration control strategies
BOD removal
DO control aims for optimal DO for aerobic processes
NH4 control optimizes nitrification process
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1) Limiting aeration: Reduce energy consumption,
increase denitrification, improve bio-P performance
2) Reducing effluent ammonia peaks:
Reduce the extent of effluent ammonia peaks
Ammonia-based aeration controlAeration control strategies
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• Nitrifiers grow slowly Rate limiting step
• Pure DO control Aeration even after ammonia is gone
• NH4 control Intermittent aeration/varying intensity to limit nitrification
NH4O2
Tailored nitrification/denitrification
1) Limiting aerationAeration control strategies
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M
DOController
ManipulatedvariablePressurized air
Reference variable(setpoint)Measured variable
(Actual value)
O2
NH4
controllerDO f(NH4)
NH4
1) Limiting aeration: Cascaded NH4/DO controlAeration control strategies
Aeration intensity control(or intermittent aeration)
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M
NH4
Controller
ManipulatedvariablePressurized air
Reference variable(setpoint)Measured variable
(Actual value)
NH4
1) Limiting aeration: Direct NH4 controlAeration control strategies
High NH4 leads to over-aeration Additional DO probe More difficult to tune
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2) Reducing ammonia effluent peaksAeration control strategies
Intensity control: Manipulate aeration intensity early to create buffer for incoming peak
Volume control: Change aerated volume by switching on/off swing zones
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Feedback versus Feedforward controlControl fundamentals
Feedback control
Controller Final Control Element
Process
Measuring Device
Reference variable / Setpoint
u
Controlled variablex
ε
Disturbancesz
Measured variabler
y
Measure process answer
Setpoint Target variable
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Fast reaction before disturbance hits the plant
Process model required
Must be complemented by feedback signal More sensors required
Feedback versus Feedforward controlControl fundamentals
Feedforward control
Controller Final Control Element
Process
Measuring Device
Reference variable / Setpoint
u
Controlled variablex
ε
Disturbancesz
r
y
System model
Measure process disturbance
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M
DOController
ManipulatedvariablePress. air
Ref. variableMeasured variable
O2
NH4
controllerDO f(NH4)
NH4
Maximum-criteria
NH4
FeedforwardController
Feedback+Feedforward controlControl fundamentals
Q
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NH4
DO
Aerator
Airflow
Air flowsystem
NH4
controllerDO
controllerAirflow
controller
NH4
setpoint
AirflowsetpointDO
setpoint
Valveopening
Gustaf Olsson, 2012
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Variable DO setpoint controlControl fundamentals
HRSD‘s Nansemond WWTPCase study
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Input 1) Dry weather conditions at average temp. of 12°C
Input 2) Dry weather conditions at average temp. of 20°C
Input 3) Dry weather conditions at average temp. of 30°C
Input 4) Ammonia peak at average temperature of 12°C
Influent / temperature scenariosCase study Nansemond
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Base case: Existing strategy: DO control
CS 1: DO probes moved
CS 2a: Ammonia feedback: continuous change of DO setpoint PID with DO setpoint 0.5-2 mgDO/L
CS 2b: 2a but DO setpoint 0-2 mgDO/L
CS 3a: Ammonia feedback: high-low / intermittent aerationOn-Off with DO setpoint 0.5/2 mgDO/L
CS 3b: 3a but DO setpoint 0/2 mgDO/L
CS 4: Feedforward+Feedback ammonia control
Control scenariosCase study Nansemond
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DO setpoint2.5 mg/L
Current DO control strategyCase study Nansemond
DO setpoint2.0 mg/L
DO setpoint1.0 mg/L
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Base control strategyCase study Nansemond
Sensor information vs. DO profile
Low DO conc. in downstream section of second aeration zone
Aeration zone 1
Aeration zone 2
Aeration zone 3
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Base control strategy
Control strategy 1 (DO probes AZ1&2 moved downstream)
Optimal DO probe locationCase study Nansemond
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NH4
Feedforward
Feed-forwardNH4 high/low
Controller
Selector
DO ControllerAAA E/F
O2M
airflow
O2 M
airflow
O2M
airflow
DO ControllerAAA E/F
DO ControllerAer4-7 a-e
Q
NH4
Feed-backNH4 PID
Controller Feedback
Control strategy (FF+FB)Case study Nansemond
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Feedforward controlCase study Nansemond
Feedforward controlonly activ at 12°C
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Feedforward controlCase study Nansemond
Feedforward control has no significant impact on effluent ammonia
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Feedforward controlCase study Nansemond
Even at extreme peak eventslimited impact of feedforward control
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Feedforward control for Limiting aerationCase study Nansemond
FB control more robust, FF requires safety factors
against model failures
Simple model may not be accurate enough, complex
model needs several inputs
Effluent ammonia concentration changes slowly
Very limited control authority at higher ammonia conc.
Increased risk
Often not required
More complex/more expensive
Not functional
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Feedforward control for Volume controlCase study Nansemond
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Min/Max blower capacityCase study Nansemond
12°C
30°C
20°C
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Aeration zone 3, 20°C
Aeration zone 2, 20°C
Min. mixing requirementbased on 0.12 scfm/ft2
(re-suspension)
Minimum mixing requirementsCase study Nansemond
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Control scenario 1, 20°C
Control scenario 2a, 20°C
Air flow per diffuserCase study Nansemond
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Conclusions I/IIAmmonia-based aeration control has two objectives: Limiting aeration to prevent complete nitrification Reduce ammonia effluent peaks
Limiting aeration:o energy savings, improved denitrification / bio-P, less carbon additionHigh control authority to limit nitrificationDoes NOT increase nitrification capacity when DO > 1.5 mg/L
Reducing ammonia effluent peaks:o Ammonia effluent peaks often due to limited mass of nitrifiers Kinetic constraint and cannot be solved by more airVery low control authority of aeration intensity controlSwing zones to control ammonia peaks
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Conclusions II/IIAmmonia-based aeration control:
What is the control objective ? Is FF really necessary ? (home-made peaks) Feedforward aeration control often
o involves higher riskso is more complex / more expensiveo has limited control authority (intensity control)
To reduce effluent ammonia peaks, use volume control(swing zones)
Use dynamic simulation as a tool to design yourprocess control system !
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Leiv RiegerPh.D., P.Eng.
inCTRL Solutions Inc.Canada
Email: [email protected]
Presenter contact information
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WEF WebcastLow Energy Process Control
Efficient Nutrient Removal under Low Dissolved Oxygen Concentrations
Jose Jimenez, Ph.D., P.E.Brown and Caldwell
1/22/2013
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Overview
Nitrogen removal
What do we know about SND?
Factors affecting SND for N removala. Available carbon
b. Dissolved oxygen
c. Sludge bulking
Applicability and Implications
Conclusions
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• N and P removal generally are carried out with physically separated anaerobic, anoxic and aerobic zones
• N removal relies primarily on autotrophic nitrification and heterotrophic denitrification
Conventional Biological Nutrient Removal
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• Biological process where nitrification and denitrification occur concurrently in the same aerobic reactor (or in the same floc).
• Sludge settling characteristics are a real concern
• SND relies on achieving a dynamic balance between nitrification and denitrification
Simultaneous Nitrification-Denitrification
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• SND depends on:
– Micro environment
– Macro environment
– Bulk DO concentration
– Carbon availability
• Presence of novel microorganisms
Potential Advantages
• Elimination of separate tanks and internal recycle systems for denitrification
• Simpler process design
• Reduction of carbon, oxygen, energy and alkalinity consumption
Simultaneous Nitrification-DenitrificationPotential Disadvantages
• Limited controlled aspects of the process such as:
– floc sizes
– internal storage of COD
– DO profile within the flocs
• Sludge bulking; primarily because of the excessive growth of filamentous bacteria
• Complex instrumentation
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Complete Nitrification-Denitrification
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Complete Nitrification-Denitrification
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4.57 mg O2/ mg ammonia-N nitrified(-) 2.86 mg O2/mg N denitrified
________________________________1.71 mg O2/mg-N removed
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Nitritation-Denitritation
Nitritation-Denitritation3.43 mg O2/ mg ammonia-N nitrified
(-) 1.72 mg O2/mg N denitrified________________________________
1.71 mg O2/mg-N removed
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Factors affecting SND for N removal
• To accomplish denitrification in any process, the availability of readily biodegradable organic carbon is essential
Effect of Influent Carbon on SND
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Jimenez et al. (2010) Jimenez et al. (2011)
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• Control of bulk DO concentration in the system is essential for achieving a high degree of SND
Effect of DO on SND
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DO constraints – nitrification kineticsNitrification fundamentals
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• Nitrification rate at low DO remains at 85% of the maximum value after adaptation
Effect of DO on Nitrification
57Giraldo et al. (2011)
New Tools for SND Control
• Ammonia-based Aeration Control– Allows stringent control over DO provided– Control aerobic SRT to be as long as needed
• NOB Repression– Rapid transient anoxia seems to be the key– Mechanisms?
• AOB always at maximum growth rate (aerobic SRT control with excess NH4 available)
• NOB enzyme expression delay• Aerobic SRT controlled• Nitrite availability delay• Oxygen affinity• Free ammonia (NH3) inhibition of NOB
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Low DO Bulking and SND
Lu-Kwang et al. (2006)
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Plant SVI (mL/g)
Iron Bridge 115/165
Eastern Reg. 120/160
Snapfinger 200/300
Central 140/180
Winter Haven 130/190
Mandarin 150/180
Marlay Taylor 170/280
Stuart 212/350
Smith Creek 200/245
SND – Constant Aeration (Continues Flow)
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Bulk DO Controlled to 0.5 mg/L
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SND – Constant Aeration (Batch Reactor)
SND – Cyclical Aeration(Continues Flow)
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SND – Cyclical Aeration(Batch Reactor)
• AOB rates are not significantly affected during SND
• NOB seems not to be inhibited by low DO conditions during SND
• NOB rate slow down during cyclical aeration
• Possible Nitrite Shunt
• NO2 from nitritation can be used for denitritationbyHeterotrophs and convert to N2
• Less carbon might be required to convert N to N2
during SND
Batch Tests Results Seem to Indicate:
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SND - Nitrate vs. Nitrite
• High C:N Ratio
• SND (via Nitrate)
• NH4-based aeration control
• Lower energy generation potential (WAS-only anaerobic digestion)
SND - Nitrate vs. Nitrite
• PST reduces C:N Ratio; hence, possible C limitations for denitrification
• SND or Nitrite-Shunt – selection based on C:N Ratio
• NH4-based aeration control
• Good energy generation potential
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• Low C:N Ratio for denitrification
• Nitrite-Shunt required for N removal
• NH4-based aeration control
• Nitrite-Shunt compatible with mainstream Anammox
• High energy generation potential
SND - Nitrate vs. Nitrite
• The application of SND processes may be based and limited by:– Influent C:N ratio
– Sludge bulking issues due to the excessive growth of filamentous bacteria
– Instrumentation and control requirements
– The operator has limited control over important parameters impacting SND
Conclusions (I/III)
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• COD:N ratios of at ~ 8 and ~ 5 are required for SND and Nitrite-Shunt.
• Optimum bulk DO from 0.2 mg/L to 0.7 mg/L
• SND is more susceptible to nitrification limitations (DO) and denitrification limitations (carbon).
• Advantage of cyclical aeration resulted from the more ready availability of NO2 and NO3 (generated during nitrification) for denitrification
• Under constant low DO, denitrification would rely on the slow diffusion of NO2 and NO3 from the outer nitrification zone of the flocs into the inner denitrification zone
Conclusions (II/III)
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• Nitrite shunt might be possible during SND systems with transient anoxia.
• The results suggested that the nitrite shunt might take place mainly because of the disrupted nitrification at low DO conditions and pressure to the NOB
• Cyclical aeration seems to be more effective than constant aeration in avoiding low DO bulking
Conclusions (III/III)
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Jose JimenezPh.D., P.E.
Email: [email protected]
Presenter contact information
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WEF WebcastLow Energy Process Control
High‐Rate Activated Sludge System for Carbon Removal
Jose Jimenez, Ph.D., P.E.Brown and Caldwell
1/22/2013
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Overview
Evolution of high-rate activated sludge (HRAS) systems
Fundamentals and design considerationsSolids Retention Time (SRT)
Dissolved Oxygen (DO)
Case study – Strass WWTP, AT
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• Dr. Charles Bott, HRSD
• Mark Miller, VT
• Dr. Sudhir Murthy, DC Water
• Dr. Bernhard Wett, ARA Consult
Acknowledgement
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• HRAS process uses high F/M ratios and low SRT with short HRTto remove organics from wastewater.
• Current application of this process recognizes:– Particulate and colloidal organics are
removed by bio-flocculation (adsorption into the biological floc) and subsequent solids-liquid separation
– Soluble organics can be removed by intracellular storage, biosynthesis or biological oxidation
Evolution of HRAS Systems
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Chase, ES and Eddy, HP (1944), Sewage Works Journal, Vol. 16, No. 5, pp. 878-885
• The issue with aerobic treatment is that electrical energy needed for aeration is used to remove chemical energy.
• This practice is needed by current technology limits for carbon removal in secondary plants.
• Aerobic treatment is currently the only reliable means to remove carbon to meet secondary limits.
Evolution of HRAS Systems
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• HRAS systems can be designed and operated as:
• Carbon oxidation (energy intensive) systems to meet secondary effluent standards.
• Carbon adsorption processes (less energy intensive) when use as the first step in a two-stage process.
Evolution of HRAS Systems
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Common design parameters:
SRT < 3.5 daysF:M:0.5-1.0 g BOD per g VSSDetention Time: 0.3 – 3 hoursMLSS: 1,000 to 3,000 mg/LDO: > 2.0 mg/L
SRT < 0.5 dayF:M: 2.0-10 g BOD per g VSSDetention Time: ~ 0.5 hoursMLSS: 1,000 to 3,000 mg/LDO: < 1.0 mg/L (intermittent aeration)
• HRAS process is operated to minimize the aeration energy needed and to maximize the carbon sorption onto biomass, which is subsequently sent to anaerobic digestion for energy recovery.
A/B Process Alternative
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Figure provided by Dr. Charles Bott, HRSD
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• When a HRAS system is the first step in a two-stage process, the picture is substantially different. – By operating at low SRT and low DO, carbon oxidation
should be minimized and biological flocculation and intracellular storage of soluble substrate (carbon sorption) should be maximized.
– The transfer of organics from the liquid train to the anaerobic digesters is maximized; hence, energy generation potential can be maximized.
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Evolution of HRAS Systems
Process Control Variables Affecting COD Removal in the HRAS Process
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Impact of the System SRT on WAS VS Content
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Impact of SRT on the C Removal Efficiency
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Impact of DO on the COD Removal Efficiency
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Impact of SRT on the Specific Aeration Requirement
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At Lower SRT, the SAR decreases indicating possible
C adsorption and storage.
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Process Control Variables Affecting COD Removal in the HRAS Process
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HRSD’s A-Stage Pilot Plant
Figure provided by Mark Miller, VT/ HRSD 86
High CO2 PR = High OHO activity which may indicate C
oxidation(energy intensive process)
Low CO2 PR = Lower OHO activity which may indicate C adsorption and
storage(less energy intensive process)
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• Two-stage BNRplant (A/B plant)
• Load variations from 90,000 to 230,000 PE weekly average
Case Study - Strass WWTP, AT
87Data provided by Dr. Wett
Case Study - Strass WWTP, AT
88Figure provided by Dr. Wett
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• A-Stage: 0.5 days SRT
– 55-65% COD removal • B-Stage: 10 days SRT
– Pre-denitrification, on-line NH4-N controlled intermittent aeration
Brown and Caldwell 89Data provided by Dr. Wett
Strass WWTP - High Gas Potential in A-Stage Sludge Compared to B-Stage Sludge
90Data provided by Dr. Wett
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Strass WWTP - Maximize Transfer of Organics from Liquid Train to the Digesters Means Operation at Low SRT or High F/M Ratio
91Data provided by Dr. Wett
Strass WWTP - Multi-Step Optimization Process both in Energy Consumption and Production
Data provided by Dr. Wett 92
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Conclusions
• Carbon oxidation = energy intensive system.
• Carbon adsorption processes = less energy intensive system.
• The proper selection of SRT (F:M), HRT and DO, bio-flocculation and intracellular storage of carbon should be maximized.
• The transfer of organics from the liquid train to the anaerobic digesters is maximized; hence, energy generation potential can be maximized.
Jose JimenezPh.D., P.E.
Email: [email protected]
Presenter contact information
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Questions?