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Low Energy Process Control

January 23rd, 2013

1/22/2013

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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 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 control has no significant impact on effluent ammonia

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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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Efficient Nutrient Removal under Low Dissolved Oxygen Concentrations

Jose Jimenez, Ph.D., P.E.Brown and Caldwell

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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

55Jimenez et al. (2010)

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)


• PST reduces C:N Ratio; hence, possible C limitations for denitrification

• SND or Nitrite-Shunt – selection based on C:N Ratio


• Good energy generation potential

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• Low C:N Ratio for denitrification

• Nitrite-Shunt required for N removal


• Nitrite-Shunt compatible with mainstream Anammox

• High energy generation potential


• 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.



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High‐Rate Activated Sludge System for Carbon Removal

Jose Jimenez, Ph.D., P.E.Brown and Caldwell

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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.


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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.


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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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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

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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


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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


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.



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Questions?

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