An Overview of Ammonia and Nitrogen Removal in · PDF fileAn Overview of Ammonia and Nitrogen...

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An Overview of

Ammonia and Nitrogen Removal

in Wastewater Treatment

Tim Constantine, CH2M HILL Canada

February 19th, 2008

2

Presentation Overview

• Ammonia Removal

– Why remove it?

– Regulations

– Nitrification biochemistry and factors

– Design considerations and example

• N-Removal

– Why remove it?

– Regulations

– Denitrification biochemistry and factors

– Design considerations and example

• Return liquors from dewatering

3

Ammonia Removal

4

Introduction

• Why removal ammonia?

– A nutrient, so can promote algae growth

– Can exert oxygen demand in receiver

– Free or un-ionized fraction is toxic to aquatic life

• What technologies remove ammonia?

– Breakpoint Chlorination

– Air stripping

– Ion exchange

– Biological methods

• Focus of this presentation will be biological methods

– Uptake of ammonia during biological growth (heterotrophs)

– Biological nitrification

5

Regulations and Basis for Limits

• In certain cases, effluent ammonia limits based on

limiting oxygen demand in receiver

• More often, limits set based on toxicity to aquatic life

• “Free” ammonia fraction is toxic:

– Chronic toxicity limit ~ 0.02 mg/L (after mixing in receiver)

– Acute toxicity limit ~ 0.1 mg/L (at end of pipe)

• Dissociation constant – pKa = 9.24

• Impacted by pH and temperature

• At pH 7.5, 15oC, about 1% of TAN is NH3-N– So to achieve 0.1 NH3, need to be below 10 mgN/L

NH4+ → NH3 + H+←

6

Nitrification Biochemistry

• Nitrification is a two-step reaction:

• More about the “nitrification reaction”:– Nitrifiers do not use organic carbon as basis for growth

– Building blocks for growth from alkalinity

– 4.57 g O2 consumed per g ammonia oxidized to nitrate

– Acid formed, consumes alkalinity, 7.14 g alkalinity per g NO3

– Nitrite intermediate almost never “stable”, nitrite oxidizers grow faster than ammonia oxidizers at typical WW temperatures

NH4+ + 1.5O2 → NO2

- + 2H+ + H2O AOB

NO2- + 0.5O2 → NO3

-NOB

7

What Impacts Nitrification?

• Alkalinity– Alkalinity is the carbon source for nitrifier growth

– Do not want to go below 50 mg/L as CaCO3, or pH ↓

– For 21 mg/L ammonia nitrified, require 200 mg/L alkalinity

• 7.14 x 21 +50 ~ 200 mg/L

• pH– Optimal pH for nitrification between 7 - 8.5

– pH below 6 can lead to inhibition

• Low Oxygen– Nitrifiers are strict aerobes, inhibited at very low DO levels

– As long as DO > 2 mg/L, little impact on nitrifier growth rate

8


• Inhibitory substances– Nitrifiers can be sensitive to a number of compounds

– Can impact growth rate

– Higher concentration = Lower growth rate or complete loss

– Classic example is free ammonia and nitrous acid inhibition

• Solids Retention Time (SRT) and nitrifier growth

– Concept of minimum SRT is really important!

Min. SRT = 1

µmax,N - bN

• If operating SRT > min. SRT, nitrification takes place

• If operating SRT< min. SRT, no nitrification

9

The Washout Phenomenon of Nitrification

5

10

15

20

25

30

Eff

lue

nt

Am

mo

nia

[m

gN

/L]

0 1 2 3 4 5 6 7 8

Solids Retention Time [days]

MinimumSRT

T = 12oC

DesignSRT

10


• Wastewater Temperature– Single greatest impact on nitrification

– Nitrifiers are much more sensitive to temperature than heterotrophic bacteria

– Drives the design sizing of aeration tanks in colder climates

11

Temperature Effect on Minimum SRT

5

10

15

20

25

30

Eff

lue

nt

Am

mo

nia

[m

gN

/L]

0 1 2 3 4 5 6 7 8


MinimumSRT

LowerTemp

HigherTemp

DesignSRT

12

Temperature Effect on Nitrification

10

Wastewater Temperature [oC]

12 14 16 18 20

2

4

6

8

10

12

So

lid

s R

ete

nti

on

Tim

e [

da

ys

]

Minimum SRT

Design SRT

13

Design Considerations for

Nitrification

• What do we need to select/determine?

– Bioreactor volume requirements

– Tank configuration

– Oxygen requirements

14

Bioreactor Volume for Nitrification

• Critical parameters for bioreactor volume sizing:

– Design aerobic SRT

– Raw wastewater characteristics

– Selected operating MLSS

• Selection of Design Aerobic SRT

– Almost always based on coldest wastewater temperature

– If good historical data available, use minimum week effluent

temperature

– Aerobic SRT selection might be impacted by effluent limits, but

not normally

– Typical coldest temperature in Ontario is 10oC

– In this case, design aerobic SRT of 8-10 days is typical

15


• Raw wastewater (or primary effluent) characteristics:

– Can have major impact on sizing of tankage

– Nitrifiers only make up ~1-2% of MLSS

– TSS/BOD feed to bioreactor is most important

– Should do rigorous review of historical data to determine design

characteristics

– For bioreactor sizing, look at peak month characteristics

– At times, peak month loading can coincide with minimum

wastewater temperature

• Good example is a university town, where population is higher in

winter/spring

– Peak month loading of 1.2x average annual is typical

16


• Design MLSS concentration:

– Optimal in terms of aeration tankage and secondary clarifier

sizing is 2,500 – 3,000 mg/L

– Higher MLSS values can be used, but usually means very large

secondary clarifier to account for higher solids loading

17

Design Example (Guelph Plant 4)

• Design Conditions:

– Average design flow = 22 MLD

– Minimum week WW temperature = 10oC

• Design Aerobic SRT = 9 days

– Average Primary Effluent cBOD5 = 140 mg/L

– Peak Month Loading factor = 1.3

– Design MLSS = 2,500 mg/L (2.5 kg/m3)

• Used process model, but simple design shown below

– MLSS yield (from model) = 0.9 mgMLSS / mg cBOD5

– ML Mass Required = 0.9 * (22 MLD) * (140 mg/L) * (1.3) * (9 d)

– ML Mass Required = 32,500 kg

– Volume = 32,500 kg / 2.5 kg/m3 = 13,000 m3

– HRT = 13,000 / 22,000 m3/d = 14 hours

18

Same Secondary Clarifier Size

Plant 1

Plant 2

Plant 3

Plant 4

Bioreactor VolumePlant 1 = 4,350 m3

Plant 4 = 13,000 m3

19

Bioreactor Oxygen Requirements

• Aeration is required for:

– Supply oxygen for biological processes

– Mixing

• For mixing, minimum requirements:

– Mechanical aeration = 5 W / m3 of tankage

– Diffused air = 0.3 L/s per m3 of tankage

• Oxygen specifically used for:

– Oxidation of organic matter (BOD)

– Nitrification

– Endogenous decay

20

SRT/nitrification Impact on O2 Demand

Ac

tua

l O

xyg

en

Re

qu

ire

me

nts

[k

gO

2/d

ay]


T = 20oC

2 3 4 5 6 7 8 9 101

WithoutNitrification

IncludingNitrification

21


• Other design considerations:

– Don’t forget about recycle streams

– Need sufficient oxygen transfer for to meet peak diurnal oxygen

demand during peak day loading

– Don’t forget about minimum mixing criteria, especially in last

pass of an aeration tank

– Consider using fine bubble aeration

22

Aeration Efficiency

0.8

1.2

1.6

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Efficiency

(kg O2/kWh)

Coarse Bubble

Spiral Roll

Mechanical

Aeration

Fine Bubble

Fine bubble aeration can reduce aeration energy by 25 to 50%

23





demand during peak design period




– Consider providing DO sensors with feedback loop to blowers

24

DO Control can Provide Savings

5 10 15 20

Time (hours)

0

5

10

15

20

25

30

35

40

45

50

0

Ae

rati

on

En

erg

y R

eq

uir

ed

(kW

)

No DO Control (47 kW)

With DO Control (35 kW)

25





demand during peak design period




– Consider providing DO sensors with feedback loop to blowers

– Do provide tapering of air diffusers, typically:

• Pass 1 = 50% of O2 demand



26

Nitrogen Removal

27

Introduction

• Why remove nitrogen?

– A requirement for biological phosphorus removal

– High nitrate levels associated with methemoglobinemia (blue baby syndrome)

– A nutrient, can lead to growth “harmful algal blooms” and hypoxic conditions

• Many jurisdictions now have effluent TN limits:

– EU countries – 70-80% N removal but varies

– “Bubble Limits” in sensitive watersheds (e.g. Chesapeake Bay)

• Canada now has effluent nitrate limits

– 13 mg/L as nitrate ion after mixing

– Equivalent to 2.9 mgN/L nitrate

28

Nitrogen Removal Intro

• There are a number of technologies capable of removing

nitrogen:

– Breakpoint chlorination of ammonia

– Ion exchange (ammonia or nitrate)

– Air stripping of ammonia

– Biological removal

• How is nitrogen removed in biological systems?

– A certain amount of nitrogen is required for biomass growth

– By nitrification / denitrification

– By deammonification (Anammox)

29

Nitrogen Removal - Denitrification

• Denitrification is defined as growth of bacteria when nitrate is

used as the terminal electron acceptor:

• Different from aerobic growth in that nitrate is used instead of

oxygen

• Majority of heterotrophic bacteria in WW treatment can use

both oxygen and nitrate

• If oxygen is present, it will be used before nitrate

• Oxygen inhibits denitrification

Organic

Compound+ NO3 → Biomass + CO2 + N2

30

Consequences of Denitrification

• Reduction in total oxygen required, as part of organic

material is oxidized using nitrate

– Nitrification: 4.57 mgO2/mgN

– Denitrification: 2.86 mgO2/mgN } 63% savings

• Actual O2 savings depends on:

– Raw WW characteristics

– Process configuration and process design

• Alkalinity is formed during denitrification!

– Recover 50% of alkalinity lost through nitrification

– More stable pH, since alkalinity serves as pH buffer

31

Requirements for Denitrification

• Need nitrate to be formed

– Nitrate is formed during nitrification

– As long as system is nitrifying, this criterion is met

• Need “denit” or anoxic zone in system:

– Nitrate

– Bacteria

– Substrate

– No oxygen, but mixing to retain biomass in suspension

32

Anoxic

NO3→N2

The “Wuhrmann” process

Aerobic

NH3→NO3

• Process can virtually removal all nitrogen

• Substrate for denitrification via endogenous decay

• For high N-removal, anoxic zone very large

• In practice, re-aeration step is usually added

re-a

erat

ion

33

The “Ludzack-Ettinger” process

Aerobic

NH3→NO3

Anoxic

NO3→N2

• Readily biodegradable substrate used for denitrification

• Significantly higher denitrification rate compared to

Wurhmann

• Only removes nitrate associated with the RAS

• If RAS = Qave, 50% nitrate removal if carbon not limiting

34

“Modified Ludzack-Ettinger” process

Anoxic Aerobic

NH3→NO3NO3→N2

• MLE process includes recirculation to bring greater

quantities of nitrate back for denitrification

• Much higher levels of nitrate removal possible

• Very popular process alternative today

35

Impact of Recycle Rate on MLEN

itra

te R

em

ova

l [%

]

Recirculation Rate [% of influent flow]

100% 200% 300% 400% 500%

10%

20%

30%

40%

50%

60%

70%

80%

90%

36

4-Stage Bardenpho process

Anoxic Aerobic

• Combination of MLE and Wuhrmann process

• Very high levels of nitrate reduction possible

• External carbon source (methanol) often added to

increase second stage denitrification rate

Anoxic

re-a

erat

ion

MeOH

37

Design Considerations for

Denitrification

• Do not take away “required” aerobic SRT to provide

anoxic zones

– Unless providing only seasonal denitrification via “swing”zones

• Size anoxic zones appropriately and provide mixing

– Minimum SRT for MLE anoxic zone ~ 1.5 days

• Kinetics of denitrification are fairly complex, and very

dependent on nature of COD (readily biodegradable,

slowly biodegradable)

– Carry out raw wastewater characterization

– Use a process model to assist in design sizing

• Design “zones” to allow free flowing surface

• Minimize O2 return in recycle streams

38

Sidestream Treatment

39

Centrate Characteristics / Impacts

Parameter Centrate % of Influent

Flow: - 0.5%

TSS: 1,750 mg/L 4.0%

BOD5: 200 mg/L 0.5%

Phosphorus: 200 mg/L 19.0%

Ammonia: 1,000 mg/L 20.0%

Temperature: ~35ºC -

• Major impacts:

– Increased oxygen demand

– Increased carbon requirements for denitrification

– Can lead to bleed through of NH3 / NO3 if not equalized

40

Impact of Centrate on Performance

Se

co

nd

ary

Eff

lue

nt

Am

mo

nia

[m

gN

/L]

12:00 12:000:000:00

2

4

6

8

10

Time of Day

With Centrate addedover 8 hours

No Centrate

41

Centrate Management Alternatives

Centrate Management

Alternatives

Separate

Treatment

No Separate

Treatment

- Do Nothing

- Centrate Equalization

Biological

Treatment

Phys-Chem

Treatment

- Hot Air/Steam Stripping

- Ion Exchange

- Breakpoint Chlorination

- Struvite Precipitation

- Suspended Growth Activated Sludge

- Fixed Film

- Bioaugmentation (e.g. BABE®, InNitri®)

- Nitritation (e.g. SHARON®, AT-3, others)

- Anammox (deammonification)

42

Process Overview - Bioaugmentation

• What is it?

– Side-stream treatment process to treat centrate

– Produces enriched population of nitrifiers

– Nitrifiers seeded to mainstream plant

• Benefit:

– Reduce ammonia (nitrogen) load on mainstream plant

– Allows improved nitrification in mainstream plant

• A number of treatment processes:

– InNitri (Inexpensive Nitrification)

– BABE (Bio-Augmentation Batch Enhanced)

– Nitrification in RAS reaeration (ScanDeNi, Prague)

43

SidestreamSystem

DewateringCentrate

25oC30oC

Process Overview - Bioaugmentation

• Improved mainstream nitrification efficiency

• Typically allows 30% less bioreactor volume

• Requires supplemental chemicals

• Process still emerging – some full scale

installations

Influent

RAS

Effluent

Nitrifiers

Alkalinity/Methanol

PC Bioreactor SC

InNitri ProcessBABE Process

RAS

RAS reaeration

44

Bioaugmentation Benefits MainstreamE

fflu

en

t A

mm

on

ia [

mg

N/L

]

2

4

6

8

10

12

Mainstream Aerobic SRT [days]

1 2 3 4 5 6 7 8 9 10

Design basis: 10oC

WithoutBioaugmentation

Effluent with

Bioaugmentation

45

Process Overview - Nitritation

• What is it?

– Process that converts ammonia to nitrite (NH3 → NO2)

– “Partial nitrification” in side-stream system

• Benefits:

– Reduced O2 (25% less than full nitrification)

– Reduced chemicals (e.g. 40% less MeOH for denitrification)

– Less tankage than bioaugmentation

• A number of treatment processes:

– Solids retention time control (SHARON Process)

– Toxicity control (e.g. AT-3 process)

– Dissolved oxygen control

46

Process Overview - SHARON®

So

lid

s R

ete

nti

on

Tim

e [

da

ys

]

Temperature [oC]

10 15 20 25 30 35 40

1

2

3

4

5

AmmoniaOxidizers

NitriteOxidizers

NH3

NO3

NO2

47

Process Overview - Anammox

• Anammox:

Anaerobic Ammonium Oxidation

• New organisms discovered - Anammox bacteria

NH4-N + NO2-N N2 + 2H2O

• Notable properties of anammox bacteria:

– Very low growth rate (1/10th that of nitrifiers!)

– Inhibited by oxygen even at very low levels

• Key: Generate both NH3 and NO2

48

Process Overview - Anammox

• Efficient N-removal:

– 60% savings in oxygen, 100% savings in carbon

• Long start-up, requires good solids retention

• Full-scale: Rotterdam, Strass

Influent

RASWAS

Effluent

Alkalinity?Methanol ?

DewateringCentrate

SHARONReactor

30oCNONO22

NHNH3330oC

N2

AnammoxReactor

49

Centrate Processes - Which is best?

• Example 1:

– Limitations in nitrification or denitrification capacity

– Low cost secondary treatment expansion

– Bioaugmentation may be best

• Example 2:

– No Limitation on nitrification or denitrification capacity

– Limitation in influent carbon or want to limit operating costs

– Nitritation or Anammox may be best

• Example 3:

– No major limitations in nit/denit capacity

– Equalization

50

Summary

• There will continue to be increased needs in

providing nitrification and N-removal

• Many options available for both systems

• Good process design practice should include:

– Raw WW characterization

– Process simulation

• Centrate treatment alternatives emerging, may solve

upgrade challenges for nitrification or N-removal

Knowledge… Commitment... Integrity… at Your Service

An Overview of

Ammonia and Nitrogen Removal

in Wastewater Treatment

Tim Constantine, CH2M HILL Canada

February 19th, 2008

Thank you for your attention!

An Overview of Ammonia and Nitrogen Removal in · PDF fileAn Overview of Ammonia and Nitrogen...

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