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Overview of Wastewater Aeration

Randal W. Samstag, P.E.Carollo Engineers

Presentation Outline

1. Some definitions2. Types of aeration

devices3. What affects aeration

efficiency?4 How is aeration

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4. How is aeration efficiency measured?

5. What affects process efficiency?

6. Case study: Kinetics and aeration

Some Acronyms

TDS

SRT

Y

,SOTE SAE

d

TransferEfficiency

Site Conditions/Water Quality

BiologicalProcess

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. .

TDS

BP

T

D O3

,

Net

content VSS

NO recycled

Y

N

f

/

blower

motor

d

AD AT

e

e

Basic Equation

dt

dC

)( *LL CCaK

dt

dC

Rate of oxygen transfer (ppd)

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aKL

*C

LC

Overall mass transfer coefficient (1/day)

Equilibrium DO concentration (mg/L)

Oxygen concentration in the liquid (mg/L)

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Oxygen Transfer Rate Equation

SOTE

C

CCOTE L

T

f *20

*20

20 )(

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OTEf = In-place oxygen transfer efficiency (%)

SOTE = Standard oxygen transfer efficiency (% under Standard Conditions)

Standard Conditions

Parameter U.S. Practice European Practice

Type water Tap Tap

Water temperature 20OC 20OC

CL 0 mg/L 0 mg/L

Barometric pressure 1 atm 1 atm

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Air flow 20OC 0OC

36% R.H. 0% R.H.

γ = 0.075 lb air/ft3 ρ = 1.293 kg air/m3

= 0.01736 lb O2/ft3 = 300 g O2/m3

Standard Aeration Efficiency (SAE)

“The rate of oxygen transfer (standard conditions) per unit power input, which may be based on either

delivered power (DP) or wire power (WP)”

Units: pounds per hour per horsepower (pph/hp)

Mueller, et al., Aeration: Principles and Practice (2002)

Standard Aeration Efficiency (SAE)

In this presentation I will always use SAE as the rate of oxygen transfer per unit of electrical (wire) power!

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Types of Aeration Devices

1. Surface mechanical2. Diffused aeration3. Hybrid4 Cascade

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4. Cascade

Surface Mechanical Aeration Devices

1. High speed propeller2. Low speed mixer / aerator3. Horizontal rotors4 Discs

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4. Discs

Surface Mechanical Aeration Devices

1. High speed propeller2. Low speed mixer / aerator3. Horizontal rotors4 Discs

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4. Discs

Transfer oxygen by transporting the water into the air (or high purity oxygen).

High Speed Propeller Aerators

1. Direct drive coupled propeller aerators

2. Usually floating (not fixed mounted)

3. Used for lagoons and

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open-topped tanks4. Standard Aeration

Efficiency (SAE): 1.8 to 2.5 pph/hp (Mueller et al.)

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Low Speed Mechanical Conventional

1. Pitched-bladed turbine with lower mixing impeller for deeper applications

2 Fixed-mounted

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2. Fixed mounted3. Descendents of early

1920s designs4. Used for air and HPO5. SAE 2.5 to 3.0

pph/hp (Mueller et al.)

Low Speed Mechanical (Newer Design)

1. Flat bladed mixer/aerator

2. Top mounted (no bottom impeller)

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p )3. Recent design for

HPO upgrades4. SAE 3.2 to 3.5

pph/hp (Carollo Test)

Rotor Aerators

1. Horizontal mixing impellors

2. Used commonly in oxidation ditches

3 Developed in US and

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3. Developed in US and Europe in 1930s

4. SAE 2.5 to 3.5 pph/hp (Mueller et al.)

Disc Aerators

1. Horizontal mixing impellors

2. Used commonly in oxidation ditches

3 Often used for

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3. Often used for simultaneous nitrification denitrification (SND)

4. SAE 2.0 to 3.0 pph/hp (Mueller et al.)

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RBC Disc Aerators

1. Rotating biological contactors (RBS)

2. Mechanical drive or air sparged

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p g3. Low aeration

efficiency

Diffused Aeration

1. Coarse bubblea. Orificesb. Tray typec. Static tube

2 Medium bubble

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2. Medium bubblea. Membrane tubes

3. Fine bubblea. Porous discs or

domesb. Membrane discsc. Membrane panels

Diffused Aeration

1. Coarse bubblea. Orificesb. Tray typec. Static tube

2 Medium bubble

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2. Medium bubblea. Membrane tubes

3. Fine bubblea. Porous discs or

domesb. Membrane discsc. Membrane panels

Diffused aeration transfers oxygen by passing air through the water.

Coarse Bubble Diffused Aeration1. Orifice diffuser2. Plastic materials3. Higher alpha than

fine bubble

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fine bubble4. SOTE: 12-15%5. SAE : 1.5 – 2.5

pph/hp (Mueller et al.)

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Coarse Bubble Diffused Aeration1. Tray type diffuser2. Stainless steel3. Higher alpha than

fine bubble

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fine bubble4. SOTE: 12-15%5. SAE : 1.5 – 2.5

pph/hp (Mueller et al.)

Coarse Bubble Diffused Aeration1. Static tube aerator2. Air lift pumping

action3. Typically used in

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3. Typically used in lagoons

4. SOTE: 0.6 to 1.4 %/ft

5. SAE: 1.8 to 3 pph/hp (Mueller et al.)

Medium Bubble Membrane Tubes

1. Medium bubble tubes

2. Various materials: EPDM, PVC, ceramic other

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ceramic, other plastics

3. SOTE: 1.5 – 2 %/ft

4. SAE: 5.0 to 7 pph/hp (Mueller et al.)

Fine Bubble Aeration

1. Fine bubble porous discs or domes

2. Oldest type of aeration diffuser

3 Materials: ceramics

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3. Materials: ceramics or porous plastics

4. Requires acid cleaning

5. SOTE: 2 – 3.0 %/ft6. SAE: 5.9 to 9

pph/hp (Mueller et al.)

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Fine Bubble Aeration

1. Fine bubble membrane discs

2. Introduced in 1960s3. Materials: Earlier,

poly vinyl chloride (PVC) N

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(PVC); Now, ethylene-propylene dimers (EPDM)

4. Cleaned by hosing5. SOTE: 2 – 3.0 %/ft6. SAE: 5.9 to 9

pph/hp (Mueller et al.)

Fine Bubble Aeration

1. Fine bubble membrane panels

2. Developed by Messner in G

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Germany3. Higher pressure loss

than discs4. SOTE: 1.5 – 3.5

%/ft5. SAE: 5 to 11

pph/hp (Mueller et al.)

Hybrid Aerators

1. Submerged turbines

2. Open bladed turbine with sparger ring of air

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sparger ring of air below

3. Problems with unbalanced forces

4. SAE: 1.75 to 2.75 pph/hp (Mueller et al.)

Hybrid Aerators

1. Aspirator Aerators2. Propeller drives air

down a shaft to the propeller hub

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p p3. Mounted on floats 4. Low efficiency5. SAE: 0.6 to 1.5

pph/hp (Mueller et al.)

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Hybrid Aerators

1. Aspirator Aerators2. Submersible pump

draws down a shaft to the impeller

3 Mounted on basin

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3. Mounted on basin floor

4. Used in aerated grit chambers

5. Low efficiency6. SAE: 0.6 to 1.5

pph/hp (Mueller et al.)

Hybrid Aerators

1. Jet Aeration2. Combines liquid

pumping with gas entrainmentPi i d j t

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3. Piping and jet materials: polypropylene, fiberglass, or stainless steel

4. SAE: 2.5 – 3.5 pph/hp (Yunt)

Cascade Aerators

1. Cascade Aerators2. Pump water over

trays to encourage droplet formation

3 Used for simple water

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3. Used for simple water treatment (iron removal)

4. Trickling Filters are cascade aerators

5. Efficiency not well documented

Comparative Aeration Efficiency

Device SOTE (%/ft) SAE (pph/ehp)Diffused Air

Fine Bubble Membrane Panels 1.50 – 3.5 5.0 - 11.0

Fine Bubble Porous Disc or Dome 2.0 - 3.0 5.9 - 9.0

Fine Bubble Membrane Disc 2.0 - 3.0 5.9 - 9.0

Medium Bubble Tube 1.5 - 2.0 5.0 - 7.0

Coarse Bubble Tray 0 75 - 1 5 1 5 - 2 5

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Coarse Bubble Tray 0.75 - 1.5 1.5 - 2.5

Mechanical Aeration

Low Speed Aerators 2.5 – 3.5

Horizontal Rotors 2.5 - 3.5

Discs 2.0 - 3.0

High Speed Propeller 1.8 - 2.5

Hybrid Aeration

Jet Aeration 2.5 – 3.5

Submerged Turbines 1.75 - 2.75

Aspirator Aerators 0.6 - 1.5

Cascade ???

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A Note on Fine Bubble Diffused Aeration1. OTE depends on a number of factors

a. Solids residence time (SRT) of the MLSSb. Diffuser density (AD/AT)c. Diffuser air rate (cfm/diffuser)

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d. Diffuser distribution in the basin

2. Under some circumstances (high air rate, fine bubble diffusers not uniform on basin floor) coarse bubble diffusers can be nearly as efficient as fine bubble diffusers

How is Efficiency Measured?

1. SOTE: Clean Water Transfer Test2. OTEf : Off gas testing

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SOTE: Clean Water Transfer Test1. ASCE Oxygen

Transfer Committee Procedure

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2. De-oxygenate basin with sulfite

3. Re-aerate4. Measure DO and

shaft and motor power during re-aeration

OTEf : Off gas Testing

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1. For measurement of alphas for diffused aeration

2. Measure concentration of oxygen in a hood over the mixed liquor

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A Quick Look at Treatment Process Efficiency

The aeration tank configuration, whether plug flow or completely mixed, makes a difference

in the efficiency of treatment

Why is tank configuration important?Monod Kinetics

The rate of growth / uptake of BOD is proportional to the initial concentration

)(max

outs

out

SK

S

Rate of growth (1/day)

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Sout = BOD concentration (mg/L)

Rate of growth (1/day)

max Maximum rate of growth (1/day)

Ks = Half-saturation coefficient (mg/L)

Multiple Pass (Plug Flow) With Fine Bubble Aeration

Sample

Sample Point 2

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Sample Point 1

Plug Flow Tank

Residence Time Distribution (RTD) Soluble BOD Profile

Soluble BOD ProfileFiltered Carbonaceous BOD

54.8123.50

Four Pass Fine Bubble Tank n = 53

Two Pass Fine Bubble Tank n = 18

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Reactor 1 Reactor 2 Reactor 3 Reactor 4 Reactor 5 Reactor 6 Reactor 7 Reactor 8

CO

NC

. (m

g/L

)

5

4

3

2

1

0

1.612

1.3091.086

0.8920.74

0.636 0.571

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

0.00 0.50 1.00 1.50 2.00

/o

C /

Co .

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Oxidation Ditch Tanks (Completely Mixed)

Sample Point 1

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Sample Point 2

Completely Mixed Tanks

RTD Curve Soluble BOD Profile

Soluble BOD ProfileFiltered Carbonaceous BOD

52

Two Ditches in Series n = 1.44

Single Oxidation Ditch n = 1.00

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Reactor 1 Reactor 2 Reactor 3 Reactor 4 Reactor 5 Reactor 6 Reactor 7 Reactor 8

CO

NC

. (m

g/L

)

5

4

3

2

1

0

1.798

1.5421.369

1.249 1.173 1.121 1.084 1.056

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0.00 0.50 1.00 1.50 2.00

/o

C /

Co

Conclusion?

Plug flow tanks have a higher concentration in the front of the reactor concentration in the front of the reactor -> Higher reaction rates in systems with Monod kinetics

Case study: Comparison of upgrade choices for an

oxidation ditch for higher energy efficiency and h d i lenhanced nutrient removal

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Classic Oxidation Ditch with Rotor Aerators

Reactor 2 Reactor 3 Reactor 4

Reactor 6Reactor 7Reactor 8

Influent Reactor 1 - BrushReactor 5 - Brush

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Effluent

Waste Sludge

Consider a Number of Alternatives1. Localized fine bubble aeration and banana blade

mixers2. Plug flow upgrade with floor coverage fine

bubble aeration3 Anaerobic selector plug flow upgrade with fine

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3. Anaerobic selector plug flow upgrade with fine bubble aeration

4. Anaerobic selector plug flow upgrade with low DO simultaneous nitrification denitrification (SND)

5. Modified Ludzak-Ettinger (MLE) upgrade with fine bubble aeration

Localized Fine Bubble Upgrade with Mixer

Reactor 2 Reactor 3 Reactor 4

Reactor 6Reactor 7Reactor 8

Influent

Reactor 1

Reactor 5

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Effluent

Waste Sludge

Plug Flow with Floor Coverage Fine Bubble Aeration

Reactor 2 Reactor 3 Reactor 4

Reactor 5Reactor 6

Reactor 1

Influent

Reactor 7Reactor 8

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Effluent

Waste Sludge

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Anaerobic Selector Plug Flow Upgrade with Fine Bubble Aeration

Reactor 2 Reactor 3 Reactor 4

Reactor 5Reactor 6

Reactor 1

Influent

Reactor 7Reactor 8

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Effluent

Waste Sludge

Anaerobic Selector Plug Flow Low DO Simultaneous Nitrification Denitrification (SND)

Reactor 2 Reactor 3 Reactor 4

Reactor 5Reactor 6

Reactor 1

Influent

Reactor 7Reactor 8

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Effluent

Waste Sludge

MLE with Fine Bubble Aeration

Reactor 2 Reactor 3 Reactor 4

Reactor 5Reactor 6

Reactor 1Influent

Reactor 7Reactor 8

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Effluent

Waste Sludge

Comparison

1. Oxygen demand2. Power3. Effluent Quality

a Ammonia

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a. Ammoniab. Total inorganic nitrogen (TIN)c. Total phosphorus (Total P)

4. Phosphorus accumulating organisms (PAO)

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Oxygen Consumption

5000

6000

7000

8000

9000

Oxygen Demand, ppdOxygen Demand, ppd

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0

1000

2000

3000

4000

Ox Ditch Mixed Aerobic

Plug Flow Aerobic

Plug Flow AS AS SND MLE Plug Flow Aerobic

Power Consumption

200

250

300

350

Power Consumption, bhpPower Consumption, bhp

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0

50

100

150

200

Ox Ditch Mixed Aerobic

Plug Flow Aerobic

Plug Flow AS AS SND MLE Plug Flow Aerobic

Effluent Ammonia

0 25

0.3

0.35

0.4

0.45

Effluent Ammonia, mg/LEffluent Ammonia, mg/L

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0

0.05

0.1

0.15

0.2

0.25

Ox Ditch Mixed Aerobic

Plug Flow Aerobic

Plug Flow AS AS SND MLE Plug Flow Aerobic

Effluent TIN

8

10

12

14

Effluent TIN, mg/LEffluent TIN, mg/L

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0

2

4

6

Ox Ditch Mixed Aerobic Plug Flow Aerobic

Plug Flow AS AS SND MLE Plug Flow Aerobic

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Effluent Total P

3

4

5

6

Effluent Total P, mg/LEffluent Total P, mg/L

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0

1

2

3

Ox Ditch Mixed Aerobic Plug Flow Aerobic

Plug Flow AS AS SND MLE Plug Flow Aerobic

PAO Percentage

20.00%

25.00%

30.00%

PAO, %PAO, %

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0.00%

5.00%

10.00%

15.00%

Ox Ditch Mixed Aerobic Plug Flow Aerobic

Plug Flow AS AS SND MLE Plug Flow Aerobic

Summary Comparison

Description Ox Ditch Mixed AerobicPlug Flow Aerobic Plug Flow AS AS SND

MLE Plug Flow Aerobic

Oxygen Demand, ppd 7680 7598 8256 7268 7015 6768

Power Consumption, bhp 331 102 112 98 86 94

ffl i

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Effluent Ammonia, mg/L 0.31 0.39 0.1 0.07 0.07 0.04

Effluent TIN, mg/L 5.23 3.96 11.48 7.42 1.84 2.54

Effluent Total P, mg/L 4.97 4.98 5.05 0.84 0.79 1.01

PAO, % 0.03% 0.03% 0.03% 18.39% 27.27% 18.39%

Final Conclusions

1. There is a wide variety of aeration devices for wastewater aeration

2. Generally, full-floor coverage, fine-bubble aeration is the most energy efficient

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gy3. Plug flow tanks have a higher process

efficiency than completely mixed tanks4. Anaerobic selector and MLE configurations

can achieve TIN and P removal and good settleability

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Acknowledgements

Michael K. StenstromMichael K. StenstromRod ReardonMueller et al.King County

Olympus Terrace Sewer District

Thank you!

Questions?

Randal W. SamstagCarollo Engineers1218 3rd Avenue, Suite 1600Seattle, WA 98101206-684-6532