Stirred not Shaken Stirred not Shaken Anoxic/Anaerobic Zone Mixer SizingAnoxic/Anaerobic Zone Mixer Sizing

Peter SchauerEd KobylinskiPNCWA 2010

2

Agenda

General mixing in wastewater treatmentGross design parametersSpecific design parametersFull-scale testing

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Mixing in Wastewater Treatment

Want rapid blending of MLSS of two or more streams to intermix suspended solidsWant dissipation of incoming kinetic energyWant rapid mixing of soluble rbCOD and nitrate Want shear to expose inner floc and active microbes to rbCOD and nitrateHigh denitrification rates only possible with high rbCOD concentrations

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Mixing in Wastewater Treatment

Input energy into tank and get liquid in motion with the least amount of electrical energyMeet process objectives without transferring excessive amounts of oxygenTurnover of liquid will stimulate oxygen transfer –Do Not Want Surface Turbulence

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Typical Design Parameters

Power input to mixerHP/kcfVary between types of mixing

Manufacturer inputDirect mixer selectionGuidance on recommendationCFD analysis

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Performance Design Parameters

Basin turnoverPumping rateGThrust / Momentum

ISO 21630 – Submersible Mixers for Wastewater and Similar Applications

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Mixer Thrust / Momentum

The energy imparted to liquid can be expressed as kinetic energy. Momentum is a measure of the energy contained by mass that is in motion and through friction that energy can be imparted to other fluid massSo for the purposes of mixing, momentum imparted into the system will gradually induce other liquid into motion and liquid in motion will keep solids in suspensionThe momentum generated by the mixer pumping the fluid can be measured by the thrust (force) generated by the mixer on the mixer mounting

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Full-Scale Testing: Purpose of Study

Controlled recycle rate of MLSS from Oxic zoneAdjusted mixer speedPerformed a profile of nitrate, nitrite and soluble COD through anoxic zonesUsed on-site Hach Methods for nitrate, nitrite and COD analyses

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FacilitiesNew 4.7 mgd 5 Stage Bardenpho System

Anaerobic zoneAnoxic zoneAerobic zonePost-anoxic zoneReaeration zone

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FacilitiesTested new 4.7 mgd rated treatment trainAnaerobic zone divided into 3 cells in seriesAnoxic zone divided into 3 cells in series5 Hp top entering mixers - axial flow down-pumping impellers

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Anaerobic and Anoxic Zone Schematic

Cell 1

Anaerobic/ Fermentation

Cell 2 Cell 3

Cell 1 Cell 2 Cell 3

Anoxic MLSS recycle from aerobic

zone

Influent plus RAS

Flow back to aerobic zone

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Collection System Issues

RAS

Influent

Overflow Underflow

Overflow

Deniter gate

MLSS Recycle

Slot in Wall Top to Floor

3 Anaerobic Cells in series

MLSS Return

Flow

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Anaerobic Zone

Baffle Wall Surface Port

Baffle Wall Submerged Port

Baffle wall submerged at above average

flows

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Anoxic Zone Baffle Wall Gap from Surface to

Floor

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Mixing TestsMixers designed at 34 rpmMixers were run at three different speeds

28 rpm16 rpm8 rpm

69 inch diameter impeller

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Top Entering MixersAt 28 rpm there is little surface turbulenceLittle surface vortexingSurface DO less than 0.4 mg/L

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Anoxic Zone Cell #1 – Toughest Mixing ApplicationSurface entry from Anaerobic Zone Cell #3MLSS recycle up to 4 times influent – full depthBoth flows are opposing

Anaerobic Influent

MLSS Recycle

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Deniter Gate

Gate controls MLSS recycle3 ft wide channel, 18 ft deep

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Anoxic Zone Cell – Sample Locationspoint #6 Submergence, sample ft below surface First Anoxic Zone

1 3 Cell #2 Cell #3 Point #2 Submergence, 2 6 2 ft sample # ft below surface3 9 1 34 12 #6 #2 2 6

point #4 ft below surface 2 ft 3 91 3 4 122 6 #4

point #3 ft below surface #31 32 6 #5 #1

8 6

Point #5 Submergence, Point #1 Submergence, sample # ft below surface sample # ft below surface

1 3 1 32 6 2 63 9 3 94 12 4 12

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Anoxic Zone Cell #2 – MLSS Profile

3 6 9 121

3

5

3,4403,4603,4803,500

3,5203,540

3,560

3,580

3,600

3,620

3,640

MLSS, mg/L

Depth, ft

Location

123456

Anoxic Cell #2 - 28 rpm

Largest Variation - 120 mg/L3.35% variation

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Anoxic Zone Cell #2 – MLSS Profile

3 6 9 121

3

5

3,300

3,350

3,400

3,450

3,500

3,550

3,600

MLSS, mg/L

Depth, ft

Location

123456

Anoxic Cell #2 - 16 rpm

Largest Variation - 160 mg/L4.6% variation

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Anoxic Zone Cell #2 – MLSS Profile

3 6 9 121

3

5

3,250

3,300

3,350

3,400

3,450

3,500

3,550

MLSS, mg/L

Depth, ft

Location

123456

Anoxic Cell #2 - 8 rpm

Largest Variation - 190 mg/L5.52% variation

MM

Anoxic Cell #2 Mixer at 8 rpm. Notice MLSS floc size and some clear liquid zones

Anoxic Cell #3 Mixer at 16 rpm. More uniform floc at surface

Anoxic Cell #2 Mixer at 8 rpm. Notice MLSS floc size and some clear liquid zones

Anoxic Cell #1 Mixer at 28 rpm. Notice MLSS floc size and some clear liquid zones

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Different Mixing Conditions

28 rpm

8 rpm

16 rpm

Notice differences in floc appearance

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Surface Appearance at 8 rpm

While surface looks different, 3 ft and lower good solids

distribution

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Anoxic Zone Cell #1 Sample LocationsAnaerobic Cell #3 First Anoxic ZoneFlow to Anoxic Cell #1 Cell #1

6 7 1

43

2

5

MLSS Recycle FlowFrom Ditch thru

deniter gate

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Anoxic Zone Cell #1 MLSS Profile

3 6 9 121

3

5

7

3,200

3,250

3,300

3,350

3,400

3,450

3,500

MLSS, mg/L

Depth, ft

Location

1234567

Largest Variation - 125 mg/L

3.65% variation

Anoxic Cell #1

Mixer Speed 28 rpm

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Anoxic Zone Cell #1 MLSS Profile

3 6 9 121

3

57

3,100

3,150

3,200

3,250

3,300

3,350

3,400

3,450

3,500

3,550

MLSS, mg/L

Depth, ft

Location

1234567

Largest Variation - 290

mg/L8.63% variation

Anoxic Cell #1 - 16 rpm

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Mixing Power Turndown

At 100% speed (34 rpm) mixers draw 5 HpAt 80% speed (28 rpm) mixers draw 2.8 HpAt 50% speed (16 rpm) mixers draw 0.5 HpAt 30% speed (8 rpm) mixers draw 0.1 HpAm awaiting confirmation of power calculations from mixer manufacturerMixer liquid pumping is proportional to rpm –80% speed = 80% pumping capacity

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Nitrate and Soluble COD Profile

Nitrate and Soluble COD Profile 2/16/2009

0.4

10.97.8 8.0 7.7 6.4

36.8

11.6 10.0 8.5 10.06.2

0.05.0

10.015.020.025.030.035.040.0

AN Eff MLR AX1 Eff AX2 Eff AX3 Eff Final Eff

Location

Con

cent

ratio

n, m

g/L

NO3-N

COD

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Nitrate and Biodegradable Soluble COD Profile

Nitrate and Biodegradable Soluble COD Profile 2/16/2009

0.4

10.98.72 7.8 8.0 7.7 6.4

30.6

5.4

10.4

3.8 2.3 3.80.0

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

AN Eff MLR AN Eff+MLR AX1 Eff AX2 Eff AX3 Eff Final Eff

Location

Con

cent

ratio

n, m

g/L

NO3-N

COD

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Nitrate Removal

Soluble COD consumed in first Anoxic CellDenitrification in Cells #2 and #3 is driven by endogenous oxygen demandIt rained the night before testing beganIncoming carbon affected by fermentation in the sewer – high sewer flow strips off slimeMixing versus denitrification rate was inconclusive because of insufficient carbon

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Dual Level Mixing

High intensity mixing for floc Shear and lower level mixing for solids dispersion

AnaerobicZone

Effluent

MLSSrecycle

Rapid Mixfor Shear

Slow Mixfor Solids

Distribution

Combined FlowAnaerobic

ZoneEffluent

MLSSrecycle

Rapid Mixfor Shear

Slow Mixfor Solids

Distribution

Combined Flow

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Discussion