Mill Creek WWTP Secondary Clarifiers Assessment

2017 OWEA Technical Conference & Expo

Pete McKee, MSDGCChad Cecrle, Black & Veatch

Agenda

• Secondary Upgrades Project Background• Project goals and improvements

• Secondary Clarifier Field Testing• Field testing goals and results

• Secondary Clarifier CFD Analysis• CFD analysis goals and results

• Secondary Clarifier Follow-up Field Testing• Follow up field testing goals and results

• Final Recommendations• Recommended enhancements for achieving up to 300 mgd

wet weather treatment2MSDGC

Black & Veatch

Secondary Upgrades Project Background

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Mill Creek WWTP Secondary Clarifier Assessment

Secondary Upgrades Project Goals

• Increase secondary treatment capacity to 240 MGD• More reliable wet weather treatment

• Improve control of RAS withdrawal from clarifiers• Add dedicated RAS pumps

• Evenly distribute RAS to aeration tanks• Add RAS flow control

• Add option for step feed operation• Add new influent gates into 3rd pass of aeration tanks

Ensure reliable operation during wet weather (240+ mgd) 44MSDGC

Black & Veatch

Conceptual Design Evaluation• Rectangular with three

rotating manifolds• $35M cost estimate• 3-4 year construction

• Rectangular with chain and flight scrapers

• $45M cost estimate• 4 year construction

• Circular with rotating scraper

• $45M cost estimate• 4 year construction

Option 2

Option 3

Option 1

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Mill Creek Secondary Treatment

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Aeration Tanks

Clarifiers

1WS2WS

3WS1WM

2WM

3WM

1WN

2WN

3WN

1,2,3 E S,M,N

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Mill Creek Secondary Treatment

RAS evenly distributed to head of aeration tanks7

Aeration Tanks

Sludge Withdrawal

RAS PumpsRAS Distribution

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Mill Creek Secondary Treatment

Step feed allows 240+mgd treatment during wet weather flows8

Primary Effluent RAS3rd Pass 1st pass

Reduced Solids Loading

2W

1W

3W

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Mill Creek Secondary Treatment

Unsightly scum can be removed via sprays and skimmings troughs9

Scum Removal2W

1W

3W

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Field Testing –Clarifiers 1W & 2W

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Mill Creek WWTP Secondary Clarifier Assessment

Clarifier Field Testing Goals

• Confirm secondary treatment capacity of 240+ mgd• Stress test clarifier trains 1W and 2W up to 40 mgd

• Measure hydraulic efficiency• Dye testing at 20 mgd and 40 mgd

• Measure hydraulic profile• Drogue testing at 20 mgd and 40 mgd

• Prepare field data for CFD model• Calibration data for CFD

Testing performed in Fall 2011

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Secondary Clarifier Influent

• Influent baffle wall

• Sludge withdrawal arm

• Reaction baffle

• Corner fillets

1

2

3

4

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Dye Testing at 20 mgd and 40 mgd

Introduce dye at clarifier influent

Measure concentration at effluent

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Dye Testing Reveals Short Circuiting

Rapid rise to peak indicates high velocity current

0

20

40

60

80

100

120

140

0 50 100 150 200 250

Dye

Conc

entr

atio

n

Time (minutes)

Theoretical D.T. = 115 minutesClarifier 1W D.T. = 51 minutesClarifier 2W D.T. = 52 minutes

0

20

40

60

80

100

120

140

0 50 100 150 200 250

Dye

Conc

entr

atio

n

Time (minutes)

Theoretical D.T. = 230 minutesClarifier 1W D.T. = 75 minutesClarifier 2W D.T. = 81 minutes

( )

Clarifier 1W Clarifier 2W

Dye Curve @ 20 mgd Dye Curve @ 40 mgd

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Drogue Testing

Used to measure velocity profile along clarifierMSDGCBlack & Veatch

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Velocity profile

High velocity currents along inlet side of clarifier

25W 25E7WClarifier 1W Drogue Station: South Flow Rate: 40 mgd

-2’

-6’

-10’

O

O

O O

O

+7 +14

+3

To effluentTo influent

O

fpm

Dept

h

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Computational Fluid Dynamics (CFD) Model

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Mill Creek WWTP Secondary Clarifier Assessment

CFD Model Goals

• Is there benefit in having the reaction baffle (2W)?• 20 mgd & 40 mgd simulated flows through 1W & 2W

• Is there benefit in adding full corner fillets?• Simulated flows with and without full depth fillets

• Can flow be better distributed across clarifier with an influent baffle?• Simulated flows with and without influent baffle

CFD model calibrated using field test data18MSDGC

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Secondary Clarifier ModelEffluent (North)

Reaction baffle (2W only)

Effluent Launder

RAS Withdrawal

Inlet (South)MSDGCBlack & Veatch

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Simulated Flow with Reaction Baffle

High velocities along inlet side present in both scenarios

40 MGD Clarifier 1W (no reaction baffle)

Clarifier 2W (reaction baffle)

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Velocity Profile

More uniform streamlines. Improved flow distribution.

reamlines biased to wnstream side of annel. Poor distribution th higher velocities on ht side of clarifier.

Simulated Flow with Influent Baffle

Baffle in inlet channel improves flow distribution21

40 MGD 40 MGD

Inlet Baffle

No Influent Baffle

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Simulated Flow with Influent Baffle

Predicted 500 mg/L solids elevation 2 ft lower with baffle

40 MGD

Influent BaffleMSDGCBlack & Veatch

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Simulated Flows with Full Corner Fillets

Flow distribution improves during dry weather, worsens during wet weather flows

20 MGD

40 MGD

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Field Testing and CFD Modeling Recommendations

• Is there benefit in having the reaction baffle?• No appreciable benefit. Not recommended for remaining tanks.

• Is there benefit in adding full corner fillets?• Not recommended due to cost and adverse flow distribution

impacts during wet weather flows• Recommend regular operation of skimmings troughs

• Can flow be better distributed across clarifier with an influent baffle?• Yes. Recommend further development of influent baffle design

to implement for trial.

MSDGC implemented recommendations including trial influent baffle

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Follow Up Field Testing – Clarifiers 2E & 3E

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Mill Creek WWTP Secondary Clarifier Assessment

Influent and Intermediate Baffles Addition

Recommended baffle additions for follow up field testing26

3E

Influent Baffles

2E1E

Intermediate Baffle

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Influent and Intermediate Baffles Addition

Influent baffles – recommended from CFD modelIntermediate baffles– recommended by CPE

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Intermediate Baffle in Clarifier 3EInfluent Baffle in Clarifiers 2E & 3E

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Follow Up Field Testing Results

Recommend installing L shaped influent baffle on other four clarifiers 1W, 2W, 3W, & 1E

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Mill Creek WWTP Secondary Clarifiers Assessment

2017 OWEA Technical Conference & Expo

Pete McKee, MSDGCChad Cecrle, Black & Veatch