NC AWWA WEA Annual Conference 2014 Rapid Volume … · 2018-04-04 · mixers - slow changes in...

Rapid Volume Expansion of Digester Contents

NC AWWA WEA Annual Conference 2014

November| 2014

• What is rapid volume expansion? Is it foaming or something else?

• What factors can influence events? • Case Study at the Brightwater WWTP in Woodinville,

WA. • Design modifications to manage events. •  Take homes

Agenda

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Tennessee Case - 2012

Events Can Be Serious

What is Foam?

•  Gas bubbles entrapped in the liquid matrix

•  Surface tension and capillary forces stabilize foam bubbles

•  Foaming increases when the surface tension of the liquid is reduced (increased bubble stability)

•  Foaming agents (surfactants) increase foam formation

•  Foam bubbles collapse when liquid drains by gravity

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What is Foam?

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Why is digester volume expansion not foaming?

•  Digester volume expansion is caused by changes in digester gas holdup •  Production rate greater than

release rate

•  In some cases, an apparent foaming event is actually caused by expansion of the digester contents

•  Rapid volume expansion can present a significant risk to digester and operator safety

PIT

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During normal digester operations gas generation and evolution to the headspace are at equilibrium

CH4

CO2 H2

CO2 CH4

Digester Headspace

Digesting Sludge

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The Principal of Gas Holdup

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PIT

Typical Digester Operation: Gas production and conveyance to headspace is at steady state

PIT

When conveyance and production are out of balance

Examples of gas hold up in liquids

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Bubble Hydrodynamics – The Science of Gas Holdup • Sludge Characteristics that impact/result in slow

bubble rise rates •  Size: Small bubbles rise slower than large ones •  Viscosity: Digester sludge is viscous, behaves as a

viscoplastic •  Reduced viscosity with increase shear stress (mixing) •  Yield Stress – bubbles must overcome this

•  Surface tension: Surfactants present in digesters reduce surface tension

•  Pressure: Tall digesters have higher pressures near the bottom

•  Particles: Grit and solids in the digester will impact bubble size

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Improved digester design, increased process efficiency has made this an issue

Hyperion 1952

Hyperion 2010

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WAS and Filaments

•  Foam trapping allows accumulation of Nocardioforms

• Hydrophobic surface causes attachment to air bubbles

(Source: Parker et al, WER, Vol 86, No 6, 2014)

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Digester Operation Influences Gas Holdup • Changes in gas holdup impact the volume of

digester contents • Common causes of volume expansion are

related to digester operation: •  Digester feed rates •  Digester mixing intensity •  Power outages •  Scum / Fog addition •  And more…

• It is unlikely we can eliminate or prevent all volume expansion events

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Example - Sudden Changes in Mixing can Cause Volume Expansion • Several documented

cases of volume expansion following a sudden change in mixing

• Changes in mixing direction can cause a sudden shift in gas holdup resulting in a dramatic change in liquid level

M

M MM

M

M MM

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Case Study: Brightwater Digesters • Modern digester design • Similar to an egg shaped digester but lower

construction cost (Modified Silo) •  3 digesters, 1.25 Mgal each •  1 storage tank, 790kgal •  Diameter 59 ft •  Digester SWD 65 ft

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Brightwater WWTP digestion process

PIT PIT

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Digester-3 Level

Digester Gas Flow

Restart Mixing

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Planned Power Outage

Outage Start

Digester Level

61.3 ft

Digester Level

51.1 ft

Mixer Off Mixer On

Storage Level 21.9 ft

Gas Pressure

Feed Pump Off

Gas Production 114 cfm

Flare 1 Temp

Storage Level 28.7 ft

Outage End

Planned Power Outage

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Test Results

• Density change due to gas holdup •  Start density near water: SG = 0.92 •  End density lower than water: SG = 0.77

• Volume transferred: •  140,000 gal, 11% of digester active volume transferred to

Blending Storage tank •  400-500 gpm transfer rate

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New Research: Laboratory scale supports the full scale observations

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Source: Higgins et al (2014) Proceedings of the WEF Residual and Biosolids Conference

Measured liquid surface level in clear Plexiglas reactor

Digester Modifications During Commissioning

• Verified existing overflow capacity was sufficient – Yes, it was!

• VFD operation of digester mixers - slow changes in mixer speed and direction

• Radar level monitoring – accurate surface/liquid level monitoring

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Surface Level measurement

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Radar shows the true digester level

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Digester-2 Level-PIT

Storage Tank Level-PIT

Storage Tank Level- Radar

~10 ft = 204 kgal or about 26% of the tank volume

Recent OWASA (NC) data suggest low-specific-gravity can be normal / ongoing

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Digester Stage

Pressure (ft )

Radar (ft)

Calculated Average Specific Gravity

1 22.6 30.4 74%

2 19.2 23.3 82%

3 16.2 19.0 85%

RAD

PIT

Take Home Message • DESIGNERS:

•  Consider rapid volume expansion in future digester designs

• OPERATORS: •  Be aware of rapid volume

expansion during operations and maintenance activities

•  I would be interested in hearing about experiences from your plants

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• Chris Muller – Brown and Caldwell – Andover, MA •  Tom Chapman – Brown and Caldwell- Tucson, AZ • Steve Krugel –Brown and Caldwell- Seattle, WA

Acknowledgements

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Thank You

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Grant Sharpe, PE Brown and Caldwell, Charlotte, NC gsharpe@brwncald.com 704-373-7134