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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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3
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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