Metro Wastewater Reclamation District Digester Complex What works and what doesnt.
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Transcript of Metro Wastewater Reclamation District Digester Complex What works and what doesnt.
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Metro Wastewater Reclamation District
Digester ComplexWhat works and what doesn’t.
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MWRD Digester Complex
• Load - Design• Configuration• Feed/Recirculation• Effluent• Gas system• Gas mixing
• Digester #4 modifications
• Strategy, feed, foam• Construction effects• Digester upsets• What works and what
doesn,t
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MWRD Digester Complex
Plant Load 2002 Averages
• Flow, 140 MGD
• Suspended Solids, 145 Tons/day
• BOD, 136 Tons/day
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MWRD Digester Complex
Design Criteria
Peak 2-week loads• TS, 202.5 Tons/day• VS, 155 Tons/day• 0.13 LbsVS/cuft/day• Hydraulic, 1.129
MGD
2002 load• TS, 162
Tons/day(80%)• VS, 131
Tons/day(85%)• 0.12 #VS/cf/d (92%)• .915 MGD (81%)
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MWRD Digester Complex
January 2003
Digester data• VS 194 Tons/D (max)• VS 81 Tons/D (min)• 0.176 #VS/cf/d (max)• 0.064 #VS/cf/d (min)• Effects of varying
digester feed rate.
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MWRD Digester Complex
60
80
100
120
140
160
180
SHT#1 in Dig Complex Dewater
Tons/day
Primary SolidsSecondary SolidsDig Solids inDig Solids out
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MWRD Digester Complex
Sludge Holding Tank #1• Feed tank for digesters• Capacity, 382,000 gals• 4 progressive cavity
pumps @ 450 gpm ea.
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MWRD Digester Complex
Sludge Holding Tanks 2 and 3
• De-water and future feed
• capacity 1,500,000 gals each
• 4 disc-flo/4 PC pumps
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MWRD Digester Complex
Anaerobic Digesters• 10 digesters• Capacity, 1,650,000
gals each.@ 28 ft operating level (summer)
• 1,472,000 gals each @ 25 ft (winter)
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MWRD Digester Complex
Typical Digester Gas System
Flame Traps Vent PRV’sTo Digester Gas Header
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MWRD Digester Complex
• Digesters feed is sequenced
• Feed volume by set point
• Feed rate; manual flow set point or auto flow by SHT level
• Feeding is continuos 24 hr/d
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MWRD Digester ComplexSludge Holding Tank #1
Raw Sludge Flow
Digester #9
Electric valves
Valve opens – sludge flowinto feed box
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MWRD Digester ComplexDigester
Recirculation pump
Sludge feed enters through feed pipe that opens to atmosphere. The raw sludge blends withheated recirculated digester contents.
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MWRD Digester Complex
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MWRD Digester Complex
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MWRD Digester Complex
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MWRD Digester Complex
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MWRD Digester Complex
• Digester effluent is withdrawn through a snorkel
• Snorkels can rotate 90 degrees
• Snorkels can be adjusted to control liquid level of digester
• Snorkels plug often
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MWRD Digester ComplexDigester effluent is withdrawn through a snorkel.
The snorkel can rotate 90 degrees using a rotary joint. The digester liquid level can beadjusted between 17 – 31 feet. This allows for seasonal level control.
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MWRD Digester Complex
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MWRD Digester Complex
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MWRD Digester Complex
• Atari gas mixing system
• Gas bubble mixing
• Uses bubble cannons
• Gas compressor
• Uses digester gas for bubble generation
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MWRD Digester Complex
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MWRD Digester Complex
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MWRD Digester Complex
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MWRD Digester Complex
Gas mixing effects
• Digester contents expands
• Contents specific gravity less than water
• Entrained gas in contents causes pumping problems
• Can generate foam
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MWRD Digester Complex
Operational Precautions
• Digester contents can expand 2 - 3 feet in depth
• Standard centrifugal pumps are not effective moving this liquid
• Gas compressor piping critical
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MWRD Digester Complex
Original gas mixer piping configuration
Flame trap
To gas system
Gas cannonsGas mixer
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MWRD Digester Complex
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MWRD Digester Complex
When foam enters the gas system the flame traps become plugged.This prevents pressure build-up within digester from exiting.If pressure is not relieved by some device, auto or manually,Structural damage will occur.
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MWRD Digester Complex
Digester #4 Modifications
• Roof
• Gas compressor
• Foam trough
• Foam suppression
• Foam detection and alarm
• Pressure relief manhole
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MWRD Digester Complex
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MWRD Digester Complex
Re-designed gas mixer piping configuration
Separate withdrawal pipes
To gas system To gas system
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MWRD Digester Complex
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MWRD Digester ComplexPWC spray Foam trough
Effluent
Gas system and compressor
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MWRD Digester Complex
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MWRD Digester Complex
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MWRD Digester ComplexDigester Radar
Capacitance probeRadar sees top of foam High level alarm
Liquid level transmitter Digester contents Foam
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MWRD Digester Complex
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MWRD Digester ComplexPressure Relief Manhole CoverRelief range: 16 – 18 inches of water column (.42 - .47 psi)Digester roof: 7850 ft2 = 1,130,400 in2 X .45 psi =254.34 Tons of upward forceDigester roof weight approx. 350 TonsFoam can exit without cracking the roof
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MWRD Digester Complex
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MWRD Digester Complex
Foam Strategy
• Lower digester levels to 25 feet - 6 feet headspace
• Control digester feed to meet 15 day SRT
• Pump foam out of digester to sludge holding tank
• Deal with foam in sludge holding tank
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MWRD Digester Complex
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MWRD Digester Complex
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MWRD Digester Complex
Digester Feed Strategy
• Maintain constant flow rate
• Do not use SHT level as a pump control
• Control WAS solids / primary solids ratio
• Maintain continuos de-watering operations
• Maintain continuos even digester effluent flow
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MWRD Digester Complex
Foam Pumping Strategy
• Find a pump that will pump foam - a double diaphragm air actuated works well
• Pump foam to SHT and de-water
• Pump foam to digester #4 - use suppression equipment
• Pump foam back into itself
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MWRD Digester Complex
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MWRD Digester Complex
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MWRD Digester Complex
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MWRD Digester Complex
Aggressive Construction Schedule
• Construction caused process interruptions
• Holding solids back in process areas
• Backing up digestion process
• Increases load to digesters
• Causes overload foaming (upset foam)
• Backed up secondary Nocardia foam
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MWRD Digester Complex
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MWRD Digester Complex
Upset Approach
• Stop all scheduled process interruptions
• Control digester feed rate
• Control WAS / Primary solids ratio
• Chlorinate RAS to control Nocardia
• Clean up mess daily until process stable
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MWRD Digester Complex
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MWRD Digester Complex
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MWRD Digester Complex
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MWRD Digester Complex
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MWRD Digester Complex
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MWRD Digester Complex
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MWRD Digester Complex
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MWRD Digester Complex
What Works• WAS/PRI solids
control• Digester temperature
control• Steady continuos
digester flow • Pressure relief
manholes
What Doesn’t• Too much WAS not
enough Primary• 1 degree F swing/24Hr
more than 5 days• Too many process
interruptions• No pressure relief