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Metro Wastewater Reclamation District. Digester Complex What works and what doesn’t. 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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metro wastewater reclamation district

Metro Wastewater Reclamation District

Digester Complex

What works and what doesn’t.

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

MWRD Digester Complex
mwrd digester complex1
MWRD Digester Complex

Plant Load 2002 Averages

  • Flow, 140 MGD
  • Suspended Solids, 145 Tons/day
  • BOD, 136 Tons/day
mwrd digester complex2
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%)

MWRD Digester Complex
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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

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 [email protected] 28 ft operating level (summer)
  • 1,472,000 gals each @ 25 ft (winter)
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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 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  • Digester effluent is withdrawn through a snorkel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 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

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

Digester #4 Modifications

  • Roof
  • Gas compressor
  • Foam trough
  • Foam suppression
  • Foam detection and alarm
  • Pressure relief manhole
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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

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

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

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
mwrd digester complex58
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

MWRD Digester Complex
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