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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


Mwrd digester complex3
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.



Mwrd digester complex5
MWRD Digester Complex

Sludge Holding Tank #1

  • Feed tank for digesters

  • Capacity, 382,000 gals

  • 4 progressive cavity pumps @ 450 gpm ea.


Mwrd digester complex6
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


Mwrd digester complex7
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)



Mwrd digester complex9
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








Mwrd digester complex16
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  • Digester effluent is withdrawn through a snorkel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





Mwrd digester complex20
MWRD Digester Complex

  • Atari gas mixing system

  • Gas bubble mixing

  • Uses bubble cannons

  • Gas compressor

  • Uses digester gas for bubble generation





Mwrd digester complex24
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


Mwrd digester complex25
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





Mwrd digester complex29
MWRD Digester Complex

Digester #4 Modifications

  • Roof

  • Gas compressor

  • Foam trough

  • Foam suppression

  • Foam detection and alarm

  • Pressure relief manhole












Mwrd digester complex40
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




Mwrd digester complex43
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


Mwrd digester complex44
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





Mwrd digester complex48
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



Mwrd digester complex50
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