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Metro Wastewater Reclamation District
Digester ComplexWhat 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
Plant Load 2002 Averages
• Flow, 140 MGD
• Suspended Solids, 145 Tons/day
• BOD, 136 Tons/day
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%)
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 Complex
60
80
100
120
140
160
180
SHT#1 in Dig Complex Dewater
Tons/day
Primary SolidsSecondary SolidsDig Solids inDig Solids out
MWRD Digester Complex
Sludge Holding Tank #1• Feed tank for digesters• Capacity, 382,000 gals• 4 progressive cavity
pumps @ 450 gpm ea.
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 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 Complex
Typical Digester Gas System
Flame Traps Vent PRV’sTo Digester Gas Header
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 ComplexSludge Holding Tank #1
Raw Sludge Flow
Digester #9
Electric valves
Valve opens – sludge flowinto feed box
MWRD Digester ComplexDigester
Recirculation pump
Sludge feed enters through feed pipe that opens to atmosphere. The raw sludge blends withheated recirculated digester contents.
MWRD Digester Complex
MWRD Digester Complex
MWRD Digester Complex
MWRD Digester Complex
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• Digester effluent is withdrawn through a snorkel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MWRD Digester ComplexDigester effluent is withdrawn through a snorkel.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.
MWRD Digester Complex
MWRD Digester Complex
MWRD Digester Complex
• Atari gas mixing system
• Gas bubble mixing
• Uses bubble cannons
• Gas compressor
• Uses digester gas for bubble generation
MWRD Digester Complex
MWRD Digester Complex
MWRD Digester Complex
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 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 Complex
Original gas mixer piping configuration
Flame trap
To gas system
Gas cannonsGas mixer
MWRD Digester Complex
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.
MWRD Digester Complex
Digester #4 Modifications
• Roof
• Gas compressor
• Foam trough
• Foam suppression
• Foam detection and alarm
• Pressure relief manhole
MWRD Digester Complex
MWRD Digester Complex
Re-designed gas mixer piping configuration
Separate withdrawal pipes
To gas system To gas system
MWRD Digester Complex
MWRD Digester ComplexPWC spray Foam trough
Effluent
Gas system and compressor
MWRD Digester Complex
MWRD Digester Complex
MWRD Digester ComplexDigester Radar
Capacitance probeRadar sees top of foam High level alarm
Liquid level transmitter Digester contents Foam
MWRD Digester Complex
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
MWRD Digester Complex
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 Complex
MWRD Digester Complex
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 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 Complex
MWRD Digester Complex
MWRD Digester Complex
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 Complex
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 Complex
MWRD Digester Complex
MWRD Digester Complex
MWRD Digester Complex
MWRD Digester Complex
MWRD Digester Complex
MWRD Digester Complex
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