metro wastewater reclamation district digester complex what works and what doesnt

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


Plant Load 2002 Averages

• Flow, 140 MGD

• Suspended Solids, 145 Tons/day

• BOD, 136 Tons/day


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%)


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.


60

80

100

120

140

160

180

SHT#1 in Dig Complex Dewater

Tons/day

Primary SolidsSecondary SolidsDig Solids inDig Solids out


Sludge Holding Tank #1• Feed tank for digesters• Capacity, 382,000 gals• 4 progressive cavity

pumps @ 450 gpm ea.


Sludge Holding Tanks 2 and 3

• De-water and future feed

• capacity 1,500,000 gals each

• 4 disc-flo/4 PC pumps


Anaerobic Digesters• 10 digesters• Capacity, 1,650,000

gals each.@ 28 ft operating level (summer)

• 1,472,000 gals each @ 25 ft (winter)


Typical Digester Gas System

Flame Traps Vent PRV’sTo Digester Gas Header


• 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.


• Digester effluent is withdrawn through a snorkel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MWRD Digester ComplexDigester effluent is withdrawn through a snorkel.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.


• Atari gas mixing system

• Gas bubble mixing

• Uses bubble cannons

• Gas compressor

• Uses digester gas for bubble generation


Gas mixing effects

• Digester contents expands

• Contents specific gravity less than water

• Entrained gas in contents causes pumping problems

• Can generate foam


Operational Precautions

• Digester contents can expand 2 - 3 feet in depth

• Standard centrifugal pumps are not effective moving this liquid

• Gas compressor piping critical


Original gas mixer piping configuration

Flame trap

To gas system

Gas cannonsGas mixer


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.


Digester #4 Modifications

• Roof

• Gas compressor

• Foam trough

• Foam suppression

• Foam detection and alarm

• Pressure relief manhole


Re-designed gas mixer piping configuration

Separate withdrawal pipes

To gas system To gas system

MWRD Digester ComplexPWC spray Foam trough

Effluent

Gas system and compressor

MWRD Digester ComplexDigester Radar

Capacitance probeRadar sees top of foam High level alarm

Liquid level transmitter Digester contents Foam

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


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


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


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


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


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


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

metro wastewater reclamation district digester complex what works and what doesnt

Documents

tank slide

tonsday slide

hrd slide

critical slide

digester effluent flow

digester gas

process stable slide

bubble generation slide