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Metro Wastewater Reclamation District

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

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  1. Metro Wastewater Reclamation District Digester Complex What works and what doesn’t.

  2. 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

  3. MWRD Digester Complex Plant Load 2002 Averages • Flow, 140 MGD • Suspended Solids, 145 Tons/day • BOD, 136 Tons/day

  4. 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

  5. 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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  7. MWRD Digester Complex Sludge Holding Tank #1 • Feed tank for digesters • Capacity, 382,000 gals • 4 progressive cavity pumps @ 450 gpm ea.

  8. 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

  9. 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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  11. 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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  18. MWRD Digester Complex • Digester effluent is withdrawn through a snorkel • Snorkels can rotate 90 degrees • Snorkels can be adjusted to control liquid level of digester • Snorkels plug often

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  22. 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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  26. 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

  27. 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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  31. MWRD Digester Complex Digester #4 Modifications • Roof • Gas compressor • Foam trough • Foam suppression • Foam detection and alarm • Pressure relief manhole

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  42. 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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  45. 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

  46. 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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  50. 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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