1. Metro Wastewater Reclamation District Digester Complex What works and what doesnt.

2. 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

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

4. 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%)

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.

6. MWRD Digester Complex

7. 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)

10. MWRD Digester Complex

11. 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

12. MWRD Digester Complex

18. Digester effluent is withdrawn through a snorkel Snorkels can rotate 90 degrees Snorkels can be adjusted to control liquid level of digester Snorkels plug often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. Atari gas mixing system Gas bubble mixing Uses bubble cannons Gas compressor Uses digester gas for bubble generation

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26. 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. Digester #4 Modifications Roof Gas compressor Foam trough Foam suppression Foam detection and alarm Pressure relief manhole

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42. 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. 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. 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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52. 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

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60. What Works WAS/PRI solids control Digester temperature control Steady continuos digester flow Pressure relief manholes What Doesnt Too much WAS not enough Primary 1 degree F swing/24Hr more than 5 days Too many process interruptions No pressure relief