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Folklore in Activated Sludge Treatment Plant Operations

Rocky Mountain Water Environment Association 65 th Annual Meeting September 17, 2002. Folklore in Activated Sludge Treatment Plant Operations. Eric J. Wahlberg. Folklore Defined. Folklore Defined. “An unsupported notion, story, or saying that is widely circulated.”. Learning Objectives.

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Folklore in Activated Sludge Treatment Plant Operations

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  1. Rocky Mountain Water Environment Association 65th Annual MeetingSeptember 17, 2002 Folklore in Activated Sludge Treatment Plant Operations Eric J. Wahlberg

  2. Folklore Defined Folklore Defined “An unsupported notion, story, or saying that is widely circulated.”

  3. Learning Objectives • Myth 1 • Myth 2 • Myth 3 • Myth 4 • Myth 5

  4. What’s In It For You?

  5. Folklore Defined Where Did I Get This Title?I Stole It. Richard I. Dick (1976). “Folklore in the design of final settling tanks,” Journal Water Pollution Control Federation, 48, pp. 633-644.

  6. Myth 1 Blankets are necessary for secondary clarifiers to work properly.

  7. How Often Has It Been Said Activated sludge secondary clarifiers perform two functions: 1. Clarification of the overflow 2. Thickening of the underflow

  8. How Often Has It Been Said Activated sludge secondary clarifiers perform two functions: 1. Clarification of the overflow 2. Thickening of the underflow Yuk!

  9. Thickening Role is Root Cause of Many Problems • Operators try to achieve unachievable RAS concentrations • Denitrification • Phosphorus release • Deterioration in sludge quality

  10. Just Say No Thickening in secondary clarifiers

  11. RAS Concentration a Fixed Multiple of MLSS XRAS = XML Q + QRAS QRAS

  12. Secondary Clarifier as a Reactor?

  13. Secondary Clarifier as a Reactor?

  14. Secondary Clarifier as a Reactor?

  15. Secondary Clarifier as a Reactor?

  16. Secondary Clarifier as a Reactor?

  17. Secondary Clarifier as a Reactor?

  18. Secondary Clarifier as a Reactor?

  19. Truth 1 Blankets are not necessary for secondary clarifiers to work properly, and better performance will result when no blankets are kept in secondary clarifiers.

  20. Myth 2 The hydraulic detention time in the aeration basin is decreased by increasing the RAS flow rate.

  21. Proper Response to an Over-loaded Clarifier: Increase RAS Increase RAS flow

  22. The Basic Activated Sludge Process Effluent Influent Mixed liquor Secondary clarifier Aeration basin WAS RAS

  23. Control Volume Shows Two Flows Into The Aeration Basin Control Volume

  24. This Is How The Myth Got Started • HRT = VAB/(Q + QRAS)

  25. This Is How The Myth Got Started • HRT = VAB/(Q + QRAS) • But this is not correct. Why?

  26. Control Volume Shows Two Flows Into The Aeration Basin Control Volume

  27. Shrinking the Clarifier the RAS Flow Is Completely Internal Control Volume

  28. The Basic Activated Sludge Process Effluent Influent Mixed liquor Secondary clarifier Aeration basin WAS RAS

  29. Tanks Are Full; No Influent, Effluent, WAS Mixed liquor Secondary clarifier Aeration basin RAS

  30. Truth 2 As Glen Daigger stated Sunday: “Bugs only know SRT; they do not know HRT”. The hydraulic detention time in the aeration basin is not affected by the RAS flow rate.

  31. Myth 3 The MLSS concentration can be controlled by the RAS flow rate.

  32. “No” Blankets In Under And Critically Loaded Clarifiers

  33. Blankets Form In Over Loaded Clarifiers Decrease RAS flow

  34. System Response: Blanket Comes Up, MLSS Goes Down

  35. d) 40 2 30 20 Solids Flux (lb/ft 10 0 Over Loaded To Under Loaded, MLSS Goes Up 0 5 10 15 Solids Concentration (g/L)

  36. Truth 3 The MLSS concentration can be controlled by the RAS flow rate, but you don’t want to because it’s limited and it requires blankets in the clarifiers (see Truth 1).

  37. Myth 4 Process control means keeping the MLSS concentration or the F:M ratio or the SRT constant.

  38. What Are We Trying to Accomplish?

  39. Settles well Compacts well Flocculates well In Essence Our Jobs Are Very Simple As David Jenkins almost said on Sunday, we want a sludge that:

  40. You Cannot Control What You Do Not Measure Process control should be based on maintaining stable, optimal sludge quality.

  41. Truth 4 Process control means maintaining stable, optimal sludge quality (that meets permit).

  42. Myth 5 The MLSS concentration, the F:M ratio, and the SRT can be controlled simultaneously .

  43. Engineers’ Design Data Keep This Myth Going

  44. Equation For MLSS Concentration XML = {XINF-ISS + [Yg( )]} So – S SRT HRT 1 + bSRT

  45. Truth 5 Fix the SRT to achieve permit compliance and let the MLSS and F:M “drift” to wherever they want to.

  46. Thank you! • Myth 1 • Myth 2 • Myth 3 • Myth 4 • Myth 5

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