1. Folklore in Activated Sludge Treatment Plant Operations
Rocky Mountain Water Environment Association
65th Annual Meeting September 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. Where Did I Get This Title? I Stole It.
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

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
Q + QRAS
XRAS = XML
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

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

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

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

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

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
So – S
SRT
XML = {XINF-ISS + [Yg( )]}
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