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Explore common folklore in activated sludge treatment plants and debunk myths for optimal performance and efficiency. Learn actionable insights for better plant management.
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Rocky Mountain Water Environment Association 65th Annual MeetingSeptember 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 • Myth 1 • Myth 2 • Myth 3 • Myth 4 • Myth 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.
Myth 1 Blankets are necessary for secondary clarifiers to work properly.
How Often Has It Been Said Activated sludge secondary clarifiers perform two functions: 1. Clarification of the overflow 2. Thickening of the underflow
How Often Has It Been Said Activated sludge secondary clarifiers perform two functions: 1. Clarification of the overflow 2. Thickening of the underflow Yuk!
Thickening Role is Root Cause of Many Problems • Operators try to achieve unachievable RAS concentrations • Denitrification • Phosphorus release • Deterioration in sludge quality
Just Say No Thickening in secondary clarifiers
RAS Concentration a Fixed Multiple of MLSS XRAS = XML Q + QRAS QRAS
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.
Myth 2 The hydraulic detention time in the aeration basin is decreased by increasing the RAS flow rate.
Proper Response to an Over-loaded Clarifier: Increase RAS Increase RAS flow
The Basic Activated Sludge Process Effluent Influent Mixed liquor Secondary clarifier Aeration basin WAS RAS
Control Volume Shows Two Flows Into The Aeration Basin Control Volume
This Is How The Myth Got Started • HRT = VAB/(Q + QRAS)
This Is How The Myth Got Started • HRT = VAB/(Q + QRAS) • But this is not correct. Why?
Control Volume Shows Two Flows Into The Aeration Basin Control Volume
Shrinking the Clarifier the RAS Flow Is Completely Internal Control Volume
The Basic Activated Sludge Process Effluent Influent Mixed liquor Secondary clarifier Aeration basin WAS RAS
Tanks Are Full; No Influent, Effluent, WAS Mixed liquor Secondary clarifier Aeration basin RAS
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.
Myth 3 The MLSS concentration can be controlled by the RAS flow rate.
Blankets Form In Over Loaded Clarifiers Decrease RAS flow
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)
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).
Myth 4 Process control means keeping the MLSS concentration or the F:M ratio or the SRT constant.
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:
You Cannot Control What You Do Not Measure Process control should be based on maintaining stable, optimal sludge quality.
Truth 4 Process control means maintaining stable, optimal sludge quality (that meets permit).
Myth 5 The MLSS concentration, the F:M ratio, and the SRT can be controlled simultaneously .
Equation For MLSS Concentration XML = {XINF-ISS + [Yg( )]} So – S SRT HRT 1 + bSRT
Truth 5 Fix the SRT to achieve permit compliance and let the MLSS and F:M “drift” to wherever they want to.
Thank you! • Myth 1 • Myth 2 • Myth 3 • Myth 4 • Myth 5