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

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

What’s In It For You?

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. 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 Q + QRAS XRAS = XML QRAS

Secondary Clarifier as a Reactor?

Secondary Clarifier as a Reactor?

Secondary Clarifier as a Reactor?

Secondary Clarifier as a Reactor?

Secondary Clarifier as a Reactor?

Secondary Clarifier as a Reactor?

Secondary Clarifier as a Reactor?

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

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

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 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”.

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

“No” Blankets In Under And Critically Loaded Clarifiers

Blankets Form In Over Loaded Clarifiers Decrease RAS flow

System Response: Blanket Comes Up, MLSS Goes Down

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)

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.

What Are We Trying to Accomplish?

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

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 .

Engineers’ Design Data Keep This Myth Going

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