Folklore in Activated Sludge Treatment Plant OperationsFolklore in Activated Sludge Treatment Plant Operations

Eric J. WahlbergEric J. Wahlberg

Rocky Mountain Water Environment AssociationRocky Mountain Water Environment Association6565thth Annual Meeting Annual MeetingSeptember 17, 2002September 17, 2002

Rocky Mountain Water Environment AssociationRocky Mountain Water Environment Association6565thth Annual Meeting Annual MeetingSeptember 17, 2002September 17, 2002

Folklore Defined

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

Folklore Defined

Myth 1

Myth 2

Myth 3

Myth 4

Myth 5

Learning Objectives

What’s In It For You?

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MODIFIED FST No. 13

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UNMODIFIED FST No. 23

Folklore DefinedWhere 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

Thickeningin secondary

clarifiers

RAS Concentration a Fixed Multiple of MLSS

XRAS = XML

Q + QRAS

QRAS

Secondary Clarifier as a Reactor?

Reaction My Answer The Reason Why

COD removal? No Needs O2 and mixing

Secondary Clarifier as a Reactor?

Reaction My Answer The Reason Why

COD removal No Needs O2 and mixing

Nitrification? No Needs O2 and mixing

Secondary Clarifier as a Reactor?

Reaction My Answer The Reason Why

COD removal No Needs O2 and mixing

Nitrification No Needs O2 and mixing

Denitrification? No Causes floating sludge

Secondary Clarifier as a Reactor?

Reaction My Answer The Reason Why

COD removal No Needs O2 and mixing

Nitrification No Needs O2 and mixing

Denitrification No Causes floating sludge

P uptake? No Needs O2

Secondary Clarifier as a Reactor?

Reaction My Answer The Reason Why

COD removal No Needs O2 and mixing

Nitrification No Needs O2 and mixing

Denitrification No Causes floating sludge

P uptake No Needs O2

P release? No Increases effluent P

Secondary Clarifier as a Reactor?

Reaction My Answer The Reason Why

COD removal No Needs O2 and mixing

Nitrification No Needs O2 and mixing

Denitrification No Causes floating sludge

P uptake No Needs O2

P release No Increases effluent P

Filtration? No Can’t control flow

Secondary Clarifier as a Reactor?

Reaction My Answer The Reason Why

COD removal No Needs O2 and mixing

Nitrification No Needs O2 and mixing

Denitrification No Causes floating sludge

P uptake No Needs O2

P release No Increases effluent P

Filtration No Can’t control flow

Compression/Thickening?

No Not predictableUncouples tie to 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

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Increase RAS flow

The Basic Activated Sludge Process

Influent

RAS

WAS

Mixed liquorEffluent

Aeration basin

Secondary clarifier

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

Influent

RAS

WAS

Mixed liquorEffluent

Aeration basin

Secondary clarifier

Tanks Are Full; No Influent, Effluent, WAS

RAS

Mixed liquor

Aeration basin

Secondary clarifier

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

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Blankets Form In Over Loaded Clarifiers

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Decrease RAS flow

System Response: Blanket Comes Up, MLSS Goes Down

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Over Loaded To Under Loaded, MLSS Goes Up

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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

Settles well

Compacts well

Flocculates well

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 .

Engineers’ Design Data Keep This Myth Going

Parameter Process goal/target

MLSS 4,500 mg/L

MLVSS 3,375 mg/L

SRT 22 days

Equation For MLSS Concentration

XML = {XINF-ISS + [Yg( )]}1 + bSRTHRTSo – SSRT

Truth 5

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

Myth 1

Myth 2

Myth 3

Myth 4

Myth 5

Thank you!