Rocky Mountain Water Environment Association 65 th Annual

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

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

Learning Objectives n Myth 1 n Myth 2 n Myth 3 n Myth 4 n Myth 5

What’s In It For You?

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 n Operators try to achieve unachievable RAS concentrations n Denitrification n Phosphorus release n Deterioration in sludge quality

Just Say No Thickening in secondary clarifiers

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

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 Overloaded Clarifier: Increase RAS flow

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

Control Volume Shows Two Flows Into The Aeration Basin Control Volume

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

This Is How The Myth Got Started n HRT = VAB/(Q + QRAS) n 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 Mixed liquor Effluent Secondary clarifier Aeration basin RAS WAS

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

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

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 + b. SRT

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

Thank you! n Myth 1 n Myth 2 n Myth 3 n Myth 4 n Myth 5