COLLEGE OF ENGINEERING Chemical, Biological & Environmental Engineering

Optimization of Secondary Clarifier Draft Tube

Configuration for the City of CorvallisStephanie Rich, Shumin Lu, Jon Curry

Wastewater Treatment

Future Work

Acknowledgments

Draft Tube Configurations

Experimental Metrics

RAS Concentration• Measured using centrifuge

• Sample taken from feed well

• Optimally 10,000-17,000 mg/L

Settlometer• Settleometer used to determine

Sludge Volume Index (SVI)

• Optimal SVI <100 mL/g

0

5

10

15

20

25

30

0

10

20

30

40

50

60

3/2 3/7 3/12 3/17 3/22 3/27

Infl

uen

t F

low

Ra

te (

MG

D)

Slu

dg

e B

lan

ket

Th

ick

nes

s (i

n)

Date

Sludge Blanket Thickness

Influent Flow Rate

Sludge Blanket Height• Measured using Sludge Judge

• Another evaluation of sludge

settling quality

• Optimally less than 3 ft to show

good compression settling

Pressure Head• Differential gauge to measure

the change in water height

between feed well and outside

clarifier

• Typically between 4-8 inches at

the Corvallis Wastewater

Reclamation Facility

• Include friction loss due to piping to improve

current theoretical model

• Apply Poiseuille’s Equation to determine head

∆𝑃 =128 𝜇𝐿𝑣

𝜋𝐷4

• Predict maximum influent flow to clarifier

• Measure sludge viscosity to incorporate in model

A “stress test” is when a process is pushed to the

point of failure. We used this test to determine a

relationship between pressure head and influent

flow rate as show below.

Thank you to the Corvallis Wastewater Reclamation

Facility Operations Department: Stan Miller, Gene

Freel, Matt Mead, Jim Green, James Hughes, Les

Wiensz, and Mark Lankford, Utilities Division

Manager Tom Hubbard, and Dr. Philip Harding.

vs

4-12 MGD 12-18 MGD

Biological processes use organisms to degrade

pollutants in municipal wastewater. These processes

include an aeration basin for waste degradation, and

a secondary clarifier to settle out sludge and

concentrate it for further use.

RAS: Return Activated Sludge from secondary

clarifier to the aeration basin

To improve secondary clarifier performance by

predicting optimal draft tube configurations using

experimental data and theoretical modeling.

Model Improvement: Stress Test

Conclusions

0

10

20

30

40

0 5 10 15 20

∆H

(in

)

Inflow (MGD)

Theoretical

Experimental

Pressure head predicted as a function of open draft tube area and

RAS pump rate vs experimentally measured values. The model is

only accurate for low flows, and needs to be further improved for

influent flow rates above 12 MGD.

• RAS concentrations were increased with

proposed high flow configuration

• Low flow configuration needs improvement

• Clarifier performance depends on many process

parameters in addition to draft tube configuration

8000

9000

10000

11000

12000

13000

14000

15000

16000

2/25 3/2 3/7 3/12 3/17 3/22 3/27 4/1 4/6

RA

S C

on

cen

tra

tio

n (

mg

/L)

Date

Change on 3/10/15

Sludge blanket thickness compared to influent flow rate. The draft tube

configuration was changed on March 10th, and no subsequent effect in the

thickness was observed. This figure indicates a strong relationship between

influent flow and sludge blanket thickness instead.

Secondary Clarifier Modeling

Profile view of secondary clarifier with 4 draft tubes on each side. Sludge is collected from the bottom of the clarifier and sent through draft tubes in the feed well.

0.0

0.5

1.0

1.5

2.0

RA

S F

low

(M

GD

)

Plant Inflow (MGD)

Q(RAS,in)

Q(RAS,out)

4 9 126 18 24

Figure of inflow and outflow predictions based on theoretical

assumptions shown above. The model is accurate for low

flows, but not for high flows.

Feed Well

Sludge Blanket

Center box contains 8

draft tubes.

A1A2A4 A3 B1 B2 B3 B4

Assumptions:• Potential energy from pressure head is equal to kinetic

energy driving the sludge to flow in the draft tubes

• Two clarifiers have identical performance

• Sludge density = 1400 kg/m3

• Friction loss due to piping is negligible

Equations:• 𝑄𝑅𝐴𝑆,𝑖𝑛 = 𝑄𝑅𝐴𝑆,𝑜𝑢𝑡• 𝑄𝑅𝐴𝑆,𝑖𝑛 = 𝑓(∆𝐻, 𝐴𝑜𝑝𝑒𝑛)

• 𝑄𝑅𝐴𝑆,𝑜𝑢𝑡 = 𝑃𝑢𝑚𝑝 𝑅𝑎𝑡𝑒 ∗ 𝐼𝑛𝑓𝑙𝑢𝑒𝑛𝑡 𝐹𝑙𝑜𝑤

RAS concentration increased after the draft tube configuration was changed

according to high flow conditions. Average influent flow rates ranged from

6-25 MGD during this period. Higher RAS concentration indicates a

successful configuration due to process improvement.

Open AreaSludge Inlet

Sludge inlet from bottom of clarifier

leading into feed well via sludge pipes.A single draft tube taken out from the feed

well for demonstration of open area.

Channel of wastewater flow going to disinfection basin. High flow rates were

observably more turbulent—pushing the clarifier closer to its maximum limit.

Objective