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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
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 model
• Darcy-Weisbach Equation for
friction loss
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
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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
• Sludge flow can be modeled as turbulent pipe flow
• Clarifier performance depends on many process
parameters in addition to draft tube configuration
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
0
5
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0
2000
4000
6000
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10000
12000
14000
16000
18000
20000
1/1 1/11 1/21 1/31 2/10 2/20 3/2 3/12 3/22 4/1 4/11 4/21P
lan
t F
low
(M
GD
)
RA
S C
on
cen
tra
tio
n (
mg
/L)
Date
Before Our Configuration
After Our Configuration
Plant Inflow (MGD)
ℎ𝑓 = 𝑓𝐷𝐿
𝐷
𝑣2
2𝑔
∆𝑃
𝜌𝑔= ℎ𝑓 + ∆𝑧
Head Loss (ΔH), Sludge Velocity (v), Open Area of Draft Tubes (Aopen), Pipe
Lengths (Lpipe ), Friction factors (ff). Sludge Viscosities (µ), Reynold’s Numbers
(turbulent flow), Pipe material (PVC), Pump Rate (%), Influent Flows (Qin)
Parameters Influencing Clarifier Performance: