Transcript Slide 1

A New Activated Sludge Process
Control Test Uncovers Secondary
Clarifier Inefficiencies
by
E.J. Wahlberg, S. Biesterfeld,
G. Farmer, D. Kinnear, L. Robb
The Littleton/Englewood WWTP
• Constructed in 1977 as 20 mgd pure oxygen
activated sludge plant
• Modified in 1993 to a 32 mgd trickling filter/
activated sludge plant (TF/AS)
• Upgraded in 1999 to a 36.3 mgd trickling
filter/solids contact plant (TF/SC)
• Currently in pre-design for 50 mgd TF/SC
L/E WWTP Process Schematic
Process Objectives in TF/SC and
Activated Sludge
To grow a biomass that:
• Flocculates
• Settles
• Compacts
Typical Approach to Activated
Sludge Control Inappropriate
• “Performance in the activated sludge process is
typically measured by monitoring secondary
clarifier effluent for TSS, BOD, COD….”
• Inappropriate: effluent quality includes secondary
clarifier inefficiencies.
Modified Settleometer Test
• Collect secondary clarifier and
mixed liquor effluent samples
• Pour mixed liquor sample into
square flocculation jar
• Mix for 30 minutes at 50 rpm
• Settle for 30 minutes
• Record 5 and 30 minute SSV
• Collect supernatant sample
• Analyze supernatant and effluent
samples for TSS
Sludge Quality Measurements
• SSV5: Settling characteristic
• SSV30: Compacting characteristic
• Supernatant TSS: Flocculating
characteristic
Common Sludge Quality
Parameter Highly Variable
350
SSV30 (mL)
300
250
200
150
100
50
0
10/25/99
2/22/00
6/21/00
10/19/00
2/16/01
6/16/01
SRT (d)
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Flocculated Suspended Solids
• Supernatant TSS (FSS) indicates
effluent TSS possible with a perfect
secondary clarifier.
• Difference between FSS and TSS
measures clarifier inefficiency
ESS and FSS (mg/L)
Change Stabilizes Sludge But
Not Effluent Quality
24
20
Change in
Operational Strategy
ESS
16
12
8
FSS
4
0
10/25/1999 2/22/2000
6/21/2000 10/19/2000
2/16/2001
6/16/2001
24
24
Temp
20
ESS
22
16
20
12
8
18
FSS
4
16
0
14
10/25/1999 2/22/2000
6/21/2000 10/19/2000
2/16/2001
6/16/2001
Effluent Temp (degC)
ESS and FSS (mg/L)
Wastewater Temperature Affects
Secondary Clarifier Performance
Conclusions
• SSV30 (SVI) may not be appropriate for
control
• FSS better control parameter than TSS
• Secondary clarifier inefficiencies result in
TSS increase between 2 and 10 mg/L
• Secondary clarifier inefficiency increases
with decreasing wastewater temperature