LOW D.O. OPERATION:
EFFECTS ON BIOLOGICAL PHOSPHORUS
REMOVAL, OXYGEN TRANSFER EFFICIENCY,
DENITRIFICATION, AND ENERGY SAVINGS
• IN 1980 THE PLANT WAS DESIGNED
FOR 50 MGD NITRIFICATION
• IN 1996 THE PLANT WAS MODIFIED
FOR BIO-P
• ABOUT 20% OF THE TANKAGE WAS
REMOVED FROM AERATION
• IT WAS STILL ESTIMATED THE PLANT
CAPACITY WAS 50 MGD
• TOO GOOD TO BE TRUE???
2002 UW SPONSORED STUDY
• STUDY POSSIBILITY FOR INCREASED
SIMULTANEOUS NITRIFICATION AND
DENITRIFICATION
• INCREASED TANK LOADINGS TO
ACCOMPLISH GOAL
• PROVIDED THE OPPORTUNITY TO
EVALUATE PLANT CAPACITY
NITRIFICATION/DENIT
UW STUDY IN ‘83 & ‘84
•LOW D.O. WAS MAINTAINED IN
AERATION TANKS (<0.5 MG/L)
•DENITRIFICATION WAS THE GOAL
•FILAMENTOUS GROWTH (TYPE 021N)
AND NOCARDIA INTERRUPTED STUDY
•FILAMENTOUS CONTROL WAS A
CONSTANT BATTLE UNTIL BIO-P
-The plant is a Modifed UCT process without mixed liquor recycle
-Prior to study the D.O. setpoint for pass 2 was set at 1.5 mg/l and air flow
controlled in passes 1 & 2 to meet it. The study setpoint was set at 0.3
mg/l
-The D.O. setpoint in pass 3 controlled air flow to pass 3 and was
normally set at 2.5 mg/l. The study setpoint was 1.0 mg/l.
In August, ‘01 one half of plant 4 was removed from service in order
to increase the organic loading and enable operating at lower D.O.’s
Plant 3 was the control plant with the same volume of tankage in service
during both time periods.
The goal was to maintain the same sludge age in both plants for both time periods.
The higher organic loadings in plant 4 allowed operating at lower D.O.’s. The
effluent TN was about 3 mg/l lower in plant 4 than plant 3 and the TP levels about
the same. The air usage/lb of BOD removed was 40% less in plant 4 than plant 3.
Operating D.O. Levels & Oxygen
Transfer Efficiency
The District owns an off-gas apparatus for testing oxygen transfer efficiency. This
tool has been used for several research projects with the UW on fine bubble diffusion.
The purpose for these investigations was to check if alpha was suppressed in the
tankage as a result of maintaining lower D.O.’s.
The points 1-1 thru 3-6 are points where the off-gas testing was carried out,
in both plants 3 & 4, before and during the test period.
-D.O. measurements are continuously recorded and compressed to hourly
averages.
-Prior to the test period the D.O. setpoint at the first point could be maintained
to + or - 0.5 mg/l about 70% of the time, but because of the low organic loadings
the D.O.’s were always much higher at the tail end of the tank than the setpoint.
-This was because minimum air flow setpoints controlled.
During the test period the D.O. at the first probe in the tank could be maintained very
close to the setpoint of 0.3 mg/l and at the tail end of the tank was within 0.2 mg/l of
the 1.0 mg/l setpoint 75% of the time. The other 25% of the time were low loading
periods and minimum air flow setpoints still controlled.
The profile of alpha in the plant 4 tanks was approximately the same with normal,
low loadings and high D.O.’s as during the test with higher loadings and low D.O.’s
The D.O. profiles were much different with the two conditions
D.O.'s During 2002 Off-Gas Testing
D.O. Plt4 7/14 (Normal Ops)
8
D.O. Plt4 8/28 (Test)
7
6
D.O. (mg/l)
5
4
3
2
1
0
1-1
1-2
2-1
2-2
2-3
2-4
2-5
2-6
3-1
3-2
3-3
3-4
3-5
3-6
-The field transfer efficiencies were much different because of the D.O. profiles.
-If the demand can be satisfied in passes 2 & 3 and take advantage of low D.O.’s
and high alpha values, significant air can be saved.
Jan-20
Jan-19
Jan-18
Jan-17
Jan-16
Jan-15
Jan-14
Jan-13
Jan-12
Jan-11
Jan-10
Jan-09
120000
Jan-08
Jan-07
Jan-06
Jan-05
Jan-04
Jan-03
Jan-02
Jan-01
Jan-00
Jan-99
Jan-98
Jan-97
Jan-96
Jan-95
Jan-94
Jan-93
Jan-92
Jan-91
Jan-90
Jan-89
Jan-88
Jan-87
Jan-86
Jan-85
BOD Loading (lbs/day)
BOD Loadings have increased 62% from Jan, ‘88 to Jan, ‘06
Avg Monthly BOD Loadings
y = 138.68x - 93473
R2 = 0.8803
100000
80000
60000
40000
20000
0
Jan-20
Jan-19
Jan-18
Jan-17
Jan-16
Jan-15
Jan-14
Jan-13
Jan-12
Jan-11
Jan-10
Jan-09
Jan-08
Jan-07
Jan-06
Jan-05
Jan-04
Jan-03
Jan-02
Jan-01
Jan-00
Jan-99
Jan-98
Jan-97
Jan-96
Jan-95
Jan-94
Jan-93
Jan-92
Jan-91
Jan-90
Jan-89
Jan-88
Jan-87
Jan-86
Jan-85
TKN (lbs/day)
TKN Loadings have increased 56% from Jan, ‘88 to Jan, ‘06
Average Monthly TKN Loadings
18000
16000
14000
12000
10000
8000
6000
4000
2000
0
Air use has increased 45% from Jan, ‘88 to Jan, ‘06, much less than 60%
’02
The drop in air use was due to changing control strategy following this
study--a savings of about $32,000 in ‘03
‘85-’88 Reduced air use was due to replacement of coarse bubble diffusers with
fine bubble and implementation of D.O. control
Air Flow vs. Time
60.00
40.00
30.00
20.00
10.00
Oct-05
Oct-04
Oct-03
Oct-02
Oct-01
Oct-00
Oct-99
Oct-98
Oct-97
Oct-96
Oct-95
Oct-94
Oct-93
Oct-92
Oct-91
Oct-90
Oct-89
Oct-88
Oct-87
Oct-86
0.00
Oct-85
Air Flow (mil CF/D)
50.00
-Air flow is maintained constant at locations 1-1 & 1-2 regardles of D.O.’s. at
about 1.3 cfm/diffuser
-The D.O. setpoint for the second probe is 0.5 and controls air to pass #2
-The D.O. setpoint for the final probe is 3.0 mg/l, controls air in the last pass,
and is set as such to assure an effluent D.O. of 5.0 mg/l
A real time process calculator/simulator in the plant process control system was
used to project plant capacity with the knowledge gained as a result of the study.
The aeration capacity of the plant likely won’t be reached until the average
Annual daily flow exceeds 60 MGD.
Plant clarifier capacity may be reached sooner, when the average annual
daily flow exceeds 50 MGD.
Operational Observations
• Diffusers have shown no more of a
tendency to plug since Bio-P
• Occassionally there has been minor foaming
organism growth on the tanks
• A related process, UV disinfection, has
benefited from Bio-P; quartz tube are acid
cleaned once/year versus every 3-4 weeks
prior to Bio-P
SUMMARY
• Plant was capable of N & P removal at
higher loadings and much lower D.O.
concentrations
• Settling remained good
• Oxygen Transfer efficiency remained high
at lower operating D.O. level
• It appears optimistic projections for future
plant treatment capacity look good
A paper prepared and presented at
Central States on this project can be
found at www.cswea.org, click on
Published Papers,
“Effect of Biological Phosphorus
Removal On Plant Operations and
Capacity at the Nine Springs WWTP