Duffany, Fountain, Julian
Optimizing Nitrogen Removal in a Sequencing Batch Reactor
CEE 453
Findings Report:
Optimizing Nitrogen Removal in a
Sequencing Batch Reactor
PI’s:
Matthew Duffany
Matthew Fountain
Timothy Julian
5/11/04
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CEE 453
Table of Contents:
Topic
Page
Abstract ............................................................................................... 3
Introduction ......................................................................................... 3
Objectives ............................................................................................ 4
Methods ............................................................................................... 4
Nitrate Probe Analysis ........................................................................ 6
Nitrogen Removal Discussion............................................................ 13
Appendix 1: Plant Setup ..................................................................... 17
Appendix 2: Reactor Setups .............................................................. 18
Appendix 3: Waste Characteristics ................................................... 19
Appendix 4: Academic / Industrial Designs ...................................... 20
Appendix 5: Nitrate Probe Calibration ............................................... 22
Appendix 6: Aeration Level ................................................................ 23
Works Cited ......................................................................................... 25
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Abstract:
The initial goal of this project was to establish a system for optimized nitrogen
and BOD removal from a wastewater stream in an activated sludge batch reactor. The
plan for optimization included the implementation of online nitrate measurement
combined with Process Controller V2 LabView software to manage reactor state changes.
However, initial testing of the nitrate probe (an Analytical Sensors Inc gel filled LogIT
ion selective electrode) suggested that the device was unsuitable for the current project
intentions. At this point experiments were run to confirm probe difficulties and research
was begun to establish industry and academic criteria for nitrification and denitrification
in process. This report presents a summary of findings into the difficulties encountered
with the online probe (ranging from voltage fluctuations under varying conditions to
ionic interference in different conditions) and condenses the state of the art on nitrogen
(biological and chemical aspects, sensitivities, kinetics and processes) removal as it
pertains to lab tested small scale batch reactors.
Introduction:
Much ongoing research is focused on the analyses of varied waste water treatment
strategies. One of the increasingly important criteria is nitrogen concentrations in waste
effluent. The major sources of nitrogen in municipal wastewater are urea and fecal
matter. Nitrogen is important to treat as ammonia nitrogen is a serious fish poison even
in small concentrations1, nitrate nitrogen can lead to eutrophication of coastal (nitrogen
limited) ecosystems even in low effluent concentrations2, and nitrite creates human health
concerns by causing blue baby syndrome in children (nitrite in concentrations of 1 mg/L).
The nitrogen can be present as any of these species due to the bacterial pathways:
Nitrosomonas
2 NH 4  3O2 

 2 NO2  2H 2 O  4 H 
Nitrobacter
2 NO2  O2 
 2 NO3
To this extent, drinking water is regulated by the EPA at 1 mg/L for nitrite-nitrogen and
10 mg/L for nitrate-nitrogen.
However, while the nitrification cycle is simply described, its biological kinetics
forms a slightly more complicated process. To begin with, a typical nitrogen removal in
bio-solids is the volatilization of ammonia, this method is difficult to achieve in
wastewater treatment as very small amounts of gaseous ammonia will be lost at pH
values less than 8.5 as hydrogen ions are a component in the equilibrium3:
[ NH 4 ]  [ NH 3 ]  [ H  ]
1
Mogens Henze [et al.].
Water Pollution Control Federation. Task Force on Nutrient Control
3
ibid
2
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CEE 453
Therefore, the ammonia must instead be oxidized and taken through the reactions shown
above to nitrate so that denitrification can later be used to remove the nitrogen to a
harmless form of diatomic nitrogen gas.
Nitrification is a relatively specialized process that will only be accomplished
through the Nitrosomonas and Nitrobacter species of bacteria. These bacteria are
obligate aerobes (their populations will only grow under aerobic conditions) but they can
survive long periods of anaerobic conditions. They are both autotrophs that oxidize
ammomia to provide energy for cell synthesis, but Nitrosomonas kinetics is the limiting
rate in the process, leading to solutions typically low in nitrite compared to ammonia or
nitrate concentrations4.
Once the nitrogen has all been converted to nitrate the next step is to induce the
action of denitrifying bacteria. These bacteria come from a range of diverse species
including Pseudomonas, Micrococus, Archromobacter, Thiobacillus, and Bacillus.
These species are mostly facultative (able to use either oxygen or nitrate as the terminal
electron acceptor in respiration), and thus oxygen concentrations must be kept low as
oxygen provides greater energy and is thus preferred by the organisms (oxygen
respiration of glucose yields 686 Kcal / mol while nitrate yields only 570 Kcal / mol).5
More specific analyses of these processes follow below.
Objectives:
The initial goal for this project consisted of establishing an online control of an
activated sludge batch reactor to optimize both nitrogen and BOD removal. To this aim
funding was requested and received for an online nitrate probe. However, the nitrate
probe proved unsuitable due to extreme fluctuations for the online control of the plant.
At this point the project was shifted to an analysis of effects on the nitrate probe and what
factors may be causing these variations, and whether or not they were correctable. The
experimental studies on the probe also led us to delve into the literature surrounding the
difficulties with both nitrification and denitrification, and the eventual report became a
combination of analysis on the probe and a study on applying industrial and academic
techniques of nitrogen removal in a sequencing batch reactor in the hopes that future
groups would be able to advance our work into a fully functional model.
Methods:
An activated sludge sequencing batch reactor was set up to simulate the activity in
an activated sludge single tank treatment plant. While treating waste as a batch process is
unlikely in an industrial or municipal setting where extremely large volumes would be
neccessary, the approximation served adequately to experiment with nitrogen removal as
4
5
ibid
Halling-Sørensen, B.
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it related to oxygen and timing. If specific methods were to be devised, a model of a
continuous flow reactor would need to be constructed and tested as well. The details of
the plant design can be found in Appendix 1: Plant Setup, and the precise volumes,
timings, and alternative inputs of the multitude of experiments run in this project can be
found in Appendix 2: Reactor Setups. The characteristics of the waste flows can be
found in tables provided in Appendix 3: Waste Characteristics. In a general sense the
plant was constructed, tested for structural integrity and leaking, and then run
continuously (other than a 2 day downtime due to computer virus infection of the lab) for
a six week period. During this period the different reactor setups were tested to
determine different information about the nitrate probe and BOD levels. The reactor was
run according to Process Controller v2 software developed by Monroe Weber-Shirk, as a
piece of LabView code which controlled different states of the reactor as shown in the
appendix through a series of solenoid valves controlling waste input and air input.
Oxygen was introduced to the reactor through a ceramic diffuser to maximize dissolution
into the waste solution. The reactor was also brought into effectiveness by the
introduction of 4 L of activated sludge from the Ithaca Waste Water Treatment Facility.
It was also important to refrigerate the waste so that no degradation would occur in the
waste bottle which would falsely serve as a “pretreatment.” Furthermore, it was
necessary to load the waste into the tank before the water so that the water could be used
to flush the pipes that were outside the refrigerator and prevent a bioaccumulation from
growing on the waste in the piping and possibly blocking future flow. The different
states run in the reactor can be summarized as the following:
Add Waste: input of 140 mL of 20x concentration stock waste from the
refrigerator; controlled through flowrate of the peristaltic pump.
Add Water: input water (with stock 2 and 3) into the reactor until 4 L of total
volume is recorded by a pressure sensor (calibrated to account for altered density
of water due to waste and sludge).
Aerate: air pumped into the ceramic diffuser at air rate controlled by solenoid
valves while the reactor is stirred by a stirbar on a stir plate set at 8. Operated on
a timer system.
Anaerobic: no aeration occurs but stirring continues at setting number 8. Operated
on a timer system.
Settling: ceasing of aeration and stirring, sludge allowed to settle to the bottom of
the tank. Operated on a timer system.
Discharge: with no stirring or aeration the water is removed from the top of the
settled sludge though a port at 1.2 L height in the tank, along with wasted sludge
that allows mass balance with growth to retain constant sludge volume.
Controlled by pressure sensor recording minimum volume.
Repetition of steps above
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Nitrate Probe Analysis:
The nitrate probe was calibrated as shown in Appendix 5. The nitrate probe was
first soaked in water for 15 minutes, then a diluted nitrate solution for 2 hours. At this
point the probe was washed clean and placed into distilled water with different
concentrations of nitrate in order to measure the corresponding voltages. The probe
values show a distinctively curved calibration line, implying a similar voltage to
interpreted value ratio as a pH meter. This result seems sensible as both meters use
voltage from the diffusion of a solution through a membrane.
One of the major problems with the nitrate probe is that the calibration was
accomplished in distilled water with only nitrate ions present. In the activated sludge
reactor there are many other chemicals present. The probe will suffer from interference
in solutions with high ionic strength, and this was one issue that had to be tested in the
reactor.
The initial testing of the nitrate probe consisted of measuring the voltage output
from the probe while it was immersed in the tank during our standard reactor cycle of
adding waste and water, aerating, letting sit anaerobically, settling, and discharging
effluent. By applying the calibration curve to the voltage received from the probe while it
was immersed in the reactor, the nitrate concentration can, hypothetically, be determined.
This concentration is inversely related to the voltage received. This relationship is shown
in Figure 1, where nitrate voltage is measured during one complete cycle and is converted
into nitrate concentration. The two graphs are mirror images of each other due to the
inverse relationship between voltage and nitrate concentration. In addition, variations in
the voltage of approximately 0.02 volts correspond to variations in nitrate concentration
of roughly 0.18 ppm/100. This demonstrates the magnification effect that occurs when
converting from voltage to concentration of nitrate.
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CEE 453
Concentration / Voltage vs Tim e Over One Period
Discharge
Anaerobic
Fill
0.7
0.18
Aerate
Settle
0.16
0.6
0.14
0.5
0.4
0.1
volts
concentration
0.12
4/30
0.08
0.3
Nitrate (ppm/100)
Nitrate (Volts)
0.06
0.2
0.04
0.1
0.02
0.34
0.33
0.31
0.29
0.27
0.25
0.23
0.22
0.2
0.18
0.16
0.14
0.13
0.11
0.09
0.07
0.05
0.04
0
0
0.02
0
tim e (day fraction)
Figure 1: This figure shows the voltage read by the nitrate probe and corresponding nitrate concentration
over the period of one wastewater treatment cycle. This demonstrates the indirect relationship between
voltage output by the probe and nitrate concentration in the reactor.
Once the relationship between the voltage and the nitrate concentration was determined,
the nitrate concentration could be constantly measured and monitored in the reactor. This
was a vital component of our proposed research as our initial goal was to use the nitrate
probe as a real-time feedback monitor of conditions in the reactor (online reactor control).
However, the measurements provided by the nitrate probe were not always accurate. The
magnitude that the nitrate concentration wavered, as measured by the probe, inhibited its
use as a real-time feedback monitor of conditions in the tank. While the reactor was run
for multiple cycles under the typical Wuhrman Configuration conditions of aerobic,
anaerobic, settle, discharge / fill, the nitrate was measured. The results were then
compared to expected results and it was discovered that the nitrate probe was displaying
wavering concentrations of nitrate that did not follow the expected conditions. This is
demonstrated in Figure 2, which shows the nitrate concentrations in two complete cycles,
along with idealized expected nitrate concentrations and dissolved oxygen in the tank.
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CEE 453
Actual Nitrate Vs. Predicted Nitrate Over Two Cycles
Discharge / Fill
Discharge / Fill
1.2
2
Settle
Anaerobic
Aerobic
Aerobic
Anaerobic
Settle
1.8
1
1.4
DO (mg/L)
0.8
1.2
0.6
1
0.8
0.4
0.6
Nitrate (ppm/100)
1.6
0.4
0.2
0.2
0.8
0.78
0.76
0.74
0.72
0.71
0.69
0.67
0.65
0.63
0.61
0.57
0.59
0.56
0.54
0.5
0.52
0.48
0.46
0.44
0.42
0.41
0.39
0.37
0.33
0.35
0.31
0.29
0.27
0.26
0.24
0.2
0.22
0.18
0
0.16
0
Time (day fraction)
DO (mg/L)
Expected Nitrate
Actual Nitrate (ppm/100)
Figure 2: This figure shows the expected nitrate concentration along with the actual nitrate and dissolved
oxygen concentrations as demonstrated by the nitrate and oxygen probes. While the oxygen probe is
demonstrating expected results, the nitrate probe wavers significantly throughout the cycle with only small
correlation to the expected nitrate concentration trends. This demonstrates that the nitrate probe is not
yielding expected results.
The project direction (of the laboratory module) was now devoted to discerning the cause
for the wavering nitrate readings so that the probe could be used to in conjunction with
the rest of the reactor to optimize waste treatment.
To understand whether the nitrate probe was recognizing nitrate concentration or it was
recognizing concentration of ion interference, a burst of nitrate (0.971 g NaNO3) was
added to the tank while the wastewater treatment was in the anaerobic state. The
concentration of nitrate was meanwhile measured using the probe. A nitrate
concentration spike was demonstrated, as expected, by the probe as shown in Figure 3
while the reactor was being aerated. A burst of dextrose (10 g) was then added to the
tank two hours after the nitrate was added. The purpose of this was to provide the
activated sludge with a source of chemical oxygen demand that would increase the rate of
denitrification. The expected result of this would be a sharp decline in the concentration
of nitrate.
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CEE 453
Nitrate and Dextrose Spikes
Aerobic
8
9
Anaerobic
Aerobic
Anaerobic
8
7
6
DO (mg/L)
5
Nitrate Added
Dextrose Added
5
4
4
3
Nitrate (ppm/100)
7
6
DO (mg/L)
Nitrate (ppm/100)
3
2
2
1.147
1.137
1.126
1.116
1.105
1.095
1.084
1.074
1.064
1.053
1.043
1.032
1.022
1.012
1.001
0.991
0.980
0.970
0.959
0.949
0.939
0.928
0.918
0.907
0.897
0.887
0.876
0
0.866
0
0.855
1
0.845
1
Time (day fraction)
Figure 3: This figure shows the nitrate concentration in the reactor after a spike of 0.971 g of sodium nitrate
(NaNO3) and 10 g dextrose were added to the tank two hours apart. The concentration of nitrate spikes,
and then slowly declines while in the anaerobic state. As soon as the dextrose is added, the nitrate
concentration drops dramatically and remains low even when the reactor switches to the aerobic stage.
The results demonstrated that the nitrate probe was recognizing changes in nitrate
concentration. However, the nitrate concentration that was being measured by the probe
must clearly not have been the only contributor to the measurement because, as shown
above, the nitrate probe was not giving expected results in the full cycle controlled
situation. Because this experiment was run in only in aerobic / anaerobic conditions, it
was postulated that electrical interference caused by the complete cycle run through may
have caused the wavering nitrate readings.
At this point, further experiments were run. To test this electrical hypothesis, the nitrate
probe was removed from the tank and placed in a separate beaker with a steady diluted
nitrate concentration while the wastewater reactor was run. If the nitrate concentration
changed even slightly over the period of one cycle, this would demonstrate that state
changes were causing electrical interference. Because the nitrate concentration remained
steady over the period of one cycle, electrical interference was ruled out as the cause for
error. However, as stated, the nitrate concentration in the beaker was composed entirely
of distilled water and sodium nitrate, with no other ionic interference present.
The next possible source of error examined was the ionic or electrical interference
created from the dissolved oxygen probe which was measuring dissolved oxygen only a
few centimeters from the nitrate probe, as shown in the lab set-up in Appendix 1. The
dissolved oxygen probe was removed from the tank in order to determine any difference
in the nitrate probe readings, but there was no change in the nitrate readings. The
dissolved oxygen concentration displayed by the DO probe equilibrated, but the nitrate
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CEE 453
probe reading did not change. This demonstrates that the proximity to the working
dissolved oxygen probe did not influence the nitrate probe.
Because the nitrate probe was carefully calibrated using a solution of diluted nitrate, we
determined that the activated sludge (or other compounds) in the wastewater tank was a
possible source of interference. In order to test this hypothesis, we ran four separate sets
of cycles while measuring nitrate, slowly removing different aspects of the wastewater
treatment plant. If the removal of one of the components in the reactor resulted in
expected unwavering results, we could rule that it was the cause of interference. The first
cycle tested was the normal running mode of the tank, in which every aspect of the
reactor was included: water, waste, and activated sludge. The second cycle tested
removed the activated sludge and ran the reactor with only water and waste. The third
cycle tested consisted of running only water with a nitrate spike added at the beginning.
The results are shown as follows, where Figures 4, 5 and 6 demonstrate the results from
water – water and nitrate spike -- water and waste – and -- water, waste and activated
sludge respectively.
Nitrate Spike Added to Water / Cycle Run
Discharge
Fill Water
12
8
Aerobic
Anaerobic
Settle
7
DO concentration (mg/L)
10
6
8
5
6
4
3
4
2
2
1
0
0.
95
0
0.
96
7
0.
98
5
1.
00
2
1.
01
9
1.
03
7
1.
05
4
1.
07
1
1.
08
9
1.
10
6
1.
12
4
1.
14
1
1.
15
8
1.
17
6
1.
19
3
1.
21
0
1.
22
8
1.
24
5
1.
26
2
0
Day Fraction
Nitrate Spike
DO (mg/L)
Nitrate (ppm/100)
Figure 4: This figure demonstrates the results from running through a cycle with water after a spike of
nitrate was added. Because there is no activated sludge in the tank to treat the nitrate, the concentration
should remain relatively stable. Though this is true for the aerobic state, the measured nitrate spikes in the
anaerobic state and remains high.
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The nitrate spike shown at the beginning of Figure 4 corresponds with the results
demonstrated in Figure 3 that the nitrate probe recognizes an increase in nitrate level.
However, the nitrate level is supposed to slowly increase in the aerobic stage, and then
decrease in the anaerobic stage as expected in Figure 2. Clearly, the multiple spikes that
occur in the anaerobic stage do not conform to this projection. After the multiple spikes
occur, the nitrate concentration remains high until the reactor discharges the waste and is
refilled. As the only components present in the tank are water and the initial spike of
nitrate, the switch to anaerobic conditions should have no effect on the nitrate levels
whatsoever. This, however, was not demonstrated by Figure 4.
Next, the waste was added to the cycle as shown in Figure 5. The waste provided a
source of chemical oxygen demand, but did not provide any nitrate. Again, because there
was no activated sludge included in the tank, the nitrate levels should have remained
steady at close to zero (since the majority of the previous spike of nitrate was
discharged). This was not demonstrated, however, as the nitrate was measured to be
wavering around 0.4 ppm / 100. Clearly, a problem with the nitrate probe is
demonstrated in Figure 5.
Cycle with Water and Waste Only
Fill
Discharge
12
0.7
Anaerobic
Anaerobic
Settle
0.6
10
0.5
DO mg/L
8
0.4
6
0.3
4
Nitrate (ppm/100)
Aerobic
0.2
2
0.1
0
0.
50
6
0.
52
0
0.
53
4
0.
54
7
0.
56
1
0.
57
5
0.
58
9
0.
60
3
0.
61
7
0.
63
1
0.
64
5
0.
65
9
0.
67
2
0.
68
6
0.
70
0
0.
71
4
0.
72
8
0.
74
2
0.
75
6
0.
77
0
0.
78
4
0.
79
7
0.
81
1
0
Time (day fraction)
DO (mg/L)
Nitrate (ppm/100)
Figure 5: This figure shows the results for nitrate and dissolved oxygen levels as obtained in the reactor
running with the water and waste running without the activated sludge. In the aerobic stage, just after
waste was added, the nitrate level spikes and slowly decays but still remains high. In the anaerobic stage,
however the nitrate again spikes and slowly decays. Because there is no activated sludge in the tank, the
cause of the spike / decay in the anaerobic stage is not understood.
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Though a clear problem is shown in the attempt to measure nitrate in a reactor containing
only water, we added the activated sludge to the tank to repeat the previous experiment of
running a complete cycle and measuring the nitrate. Unsurprisingly, results similar to
Figure 2 were measured as shown in Figure 6. Those results show a spike of nitrate
during the discharge / fill stage, and then show a wavering line around a constant
concentration. What was surprising, however, was that the concentration of the nitrate
was lower (around 0.08 ppm / 100) and that the nitrate concentration did not waver as
much as it previously had. Though these two experiments were identical, except that
Figure 2 demonstrates an experiment that had been running for a couple of days while
Figure 6 was re-added activated sludge. A possible cause for this was that the activated
sludge may have been less active than their predecessors because they had been sitting
un-aerated and without waste for a period of two days.
Fill
Cycle with Water, Waste, and Activated Sludge
Discharge
9
1
Aerobic
Settle
Anaerobic
0.9
8
0.8
7
DO (mg/L)
0.6
5
0.5
4
0.4
3
0.3
2
0.2
6
0
5
9
3
8
2
2
0.
64
0.
62
0.
61
0.
59
0.
57
0.
56
0.
54
7
0.
53
5
0
4
8
3
7
2
1
0.
51
0.
50
0.
48
0.
47
0.
45
0.
43
0.
42
0.
40
0.
39
0.
37
0.
36
0.
34
6
0
0
0
5
0.1
9
1
0.
32
Nitrate (ppm/100)
0.7
6
Time (day fraction)
DO (mg/L)
Nitrate (ppm/100)
Figure 6: The activated sludge was re-added to the system of water and waste and was run through a
complete cycle. The nitrate results shown are more consistent than the first measurements obtained , but
are lower in magnitude and still do not follow the expected trend. This is most likely due to less active
sludge.
Though the cause of the nitrate probe’s inability to function appropriately was never
determined, we postulate that interference from ions in the water, waste, and activated
sludge are possible sources. The nitrate probe seemed to work best in distilled water /
sodium nitrate mixtures. For this reason, we do not recommend using this nitrate probe
in the future for work in an activated sludge reactor due to the high concentration of
impurities.
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Though our original goal was to use the nitrate probe as a real-time feedback monitor that
enabled us to change reactor states based on nitrate levels, the high fluctuation in nitrate
measurements made this task impossible. A possible suggestion for the future would be
to test other nitrate probes available on the market, and rely on this nitrate probe only for
pure concentrations of nitrate.
Nitrogen Removal Discussion:
As explained in the introduction, the removal of nitrogen from the wastewater
effluent requires both nitrification and denitrification. There are a number of difficulties
that must be overcome to make these processes dominant in a reactor designed to remove
BOD from waste.
The first difficulty in nitrogen removal occurs in the oxidation of ammonia into
first nitrite then later nitrate. The microorganisms that oxidize the ammonia are obligate
aerobes, so for growth must be exposed to oxygenated waters with concentrations of
nitrogen. However, with the raw waste that is input into the reactor, the same aeration
cycle will also provide oxygen for the heterotrophic microorganisms that degrade the
organic carbon. These two groups of organisms are in direct competition with each other
for the available dissolved oxygen, and those that decompose organic carbon have growth
kinetics that allow them a higher actual growth rate than the maximum possible growth
rate for the nitrifying bacteria. This disparity in rates means that no matter how
unbalanced the growth becomes, the carbon utilizing bacteria will dominate the
population of the reactor as long as any significant quantities of organic carbon remain.
This order means that nitrification will not occur until after the degradation of
carbonaceous oxygen demand, and thus when oxygen levels begin to rise, the nitrogen
may not have been oxidized as of yet.
This significant lag time in nitrification has been observed this year by the work
done in Rudi Sheck’s laboratory group. The lag time occurs not only because the
nitrifiers must wait for oxygen to be available for their processes, but also must wait once
that oxygen is available for a significant increase in their population for them to impact
the tank environment. The nitrifier’s population kinetics can be modeled as a dual form
of the Monod equation6. This lag means that the reactor management must account for
an increase in dissolved oxygen levels before nitrification and not mistakenly identify the
increase of dissolved oxygen to imply that all oxygen demand has been met by the
aeration step. Finally, due to diffusion not only into the water, but also from
heterotrophic biomass inhibiting diffusion from the water into the floc, oxygen levels
must be kept at greater than 3 mg/L for nitrification whereas COD reactions require a
mere 0.5 mg / L.7
6
7
Henze, et al., 1986
T.J. Casey.
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The next difficulty when removing nitrogen comes in the sensitivity of nitrifying
bacteria to heavy metals and synthetic organic chemicals (to a much greater extent than
the heterotrophic bacteria). Some of the metals that these bacteria are sensitive to are
cobalt in concentrations of less than 0.08 ppm, as well as copper and zinc, all of which
are included at small concentrations in the water stream added to the reactor (reference
Appendix 3: Waste Characteristics).8 Even if these concentrations are not high enough to
cause acute toxicity (near instant death of a population) they may over time lead to
chronic toxicity with bioaccumulation of heavy metals inhibiting nitrification. The
impact of chronic toxicity depends on both concentrations in the waste water as well as
the sludge retention age.9
The concept of sludge retention age is one of the most important controls on
nitrification and denitrification of a waste. In order for microbial populations to reach
sufficient biomass, literature shows that it is proven necessary for the sludge age in a
reactor to be approximately 9-12 days.10 Sludge age is related to hydraulic retention time
in that it is simply the measure of:
Mass Sludge In System
SRT 
Mass Sludge Leaving System Per Day
Another difficulty in the process of nitrification arises in pH control. Nitrifying
bacteria become inhibited at pH below 7, with optimal pH values of approximately 8.311
However, the nitrification reaction produces hydrogen ions, thus decreasing the pH:
NH 4  2O2  NO3  2 H   H 2 O
In fact, if pH is to remain balanced, 7.14 mg of CaCO3 alkalinity will be destroyed per 1
mg of NH4+-N oxidized. Therefore it is important to ensure that sufficient alkalinity
exists in the reactor to control pH levels.
After the troubles of nitrification, denitrification seems a relatively
straightforward process. Where there are only a few specialized bacteria that can act as
nitrifiers, denitrifiers have a much larger possible grouping of species as discussed in the
introduction. The main concern with denitrification is that most of these microorganisms
are facultative denitrifiers, and thus will prefer to use oxygen if it is available. Therefore
oxygen concentrations must be kept as close as possible to zero for maximum
denitrification rates. To some extent this pH change can be cancelled by using a high
enough aeration rate that CO2 is stripped from the solution.
One of the most difficult factors of denitrification to deal with in a batch reactor is
the demand of denitrifiers for organic carbon. This need is a problem since as discussed
above, the oxidation of the ammonia to provide the nitrate needed for denitrification first
eliminated the organic carbon from the waste. As a result, many municipal waste water
treatment plants will add methanol to stimulate denitrification at concentrations of 3 kg of
8
Mogens Henze, et al.
Randy Junkins, et al.
10
Halling-Sørensen, B.
11
ibid
9
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methanol: 1 kg NO3—N.12 This addition can become very expensive as well as tedious.
Some wastewater treatment facilities have tried to reintroduce raw waste to later parts of
the reactor to provide this carbon without incurring additional cost, but this often leads to
lower quality effluent, or the need for further aeration steps. If too many cycles are used,
microorganisms that have an assimilatory nitrate reduction (they reduce nitrate to
ammonium for use in protein synthesis) may become a concern.
Appendix 4: Academic / Industrial Designs, details some of the proposed methods
for a treatment of a single waste stream (nitrate and oxygen demand treated in same
process) with an activated sludge system.
The Wuhrmann configuration tries to defeat the need for adding organic carbon
that was discussed earlier by utilizing the death of microorganisms to provide the
necessary carbon. However, this process will proceed extremely slowly as a result of low
growth rates and therefore not produce substantial nitrogen removal without very long
residence times.
The Ludzack Ettinger configuration partially joins an anoxic tank with an aerobic
tank. Mixing occurs between these two tanks providing both nitrification and
denitrification in environments with plenty of organic carbon. The main problem with
this tank is that the unpredictable partial mixing creates varied results with waste output
and carbonaceous oxygen demand may come to dominate the aerobic tank.
The modified Ludzack Ettinger process recycles mixed liquor from an aerobic
environment back into an anoxic tank that is the first step in the reactor. By using an
anoxic environment first there is plenty of organic carbon for denitrification to occur.
However, without an initial aerobic step, there will be significant quantities of ammonia
that are unaffected by the first reactor and move directly into the second reactor where
they are oxidized into nitrate. The problem with this reactor setup is that with CSTR
tanks, there will be significant flow that is not recycled after being oxidized, and thus
nitrate will be present in the discharge at significant levels.
The final process this report will examine is the Bardenpho model. This model
provides nearly 100% removal of nitrate from the effluent when successfully
implemented (with any CSTR process some influent will immediately become untreated
effluent, so perfection is impossible with this general model type). The configuration
consists of an anoxic tank followed by an aerobic tank, with another anoxic tank, and
finally a reaeration tank. Mixed liquor transport occurs from the aerobic tank into the
first anoxic tank. This setup excels in that it provides initial anoxic denitrification of any
nitrate in the original waste water, then an aerobic step allows nitrification and the new
nitrate proceeds into a tank of anoxic conditions for further denitrification. A final pass
through a reaeration tank brings dissolved oxygen levels of the effluent up to acceptable
standards and aids in stripping CO2 from the effluent to increase pH levels. The recycle
of mixed liquor from the first aerobic tank to the first anoxic tank is also important in that
12
Water Pollution Control Federation. Task Force on Nutrient Control
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it allows sufficient quantities of nitrifying bacteria to accumulate by providing longer
residence times.
The only real problem with the Bardenpho model is that it is very difficult to
modify into a sequencing batch reactor design. The recycle of aerobic liquor to an anoxic
state that contains raw waste could be attempted with “mini-cycles” of fresh effluent
brought into the reactor, but at this point mass balance becomes very difficult if you want
to preserve effluent quality. The intermediate anoxic state is easy in that once the first
two tanks have been simulated it is simply a true sequencing batch reactor. The final
reaeration step would be a simple matter of setting the ceramic diffuser for a very high
airflow rate temporarily. It would be a valuable experiment to modify this process to
different setups of a SBR and see what specific configuration provides the best results,
and if they approach that from a true Bardenpho model. Of all possible offshoots from
the research of this report this is the avenue that provides the most interest to the authors
of this report and with a functional nitrate probe may have been the path they would have
taken.
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Appendix 1: Plant Setup
Laboratory Setup
Key to Laboratory Setup
= 100 kPa input airline
= solenoid valve controlling air entering accumulator
= accumulator
= solenoid valve controlling air leaving accumulator
= needle valve to fine-tune airflow rate
= peristaltic pump
= concentrated waste/tap stock input
= stirrer with 6 L tank
= dissolved oxygen probe
= Fisher Scientific Student Nitrate Probe
= 1.2 L of activated sludge mixed with 2.8 L of tap stock
Key to More Laboratory Setup
= refrigerator at 5 degrees Celsius
= solenoid valve to control waste leaving diluted waste bottle
= solenoid valve to control entering water with metals mixture
= 1 L bottle containing 1 part 100X concentrated waste and 4 parts tap water
= 1 L bottle containing 100X concentrated waste
Environment for Lab Setup
• Wastewater Treatment Plant run at 22
degrees Celsius
• Stir-bar set to the 8th setting when running
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Appendix 2: Reactor Setups
Click to open attached PowerPoint Presentation on Reactor Setups:
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Appendix 3: Waste Characteristics
Molecular
Chemical
Weight Concentration
Compound
Formula
g/mol
mg/L
Starch
84.40
40,000
Casein
125.00
30,000
Sodium acetate
C2H3O2Na3H20
136.1
31.90
Capric acid
C10H20O2
172.3
11.60
Ammonium chloride
NH4Cl
53.5
75.33
Potassium phosphate
K2HPO4
174.2
6.90
Sodium hydroxide
NaOH
40.0
175.00
Figure 1. Synthetic feed composition of the refrigerated waste stock (stock 1).
Figure courtesy of CEE 453 website: “Introduction to NRP”.
Molecular
Chemical
Weight Concentration
Compound
Formula
g/mol
mg/L
Magnesium sulfate
MgSO47H2O
246.5
69.60
Sodium molybdate
NaMoO42H2O
241.9
0.15
Manganese sulfate
MnSO4H2O
169.0
0.13
Cupric sulfate
CuSO44H2O
249.7
0.08
Zinc suflate
ZnSO47H2O
287.5
0.48
Calcium chloride
CaCl22H2O
147.0
22.50
Iron chloride
FeCl36H2O
270.3
18.33
Figure 2. Metals and their concentrations that are added to tap water for the purpose of
diluting the synthetic waste (combination of stock 2 and stock 3). Figure courtesy of CEE
453 website: “Introduction to NRP”.
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Appendix 4: Academic / Industrial Designs13
Ludzack-Ettinger Configuration
(1962)
influent
anoxic
aerobic
Wuhrmann Configuration (1964)
influent
13
aerobic
anoxic
Halling-Sørensen, B.
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Modified Ludzack-Ettinger
Process
Mixed Liquor
influent
anoxic
aerobic
Bardenpho Model
Mixed Liquor
anoxic
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Appendix 5: Nitrate Probe Calibration
water volume
std concentration
100
1000
amount std (ml)
test solution
concentration
(ppm)
10.0
20.0
35.0
50.0
1.0
2.0
3.5
5.0
ml
ppm
voltage
(volts)
0.15
0.129
0.11
0.1
60.0
NO3- (ppm)
50.0
y = 0.0061x -3.9256
R2 = 0.9961
40.0
30.0
20.0
10.0
0.0
0.1
0.11
0.12
0.13
0.14
0.15
voltage
Appendix 5: Nitrate Probe Calibration Curve for use in converting raw sensor data to
nitrate concentrations. Calibrated in distilled water with a known nitrate solution at
1000 ppm.
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Appendix 6: Aeration Level
As experimental conditions changed throughout the course of our Nutrient
Removal Project, oxygen uptake and utilization did as well. The Aeration Lab was
referenced frequently to obtain the equations necessary to determine various airflow rates
into our system. An example calculation to determine airflow rate is as follows:
Wastewater BOD: 325 mg/L
C*: 8 mg/L
Wastewater Flow Rate: 16 L/day
C: 2 mg/L
dC ˆ
 k v ,l (C *  C )
dt
dC

dt
0.015
325
eq. 1
mg
L
*16
mg
L
day
 1300
4L
L  day
mg
mg
 kˆv ,l (6
)
Ls
L
eq. 3
kˆv,l  0.00251 / s
Where:
eq. 2
eq. 4
C* is the equilibrium dissolved oxygen concentration
C is the target dissolved oxygen concentration
kv,l is the overall volumetric gas transfer coefficient
The kv,l value found above corresponds to an airflow rate found through
interpolation of the experimental data in Figure 3 of approximately 450 μM / s. Other
airflow rates can be found using the same method. A value of 1000 μM / s was used
throughout our experimentation in order to provide certainty that sufficient aeration
occurred to meet NOD and not merely COD.
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0.012
0.01
Kvl (1/s)
0.008
0.006
Kvl
0.004
0.002
0
0
2000
4000
6000
8000
10000
flowrate ( μM/s )
Figure 3: Kvl vs Flowrate
Note: Figure 3 taken from CEE 453 Aeration Lab by Matthew Fountain, Timothy
Julian, and Matthew Duffany, Spring 2004
Figure 3 shows the asymptotic relationship between kv,l and the flowrate. As
flowrate increases, the kv,l initially increases as more air bubbles are created. However,
as flowrate increases, the air bubble size increases, thus reducing their surface area until
the bubbles eventually resemble a column of air that is the upper limit of surface area.
Because kv,l is a function of interfacial surface area, kv,l must also have an upper limit.
This relationship of bubble diameter to flow rate helps to explain the asymptotic nature of
the graph.
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Works Cited:
Task Force on Nutrient Control under the direction of the Municipal Subcommittee of the
Technical Practice Committee, Biological and chemical systems for nutrient
removal Alexandria, VA : Water Environment Federation, 1997.
B. Halling-Sørensen and S.E. Jørgensen, The removal of nitrogen compounds from
wastewater, Amsterdam ; New York : Elsevier, 1993.
Clair N. Sawyer, Harry E. Wild, Jr. and Thomas C. McMahon, representing Metcalf &
Eddy, inc. Nitrification and denitrification facilities: wastewater treatment.
Washington] Environmental Protection Agency, Technology Transfer, 1973 [i.e.
1974]
Task Force on Nutrient Control ; Orris E. Albertson, chairman. Nutrient control
Washington, D.C. : Water Pollution Control Federation, 1983
John Roberts, Bubble aeration and an assessment of biochemical reactor performance.
New Castle, Australia : University of New Castle, [Dept. of Chemical
Engineering], 1978.
Poduska, Richard Alan, Nitrogen removal from wastewaters by combining controlled
activated sludge nitrification and anaerobic filter denitrification. [Ithaca, N.Y.]
1969
Harman David Stensel, Biological kinetics of the suspended growth denitrification
process, [Ithaca, N.Y.] 1971.
T.J. Casey, Unit treatment processes in water and wastewater engineering Chichester ;
New York : Wiley, c1997.
Randy Junkins, Kevin Deeny, Thomas Eckhoff, The activated sludge process :
fundamentals of operation, Boston : Butterworth, c1983.
Mogens Henze ... [et al.], Wastewater treatment : biological and chemical processes,
Berlin ; New York : Springer, c2002.
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