Transcript Slide 1

Potential for using anaerobic settling tanks to
optimize denitrification in an ecologicallyengineered wastewater treatment system
Ed Anderson, Anna Brunner, and Elizabeth Fabis
Research Project for Systems Ecology (ENVS 316)
Introduction
The Living Machine at Oberlin College is an ecologically
engineered wastewater treatment system designed to remove organic
matter, harmful nutrients, and pathogens. Influent moves through a
series of tanks, each containing organisms and conditions that provide
specific wastewater treatment functions (Fig. 1).
Results
Figure 2. Decrease in K and NO3 over time in (a) AN1, (b) AN2, (c) CA2.
Best fit determined by moving average.
a. AN1
Anaerobic
K
NO3
3
2
Open Aerobic
1
Restrooms
Recycling Line
0
Figure 4a. CBOD and NBOD of water samples collected immediately
prior to the addition of nitrate on 11/6/07 and ten days later. Standard
error, n=3. Figure 4b. Ammonium concentration as measured by Orion
probe. Best fit line is moving average.
Clarifier
Influent enters the system’s anaerobic settling tanks (AN1 and
2), where organic nitrogen, present in human waste, is converted to
ammonium by ammonification. Water then flows into the closed aerobic
tanks (CA1 and 2), where nitrification converts the ammonium to nitrate.
Later in the Living Machine, an artificial wetland is supposed to remove
the nitrate through denitrification. This wetland has the necessary
anaerobic environment to convert nitrate to harmless nitrogen gas, but
lacks sufficient carbon for the reaction to take place.1 As a result, the
effluent has higher nitrate levels than desired.
High levels of nitrate can cause algal blooms in aquatic
ecosystems and “blue baby” syndrome in humans, so it is important that
wastewater treatment systems adequately decrease nitrate
concentrations.2 Recycling effluent through the AN tanks, an ideal
environment for denitrification, may remove more nitrate from the
system.3 One potential consequence is a phenomenon
known as
“popping,” where nitrogen gas causes the bottom sludge to float,
introducing more carbon- and nitrogen-rich material into the water.
To test the ability of the AN tanks to remove high concentrations
of nitrate, we added potassium nitrate (KNO3) to the AN tanks and
observed changes in concentration over time.
Methods
After taking a set of initial samples, we added 4.10 kg of
technical grade KNO3, in solution, to AN1 and AN2, with 2.05 kg added
to each tank. We regularly took samples from AN1, AN2, CA2, and the
clarifier to assess nitrate removal. Samples from AN1 and AN2 were
taken after mixing the surface of each tank with the sampling pole to
reduce spatial variability. All samples were then filtered through cheese
cloth to remove particulates.
Nitrate and potassium concentrations were measured on a
Dionex ion chromatograph. Potassium was used as a passive tracer to
observe relative nitrate concentration decrease due to dilution and
mixing, as it is biologically inert. In addition, we measured ammonium
with an Orion probe to assess whether nitrification (the conversion of
ammonium to nitrate) was occurring as an adverse result of our project.
Biological oxygen demand (NBOD and CBOD) were also calculated,
using standard methods, to determine the amount of nitrogenous and
carbonaceous matter present in the water.
3
2
a.
b.
80
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11/6/07
11/16/07
1
60
mg/L
Artificial
Wetland
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c. CA2
40
3
20
2
0
AN1 AN1 AN2 AN2
CBOD NBOD CBOD NBOD
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AN1
CA2
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0 //
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11/09
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0
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11/9
11/13
As Fig. 2 demonstrates, nitrate is decreasing faster than
potassium, which means that more nitrate is being lost than can be
accounted for by dilution alone. The discrepancy is accounted for by
denitrification. We used this data to calculate the amount of nitrate
removed by dilution and denitrification. Fig. 3 demonstrates this for
AN1.
Figure 3. Cumulative NO3 loss in AN1. Loss due to denitrification equals
total loss minus loss attributed to dilution.
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1 Haineswood
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Time since addition (hours)
11/15
Date
References
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0
11/13
Our measurements show that a significant amount of denitrification
occurred in the AN tanks. However, our experiment suggests recycling could
increase, rather than decrease, the amount of nitrate in the effluent. The
production of nitrogen gas caused by denitrification increased the ammonium
in the AN tanks. This increase lead to more nitrification in the CA tanks, and
thus a greater amount of nitrate in the system. Therefore, we can conclude
denitrification at high concentrations in the AN tanks will ultimately increase
nitrate in the Living Machine. However, this phenomenon may not occur with
the smaller amount of nitrate expected from an effluent recycling loop. Further
research should be done to quantify this effect at lower concentrations of
added nitrate. Without further study we cannot recommend a recycling loop.
Total NO3 loss
NO3 loss due to
denitrification
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11/11
Conclusion
11/17
[NO3] (mM)
Pressure
Booster Tank
Effluent
Holding
Tank
Concentration (mM)
b. AN2
UV Filter
Ammonium, NBOD, and CBOD levels increased as an indirect
result of nitrate addition. In both AN1 and AN2, both CBOD and NBOD
levels approximately doubled over the course of our experiment (Fig.
4a). This indicates that “popping” of nitrogen gas may have occurred,
subsequently disrupting the bottom sludge and leading to the
ammonium increase in AN1 seen in Fig. 4b.
However, the increased ammonium from the AN1 tank was
relatively absent in CA2 (Fig. 4b). This indicates that ammonium, upon
entering the aerobic environment of the CA tanks, was converted to
nitrate through nitrification. The increase in nitrate concentration in the
CA2 tank over time supports this explanation (Fig. 2c).
Concentration (mM)
Flow
KNO3 addition (11/6)
4
Figure 1. Diagram of Oberlin College’s Living Machine.
Closed
Aerobic
Results (cont.)
120
and Morse. 2003. Low Organic Carbon Limits Denitrification in
the Marsh of an Ecologically Engineered Wastewater Treatment Facility
at Oberlin College. ENVS316 Research Project
2 US EPA, Toxicity and Exposure Assessment for Children’s Health, “Nitrates
and Nitrites; TEACH Chemical Summary,” US EPA.
<http://www.epa.gov/teach/chem_summ/Nitrates_summary.pdf>
3 Operations and Maintenance Manual: A Living Machine for Oberlin College.
2000. Living Technologies, Inc.: Burlington.
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K
NO3
2
Concentration (mM)
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3
2
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11/17
K and NO3 Concentrations in AN2
160
100
K observed
K predicted
NO3 observed
NO3 predicted
80
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Concentration (mg/L)
60
AN1
CA2
40
20
0
//
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Date
80
40
0
11/6
11/8
Nitrogen loss due to denitrification and dilution
50
Concentration (mM)
Ammonium Concentration (mg/L)
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//
10/31
Dilution
Denitrification
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Date
11/10
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11/12
Date
11/14
11/16