Transcript No Slide Title

Measuring Phosphorus Retention Capacity in the Marsh Substrate of an
Ecologically Engineered Wastewater Treatment Facility at Oberlin College
112.3
Joy Cernac, Apostol Dyankov and Michael Pennino
Systems Ecology (ENVS316) Fall ‘04, Oberlin College, Oberlin, OH
The Living Machine marsh is the final stage of removal of
inorganic nutrients, such as phosphorus and nitrogen.
Removing phosphorus and nitrogen is necessary to
reduce cultural eutrophication and prevent harm to
downstream ecosystems.
po4nerds2.jpg
LM Marsh Diagram: shows direction of wastewater flow
and includes tanks used in earlier stages of treatment.
Phosphorus removal is difficult because it cannot be
converted into a gas. In some conventional treatment
plants phosphate (PO42-) is removed from the water using
iron oxides or salts. Phosphate in the LM wastewater is
removed by adsorption to positively charged sites on the
surface of the gravel in the marsh.
This adsorption removal mechanism poses a problem
because eventually the rock surface will be saturated and
no more phosphate will be adsorbed. Determining the
phosphorus retention capacity of the marsh (the total
amount of phosphorus that can be adsorbed) and the time
to saturation is important for the future management of
the Living Machine.
Sampling of substrate:
• We collected about 4 kg of rock substrate from the surface and a
depth of 12 inches near the point of inflow, outflow, and the middle
of the LM marsh and submerged the rocks in marsh water from the
outflow point.
Preparation and Phosphate adsorption experiment:
• We prepared three different one-liter phosphate (PO42-) solutions
with concentrations of 2.0 mg/L, 100mg/L, and 200mg/L and to
each we added 250mL of surface rocks near the point of marsh
outflow (we assumed that similar sized rocks with the same volume
would have approximately equal surface areas).
• Using a side-to-side shaker we agitated the rocks for 12 hours
and took samples at regular time intervals (at 5min, 10min, 20min,
40min, 1hr, and every hour thereafter).
•We filtered our samples to remove particulate matter.
Ion Chromatography:
• We measured phosphorus concentrations in our samples using
an Ion Chromatograph - a device which separates and measures
different anions based on their charge affinities.
Statistical Analysis:
The IC gave us the change in PO42- concentration for our solutions
over time. From this we were able to calculate the change in
amount adsorbed over time. Using MS Excel we fitted a MichaelisMenton type hyperbolic curve to our data, estimating the total
uptake of our rock samples.
Left: Shaker with
sample bottles
containing marsh
rocks in P042solution.
Right: Dionex-500
Ion Chromatograph
used for measuring
phosphate
concentrations.
100
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• The amount of phosphorus that can be adsorbed
increases proportionally with increasing amounts of
phosphorus in the surrounding solution.
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y = 0.3922x + 6.7057
R2 = 0.9144
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0
0
50
100
150
200
250
Initial Phosphate Concentration (mg/L)
Figure 2. The total amount of phosphorus adsorbed
to the rock surface during the 12-hour experiment as
a function of initial phosphate concentration.
The R2 value of 0.9144 for the trendline fitted to our data in
Figure 2 indicates a significant relationship in our data: the
total amount of phosphorus removed from solution by the
marsh rocks increases with increasing initial phosphate
concentration. The slope for the equation fitted to the data
shows that an increase in the initial phosphate
concentration will correspond to an increase in the total
phosphate uptake by a factor of 0.4.
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59.64
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50
39.2
GOALS
• To establish a method for measuring total
phosphorus retention capacity of the substrate in the
Living Machine marsh.
Using a Mechaelis-Menton type
equation, we found that the
rocks submersed initially in 2.0
mg/L phosphate solution
reached saturation once they
had adsorbed 0.56 mg of PO42-.
• To compare phosphorus retention capacities at
different locations and depths in the marsh.
• To estimate the residual phosphorus retention
capacity of the LM marsh and the remaining time
until saturation.
Rocks submersed initially in
100 mg/L phosphate solution
reached saturation when they
had adsorbed 59.6 mg of PO42-.
HYPOTHESIS
We hypothesize that mechanically agitating LM
marsh rocks in phosphate solution will lead to a
decrease in the phosphorus concentration of the
solution over time due to adsorption to the rock
surface. We expect the rate of phosphate uptake
to decrease over time as the adsorption sites
become saturated. This will demonstrate that
there is a limit to the amount of phosphorus that
the marsh can remove.
Rocks submersed initially in
200 mg/L phosphate solution
reached saturation when they
had adsorbed 78.4 mg of PO42-.
Figure 1. The amount of phosphate adsorbed onto rock surfaces as a function of
time. The blue diamonds correspond to IC measurements converted to the total
amount of phosphorus adsorbed after each sampling. The pink hyperbolic curve
represents a Mechaelis-Menton type saturation equation fit to our data.
• Correspondingly, if the amount of phosphate in the
wastewater stream flowing through the LM marsh
increases, the rock substrate will be able to adsorb more
phosphate.
• The time of saturation is later for higher initial
concentrations in all our samples. This indicates that if the
concentration of phosphates flowing through the LM marsh
is increased, the phosphate uptake efficiency will also
increase. Specifically, more phosphorus can be adsorbed
and there will be a greater time before saturation with higher
initial concentrations.
• There must be an optimal concentration at which the
marsh substrate adsorbs the greater percentage of
phosphate input. We infer this from observing that the
largest percentage ratio of phosphate adsorbed to initial
concentration is greatest at the intermediate concentration
of 100 mg/L.
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10
0
2
100
200
Initial Phosphorus Concentration (m g/L)
Figure 3. Comparison of the percentage of total
phosphorus adsorbed to the initial concentration.
FUTURE
Directions for Future LM Marsh Phosphorus Retention Studies:
ANALYSIS
PHOSPHATE ADSORPTION RESULTS
CONCLUSIONS
• The marsh rocks will achieve a point of saturation
where they can no longer effectively adsorb phosphorus.
Phosphate Saturation Comparison
% Absorbed
The Living Machine (LM) is an ecologically engineered
ecosystem, which cleans and recycles wastewater for the
AJLC Building at Oberlin College.
METHODS
Saturation Amount (mg)
INTRODUCTION
PO42- UPTAKE COMPARISON
• The leveling off of the amount of phosphorus adsorbed
over time, as shown in Figure 1, for the rock solutions
containing 2.0 mg/l initial concentration demonstrates that
there is a limit to which the rocks can adsorb phosphorus.
• The saturation of rocks in the 2.0mg/L initial concentration
phosphate appeared to occur between three and four
hours of shaking.
• The data in Figure 1 for the phosphate solutions
containing 100 mg/L and 200mg/L demonstrate that a full
saturation was not achieved during the 12 hour experiment.
Nevertheless, the curves are approaching a saturation
point indicating that a leveling off of the amount of
phosphate absorbed over time will eventually be achieved.
• We found a surprising trend in our data: the time to reach
saturation is longer with greater initial phosphate
concentrations. One would think that with greater initial
phosphate concentrations, saturation would occur quicker.
This indicates that more complex mechanisms may be
acting upon phosphorus uptake.
• Phosphorus is adsorbed more efficiently by the rocks
when the phosphate concentration is 100 mg/L, as shown
in Figure 3.
• In order to determine the remaining time before
phosphorus saturation occurs we would need to estimate
the total surface area of the LM marsh rocks, determine the
flow path of the waste stream and do more experiments.
As it is, our results would not give an accurate estimate of
the LM marsh’s remaining phosphorus capacity.
• Samples from all locations and depths of the LM marsh
should be analyzed. Compounding factors such as acidity and
SOM of the marsh substrate should be taken into account
when comparing different locations and depths.
• This experimental method can be used to improve the
phosphorus removal quality of future wastewater treatment systems
once more data is collected for the dependence of adsorption
capacity on the location of the rocks in the marsh and on the
compounding factors such as pH and SOM.
•Better techniques for estimating total surface area of sample
rocks and of the LM marsh’s effective surface area (the portion
of the marsh which receives wastewater flow) should be
devised. Past and future experiments on LM marsh flow
patterns should be taken into account.
• Different agitation frequencies and/or initial phosphate
concentrations could be tested to determine the optimal
phosphorus input that the LM marsh substrate gravel can treat.
• A more realistic simulation study of the wastewater flow using
a horizontal flow-through column and a peristaltic pump could
be performed.
ACKNOWLEDGMENTS
We would like to thank Professor John Petersen for his continuing
support of our research project and the guidance he provided to our
Systems Ecology class.
Thanks to the environmental studies department for providing the
material and funding for carrying out our experiment.
Thanks to our classmates for their peer reviews, feedback, and
camaraderie.