112.3

 

PHOSPHATE ADSORPTION RESULTS

Measuring Phosphorus Retention Capacity in the Marsh Substrate of an Ecologically Engineered Wastewater Treatment Facility at Oberlin College

Joy Cernac, Apostol Dyankov and Michael PenninoSystems Ecology (ENVS316) Fall ‘04, Oberlin College, Oberlin, OH

po4nerds2.jpg

INTRODUCTION CONCLUSIONS

GOALS

HYPOTHESIS

ACKNOWLEDGMENTS

METHODS

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 (PO4

2-) 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 PO4

2- 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 Michaelis-Menton type hyperbolic curve to our data, estimating the total uptake of our rock samples.

ANALYSIS

PO42- UPTAKE COMPARISON

• The marsh rocks will achieve a point of saturation where they can no longer effectively adsorb phosphorus.

• The amount of phosphorus that can be adsorbed increases proportionally with increasing amounts of phosphorus in the surrounding solution.

• 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.

The Living Machine (LM) is an ecologically engineered ecosystem, which cleans and recycles wastewater for the AJLC Building at Oberlin College.

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.

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 (PO4

2-) 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.

• 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.

• To establish a method for measuring total phosphorus retention capacity of the substrate in the Living Machine marsh.

• 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.

 

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.

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 PO4

2-.

Rocks submersed initially in 100 mg/L phosphate solution reached saturation when they had adsorbed 59.6 mg of PO4

2-.

Rocks submersed initially in 200 mg/L phosphate solution reached saturation when they had adsorbed 78.4 mg of PO4

2-.

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.

Phosphate Saturation Comparison

y = 0.3922x + 6.7057

R2 = 0.9144

0

20

40

60

80

100

0 50 100 150 200 250

Initial Phosphate Concentration (mg/L)

Satu

rati

on

Am

ou

nt

(mg

)

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.

28.1

59.64

39.2

0

10

20

30

40

50

60

70

2 100 200

Initial Phosphorus Concentration (mg/L)

% A

bso

rbed

Figure 3. Comparison of the percentage of total phosphorus adsorbed to the initial concentration. FUTURE

Directions for Future LM Marsh Phosphorus Retention Studies:

• 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.    

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.

Left: Shaker with sample bottles containing marsh rocks in P04

2- solution.

Right: Dionex-500 Ion Chromatograph used for measuring phosphate

concentrations.