Comparison of Soluble Reactive Phosphorus Sorption in Water

Treatment Residuals and Limestone Bedding Sand

Alyssa Johnson, Aaron Krymkowski, Kate Bullock, Sean Breen, and Laurel Martinez

Picture 1. A sunset over Lake Champlain (photo by S. Breen)

University of VermontRubenstein School of the Environment and Natural ResourcesENSC 160: Pollution Movement through Air, Land, and Water

AbstractPhosphorous pollution threatens to have deleterious effects on the health and stability of

Lake Champlain. Agricultural practices in Vermont are responsible for 66% of the phosphorus load, and regulations have resultantly been fashioned in order to reduce the amount of phosphorus being discharged into the lake (Budd and Meals 1994). The agricultural runoff responsible for this pollution has traditionally been highly disparate, making reductions challenging within current frameworks. With recent increases in the use of tile drainage systems, there are opportunities for addressing this specific source of agricultural phosphorus.

This experiment measured the effectiveness of two medias in their abilities to remove soluble reactive phosphorus (SRP) from the water supply. The two medias utilized were water treatment residuals (WTR) and a limestone bedding sand.

Experimentation was conducted by continuously running a phosphorus solution through two troughs, each of which were filled with one of the medias. Samples were taken each day and tested for orthophosphate concentration. The results of this experiment confirmed that both the WTR and the limestone bedding sands were highly successful in sorbing phosphorus and removing it from water. Although the data was promising, further experimentation is necessary in order to determine if similar results are produced in an actual tile drainage system.

Introduction Anthropogenic nutrient additions are a major driver of ecological change in freshwater

systems. These changes are largely attributed to nitrogen and phosphorus inputs which both lead to shifts towards algae dominated ecosystems. In Lake Champlain, phosphorus pollution particularly contributes to ecological decline. 71% of the phosphorus in the lake comes from nonpoint source pollution, which makes it is difficult to regulate and treat (Budd and Meals 1994). Concern regarding increased phosphorus levels has led to the establishment of new regulations that aim to reduce phosphorus in the lake by 51.5% (Dolan 2015).

One of the principal ways that phosphorus causes environmental change is through the eutrophication of the lake. Because phosphorus is a limiting resource for photosynthetic organisms like algae, their population numbers explode when it is readily available. After a boom in population, algae die and their mass sinks to the bottom of the lake. The decomposition of algae by bacteria and microbes is fueled by respiration; a process which consumes dissolved oxygen and release carbon dioxide. This leads to hypoxic conditions, which cannot support a wide variety of life. A healthy and biodiverse lake ecosystem can quickly be reduced to a few select species that can survive low oxygen and polluted environments (Budd and Meals 1994).

Agriculture represents 66% of the total phosphorus load to Lake Champlain, primarily through the use of chemical fertilizers and manure waste (Budd and Meals 1994, Vermont Agency of Natural Resources 2010). Phosphorus sources from agriculture are particularly detrimental because they typically come in the form of orthophosphate, which is a type of inorganic phosphorus that is readily absorbed by plants. This has led orthophosphate to be

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referred to as soluble reactive phosphorus (Murphy 2007). These pollutants are also easily lost via subsurface drainage and runoff to waterways, which then enter the lake. Because agriculture is such a large contribution of nonpoint phosphorus pollution in the lake, it is of vital importance to control these sources (Sharpley 2000).

One way that runoff from agricultural fields enters waterways is through tile drainage. This is a form of subsurface drainage that removes excess water from fields to increase crop yields, especially during spring flooding (Wright & Sands 2001). Tile drainage can be useful for reducing agricultural phosphorus pollution because it concentrates runoff from the fields into individual drains which can then be treated as a point source. Because of phosphorus reduction regulations, finding an effective and inexpensive method for removing phosphorus in agricultural fields is essential (Dolan 2015). Utilizing tile drains may provide an effective solution for farmers to reduce phosphorus outputs with relative ease and improve the overall health of the lake. Through this study, we hope to provide a cost effective method for removing phosphorus in water passing through tile drains.

The two media that were chosen for this experiment were water treatment residuals (WTR) and a fine limestone based bedding sand. These two substances were selected because they both have been proven to be effective in treating phosphorus and are additionally inexpensive.

Water treatment residuals are composed of the substances removed during the water treatment process. The media utilized for our experiment was an aluminum-sulfate based water treatment residual, which is particularly abundant in urban settings. This material is typically disposed of in landfills at high costs because it is not considered to be reusable (Babatunde 2009). The aluminum oxides and other amorphous materials in the WTR promote the sorption of orthophosphates due to their ability to generate an anion exchange capacity. This resultantly allows the compound to form bonds with phosphate. (UH 2014). Although some sources contend that WTR are not likely to contain toxic substances (Babatunde 2009), there are concerns about using this material in tile drainage systems. One worry originates from the notion that other heavy metals may be present in the WTR. The presence of heavy metals in WTR would promote the introduction of these metals into the environment, which would create new sources of pollution. Despite concerns about heavy metals, the high availability of this material combined with its strong affinity for phosphorus make this media an ideal candidate for phosphorus removal.

The second media used was a limestone based bedding sand. Limestone is a sedimentary rock that is mainly composed of calcium carbonate and contains the mineral calcite (Mateus 2012). The calcium components of limestone have the ability to bind to orthophosphate, resultantly making it unavailable for plant uptake. This process does not actually remove phosphorus from the environment, but can be effective because it inhibits plants from utilizing the compound and thus reduces their ability to grow. Previous experiments have proven limestone to be a cheap and prosperous material for phosphorus removal (Mateus 2012). Vermont has abundant limestone deposits, making it an inexpensive material that can be

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utilized for numerous purposes. Limestone is commonly used for agriculture in Vermont because it is a compact material that can drain easily. The limestone bedding sand used for this experiment was obtained from the Rainville Farm in Swanton, Vermont.

MethodsSet-up Construction

The basic setup for this experiment was a simulation agricultural ditch, a catch basin, a feed basin, and a centripetal pump for the continuous cycling of water.

Two simulation agricultural ditches were built. Using a SAWZALL power tool a 7’ section of culvert pipe was cut in half to act as two trenches. 8’ x 2” PVC pipe was cut into 4 ‘ sections and ½” slits were cut at 1” intervals. The cut PVC was wrapped in 4’ x 1’ filter fabric and affixed using zip ties. These covered pipes were then laid down in the base of the previously cut culvert pipe. At one end of each culvert pipe section a 1.5’ x 1.5’ piece of chicken wire was attached acting as a catch for media. A 1.5’ x 1.5’ section of filter fabric was then wrapped on the inside of the chicken wire to prevent small diameter media from entering the catch basin due to advection. The completed simulation troughs were placed on a slight angle using 1 cinder block.

The feed basins were made from repurposed 35-gallon trashcans with drip heads attached. 2” from the bottom of the trashcan a 1” hole was drilled using a cordless hand drill. In the whole a 1’ threaded PVC valve was fitted with a 1’ galvanized steel washer on the inside and outside of the trashcan and a female threaded coupler was screwed in from the inside. This created a “sandwich” that pressed the two washers into the trash can with the valve protruding from the outside of the can and the coupler in the inside of the can. Silicon caulk was used on the inside and outside of the trashcan around the washers to ensure a watertight fit. On the other side of the valve a threaded 1” PVC female T-joint was screwed threaded on. On each end of the joint 10” sections of 1” PVC were screwed in. These 10” sections then had 10 holes drilled at ½” increments using a 7/64th” drill bit. At each open end of the PVC sections a 1” end cap was caulked into place using silicone caulk. The two basins were then elevated using cinder blocks so that the drip head had adequate clearance from the simulation troughs.

On the ground at the bottom of each trough a re- purposed 15-gallon trash can was placed to act as a catch basin. No modifications were necessary.

To continuously circulate the water in the system two provided centripetal pumps were used. Two 10’ sections of 1” plastic tubing were cut and affixed to the “outflow” side of the pump line using plastic connectors. Two 3’ sections of rubber tubing were connected to the “intake” side of the pump line using plastic connectors. The “intake” side of the line was placed in the catch basin and the “outer” side of the line was placed in the feed basins. A small amount of Duct Tape was used to restrain the “outflow” tubing along the feed basin upper lip to prevent the tubing from falling out of the feed basin due to vibrations from the pumps.

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Once each setup was built two 5-gallon buckets of media were used to fill each of the troughs. Phosphorus solution was prepared by calculating the concentration of Phosphorus water that was desired and the provided fertilizer water was diluted using tap water to result in the desired concentration. Once the dilution was performed 35 liters of solution was added to the feed basin of each trough. Note: it was important that each valve was closed when adding water. Sampling and Experimentation

To begin testing, each valve was opened 1/3 of the way to allow optimal flow without excessive pooling in the troughs. Water samples were taken from the feed basins at the beginning of the flow of water to act as the initial concentration. Once water began to flow the pumps were turned on and their return rate was adjusted to prevent more than 1 cm of water to be in the bottom of the catch basins at any time. Sampling continued over a two week period at irregular intervals. During the testing phase 25 liters of solution was required in each feed basin. These additions were necessary due to evaporation and saturation of media. These additions are visible on the graphs and are responsible for the two spikes at hours 96 and 202.

Sampling consisted of collecting roughly 50 mL of water from the catch basins. When sampling occurred the time was noted to allow for a time/ concentration correlation. Over the course of the experiment 22 samples were taken in total (11 from each trough).

All samples were immediately frozen after sampling to preserve sample integrity and prevent conversion of orthophosphate into other forms of phosphorus. Once the samples were collected, orthophosphate was analyzed using gas spectrometry. The results were then compiled into graphs.

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Results

Figure 1: SRP concentration change over time.

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Figure 2: SRP concentration over time in bar graph form.

Figure 3: Note broken Y-axis scale, to show acute change.

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Discussion

The results indicated that both the water treatment residuals and the limestone bedding sand have a distinct ability to sorb SRP from solution, and are able to do so repeatedly. The experiment itself was limited. It did not simulate the continuous load of fresh SRP that would occur in a real-world storm event. It also had a much longer residence time within the media than would be feasible using such a simple construction in the field. Solution in real world applications will not have a 24hr plus residence time in the remediation media.

Most promising in the results is the fact that both media continued to sorb very effectively after repeatedly loads of SRP solution. The water treatment residuals, containing alum, were especially encouraging. The residuals showed noticeably less loss in ability to sorb phosphorus after repeated exposure when compared to the limestone sand. The limestone results were by no means discouraging, as they continued to absorb well throughout the experiment.

The results suggest that the aluminum phosphate in the residuals acted as a better primary sorption media than the calcium carbonate. Further tests with less time between sampling, combined with more runs of SRP solution through the media would provide both better data on the time the media requires to sorb phosphorus and on the media’s ability to sorb phosphorus over multiple exposures to SRP.

Tests involving higher concentrations with carefully measured amounts of media would provide information on the media’s total ability to absorb SRP from solution. Tests on other forms of residuals would also be valuable, the residuals used in this experiment contained aluminum phosphate.

This potential method of SRP remediation is outstanding because of it’s high level of

affordability. There is little financial burden in creating a drainage trench and filling it with water treatment residuals, a freely available waste, or the inexpensive limestone sand. The cost of in-ground treatment tanks or other more complicated forms of media-based SRP remediation are cost-prohibitive.

With a majority of Vermont’s phosphorus pollution being sourced from agricultural, primarily run-off, a cost-effective system to reduce SRP loss from tile-drained agricultural land may easily have large effect on ameliorating the phosphorus pollution issue in Vermont’s waterways and lakes.

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Appendices

Appendix 1: Complete Results

Trial elapsed time (hrs)

Continuous elapsed time (hrs)

SandSRP (mg/L)

WTRSRP (mg/L)

0 0 0.328 0.234

65 65 0.013 0.005

71 71 0.009 0.007

94 94 0.006 0.003

0 94 0.194 0.219

22 116 0.011 0.005

66 160 0.012 0.004

108 202 0.01 0.007

0 202 0.14 0.302

67 269 0.012 0.008

115 317 0.012 0.009

Table 1: Complete result data.

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Appendix 2: Photographs

Picture 1: Limestone bedding sand source

Picture 2: A close-up of a feed basin. Picture 3: Perforated drainage pipe, pre fabric

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Picture 4: Complete testing setup. Water Treatment Residuals to the right, Bedding sands to the left.

Picture 5: Limestone bedding sand during testing.

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Picture 6: Water Treatment Residuals during testing.

References

Babatunde, A.O. (2009). Characterization of aluminum-based water treatment residuals for potential phosphorus removal in engineered wetlands. Retrieved December 3, 2015 from: http://www.sciencedirect.com/science/article/pii/S0269749109002103

Budd, L., & Meals, D. (1994). Lake Champlain Nonpoint Source Pollution Assessment (Technical Report No. 6A). Lake Champlain Management Conference. Dolan, K. (2015, October 8). Introducing the Vermont Clean Water Initiative: Act 64, Lake Champlain Phosphorus TMDL and Vermont’s Clean Water Goals. Retrieved December 14, 2015, from http://cleanwater.vermont.gov/cwvt/docs/2015-09-03-Act-64-and-TMDL- Implement-Plan.pdf

Mateus, D. (2012). Fragmented limestone wastes as a constructed wetland substrate for phosphorus removal. Retrieved December 2, 2015 from: http://www.sciencedirect.com/science/article/pii/S0925857412000365

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Sharpley, A. (2000). Sources and Transport of Agricultural Phosphorus within the Chesapeake Bay Watershed. In Agriculture and Phosphorus Management: The Chesapeake Bay. Boca Raton, Florida: Lewis. Vermont Agency of Natural Resources (2003).Vermont's Accepted Agricultural Practices and the Non-Point Source Pollution Reduction Program. Retrieved December 4, 2015, from http://www.watershedmanagement.vt.gov/erp/htm/agriculture.htm Wright, J., & Sands, G. (2001). Planning an Agricultural Subsurface Drainage System. Retrieved from http://www.extension.umn.edu/agriculture/water/planning-a-subsurface-drainage- system/

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