Graphical Abstract
During the first part of the year, students create and conduct an independent research project. STEM I focuses on the skills needed to conduct experiments, including research, developing projects, and conveying findings through writing and presentations.
During periods without rain, pollutants build up on impervious surfaces, such as roads or roofs. When it rains, urban runoff carries these pollutants into bodies of water, harming the environment. Current methods for filtering urban runoff are too large to be incorporated into dense urban areas. For my project, I designed, built, and tested a prototype to filter pollutants that is as effective as current methods but has a small footprint and can be incorporated into urban areas.
Nonpoint source pollution, including urban runoff, is reported to be the leading cause of inadequate water quality, leading to the destruction of aquatic ecosystems, contamination of drinking water, and disruption of recreation. Grassy swales are one of the most effective current methods of urban runoff filtration. However, the total surface area of these swales must be at least one percent of the area that drains into them in order to be effective. In some cities, there is not enough undeveloped land to place grassy swales for filtering pollutants. A prototype was created that is effective at filtering common pollutants when compared to grassy swales, has a small footprint, is applicable in an urban setting, is affordable and durable, requires low maintenance, and can handle large quantities of water. Three of the most common pollutants in urban runoff were tested. Methylene blue, simulating heavy metals, was measured with a spectrophotometer, suspended solids in the form of sediment were measured using a turbidity sensor, and the volume of motor oil was measured after separation from the water. Levels of pollutants in water were measured before and after running through the device, and the percentage removed was compared to that of grassy swales to ensure effectiveness. This device could be incorporated into urban areas to filter pollutants from runoff, helping lower water pollution in cities.
To get to my research proposal, click here.
Runoff in urban areas pollutes bodies of water with suspended solids, motor oil, heavy metal particles, and other harmful materials
Design a plant-based device to filter out pollutants in water from urban runoff before they enter waterways that has a small footprint and can be incorporated into dense urban areas
In recent years, a movement toward greener and more sustainable architecture, especially in large cities, has become increasingly important. Sustainable architecture and design contains five major elements: sustainable site design, energy, indoor environmental quality, conservation of materials, and water conservation and quality (Ragheb, 2016). For my project, I focused on improving the quality of water that is entering aquatic ecosystems in urban areas.
Water pollution worldwide is an increasing issue, as it leads to the destruction of aquatic ecosystems, contamination of drinking sources, and disruption of recreational activities, like swimming or fishing. Nonpoint source pollution is defined as pollution from sources that are not confined or distinct, for example, a specific pipe or ditch. Urban runoff falls under this category. Urban runoff occurs when pollutants, such as sediment, motor oil and grease, and heavy metals, from impervious surfaces enter delicate aquatic ecosystems, harming fish and wildlife populations and killing native vegetation (United States Environmental Protection Agency, 2021). During periods with little to no precipitation, these pollutants build up on impervious surfaces, or surfaces that cannot absorb water. Impervious surfaces include roads, buildings, roofs, and sidewalks. When a storm event occurs, large quantities of pollutants accumulate in runoff and are carried to storm drains or gutters, where they later enter larger bodies of water. Nonpoint source pollution is reported to be the leading cause of inadequate water quality in the United States (United States Environmental Protection Agency, 2022).
Thankfully, there are some current solutions to filtering urban runoff. Grassy swales are long, shallow channels with dense grass or other vegetation, and are one of the most common forms of stormwater control (Grassed Swales Maintenance, 2022). Grassy swales and other bioswales use soil, plants, and other materials, like wood chips or sand, to filter runoff and slow the flow of water (WPI, n.d.). During a storm, runoff water flows to the area, where the grass acts as a biofilter by removing pollutants from the water before it reaches larger bodies of water (Grassed Swales Maintenance, 2022). One study reports that grassy swales remove about 74% of sediment, 88% of oil and grease, and 35-79% of metals (Barrett et al., 1998). In addition, they reduce the speed of runoff and aid the infiltration of water into the ground.
However, grassy swales often take up large sections of land, which may not always be available in crowded urban settings where buildings and infrastructure have already been developed on land (United States Environmental Protection Agency, 1999). The minimum length for a grassy swale is 20 feet, while the width is 6 feet (Grassy Swales, n.d.). Grassy swales tend to have an even greater footprint, as the total surface area of the swale must be at least one percent of the area that drains into it in order to be effective (US Environmental Protection Agency, 1999). This is 500 square feet for every acre. In some dense cities, there is simply not enough undeveloped land to place grassy swales for filtering pollutants. Unfortunately, other solutions, like permeable roads or underground sand pits, can be very expensive and difficult to install. For my project, I designed a prototype filtration system that is as effective as grassy swales at filtering pollutants but has a small footprint and can easily be incorporated into urban areas.
The three-layer device was built of a frame of ½ inch PVC pipe with connectors and plastic containers with holes drilled in the bottom. Duct tape and pool noodles were used to reinforce the frame. The top layer was filled with equal parts wood mulch, sand, and rocks, while the bottom two layers contained grass. The bottom layer emptied into a large collection bin, where water was measured after passing though the device.
Before all tests, four cups of water were run through the device until four cups of water were emptied into the collection bin. This ensured that the device was no longer absorbing water, and all materials had been saturated. This was necessary to ensure consistency in all tests.The device’s ability to absorb water quickly was measured first. Five tests were conducted, where the time for four cups of water to completely filter through the device was taken. Complete filtration is defined as all of the water passing through all three layers of the device so that no water is visibly dripping into the collection bin.
Second, the device was tested for its ability to filter out motor oil. 14 oz of water and 2 oz of motor oil were measured into a bottle. The bottle was inverted 5 times and dumped into the top layer of the device as a stopwatch was started. After complete filtration, the timer was stopped. The filtered water was poured back into the bottle, and the volume of oil remaining was measured after it separated from the water. There were three tests.
Third, filtration of TSS, or total suspended solids, was measured over the course of three tests. Potting soil was mixed with 14 oz of water. A turbidity sensor was calibrated and was used to measure turbidity before and after filtration.
Finally, the device’s ability to filter heavy metals, simulated by methylene blue, was tested by three tests. Methylene blue was mixed with 14 oz of water. A sample of the water was taken before and after filtration for each of the tests. These samples were taken into the lab to be tested using a spectrophotometer. A concentration curve was created based on known concentrations of methylene blue in water and their levels on the spectrophotometer. This allowed me to find the concentration of methylene blue in the experimental samples and determine the percent removed by the device.
Five tests were conducted to measure the time to completely filter four cups of regular water without any pollutants. The mean time was 8 1/2 minutes.
Three tests were conducted to determine the prototype’s ability to filter out motor oil from water. It was found that a large majority of the oil was filtered out. The quantity left from all three tests was too small to be accurately measured, but an estimated 98% was filtered out. Images of the oil content in the water before and after filtration can be found here with additional pictures of the testing process. It was observed that the majority of oil was filtered by the first layer of the device. The times to filter each test were 13 minutes, 15 minutes, and 20 minutes.
In addition, the majority of suspended solids were filtered out as well. The first test had an initial turbidity of 615.3 NTU and a final turbidity of 15.9 NTU. The second and third tests had similar results, with the majority of TSS filtered out. The average percent of suspended solids filtered out was 97.54.
The filtration of heavy metals was simulated by using methylene blue. For the first test, the inital value of methylene blue in the water was xxx, while the final was xxx. Using the data from the later two tests, I determined that the average percent of methylene blue removed was xxx.
The primary criteria were that the device was effective at filtering the pollutants and had a small footprint to be implemented easily in an urban setting. The device was very effective at filtering all three types of pollutants, and therefore satisfied the criteria that it was effective at filtering pollutants from urban runoff. In addition, this prototype had a significantly smaller footprint than current methods of runoff filtration, specifically grassy swales. The three-layer design allows a greater amount of filtration in a smaller footprint, making it more applicable in urban areas where there is little space available. There are multiple ways that this design could be integrated into urban areas. For example, it could be attached to the side of a retaining wall over a body of water. This design allows runoff filtration to be implemented more easily in dense urban areas to prevent pollutants from entering bodies of water and causing environmental damage.
The secondary criteria were that the device was affordable, durable with low maintenance, and could absorb large amounts of water. This prototype cost less than $40 to build. With more durable materials for a permanent installation, the price may increase. However, this design is much more cost effective than other methods, such as underground sand pits, which have an expensive installation process. This design also uses affordable materials (woodchips, sand, rocks, and grass), when compared to other methods that may use chemical processes to remove pollutants. Therefore, it satisfies the criteria of affordability.
Second, this device may require some maintenance, as it is believed that the oil clogged the top layer, making complete filtration take longer over the course of the three tests. In addition, leaves, litter, and other debris in a real storm event may increase the need for maintenance to ensure the device does not overflow. However, this design would likely only require maintenance of the top layer, as the grass layers did not show signs of deterioration. Specifically, only the sand and rocks layers showed signs of being clogged, and only after filtering large quantities of oil and solids. It is believed that this device could handle multiple large storm events without needing maintenance. To prevent the risk of overflow even more, a small redesign may be needed to ensure large quantities of pollutants in overflow are not dumped into the body of water. As such, the device could use some minor modifications in order to better satisfy the criteria of low maintenance. Finally, the design was able to take in almost half a cup of water per minute in its one square foot area. This device takes in more water per area than grassy swales, which absorb ½ an inch of water per hour (Pennsylvania Stormwater Best Management Practices Manual, 2006). As the device is scaled up, it will be able to filter more water in a shorter time. In addition, the depth of water sitting in the top layer impacts the rate of filtration. With a larger depth, water filters faster. This means that larger walls of the top layer container would allow more water to filter faster. This small redesign could allow the device to better satisfy the criteria.