SummaryWaste disposal has been an ongoing societal problem since medieval times. In this lesson, students learn about the three methods of waste disposal in use by modern communities. They also investigate how engineers design sanitary landfills to prevent leachate from polluting the underlining groundwater.
Engineers develop creative technologies to dispose of the enormous amount of trash produced in the U.S. They design sanitary landfills to prevent groundwater, soil and air pollution. Sanitary landfills have liners to keep contaminants from leaking into underlying groundwater, leachate collection systems that collect and treat the precipitation coming off of the garbage, and capping systems that keep the landfill from releasing harmful gases into the air.
Each TeachEngineering lesson or activity is correlated to one or more K-12 science,
technology, engineering or math (STEM) educational standards.
All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN),
a project of D2L (www.achievementstandards.org).
In the ASN, standards are hierarchically structured: first by source; e.g., by state; within source by type; e.g., science or mathematics;
within type by subtype, then by grade, etc.
Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards.
All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN), a project of D2L (www.achievementstandards.org).
In the ASN, standards are hierarchically structured: first by source; e.g., by state; within source by type; e.g., science or mathematics; within type by subtype, then by grade, etc.
- Define the criteria and constraints of a design problem with sufficient precision to ensure a successful solution, taking into account relevant scientific principles and potential impacts on people and the natural environment that may limit possible solutions. (Grades 6 - 8) Details... View more aligned curriculum... Do you agree with this alignment? Thanks for your feedback!
Students should have a basic understanding of how groundwater flows (see Lesson 4, Who's Down the Well?).
After this lesson, students should be able to:
- Describe advances in waste disposal from a historical context to modern times.
- Explain how landfills can pollute groundwater.
- Describe national regulations involved in sanitary landfill design.
- Explain how engineers are involved in the design of sanitary landfills.
During medieval times, people would just throw their trash out the window. Generally, this was not a problem if there were few people and plenty of land. However, with an increase of population and a decrease of available land — such is the case in many cities — the over-abundance of trash became a problem. In London, England, for example, when the population doubled from the beginning to the middle of the 19th century, the streets became filled with trash. One Londoner describes a street in London as, "This is a duster of miserable houses; the gutter fronting them is full of mostly foetid (old English word meaning "smelly"), muddy filth; dust, and garbage-heaps are common." Inadequate disposal of garbage — coupled with over crowded houses and lack of sanitation — led to much disease and death in 18th century London, which eventually caused city officials to create sanitation laws and a trash removal system.
What to do with our profusion of trash is still a problem we face today. From 1948 to 2001, New York City shipped approximately 11,000 tons of city garbage to the Fresh Kills Landfill on Staten Island; the transport of the garbage ceased when the landfill became too large. In 2001, the city was forced to finally listen to the complaints of nearby neighborhoods and close the landfill.
In 1948, Fresh Kills Landfill started as an open dump placed on a wetland. At the time, no one thought about what would happen 50 years into the future. As a result, the landfill continually releases toxic chemicals into the surrounding water and noxious fumes into the air. Today, engineers understand the consequences of dumping large amounts of garbage in one place and design landfills that better protect the environment.
Three main types of waste disposal are used in the US: composting (recycling), incineration (burning trash) and dumping (depositing in landfills). Two types of landfills exist: open dumps, where garbage is placed in an open pit or on existing land and sanitary landfills. In this lesson, we are going to focus on sanitary landfills. Sanitary landfills are designed by environmental engineers and can include special liners (layers of plastic and clay that keep the contaminants from leaking into the groundwater), leachate collection systems (a method by which the water coming off of the garbage from precipitation is collected and treated) and special capping systems (which close off the landfill from leaking harmful gases into the air).
The U.S. Environmental Protection Agency (EPA) has also helped create regulations for landfill design and use. Landfill liners must have several layers, including a one of soil, then a geotextile (a synthetic permeable membrane) layer that lets water through but filters out any small pieces of trash, a leachate (water) collection system, a plastic liner layer in order to prevent leachate from reaching the groundwater, and an impermeable clay layer to stop any water that made it through the plastic. So, with all of these safeguards, how does leachate still make it through all these layers? Well, holes can be punctured in the liner during construction or by the sheer presence of tons of garbage, seams can leak, or the liner may deteriorate over time. These are some of the challenges that environmental engineers face.
Lesson Background and Concepts for Teachers
What Do We Do with Our Trash?
According to the EPA, the US is expected to produce 220 million tons of garbage per year. So what do we do with it? Three main types of waste disposal are: composting, incineration and dumping. Composting is not currently used much in the US, but is a form of recycling that involves taking organic matter, such as food or yard waste and letting it biodegrade into a soil that can be used as fertilizer. Benefits of composting are that the garbage becomes reusable, and it is an inexpensive method of disposal. Incineration involves burning trash at very high temperatures. Incineration is a favored method of trash disposal because it can reduce the volume of waste by 75% to 95%. Also, the heat created from incineration can be used for energy. However, on the downside, incineration contributes to air pollution. Combusting (or burning) trash — like combusting gasoline or coal — can emit nitric and sulfuric oxides, which are known to cause acid rain. In addition, a great deal of controversy exists about increased asthma and other respiratory diseases in communities near incinerators caused by inhaling the ash spewed from incinerators. Fortunately, engineers have designed systems to reduce the pollutants emitted by incinerators; however, these practices are usually expensive to implement. Also, open dumps and sanitary landfills are used for trash disposal. In an open dump, waste is simply piled on any available land, without any engineered systems to protect the surrounding area from contamination. Open dumps are the oldest — and arguably the easiest — form of waste disposal, but are no longer used in the US because of the pollution problems they create. Instead, we design sanitary landfills for most of our garbage. Sanitary landfills include engineered systems such as liners, leachate collections systems, and caps once the landfill is closed to help prevent air and water pollution.
Pollution from Dumping
Landfills, even well engineered ones, can cause air and water pollution. The predominant concern is groundwater and surface water pollution from runoff. Leaching is the process of water (usually in the form of precipitation, such as rain) filtering through a landfill, which then carries whatever contaminants that are in the landfill out with the ground or surface water. After water has filtered through a landfill, it is called leachate. Leachate commonly contains high concentrations of chemicals, heavy metals and microbial life. Chemicals come from many different industrial wastes that are dumped at landfills or agricultural waste that contains large amounts of pesticides. Heavy metals, such as lead, can come from any number of items we throw away, including lead-based paint. The leachate then either flows over the ground into a nearby stream or lake or it filtrates through the ground into the groundwater bringing all the bad contaminants from the landfill with it. If leachate reaches the groundwater, it can migrate into the groundwater and create a plume of polluted water. If the plume reaches a nearby well that is a source for local drinking water, people subsequently drink the contaminated water, which can cause many adverse effects for humans.
Another source of pollution from landfills is methane gas. Methane is created when microorganisms break down (or biodegrade) the organic matter that is contained in landfills. Methane gas often smells bad and can cause some respiratory problems for humans. In this lesson, we are going to concentrate on the groundwater pollution from landfills.
Because of the many old landfills that still exist — such as the Fresh Kills Landfill on Staten Island — that have caused a significant amount of pollution, the EPA has set up regulations for municipal solid waste landfills, including measures that prevent or minimize pollution from leachate and methane gas. The regulations include: location, design, operating, groundwater monitoring, closure/post-closure care and corrective action and financing.
Location Restriction: Placement of a landfill is a definite problem. The EPA has restrictions based on the geology and ecology of the surrounding area. Following are the six total restrictions. A landfill cannot be placed in: 1) a floodplain, 2) a wetland, 3) a fault zone, 4) areas with high risk of earthquakes, 5) unstable areas, and finally 6) the immediate area of an airport. In addition to the above regulations, the hydrogeology and the demographics of the surrounding area must be considered. No one really aspires to live near a landfill, so it is important to examine how many people live near a potential landfill location and what affects that landfill may have on the community, such as whether or any nearby drinking water wells could potentially be polluted. Favorable hydrogeology keeps any pollution created from migrating or reaching any nearby wells. Factors that should be considered when choosing a landfill are topographic relief, location of the water table, amount of precipitation, type of rock and soil and location of the disposal zone in the surface water and groundwater flow systems.
Design: Modern landfills must be designed with pollution prevention in mind. Modern sanitary landfills must include a liner system, a runoff collection system, a methane collection system, a groundwater monitoring system, and after closure, a cap. Of particular importance is the landfill liner, as seen in Figure 1. Landfill liners have several layers: first, a layer of soil as a foundation for the waste; then, a geotextile layer that lets water through but filters out any little pieces of trash; then, a leachate collection system, which consists of gravel that allows water to filter down to pipes where it is collected; under the leachate collection system, liners are required to have a plastic layer in order to prevent leachate from reaching the groundwater; and finally, liners have an impermeable clay layer to stop any water that may make it through the plastic.
Operating: Rules also exist for how landfill owners must operate their landfills.They must compact and cover the waste each day. A well-designed landfill has a methane and water collection system, which are also part of the daily landfill operations. Finally, landfill owners must take measures to ensure that the area is restricted so no one can illegally dump hazardous waste.
Groundwater Monitoring: The groundwater down gradient of a landfill must be continually monitored for contamination. If the presence of a contaminant is detected, it might indicate a liner leak or that some surface water has seeped through the collection system.
Closure/Post-Closure Monitoring and Corrective Action: Once an owner or operator closes a landfill, s/he is still responsible for any pollution it may cause. Therefore, owners and operators are required to monitor landfills even after closure.
Financing: Owners and operators must prove that they can successfully finance the landfill.
When landfills are constructed today, a liner — specified by the EPA — must be installed in the landfill. The liners are engineered to stop water from filtering through the soil to the groundwater below the landfill, but in all sanitary landfills, water is found in the leachate collection system below the liner. Some landfill owners go above and beyond the call of duty and install a second leachate collection system below the clay since water sometimes still manages to find its way into the clay layer. How can that be? With plastic and impermeable clay layers, how can liquid still manage to get through? Well, many things can happen to cause this leakage. First, imagine laying down plastic over an entire football field. Plastic that wide does not exist, so smaller pieces must be pieced together. Engineers take great care to make the seams as tight and leak-proof as possible using special machines (as shown in Figure 2, right), but as students will discover in the associated Eek! It Leaks! activity, it is very hard to create perfect seams. Furthermore, piling tons and tons of garbage on top of liners creates a lot of pressure, and the seams could stretch and leak over time. Also, the liner is too big for a few people to just roll out and put together. Construction engineers use big trucks, such as the one shown in Figure 2 (left). Although necessary for installation, trucks are heavy and sometimes puncture or weaken the liner, making a little hole through which liquids can leak. Finally, some chemicals that may be dumped in the landfill can deteriorate the liner material, causing leakage. And, over time, most liners deteriorate.
bedrock: Soil rock that underlies an area with soil on top.
clay: Very fine-grain soil; has a very low porosity.
leachate: Water that is produced by leaching or filtering through a landfill. Leachate often contains high amounts of organic matter and toxic chemicals.
permeability: A measure of how easily water flows through a porous material like soil.
porosity: A ratio of the volume of pore space of a soil to the total volume of the soil.
sand: Coarse-grain soil; has high porosity.
sanitary landfill: Landfills that have engineered systems, such as a liner, to stop pollution from leaving the landfill
water table: The highest point to which groundwater rises.
- Eek, It Leaks! - Students construct model landfill liners using tape and strips of plastic, attempting to create a bag that does not leak. This is similar to the challenge environmental engineers face when piecing together gigantic liners for real landfills.
- Design, Build and Test Your Own Landfill - Teams design and build model landfills using materials similar to those used in full-scale landfills. Their small-size landfills are "rained" on and subjected to other erosion processes. Students analyze their models to see if leachate was contained.
What happened to trash in medieval times? (Answer: People just threw their trash out the window.) Why did this method become unacceptable to citizens? (Answer: Waste was piling up in the streets; the smell was unbearable; lack of sanitation eventually led to disease and death.) When you throw something "away," where does it go? How do we handle it? (Answers: Burn it in incinerators, bury it in sanitary landfills, or compost or recycle it.) What happens to trash when it is put in a landfill? (Answer: Some of it biodegrades, but most of it just stays there; contaminants may leak into the groundwater; engineers design liners to protect groundwater and drinking water sources from garbage run-off.) What are the pros and cons to each method of garbage disposal? If you were an engineer working to design a community's waste disposal system, which method would you choose?
Brainstorm: Begin a class discussion by asking students: What we should do with the garbage we produce every year. Have students answer generally and write all ideas on the board. (If your class is not used to open discussion, go around the room and get one idea from every student.) Repeat answers are permitted; just do not repeat the idea on the board. Once you have made a lengthy list of ideas, go through and eliminate ideas that are not practical, such as sending garbage to the sun (most classes come up with this idea, and it is too expensive). Also, eliminate ideas that pollute the Earth, such as do nothing or dump garbage into the sea. The idea is to come up with a list of what we might actually do with garbage, including incineration, sanitary landfills, compositing and open dumps.
Question/Answer: Ask students the following questions. Call on different students to answer the questions.
- What is the difference between a sanitary landfill and an open dump? (Answer: Sanitary landfills include engineered systems to prevent pollution, while open dumps are just piles of waste on open land.)
- What are the required elements of an EPA-regulated landfill liner? (Answer: A geotextile layer that only allows water through, a permeable sand layer, a leachate collection system, a plastic liner and a layer of impermeable clay material.)
- Do engineered landfill liners still leak? (Answer: Yes)
- What are some things that cause landfill liners to leak? (Answer: Holes punctured during installation, leaking seams, chemical deterioration, and an aging liner that wears out.)
Lesson Summary Assessment:
Engineer a Landfill: Have students write letters to an EPA regulator from the viewpoint of an environmental engineer, explaining what they have done on their landfills to ensure that pollution is minimized. Students must included three things they do or have done at their landfills to minimize pollution. Some ideas: installed a liner with all the different layers; monitored the groundwater and collected the leachate everyday; chose a site that had impermeable soil and was not near communities or homes.
Lesson Extension Activities
Investigate the geology of your area and decide where it would be best to site a landfill. See the following website: http://www.epa.gov/epawaste/index.htm .
ContributorsMalinda Schaefer Zarske; Janet Yowell; Melissa Straten
Copyright© 2005 by Regents of the University of Colorado.
Supporting ProgramIntegrated Teaching and Learning Program, College of Engineering, University of Colorado Boulder
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