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Ecology of the Dump

Judith Brown and Randyll Warehime
1991 Woodrow Wilson Biology Institute


Introduction:

Waste management is becoming a serious problem for land-scarce communities. It is essential that students understand that what we bury does not all disappear and that there is a real need for recycling. The following are three activities concerning solid waste management. You may choose between a short-term activity (one week) or a long-term activity (several months). The follow-up activity is suitable for both.

Resources:

Rathje, William L., Once and future landfills, National Geographic, Volume 175, No. 5 (May 1991), pp 116-134.


Short Term Activity:
A Dump in a Petri Dish

Objectives:

At the end of this activity students should:
  • Determine which types of garbage are biodegradable and which are nonbiodegradable.

Background:

The disposal of solid waste materials has become a serous problem for many communities. A common means of disposing garbage is to layer it with dirt in a landfill. Landfills are reaching capacity levels or are already filled in many parts of the country. Because we have a mounting volume of garbage being produced, new methods of disposal are needed. When developing these new methods, scientists must take into account both the effect on the environment and the cost of technology.

Materials per lab group:

4 Petri dishes, wax pencil, tape, garden soil, dropper, water, samples of garbage which could include paper, vegetable or fruit pieces, meat, bones and Styrofoam.

Procedure:

  1. Label the bottom of 4 Petri dishes from #1 to 4. Write your name and the date on each dish. Section the bottom of the dish into 3 sections using a wax pencil.
  2. Gather 12 small samples of garbage. Cut the samples into 2 centimeter squares.
  3. Put three samples in each Petri dish - one in each section.
  4. Record the nature of the sample materials in each dish. Indicate the size, color, texture, odor and any other features you think are important. Predict how each of the samples will look after one week.
  5. Cover the sample materials with garden soil. Use a dropper to add enough water to make the soil moist but not soggy.
  6. Tape the lid onto each dish and keep at room temperature.
  7. After one week, examine the dishes and record any changes in the color, odor, texture, size of the samples.

Extensions:

  1. Some landfills are sealed causing them to become an anaerobic environment. In order to simulate these conditions and demonstrate the effect of anaerobic environment on degradation rates, the following activity could be tried.

    An anaerobic environment can be devised by putting the Petri dishes with the garbage samples inside a bell jar with a burning votive candle inside. Place a set of the Petri dishes containing the garbage samples and soil inside the bell jar. Tape the jar closed. Leave for a week. Compare the amount of degradation in an aerobic environment to the anaerobic environment.

  2. For one full day, collect the garbage and trash from your house. Divide this into five plastic bags. Bag 1 - glass jars and bottles, Bag 2 - aluminum cans, Bag 3 - plastic containers, Bag 4 - paper goods, Bag 5 - wet garbage. Use a bathroom scale to find the mass of each bag. Using this data, predict the quantity of garbage your family will produce in one week, one month and one year.



Long-Term Activity:
Rubbish In, Rubbish Out?

Objectives:

At the end of this activity students should:
  • Have further practiced measuring skills, hypothesizing, taking data, and writing a lab report.
  • Have worked cooperatively to test a hypothesis.
  • Have observed the relative biodegradability of metal, plastic, and paper products.

Background:

I use this as a beginning-of-the-year and end-of-the-year activity . While observing different materials' relative biodegradability and practicing measurement skills, hypothesizing, taking data, and writing a laboratory report, students cooperate to share data but are individually accountable for their lab report, which is the major focus of this exercise. Because the decomposition takes several months, I have students turn in a preliminary lab report at the end of a beginning-of-the-year unit on scientific method and then turn in a final report towards the end of the year. The preliminary lab report is a draft, so that the end-of-year lab report is a final, revised product of which they can be proud .

While most substances decompose slightly, all paper products rapidly decompose. Regular paper is completely gone when I dig up the packages four months later.

Materials:

Plastic screening (2 pieces/student), string, some type of non-biodegradable label (Styrofoam, dymo label tape), balance, rubbish (supplied by students).

Procedure:

  1. Discuss the process of biodegradation and which materials might be biodegradable and which might not.
  2. In groups, determine what materials to test for biodegradability. I suggest that one person in the group choose a type of plastic, another a paper product, another a metal, etc., to get a variety of materials that can be compared. Then hypothesize about what you expect to happen and why. A prediction of the percent you expect to decompose and a deduction would be appropriate.
  3. Each group member brings 10 pieces of the selected rubbish from home or the campus.
  4. Weigh 5 pieces of your type of rubbish (e.g., 5 similar-sized pieces of aluminum can). Although each piece could be weighed individually and an average calculated, since some materials will readily decompose it is better to just weigh all five pieces together for a total weight. This is the control group. Similarly, weigh the experimental group.
  5. Wrap your control and experimental groups of rubbish each in a different piece of plastic screen, tie them with string, and attach a label (one that doesn't decompose, e.g., Styrofoam or metal). Screen allows water and insects to do their work but keeps the rubbish together so that it can be retrieved.
  6. Share data with your group so that you can compare a variety of materials. Turn in a preliminary lab report.
  7. The experimental rubbish packets are buried for several months about six inches deep. Make sure you mark the burial site. The control packets are kept in the room.
  8. Rubbish packets are dug up. Weigh the contents of both packets then share your results with your group (which is like a reunion, since groups have been changed several times during the year). Then turn in a final lab report.

Extensions:

Posters, letters to the editor can be made urging the use of biodegradable products, particularly in packaging.




Follow-Up Activity:
Where did all the paper go?

Objectives:

At the end of this activity students should:
  • Understand that there is a natural enzyme system in the soil which is capable of biodegrading some of the wastes of today's society.

Background:

Students are often surprised to find that the paper in their garbage samples from the above labs has disappeared. There is a cellulose degrading bacteria present in the soil which is responsible for this. A simpler model which demonstrates this process is to isolate starch degrading bacteria from the soil using starch agar plates.

Materials:

Soil, sterile distilled water, soluble starch, nutrient agar, 20 Petri dishes, spreading loop, Bunsen burner, gram iodine

Preparation:

To make starch agar:
  • Add 2.0 g of soluble starch to 400 ml of distilled water.
  • Add 9.2 g. of nutrient agar and autoclave 15 minutes at 120oC. This will make 20 plates.

To make Gram's Iodine:

  • Add 1.0 ml of iodine and 2.0 g of Kl to 100 ml of distilled water.
  • OR use Lugol's iodine, 3.0 ml in 100 ml of distilled water.

Procedure:

  1. Weigh out 1 gram of soil and add this to 99 ml of sterile distilled water. Dilute this further by removing 1 ml of this mixture and adding it to another bottle which contains 9ml of sterile distilled water. Mix by swirling.
  2. Remove 0.1 ml of this mixture and put on a Petri dish containing starch and agar. Spread this mixture evenly around the plate using the sterile spreading loops.
  3. Turn the plates over and leave overnight.
  4. The next day, examine the plate to see if clear areas are developing in the starch agar. If clear areas are seen, flood the plate with the iodine mixture. Pour off the excess iodine after 1 minute. The starch will stain blue. The starch digesting bacterial colonies will have a brown halo around them where the starch has been consumed by the bacteria.
  5. Leave the plates another 24 hours and restain. The cleared areas will have increased dramatically.

Student Analysis:

  1. Draw the pattern of colonies on your plate.
  2. What does each colony represent?
  3. What is the clear area on the plate?
  4. Explain why the plates containing starch are effective in isolating starch-degrading bacteria.
  5. What enzyme do starch-degrading bacteria degrade?
  6. How could this method be used to isolate oil-degrading bacteria?
  7. Summarize this lab experience and include possible applications of this procedure to help clean up the environment.

Safety:

Wear goggles while working with iodine.


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