Activity 3. Continued


Before conducting each experiment, construct a hypothesis that indicates which system you think will be the most efficient and why.

Design of Experiment 1 - Composting Systems

Assign a person or group to make a specific system. The table below describes the different systems. Use the same combination of bulking agent and food substance for each system.

Name of System


Oxygen Depleted

Do not provide any exchange of air in this milk bottle with outside air

Moisture Free

Do not provide any moisture to the food or bulking agent of this system

Control Group

Use the same food mixture and bulking agent; provide ventilation and moisture

Design of Experiment 2 - Compost Material

Assign a person or group to make a specific system. The table below describes the different systems. Use the same combination of bulking agent and mix together a different food substance for each system.

Name of System

Food Substance


Lettuce scraps, carrot peelings


Apple cores, banana peels


Bread crusts


Grass clippings and weeds

Construction of a Milk Jug Compost System:

  1. Using a utility knife or sharp-pointed scissors, cut the top off one jug  just below the shoulder. This will be used as the top of the compost system. It will be used to fit over the base of the system that will be made using the other jug. Cut the other jug just above the shoulder. You will use this one as the base.
  2. Place a smaller stand (roughly 4-5 cm high) upside down into the bottom of the jug that is being used as the base. This will form a stand to support the tray that will hold the compost. You can use any plastic container or object that will fit inside the bottle and provides adequate support for the Styrofoam stand and overlying compost.
  3. The next step is to make a Styrofoam plate. Trace an outline of the jug on a Styrofoam plate and cut it out, forming a piece that fits snugly inside the soda bottle. Use a nail to punch holes through the Styrofoam for aeration. Punch the same number of holes in each plate you use.
  4. Assemble the bottom of your system by placing the stand into the jug, then resting the Styrofoam plate on top of the stand. Make a mark on your bottle to indicate where the Styrofoam plate sits. Above this point is where the compost will be.
  5. Make air holes in the sides of the milk jug in the area below the mark that you made (do not make air holes in the oxygen-depleted system). This can be done with a drill or by carefully heating a nail and using it to melt holes through the plastic. Avoid making holes in the very bottom of the bottle unless you plan to use a tray underneath to collect whatever leachate (note: leachate refers to the solution of water and decomposition products that collect at the bottom of the coke bottle) may be generated during composting. Reassemble the pieces of the system, making sure that you have provided sufficient air holes to allow air to enter the jug and flow up through the stand and Styrofoam circle.
  6. Create the mixture specified for your system. Make sure to mix the bulking agent and vegetable scraps loosely enough to provide airflow. Cut the bulking agent and food scraps into roughly 1-2 cm pieces. Soak the bulking agent in water until thoroughly moist, then drain off excess water (skip this step if you are making the moisture-free system).
  7. Mix roughly equal amounts of bulking agent and food scraps, then fill your reactor. Remember that you want air to be able to diffuse through the pores in the compost, so make sure to keep your mix light and fluffy and do not pack it down.
  8. Put the top piece of the jug back on and seal it in place with tape.
  9. Cover the top hole with a piece of screen or nylon stocking, rubber banded into place (use the bottle cap as the cover for the oxygen-depleted system). Alternatively, if you are worried about potential odors, you can ventilate the system using rubber tubing out the top. Simply use the jug cover with a hole drilled through it in order to insert a piece of rubber tubing, which leads out the window or into a ventilation hood.
  10. If you want to eliminate the possibility of flies becoming a problem, you can cover all air holes with a piece of nylon stocking or other fine-meshed fabric.
  11. Optional Step:

  12. Insulate the system, making sure not to block the ventilation holes. (Because these milk jugs are much smaller than the typical compost pile, they will work best if insulated to retain the heat that is generated during decomposition.) You can experiment with various types and amounts of insulation.

Collecting the Data:

    Data and observations should be collected daily until the original materials of the systems are no longer recognizable.

    1. Take three temperature readings by inserting a thermometer down into the compost through the top of the soda bottle. Take three readings at ten-minute intervals and record the average of the three.
    2. Write down your observations of the physical appearance of your system. How much of the materials shrunk (measure it with a metric ruler from the outside of the jug)? What does it smell like? How much leachate is collected at the bottom (measure with ruler, as above)?
    3. Use a syringe to collect some of the leachate by placing the tip of the syringe through an air hole (you will not do this for the oxygen-depleted system). Expel the leachate into a dish and examine the pH using a strip of litmus paper. Write your observations on the data sheet.


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