Duration: 40 – 60 minutes for initial construction
Vocabulary: bioreactor, composting, moisture content
Summary: Students create a mini-bioreactor in which to practice composting and try out individual experiments.
Objectives: Students will gain a deeper understanding of what composting is and monitor how differences in moisture content or nutrient ratios affect compost temperatures.
Materials: Per Bioreactor: 2 two or three liter soda bottles, styrofoam plate, nail for making holes, packing tape, piece of nylon stocking, scissors, insulation material like foam rubber, thermometer, vegetable scraps, wood shavings/small pieces of cardboard
Soda bottle bioreactors are designed to be used as tools for composting research rather than as a means to dispose of organic waste. They are small and inexpensive, enabling students to design and carry out individualized research projects comparing the effect of variables such as moisture content or nutrient ratios on compost temperatures.
Ask students what they know about composting. Where have they seen it done before? Usually composting is done on a larger scale outside, but for this activity we are trying to make our own right in the classroom. How do they think this will be different?
Have students work in groups of 3 to 5. Have students cut the top of one liter bottle just above the “shoulder” of the bottom and one right below. Have students trace a circle the diameter of the soda bottle 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. The circle will form a tray to hold up the compost in the bioreactor. Beneath this tray, there will be air space for ventilation and leachate collection.
Fit the Styrofoam circle into the soda bottle, roughly 4–5 cm from the bottom. Below this tray, make air holes in the sides of the soda bottle. This can be done by carefully heating a nail and using it to melt holes through the plastic.The object is to make sure that air will be able to enter the bioreactor, diffuse through the compost, and exit through the holes at the top.
Now, have students decide what they want to put in their bioreactor. Materials that are high in carbon include wood chips or shavings, shredded newspaper, and brown leaves. High-nitrogen materials include food scraps, green grass or yard trimmings, and coffee grounds. By mixing materials from the high-carbon and high-nitrogen groups, you can achieve a successful mixture for thermophilic composting. Try to include more than just a couple of ingredients; mixtures containing a variety of materials are more likely than homogeneous ones to achieve hot temperatures in soda bottle bioreactors. Make sure whatever you put in the reactor is diced down to a small size.
Have students fill their bioreactors loosely–they want air to be able to get through the mixture! Have them put the plastic top on and tape it down, and cover the small bottle opening with the piece of nylon. Have students insulate their bioreactors while leaving the air holes open. This is necessary because of the small size of the reactor.
Have students monitor the temperature of their reactors and compare the temperature inside them to the ambient room temperature. If the temperature doesn’t change at all inside the reactor it means that conditions are not ideal. According to Cornell University, “Because soda bottles are so small, you may not end up with a product that looks as finished as the compost from larger piles or bioreactors. However, you should find that the volume shrinks by one-half to two-thirds and that the original materials are no longer recognizable. You can let the compost age in the soda bottles for several months, or transfer it to other containers or outdoor piles for curing.”
Ask students to explain how composting works. What processes break down the added materials? How can composting help the environment? Also ask students what the results were with their personal reactors.
Have students design and build a larger compost pile based on what they learned from this experiment.
Have students teach younger grade levels about composting.