Key concepts

Have you ever seen something labeled "biodegradable" or "compostable" and wondered what that means? A lot of different products—such as food containers, bags, packaging materials, and disposable spoons and forks—claim to be biodegradable or compostable, meaning they will eventually decompose naturally. But these objects are often made of different materials. Do they decompose differently? If so, which decomposes the fastest? In this science activity you will make your own indoor composter and investigate how well different biodegradable and compostable items decompose in it.

Composting is a great way to recycle material that might otherwise be thrown into a landfill. The result of composting is very beneficial—you end up with decomposed materials that can be used to feed, or fertilize, plants. Compost is rich in nutrients that plants readily devour. 

One material that cannot be composted and frequently finds its way into landfills is plastic. Many everyday items such as grocery bags, food containers, packaging materials and disposable forks and plates are made of plastic (usually polyethylene or polystyrene). Plastic is estimated to take hundreds of years to decompose. Consequently, researchers are developing new products that decompose more quickly and can be used instead of plastic products. Some of these products claim to be "compostable" or "biodegradable." Most compostable products are made from cornstarch (as a processed form, polylactic acid, or PLA), fibrous plant pulp (such as that from sugarcane, called bagasse) or starch from other plants, such as potatoes. Many biodegradable products are also made from other plant materials, such as sugarcane pulp or bamboo. New materials are being tested all the time to see if they can be transformed into biodegradable or compostable products.

• Different compostable or biodegradable products to test (Some SunChips bags are compostable—check the bag for details. Some grocery stores that sell natural and organic foods, such as Whole Foods Market, may sell compostable and biodegradable products or offer such food containers for prepared foods.)
• Scissors
• Ruler
• Synthetic string or yarn
• Brown and green composting materials (See the Preparation section below for details.)
• Buckets (optional)
• Plastic bin with lid that seals and holds at least 10 gallons
• Drill with bits, ideally three eighths or one quarter inch
• Out-of-the-way indoor location in which to keep the compost bin for several weeks (It should not receive a lot of direct sunlight.)
• Tray that fits under the bin
• Two wooden boards, ideally three quarter inch by one and a half inches. (The boards should be cut the same length as the plastic bin. Some hardware or lumber-supply stores will cut the wood for you.)
• Soil
• Compost aerator or long rubber gloves
• Digital thermometer and humidity gauge (optional)

• Try to cut your different test products into squares that are each about the same size (about four inches in length is ideal). If possible, make a few squares from each product to test. (You may want to save some extra pieces to compare with the composted squares later.) How are the products similar to each other and how are they different?
• For each square you'll test, cut a piece of string or yarn that is about twice as long as the height of the compost bin. Tightly and securely tie a string to each test item. You may need to cut a slit in the item to tie a string securely to it.
• Collect enough "brown" scraps (which are dry and rich in carbon) to fill your bin more than half full. You can use buckets for this. Good brown scraps include: dry leaves, black-and-white newspaper, dry hedge clippings, dry garden trimmings, paper towels, straw or hay, small wood chippings, crushed egg shells, tissues, untreated cardboard, coffee filters, ashes (from wood or other plant matter), old fabric made from natural fibers, sphagnum peat moss and coconut husk fibers. Collect as many or few different kinds of brown scraps as you like. How are the brown scraps similar?
• Collect enough "green" scraps (which are wet and nitrogen-rich) to fill your bin nearly half full. You can use buckets for this. Good green scraps include: fruits, vegetables, old flowers, dead plants, young weeds, manure from herbivores (such as horses, goats or cows), tea bags, coffee grounds, moist leaves, pond algae and seaweed (sparingly), and grass cuttings. Again, collect as many or as few different types of green scraps as you like. How are the green scraps similar?
• Be sure not to use the following as scraps: Anything treated with pesticides or other chemicals, nonbiodegradable plastics or synthetic fibers, meat, fish, bones, dairy products, potato peelings, garlic, watermelon rinds, oils, diseased plants, weeds with seeds, weed roots, dog or cat manure, disposable diapers, glossy paper or magazines, charcoal or cat litter.
• On the bottom of the plastic bin have an adult carefully drill 12 holes roughly evenly spaced from one another.

• In the area you picked to keep your compost bin, set down the tray. Space the two wooden boards evenly across the inside of the tray. On top of the boards place your bin so that it is stable. (If needed, move the boards slightly so air can flow through the bin's holes.)
• Add a layer about three inches deep of brown scraps to the bottom of the bin. As you add them, shred or cut the scraps so none are longer than two inches.
• Next add a three-inch-deep layer of green scraps. Again, shred or cut the scraps. If they're very wet and make a thick and clumpy layer, make it less than three inches deep. Why do you think it's important to alternate the layers?
• On top of the green layer, add another brown layer (three inches deep) with a little soil (one half inch deep) mixed into it. Why do you think it's important to add some soil?
• Continue adding layers, alternating between brown, green and brown with soil, as you just did. When you have filled the bin about half full and are making a brown layer, include your test product squares in that layer (between brown scraps). Space the squares so none are touching one another or the bin. Hang the squares' strings over the bin’s side so their ends hang over the outside. Hang strings from squares of the same product on the same side of the bin so you can keep track of them.
• Continue adding layers, alternating as before, until the bin is filled all the way to the top. Your indoor composter is now all set up!
• Mix the compost weekly, using a compost aerator or your hands with long rubber gloves. When mixing, move material from the lower part of the bin to the top, bury material that was on the top, break up clumps, and don't lose the strings. You can also carefully dig up your test squares at this time and examine them, being sure to bury them back in the compost afterward. Do the test squares look like they're decomposing at all? Do some look like they're decomposing more than others? How does the compost in general look? Over time the compost bin's volume should decrease so you'll need to continue adding scraps if you want to keep it full.
Optional: If you have a digital thermometer with a metal rod and/or a humidity gauge, you can use these to measure the progress of the composter. An indoor composter will probably be between 68 to 86 degrees Fahrenheit, which is known as a "cold" compost pile. "Hot" piles are usually outdoors and can reach 140 to 160 degrees F. The humidity should not exceed 65 percent.
Tip: If the compost is too humid, liquid may drain into the tray. Extra brown scraps or holes on the bin’s sides, should be added to decrease humidity.
• After your test squares have been in the compost for about four to five weeks or for a different period of time if desired, stop your experiment and analyze your results. Carefully dig up all of your test squares, gently remove any debris and examine them. How do the squares look compared with how they originally looked? Did they decompose much? Did some decompose better than others? Are you surprised by your results?
Extra: There are different ways you could try to quantify your results from this experiment. One is to weigh your items at the beginning and then again at the end and see how much weight they've lost. (To do this, you'll need to let your test squares dry out for a few days at the end of the activity before you weigh them.) Which items decomposed the most, retaining the smallest percentage of their original weight?
Extra: Some products give estimates for how long they need to be composted to decompose. If you can find an estimate for a product, how does it compare with your results? Does the estimate recommend certain composting conditions that are different than yours?
Extra: One efficient composting method uses worms. This method is called vermicomposting, and it can also be done indoors. How well do compostable and biodegradable products decompose using worms? Is it faster than the indoor composter you built in this activity

Observations and results
Did you find that some test squares had significantly decomposed after four to five weeks? Did some squares decompose more than others?

After being in the indoor composter for four to five weeks, some of the test squares should have clearly decomposed, although it depends on the exact products you tested and their composition. Very thin items (such as compostable food bags) may have decomposed more than thicker items (such as compostable eating utensils). Compostable bowls and deli food containers should have visibly decomposed some.

Nitrogen and water (from the green scraps) along with carbon (from the brown scraps) are the key ingredients that microorganisms (from the soil) need to make compost efficiently, which they do by breaking down what's in the compost, including the materials that comprise biodegradable and compostable products. One other important element is oxygen, which is introduced to the compost when it is "turned," or aerated, with a compost aerator or your hands. If a compost pile has these four components in the right ratios, it should efficiently turn scraps into compost.

When you are finished with your activity, you can mix your compost into the potting soil of houseplants or add it to the soil around plants in your garden. If you tested biodegradable products, however, the compost should not be used on plants that are grown for food, because biodegradable products may contain toxins that could be released into the compost and taken up by the plants.

More to explore
Composting Indoors, from Journey to Forever
Hot versus Cold Composting Instructions: What's the Difference?, from
Mushroom Packaging—Innovation Nation, from the National Science Foundation
Disappearing Act: How Fast Do Different Biodegradable & Compostable Materials Decompose?, from Science Buddies

This activity brought to you in partnership with Science Buddies