ChemMatters | OCTOBER/NOVEMBER 2017 13
Cornell Composting. Cornell Waste Management
Institute, Cornell University: http://compost.css.
cornell.edu/ [accessed Aug 2017].
Basic Principles of Composting. What is
Composting? Louisiana State University
Agricultural Center, Research and Extension:
2622compost2.pdf [accessed Aug 2017].
Sustainable Management of Food Food Recovery
Hierarchy. U.S. Environmental Protection Agency,
Feb 19,2017; https://www.epa.gov/sustainable-management-food/food-recovery-hierarchy
[accessed Aug 2017].
Greenhouse Gas Emissions Overview of Greenhouse
Gases. U.S. Environmental Protection Agency, April
14, 2017: https://www3.epa.gov/climatechange/
ghgemissions/gases/ ch4.html [accessed Aug 2017].
Julia R. Barrett is a science writer who lives in
Madison, Wis. This is her first article in ChemMatters.
hot for bacteria to survive, and
they either die or become dormant
spores that can withstand hostile
conditions. High temperatures
cause water to evaporate from the
waste more quickly, leaving the pile
dry so it’s unable to support
bacteria growth, and it introduces
the risk of spontaneous combustion and fire.
factor is having the
right amounts of
carbon and nitrogen
in the system.
In general, biological organisms need about 25 times as
much carbon (C) as nitrogen
(N). Fruit waste, such as an
apple, has a carbon-to-nitrogen
ratio of 35:1. In this case, there
is much more carbon than nitrogen, and the composting process
slows. To speed it up, the high C:N
ratio of the apple needs to be offset. For example, coffee grounds,
which have a C:N ratio of 20:1,
could be added to lower the
overall ratio of carbon.
Adding too much nitrogen
would be a mistake because it
can lead to the production of
ammonia and other nitrogen-rich products, which would
cause the composting material to smell bad. Ideally,
the C:N ratio should
be between 20:1 and
25:1. Bacterial growth doesn’t
stop if the ratio is imprecise; it
simply slows down.
With bacterial growth nurtured by appropriate oxygen
levels, at the right carbon-to-nitrogen ratio,
and within a reasonable temperature range,
the organic composting material converts into
CO2, water, and mature compost fairly quickly.
In summer, warm temperatures encourage
bacterial activity, so with frequent turning,
compost can be ready in about three months.
The mature compost can then be spread over
soil, so plants can grow and soil can hold car-
bon. This capability has an underappreciated
role in mitigating climate change. Reducing
the emissions of methane, a greenhouse gas,
is often considered critical, but it is only one
excess carbon out of the
atmosphere is the other.
Not to mention reducing
the amount of organic
material discarded in
The most common
use of food waste in
the United States is in
municipal composting programs. Cities such
as San Francisco, Calif.; Portland, Ore.; and
Seattle, Wash., have implemented programs
that divert organic waste from landfills. In
some cities, such as Seattle, it is against the
law to put food scraps in the garbage. Other
cities, such as Madison, use anaerobic com-
posting to capture methane and they use it to
fuel cars, trucks, and buses.
“We’re taking 13,000 tons of food waste
to the landfill every year, and instead of
paying to bury it, we use it to produce elec-
tricity,” says Bryan Johnson, the recycling
director in Madison.
Young people, in particular, are conscious
of protecting the environment and taking
responsibility for the products they use. Says
Johnson, “People are becoming more aware
that even if it’s out of your home, it’s still your
Compost in a Bottle
No yard? No digester? You can
still experiment with composting
using plastic soda bottles and
some plant-based scraps.
After assembling your composter following the instructions,
set it (or hang it) in a warm spot, such as a sunny window or
under a plant light. Check it daily and record observations,
such as the appearance of the composting material and any
other changes you notice. For example, consider: Are there
any odors?, does mold form?, and does it look wet or dry?
As the weeks go by, the scraps will become increasingly
less distinguishable from one another, and the overall mass will become smaller. You probably won’t
wind up with pure compost, but the material will be different from what you started with.
If you have a lot of soda bottles, conduct more than one experiment—change one factor in each system. What happens if you change the ratio of fruit and vegetable scraps to items with a lot of lignin (for
example, newspapers and wood chips)? What happens if you place composters with the same contents
in areas with different temperatures? If you add a small amount of partially composted material to fresh
material in a new composter, does the decomposition rate change?
You will need:
three soda bottles
a nail (or other object for poking holes
in the bottle)
a razor blade, or utility knife
tape, and a few other items
Complete instructions can be found
Please be cautious when using sharp objects.