Environmental Engineering Reference
In-Depth Information
It is important to distinguish among terms such as
biodegradable
,
compostable
, and
compost-
compatible
. Another term for composting is “yeasting;” it is literally translated into changing
garden while making gestures.
• A biodegradable material is capable of being broken down completely under the action of
microorganisms into carbon dioxide, water, and biomass. It may take a very long time for
a material to biodegrade depending on its environment (e.g., hardwood in an arid area), but
it ultimately breaks down completely.
• A compostable material biodegrades substantially under composting conditions, into carbon
dioxide, methane, water and compost biomass. Compost biomass refers to the portion of
the material that is metabolized by the microorganisms and which is incorporated into the
cellular structure of the organisms or converted into humic acids, etc. Compost biomass
residues from a compostable material are fully biodegradable. “Compostable” is thus a
subset of “biodegradable.” The size of the material is a factor in determining composta-
bility, because it affects the rate of degradation. Large pieces of hardwood may not be
compostable under a specific set of composting conditions, whereas sawdust of the same
type of wood may be.
• A compost-compatible material does not have to be compostable or even biodegradable.
It may biodegrade too slowly to be itself compostable, or it may not biodegrade at all.
However, it is not readily distinguishable from the compost on a macroscopic scale and
does not have a deleterious effect on the compost (e.g., it is not a biocide). Compost-
compatible materials are generally inert and are present in compost at relatively low lev-
els. Examples of compost-compatible materials include sand particles and inert particles
of plastic.
Although composting has historically focused on creating garden-ready soil, it is becoming more
important as a tool for reducing solid waste. More than 60% of household waste is recyclable or
compostable. The decomposition of material sent to landfills is a principal cause of methane, an
important greenhouse gas, so reducing the amount of waste sent to landfills is a key element of the
fight against climate change. Surveys have shown that the first reason municipalities don't compost
their waste is because they feel the process is complicated, time-consuming, or requires special
equipment. However, especially in rural areas, much of the solid waste could be removed from the
waste stream by promoting “extremely passive composting” where consumers simply discard their
yard waste and kitchen scraps on their own land, regardless of whether the material is ever re-used
as “compost.”
Many different materials are suitable for composting organisms. Composters often refer to
“C:N” requirements; some materials contain high amounts of carbon in the form of cellulose,
which the bacteria need for their energy. Air spaces are left in the compost because decomposing
the organisms requires oxygen. Other materials contain nitrogen in the form of protein, which
provides nutrients for the energy exchanges. It would, however, be an oversimplification to describe
composting as about carbon and nitrogen, as is often portrayed in popular literature. Elemental
carbon—such as charcoal — is not compostable nor is a pure form of nitrogen, even in combination
with carbon. Not only this, but also a great variety of man-made, carbon-containing products,
including many textiles and polyethylene, are not compostable—hence the push for biodegradable
plastics. Suitable ingredients with relatively high carbon content include
• Dry, straw-type material, such as cereal straws
• Autumn leaves
• Sawdust and wood chips
• Paper and cardboard (such as corrugated cardboard or newsprint with soy-based inks).
