| What
is Composting?
Composting is a natural process whereby micro-organisms transform
organic waste materials into a soil- like product called humus
(pronounced "hue-mous"). Kitchen scraps, leaves and yard waste,
paper, wood, food-processing wastes, as well as agricultural
crop wastes and animal manures, are excellent organic waste
materials that can be composted.
Composting
has a wide range of benefits: it helps to reduce the amount
of waste going to landfills and produces a valuable product,
compost, of benefit to soil health & vitality in addition to
many other attributes such as water and soil conservation. It
is estimated that about 40 to 60 percent of the total waste
stream could be composted!
For
the composting process to work best, it is important that the
micro-organisms have a continuous supply of food (i.e., organic
wastes), water and oxygen. As well, managing the temperature
of the composting material is important to make the process
work.
Although
most organic wastes supply all of the nutrients necessary for
the micro-organisms to grow, they grow best with certain levels
of carbon (C) and nitrogen (N). Paper, leaves and wood are high
in carbon, while grass clippings and vegetable trimmings are
high in nitrogen. The materials in the composting "recipe" need
to be mixed in the correct combination to create the right C:N
ratio.
Types
of Composting
Composting can be done in many different ways. Types of composting
range from residential or backyard composting to mid-scale and
central municipal or industrial composting systems. Selecting
the most suitable method depends on the amount and type of organic
materials to be composted.
Residential
or backyard composting means that an individual household composts
most of its food and yard waste in a container located outside
the home. Worm composting is a viable option to compost kitchen
waste indoors. Composting at home is the simplest and most cost-effective
method because collection, transportation and costs are avoided.
People benefit directly from their own efforts by producing
a valuable additive for their own garden soil.
However,
not all food and yard waste can be managed so simply. Organic
material from commercial sources, such as restaurants, supermarkets,
apartment buildings and food manufacturers, needs to be managed
differently. This is where mid-scale and centralized composting
fits in.
Both
mid-scale and centralized composting involves significantly
larger quantities and a larger variety of organic wastes.
Mid-scale
composting is the on-site management of organic waste generated
by a group of people, such as in an apartment complex, office
building or hospital. This avoids the transportation of organic
waste. Centralized composting involves the collection and transportation
of organic materials to a special facility where it will be
prepared and processed into compost.
Centralized
Composting Facilities
The design and set-up of a centralized composting site must
take into account such factors as the type and volume of organic
waste, waste collection methods, sorting, storage factors and
the end use for the finished compost. Quality organic waste
and good operating procedures ensure the production of high-quality
compost.
Among
the most common centralized composting process technologies,
in order of increased technology, are:
Windrows
Organic materials are placed in long triangular rows called
windrows. Windrows are turned and watered occasionally to ensure
that the micro-organisms get an adequate supply of oxygen and
that any clumps of organic material are broken up.
This
method is commonly used for composting leaf and yard waste,
commercial food wastes, or such "specialty items" as shredded
Christmas trees.
Static
Aerated Pile
Organic waste materials are formed into windrows over perforated
pipes. Rather than the windrows being turned, air is supplied
to the micro-organisms through the pipes.
Almost
all municipalities own the necessary equipment required for
centralized composting operations using windrows or static aerated
piles. This means that the capital costs are relatively low.
In-Vessel
In-vessel systems are either fully or partially enclosed, and
can handle more material in a smaller space than windrows or
static aerated piles. However, they tend to be more costly.
These systems provide better control of aeration, temperature
and the moisture in the organic materials being composted, all
of which result in faster decomposition.
If
necessary, water can be added to maintain the correct moisture
level, and air can be pumped in to provide oxygen and to control
the temperature.
Although
different in-vessel systems are available, they are generally
of three basic types: channels or troughs, containers and rotating
drums (sometimes called tube digesters).
Channels
(or Troughs)
The composting process takes place in long rectangular troughs
or channels. The organic waste materials are mixed so that the
clumps are broken up and the material is aerated.
Containers
Composting takes place in closed containers that are supplied
with air. Excess moisture and exhaust air are removed from the
containers to maintain ideal conditions for the micro-organisms
throughout the process.
Rotating
Drums (Tube Digesters)
Organic waste materials are added to a drum which is continuously
rotating. The rotation ensures that the micro-organisms are
constantly supplied with the oxygen they need and that all of
the organic waste materials are exposed to them. The material
remains in the drum for three to five days and is then transferred
to windrows for final curing.
Anaerobic
Digesters
Organic waste materials can also be digested in an oxygen-free,
or anaerobic, environment by micro-organisms that do not need
oxygen. The length of time required to digest the organic waste
material varies according to the individual technology usually
between two and twenty days. The process produces humus, methane
and carbon dioxide. The methane is captured and converted into
energy. Following digestion, the humus can be transferred to
windrows or another method for final composting.
Excerpts
from Centralized Composting Helping To Complete
The Carbon Cycle, written by Susan Antler of The
Composting Council of Canada on behalf of Environment Canada.
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