Page 163 - Soil Degradation, Conservation and Remediation

152 6 Soil Pollution

as protein prior to composting is mineralized to inorganic N (NH 4 -N and NO 3 -N),
which is then resynthesized into other forms of organic N in microbial biomass and
humic substances during the composting process. In some composting programs,
municipal wastes are separated at source into organic compostables and non-
compostables by participating residents and businesses, while others accept a mixed
stream and separate non-compostables at a centralized facility. However, some
separation is needed even when organic compostables are separately collected.
Composting programs which accept a mixed waste stream accept material more
or less as it is collected, relying on the facility separation techniques described

below. One important modification to traditional collection techniques is the estab-
lishment of a household hazardous waste (HHW) collection program. To be effective
at reducing the contaminants of concern in MSW compost, such programs must
emphasize heavy metal sources such as batteries and consumer electronics. The
remaining mixed waste (less any separately collected recyclables and HHW) then
serves as feedstock for the composting facility, where centralized separation of
non- compostable materials will occur. With this approach, 60–70 % of the solid
waste stream is typically processed into compost. The remaining 30–40 % includes

recyclables as well as rejects destined for the landfill or an incinerator and landfi ll.
The separating techniques are shown below.

Technology Materials targeted

Screening Large: film plastics, large paper, cardboard misc
Midsized: recyclables, organics, misc
Fines: organics, metal fragments, misc
Handpicking Recyclables, inerts, and chemical contaminants
Magnetic separation Ferrous plus contaminants associated with ferrous metal
Eddy current Nonferrous metals
Separation
Air classification Lights: paper, plastic

Heavies: metals, glass, organics
Wet separation Floats: organics, misc
Sinks: metals, glass, gravel, misc
Ballistic separation Light: plastic, undecomposed paper
Medium: compost
Heavy: metals, glass, gravel, misc

After separation, the materials are reduced in volume and thoroughly mixed
before delivering to the central composting facility. The composting is largely a
biological process by which complex materials are decomposed into simpler
substances with substantial mineralization, release of available nutrients, and further
complexation . Proper design and management must be based on the needs of

microorganisms if the process is to be a success. Composting is defi ned by human
intervention into the natural process of decomposition. With a combination of proper
environmental conditions and adequate time, microorganisms turn raw putres-
cible organic matter into a stabilized product. Through composting, readily
available nutrient and energy sources are transformed into carbon dioxide, water,