Environmental Engineering Reference
In-Depth Information
Composting
is a simple process in which we copy nature
to recycle some of the biodegradable organic wastes we
produce. The organic material produced by compost-
ing can be added to soil to supply plant nutrients, slow
soil erosion, retain water, and improve crop yields.
Some cities in Austria, Belgium, Denmark,
Germany, Luxembourg, and Switzerland recover and
compost more than 85% of their biodegradable wastes.
Only 5% of the paper, yard, and vegetable food waste
in U.S. MSW is composted, but studies show this pro-
portion could be raised to 35%.
Such wastes can be collected and composted in
centralized community facilities, as is done in many
European Union (EU) countries. The resulting com-
post can be used as an organic soil fertilizer, topsoil, or
landfill cover. It can also be used to help restore eroded
soil on hillsides and along highways, and on strip-
mined land, overgrazed areas, and eroded cropland.
To be successful, a large-scale composting pro-
gram must be located carefully and odors must be con-
trolled, because people do not want to live near a giant
compost pile or plant. Composting programs must also
exclude toxic materials that can contaminate the com-
post and make it unsafe for fertilizing crops and lawns.
You can easily make your own compost by collect-
ing organic wastes in a backyard bin. For details on
composting, see the website for this chapter.
17-5
RECYCLING
Science: Two Types of Recycling
Recycling is an important way to collect waste
materials and turn them into useful products that can
be sold in the marketplace.
Recycling
involves reprocessing discarded solid materi-
als into new, useful products. Households and work-
places produce five major types of materials that can be
recycled:
paper products
(including newspapers, maga-
zines, office paper, and cardboard),
glass, aluminum,
steel,
and some types of
plastics.
Materials collected for recycling can be repro-
cessed in two ways.
Primary
or
closed-loop recycling
oc-
curs when waste is recycled into new products of the
same type—turning used newspapers into new news-
paper and used aluminum cans into new aluminum
cans, for example.
In
secondary recycling,
also called
downcycling,
waste materials are converted into different products.
For example, used tires can be shredded and turned
into rubberized road surfacing and newspapers can be
reprocessed into cellulose insulation.
Scientists distinguish between two types of wastes
that can be recycled.
Preconsumer
or
internal waste
is
generated in a manufacturing process and recycled in-
stead of being discarded.
Postconsumer
or
external waste
is generated by consumer use of products. There is
about 25 times more preconsumer waste than postcon-
sumer waste. It is important to recycle both types.
In theory, just about anything is recyclable, but
only two issues matter.
First,
will the item actually be
recycled? Sometimes separated wastes collected for re-
cycling are mixed with other wastes and sent to land-
fills or incinerated, mostly when prices for recycled
raw materials fall sharply.
Second,
will businesses and individuals complete
the recycling loop by buying products that are made
from recycled materials? If we do not buy those prod-
ucts, then recycling does not work.
Switzerland and Japan recycle about half of their
MSW. The United States recycles about 30% of its
MSW—up from 6.4% in 1960. This roughly fivefold in-
crease in recycling represents an impressive achieve-
ment but the country's total amount of solid waste has
continued to increase. Studies indicate that with eco-
nomic incentives and better design of waste manage-
ment systems, the United States and other developed
countries could recycle 60-80% of their MSW.
Science: Methods for Recycling Solid Waste
There is disagreement over whether to send mixed
urban wastes to centralized resource recovery plants
or to have individuals sort recyclables for collection
and sale to manufacturers as raw materials.
One recycling approach is to send mixed urban wastes
to a centralized
materials-recovery facility (MRF).
There,
machines or workers separate the mixed waste to re-
cover valuable materials that can be sold to manufac-
turers as raw materials. The remaining paper, plastics,
and other combustible wastes are recycled or burned
to produce steam or electricity to run the recovery
plant or for sale to nearby industries or homes. Ash
from the incinerator is buried in a landfill.
Such plants are expensive to build, operate, and
maintain. They can emit toxic air pollutants, if not op-
erated properly, and they produce a toxic ash that
must be disposed of safely.
MRFs are hungry beasts that require a steady diet
of garbage to make them financially successful. Thus
their owners have a vested interest in increasing
throughput
of matter and energy resources to produce
more trash—the reverse of what prominent scientists
believe we should be doing (Figure 17-3).
To many experts, it makes more economic and en-
vironmental sense for households and businesses to
separate their trash into recyclable categories such as
Science: Composting
Composting biodegradable organic waste
mimics nature by recycling plant nutrients
to the soil.
