Recycling involves reprocessing discarded solid materials
into new, useful products. In addition to saving
resources and reducing solid waste and pollution, recycling
also reduces unsightly and environmentally
harmful litter.
Households and workplaces produce five major
types of materials that can be recycled: paper products,
glass, aluminum, steel, and some plastics. Such materials
can be reprocessed in two ways. In primary or
closed-loop recycling, these materials are recycled
into new products of the same type—turning used aluminum
cans into new aluminum cans, for example. In
secondary recycling, waste materials are converted
into different products. For example, used tires can be
shredded and turned into rubberized road surfacing,
newspapers can be reprocessed into cellulose insulation,
and plastics can be reprocessed into various items.
Engineer Henry Liu has developed a process for making
bricks from recycled fly ash produced by coal-burning
power plants. The process saves energy, reduces air
pollution, and costs at least 20% less than the cost of
making conventional bricks.
Scientists distinguish between two types of wastes
that can be recycled: preconsumer or internal waste generated
in a manufacturing process and postconsumer or
external waste generated by consumer use of products.
Preconsumer waste makes up more than three-fourths
of the total.