In modern economies, various types of
activity, including agriculture, industry and
transportation, produce a large amount of
wastes and new types of pollutants. Soil, air
and water have traditionally been used as sites
for the disposal of all these wastes. For
example, beef cattle in the United States are
estimated to produce 92 million mt/year of
manure, while dairy cattle produce 27 million
mt/year (Tan 1995). Some of this manure may
wash into nearby streams, and pollute rivers,
lakes and soil.