Open dumping of solid waste in the past caused health and environmental issues like disease, pollution of groundwater and nearby bodies of water, and gas emissions. Modern landfills are carefully engineered with liners and covers to prevent leachate contamination and collect methane gas. Waste is compacted and covered daily, while leachate and gas are treated or used. Hazardous waste requires special secure landfills or deep well injection due to its toxicity, with strict regulations around location and treatment.

1. Pollution

2.  Until the mid-20th century, solid wastes were generally collected and
placed on top of the ground in uncontrolled “open dumps,” which often
became breeding grounds for rats, mosquitoes, flies, and other disease
carriers and were sources of unpleasant odours, windblown debris, and
other nuisances. Dumps can contaminate groundwater as well as pollute
nearby streams and lakes. A highly contaminated liquid called leachate is
generated from decomposition of garbage and precipitation that
infiltrates and percolates downward through the volume of waste
material. When leachate reaches and mixes with groundwater or seeps
into nearby bodies of surface water, public health and environmental
quality are jeopardized. Methane, a poisonous and explosive gas that
easily flows through soil, is an eventual by-product of the anaerobic (in
the absence of oxygen) decomposition of putrescible solid waste
material. Open dumping of solid waste is no longer allowed in many
countries. Nevertheless, leachate and methane from old dumps continue
to cause land pollution problems in some areas.
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3.  A modern technique for land disposal of solid waste involves
construction and daily operation and control of so-called sanitary
landfills. Sanitary landfills are not dumps; they are carefully planned and
engineered facilities designed to control leachate and methane and
minimize the risk of land pollution from solid-waste disposal. Sanitary
landfill sites are carefully selected and prepared with impermeable
bottom liners to collect leachate and prevent contamination of
groundwater. Bottom liners typically consist of flexible plastic
membranes and a layer of compacted clay. The waste material—MSW
and C&D debris—is spread out, compacted with heavy machinery, and
covered each day with a layer of compacted soil. Leachate is collected in
a network of perforated pipes at the bottom of the landfill and pumped
to an on-site treatment plant or nearby public sewerage system.
Methane is also collected in the landfill and safely vented to the
atmosphere or recovered for use as a fuel known as biogas, or landfill
gas. Groundwater-monitoring wells must be placed around the landfill
and sampled periodically to ensure proper landfill operation. Completed
landfills are capped with a layer of clay or an impermeable membrane to
prevent water from entering. A layer of topsoil and various forms of
vegetation are placed as a final cover. Completed landfills are often used
as public parks or playgrounds.

4.  Hazardous waste differs from MSW and C&D debris in both form and behaviour.
Its disposal requires special attention because it can cause serious illnesses or
injuries and can pose immediate and significant threats to environmental quality.
The main characteristics of hazardous waste include toxicity, reactivity,
ignitability, and corrosivity. In addition, waste products that may be infectious or
are radioactive are also classified as hazardous waste. Although land disposal of
hazardous waste is not always the best option, solid or containerized hazardous
wastes can be disposed of by burial in “secure landfills,” while liquid hazardous
waste can be disposed of underground in deep-well injection systems if the
geologic conditions are suitable. Some hazardous wastes such
as dioxins, PCBs, cyanides, halogenated organics, and strong acids are banned
from land disposal in the United States, unless they are first treated or stabilized
or meet certain concentration limits. Secure landfills must have at least 3 metres
(10 feet) of soil between the bottom of the landfill and underlying bedrock
or groundwater table (twice that required for municipal solid-waste landfills), a
final impermeable cover when completed, and a double impervious bottom liner
for increased safety. Underground injection wells (into which liquid waste is
pumped under high pressure) must deposit the liquid in a permeable layer of rock
that is sandwiched between impervious layers of rock or clay. The wells must also
be encased and sealed in three concentric pipes and be at least 400 metres (0.25
mile) from any drinking-water supplies for added safety.