Pollution - Environmental Science - page 161

by microbes. The remnants left in the tank undergo decomposition as well. If the tank gets full, it is pumped
and the contents taken to a landfill for disposal. Septic systems are usually found in rural areas, although as
more remote locations have been developed, septic systems can now be found in areas considered to be urban
or suburban.
In a municipal sewer system, wastewater is taken by pipes from local homes and businesses and sent to a
central treatment plant where it undergoes clarification processes. During primary treatment suspended solids
are physically removed in settling tanks. The wastewater then goes through secondary treatment where oxy-
gen enters the water from continual mixing and movement, encouraging aerobic decomposition. By the end of
this process, the majority of suspended solids have been removed. Some treatment facilities also use a third
step, or a tertiary treatment, where there is additional filtration of the water. The final step is the treatment of
the clarified water with UV light treatment or chlorine to kill bacteria. The water is then discharged into a river
or an ocean. The solids (sludge) that were extracted are placed in large tanks where decomposition occurs.
When the solids dry, the remainder is incinerated, sent to a landfill, or used as fertilizer for crops. Unfortu-
nately, in many developing countries, the bulk of domestic and industrial wastewater is discharged without any
treatment, or with only limited primary treatment. Without adequate sanitation systems, ecosystems can be des-
troyed and human populations threatened.
Solid Waste
Soil can be polluted through industrial waste, agricultural runoff, acid precipitation, underground storage tanks,
and radioactive fallout. The most prominent contaminants are heavy metals, petroleum hydrocarbons, solvents,
and pesticides. For management purposes, solid waste is considered one of four types:
• Hazardous waste is flammable, corrosive, toxic, or reactive.
• Industrial waste is created during industrial processes such as agriculture, mining, consumer goods pro-
duction, and the extraction and refining of petroleum products.
• Municipal solid waste comes from homes, business, schools, hospitals, and other types of institutions.
• Also classified as waste is wastewater , which is post-consumer water that is flushed or goes down the
drain, or water that runs into sewers from streets.
Disposal
Waste material from industry, municipalities, mining, agriculture, and medical operations requires disposal in a
way that does not contaminate the soil. These solid wastes include, but are certainly not limited to, tailings and
overburden from mining processes, agricultural remnants, medical wastes (biohazards), and radioactive and
toxic substances, each of which requires its own protocol for disposal. Landfills, open dumps, or incineration
facilities are often the final destination for these materials.
Landfills are areas where solid waste is disposed of by being buried in the ground or piled in a mound. Once a
landfill is full, it is capped, or covered. In the United States, landfill regulations help to protect human health,
wildlife, and ecosystems. For example, landfills must be located away from wetlands and cannot be built on an
earthquake fault. Also, the bottoms of landfills are lined with plastic and clay to prevent leakage into the envir-
onment. Waste in a landfill experiences aerobic and anaerobic decomposition, allowing some of the waste to
break down.
In some locations, solid waste is incinerated, or burned, at high temperatures in facilities built for this purpose.
Prior to incineration, metals are removed for recycling. The ash that remains after incineration is then sent to a
landfill. This process reduces the mass and volume of waste being placed in a landfill. Unfortunately, incinera-