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be unregulated and untreated, and are considered “nonpoint” sources of water pollution. Power plants that use water for cooling tend to produce only thermal pollution, which is not treated. Industrial and urban water uses produce what we usually
think of as wastewater—that is, sewage. Sewage is water mixed with human and
industrial waste; common waste components include nutrients like nitrogen and
phosphorous, along with salt, acid, various household and industrial chemicals,
trash & solids, and trace amounts of minerals, metals, and pharmaceuticals.
Many technologies are used for wastewater treatment, depending on the quality
of the sewage and the requirements for quality of the treated wastewater. Wastewater
plants use a large amount of electricity to operate these technologies. A typical
municipal sewage plant will use a linear sequence of:
1. screening solids;
2. coarse grid removal,
3. sedimentation to remove fine solids;
4. aeration to oxidize and degrade organic matter and contamination;
5. clarification; and
6. final disinfection and polishing.
The polishing step can use high technology processes like UV treatment, ozonation, and reverse osmosis to bring the treated wastewater back up to potable water
quality standards if the wastewater will be reused. Solid sludge is produced in large
quantities and needs to be disposed of; often it is taken to landfills, or treated and
used as fertilizer for agriculture owing to its high nitrogen and phosphorous content.
Use of sewage sludge carries many risks of contamination and must be undertaken
with care. Many industrial facilities employ specialized and hazardous chemicals
and employ specialized treatment processes to remove those chemicals. Many
industrial facilities then recycle that wastewater to reduce potable water requirements and avoid the need to obtain wastewater discharge permits. Industries that
operate in cities frequently pre-treat wastewater before discharging it into municipal sewers.
Combined sewer overflow (CSO) sewers are a major infrastructure problem in
world cities. Until late in the twentieth century, wastewater treatment in cities was
uncommon, and wastewater was discharged untreated into rivers. Storm drains in
Paris, London, and New York for hundreds of years doubled as sewers, because
both flows went to the same place—the river. It is difficult to retrofit wastewater
treatment plants onto CSO’s, because wastewater and stormwater are mixed. Where
it is impossible to build separate wastewater collection sewers, infrastructure megaprojects like the tunnels under Chicago are needed to collect, store, and then treat
the combined overflow.
Reclaimed wastewater’s purple pipes are primarily used to supply irrigation
water to golf courses and public parks. The aquatic environment is the primary
receiver of wastewater, and many ecosystems have come to depend on it—including
the so-called “accidental wetlands” that occur at the outlets of wastewater plants.
Reclaimed wastewater is frequently injected into aquifers to maintain their water
balance, as in the water banks that operate in Southern Arizona.
10 Infrastructure
be unregulated and untreated, and are considered “nonpoint” sources of water pollution. Power plants that use water for cooling tend to produce only thermal pollution, which is not treated. Industrial and urban water uses produce what we usually
think of as wastewater—that is, sewage. Sewage is water mixed with human and
industrial waste; common waste components include nutrients like nitrogen and
phosphorous, along with salt, acid, various household and industrial chemicals,
trash & solids, and trace amounts of minerals, metals, and pharmaceuticals.
Many technologies are used for wastewater treatment, depending on the quality
of the sewage and the requirements for quality of the treated wastewater. Wastewater
plants use a large amount of electricity to operate these technologies. A typical
municipal sewage plant will use a linear sequence of:
1. screening solids;
2. coarse grid removal,
3. sedimentation to remove fine solids;
4. aeration to oxidize and degrade organic matter and contamination;
5. clarification; and
6. final disinfection and polishing.
The polishing step can use high technology processes like UV treatment, ozonation, and reverse osmosis to bring the treated wastewater back up to potable water
quality standards if the wastewater will be reused. Solid sludge is produced in large
quantities and needs to be disposed of; often it is taken to landfills, or treated and
used as fertilizer for agriculture owing to its high nitrogen and phosphorous content.
Use of sewage sludge carries many risks of contamination and must be undertaken
with care. Many industrial facilities employ specialized and hazardous chemicals
and employ specialized treatment processes to remove those chemicals. Many
industrial facilities then recycle that wastewater to reduce potable water requirements and avoid the need to obtain wastewater discharge permits. Industries that
operate in cities frequently pre-treat wastewater before discharging it into municipal sewers.
Combined sewer overflow (CSO) sewers are a major infrastructure problem in
world cities. Until late in the twentieth century, wastewater treatment in cities was
uncommon, and wastewater was discharged untreated into rivers. Storm drains in
Paris, London, and New York for hundreds of years doubled as sewers, because
both flows went to the same place—the river. It is difficult to retrofit wastewater
treatment plants onto CSO’s, because wastewater and stormwater are mixed. Where
it is impossible to build separate wastewater collection sewers, infrastructure megaprojects like the tunnels under Chicago are needed to collect, store, and then treat
the combined overflow.
Reclaimed wastewater’s purple pipes are primarily used to supply irrigation
water to golf courses and public parks. The aquatic environment is the primary
receiver of wastewater, and many ecosystems have come to depend on it—including
the so-called “accidental wetlands” that occur at the outlets of wastewater plants.
Reclaimed wastewater is frequently injected into aquifers to maintain their water
balance, as in the water banks that operate in Southern Arizona.
10 Infrastructure