GGRA03H3 – Week 9 – Urban Ecosystems, Pollution
- During the last 15 years, new understandings of cities as complex ecosystems have
o Cities are now seen as complex systems with their own metabolic processes
o They have huge inputs and outputs that are structured by metabolic processes of
production, delivery, consumption, and waste emission.
o Inputs and outputs can be measured and can be influenced by public policy
- It is critical to understand the nature of the inputs and outputs.
- The concept of Urban Metabolism is a model that describes the flow and transformation
of materials and energy within cities.
- Energy, goods, water, food, and materials are imported by cities, transformed
(metabolized), and other products and wastes are exported.
- Urban Metabolism describes the full set of the technical and socio-economic processes
that occur in cities, including the production of energy and disposal of wastes
Political Ecology Perspective on Water
- Political ecology starts with the assumption that cities include multiple metabolic
- Processes of socio-environmental change are never neutral, but are mediated by politics
and power, with particular arrangements providing benefits for some, and costs for
- Rapid urbanization in the 20 century meant a huge growth of demand for water, and
expansion of the water frontier of many cities.
- This has led to growing conflicts over water and waste-water in rich and poor countries.
Water Inputs and Outputs
- Water is one of the most important urban inputs
- Polluted water is one of the main urban outputs
- One of the key achievements of the 19 century ‘sanitary revolution’ was the
development of public water supply and wastewater removal infrastructure
o Sewage Systems
- Initially just pipes to dump the water downstream, later primary treatment, secondary
treatment, tertiary treatment systems were added.
o Primary – Screen filters, settling ponds o Secondary – Active digesting of nutrients with bacteria, aeration.
o Tertiary – treatment for toxic wastes, heavy metals, pharmaceuticals, etc.
Current Water Challenges
- Are partly a consequence of dominant sewer technology
- Combined sanitary and stormwater sewer systems used one set of pipes for all
wastewater in cities
- Works well in dry weather, but heavy rain events routinely overload the system and
water is released into streams, rivers, lakes
o When snow melts, it creates large bodies of water that flows into streams, rivers,
- Millions of gallons of untreated waste into water bodies.
- Gradually, cities are rebuilding systems to separate sanitary and storm sewers.
- And storage for stormwater surges.
Key Concept 1
- Cities are now seen as ecosystems, with inputs, metabolic processes, outputs
o They have huge inputs and outputs, that are structured by metabolic processes
of production, delivery, consumption, and waste emission
o Inputs and outputs can be measured and can be influenced by public policy.
o In 20 century, a big issue was water supply and waste water management, in
terms of quantity and quality.
o Now key policy issues relate to managing flows through cities, how inputs are
transformed and quality of outputs.
Current Water Challenges 2
- Point-source and non point-source
- Point sources are those where there is a clear point of discharge
o E.g. Factories, sewers, etc. with a pipe that discharges liquid wastes
- These are relatively easy to monitor, and there has been considerable progress in
- Non point-source pollution is everything else, including pesticides and fertilizers on
lawns, dog feces, and pollution on roads (antifreeze, brake fluid, gasoline, tire rubber),
particulate pollution that settles from air
o Cigarette butts are also an example
Current Water Challenges 3 - Non-point source pollutants include wastes dumped into sewer systems where the
source cannot be identified
- What besides human wastes get dumped into sewers?
- Use of municipal sewage sludge for fertilizer spread rapidly since the 1990s (as
alternative to landfill and incineration)
- But includes volatiles, organic solids, nutrients, disease-causing pathogenic organisms
(bacteria, viruses, etc.), heavy metals and inorganic ions, and toxic organic chemicals
- Problems with drugs – 70-90% not metabolized, endocrine disruptors.
Rain as Pollution
- Why would rain runoff be considered pollution?
- Problems of hard surfaces in cities, and greatly increased storm water surges.
- Overloaded treatment plants, flooding of urban rivers and creeks
- Toxic chemicals on roads
o Gasoline, antifreeze, salt, etc.
Get washed into sewers with rains or nearby creeks
- Major investments are made to reduce storm water surges
o Retention ponds, buffers along streams, green roofs, disconnect downspouts,
Key Concept 2
- Water pollution is a major concern for urban ecology
o In most of the developed countries, the focus of policy has shifted from point-
source pollution (fairly successfully regulated) to non-point source emissions
o Non point-source pollution includes pesticides, herbicides, dog shit,