2.2 The Water Cycle and Oxygen-Demanding Waste

properties of water

composition of water molecules

  • water molecules are cohesive, meaning that there are strong forces of attraction between molecules of water

water will remain a liquid over a wide variety of temperatures

  • it has a high boiling point and a low freezing point
  • it also has a high heat capacity, so liquid water changes temperature very slowly; it can store large amounts of heat without a change in temperature
  • it has a high heat of vaporization, so it takes a lot of heat to evaporate liquid water
  • water is the universal solvent and can dissolve a variety of compounds (eg. nutrients, waste, pollutants)
  • has high surface tension, meaning that its surface behaves like a membrane (its surface is often similar to that of a solid)
  • ice is less dense than liquid water — water expands when it freezes

causes for concern with water

  • too much (flooding, eg. Pakistan)
  • too little (droughts, eg. California)
  • too poor of quality (contaminated, eg. Flint)

the water cycle

  • the vast majority of water on earth is saltwater; of the freshwater, most of it is located in glaciers/permanent snow cover and groundwater

oxygen-demanding waste

  • oxygen demanding waste: organic waste that can be decomposed by aerobic bacteria
    • aerobic bacteria: bacteria that use oxygen
    • because the bacteria use so much oxygen, the level of oxygen dissolved (DO) in the water decreases
    • fish and other animals need high levels of DO to survive
  • origins of oxygen demanding waste
    • main cause — failing infrastructure
    • eg. leaking pipes, failing/overflowing sewage systems
    • results in the release of oxygen demanding waste, eg. raw sewage, manure, decomposing algae, paper factories, meat-processing plants

streams

  • stream: a body of water that flows
    • eg. rivers, creeks
  • usually recover quickly from oxygen demanding waste, but if they are overloaded, the breakdown of degradable waste by bacteria depletes DO and produces an oxygen sag
    • oxygen sag: the reduction in dissolved oxygen plotted over a distance along a water body from a point at which sewage or other pollutants have been discharged
  • biochemical oxygen demand (BOD): the amount of oxygen consumed by bacteria while they decompose organic matter

sediment

  • a pollutant
  • sediment/suspended matter: insoluble soil particles which make water turbid
    • turbid: cloudy, opaque, or thick with suspended matter

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