Chapter 4: water and its management

• How is water distributed over the Earth?

  • 97% of the Earth's water is saline and 3% freshwater
  • 68.7% of fresh water stored in ice caps and glaciers, 30.1% in ground water, 0.3 surface water and 0.9% in other sources
  • 87% of surface water is found in lakes, 11% in swamps and 2% in rivers
• The water cycle:

  • Precipitation
  • Interception
  • Through Flow
  • Surface run-off
  • Evaporation
  • Transpiration
  • Evaporation
  • Condensation to form clouds
• Why humans need water?

  Domestic use:

  * Home use for drinking, cooking and washing.

  * watering

  Industrial:

  * Mainly used for cooling in the production of electricity

  Agricultural:

  * By far the greatest use of water in agricultural is irrigation
• Main sources of freshwater for human use:

  • Water from rivers
  • Water from the ground
  • Water from the sea
• What may dams help with?

  * The generation of electricity in hydroelectric power plants

  * Flood control

  * Irrigation

  * Tourism and leisure

  * The provision of water

  * Creation of habitat for wetland species

  * Access by boat to otherwise inaccessible areas
• Disadvantages of dams:


  * Relocating people

  * Flooding land

  * Disrupting the life cycles of fish and other aquatic organisms

  * Altering the water supply for people downstream of the dam

  * Reducing the enrichment of soil downstream of the dam (which natural flooding of the original river course would have contributed to)

  * The dam may become redundant as sediment in the river sink to the bottom of the reservoir (siltation)
• Causes of water pollution:

  • Sewage treatment
  • Water treatment
  • Toxic compounds from industry
  • Acid rain
  • Eutrophication
  • Pesticide and herbicide pollution
• Sewage treatment


  1. Sewage outfall

  2. Screening tank

  3. First settling tank

  4. Oxidation pond (aerobic bacteria)

  5. Second settling tank

  6. Effluent river

  7. Sludge digester
• Sewage outfall


  Waste from homes and industries are taken to treatment plant
• Screening tank


  Large objects are removed
• First settling tank


  Solid organic waste is allowed to settle at the bottom of a tank, called sludge
• Oxidation pond (aerobic bacteria)


  Water is taken to a tank where oxygen is bubbled through, it encourages bacteria to grow that dissolves organic matter called BOD
• Second settling tank


  Bacteria settles at the bottom forming more sludge, the cleaner water overflows the sides as effluent
• Effluent river


  Effluent is discharged
• Sludge digester


  Oxygen-free conditions are created that encourage the growth of bacteria which can break down the sludge, releasing methane which can burn
• The treated sludge can be dried in sludge lagoons and used as organic fertilisers on farmland
• Water treatment process:

  1. Raw non-potable water
  2. Mixing and coagulation which causes particles to clump together
  3. Sedimentation causes clumped particles to settle out
  4. Filtration through sand removes particles
  5. Disinfection with chlorine kills bacteria
  6. Storage
• How eutrophication occurs:

  1. Nutrients enter water
  2. Stimulate algal growth, an algal bloom
  3. Algae dies
  4. Bacteria decomposes dead algae, increasing the organic matter
  5. Oxygen used up and level is lowered
  6. Fish and many other aquatic animals, including insect larvae, dies