Water Pollution

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Water Pollution
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Causes of water pollution

• Livestock waste
• Oil spillages
• Residues of pesticides
• Detergents in sewage
• Industrial effluents

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Livestock waste

• When there is an excessive amount of organic
  matter discharged into water, the existing
  micro-organisms will grow and multiply rapidly.
  Oxygen is depleted rapidly. Anaerobic
  micro-organisms begin to break down partially the
  organic matter releasing toxic gases like CH4 ,
  NH3 and H2S. The aquatic organisms will die.

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Oil spillages

• Oil clogs the feathers of sea birds and kills
  marine life. On beaches, oil drives away
  holiday-makers and spoils recreation resorts.

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Residues of pasticides

• Pesticides cause immediate toxic effects on
  aquatic life or may accumulate along the food
  chain until a toxic level is reached.
• DDT was previously used and banned in many parts
  of the world including Hong Kong.

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Detergents

• Red tides brought about by phosphates in some
  detergents.
• Foaming in rivers and lakes.

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Industrial effluents

• Vary with the industry involved.
• May contain Cd, Cr, Cu, Hg, Ni, Pb, Zn, cyanides,
  polychlorinated hydrocarbons.

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Dissolved oxygen (DO)

• As an indicator of oxygen content in water.
• Measured in mg / dm3 or saturation
• e.g. Max. conc. 10 mg dm-3 at 16oC, if a water
  sample contains 6 mg dm-3 , the saturation of
  DO is 60.
• Fishes normally require 4-6 mg dm-3 for survival.

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Biological oxygen demand(BOD)

• The amount of oxygen required to break down the
  organic matter present in a water sample.
• Five-day BOD, (BOD5)
• A sample has an initial DO of 8 mg dm-3 drops to
  2 mg dm-3 in 5 days, BOD5 6 mg dm-3
• Natural clean water has a BOD5 of about 1 to 4 mg
  dm-3

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Measures to improve water quality

• Sewage treatment
• Only part of the wastewater receives some sort of
  treatment before discharge.
• The treatment processes include
• Primary treatment screening, sedimentation
• Secondary treatment digestion of pollutants by
  micro-organisms.
• In 1995, 5 major secondary treatment plants in
  H.K., treat 20-25 of the total sewage.

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Measures to improve water quality

• Chemical Waste Treatment Centre(CWTC)
• Physical methods
• Chemical methods
• Neutralization of acids and bases
• Precipitation reactions
• Reduction reactions
• Incineration

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Sewage Treatment link