Class 7 Biology Forests: Our Lifeline Water pollution and its control

Water pollution and its control

  • Any undesirable change in the physical, chemical or biological characteristics of water is called water pollution.
  • Sewage is a domestic waste contains biodegradable organic matter, nitrates, phosphates, and other nutrients, and toxic metal ions.
  • The amount of organic matter in sewage water by measuring Biochemical Oxygen Demand (BOD).
  • Biochemical oxygen demand is the amount of oxygen required by aerobic microorganisms to break down all organic matter.
  • Harmful effects of water pollution
  • Micro-organisms involved in biodegradation of organic matter in the receiving water body consume a lot of oxygen, and as a result there is a sharp decline in dissolved oxygen downstream from the point of sewage discharge which causes mortality of fish and other aquatic creatures.
  • Sewage from our homes as well from hospitals contains many undesirable pathogenic microorganisms which can cause diseases like dysentery, typhoid, jaundice, cholera.

Causes of water pollution

  • Presence of large amounts of nutrients in waters also causes excessive growth of planktonic (free-floating) algae, called an algal bloom.


Fig. algal bloom 

  • Algal blooms cause deterioration of the water quality and fish mortality and some bloom-forming algae are toxic to human beings and animals.
  • Water hyacinth (Eichhorniacrassipes) called as ‘terror of Bengal’ is the most problematic water weed which grow abundantly in eutrophic water bodies, and lead to an imbalance in the water ecosystem.


Fig. water hyacinth 

  • Biomagnification
  • It is the increase in concentration of the toxicant at successive trophic levels.
  • Biomagnification happens because a toxic substance accumulated by an organism cannot be metabolized or excreted, and is thus passed on to the next higher trophic level.
  • Example of toxic substances- mercury, DDT.
  • Eutrophication
  • Eutrophication is the natural aging of a lake by biological enrichment of its water.
  • Natural eutrophication-
  1. Streams draining into the lake introduce nutrients such as nitrogen and phosphorus due to which aquatic organisms grow.
  2. As silt and organic debris pile up, the lake grows shallower and warmer, with warm-water organisms supplanting those that thrive in a cold environment.
  • Marsh plants take root in the shallows and begin to fill in the original lake basin.
  1. Lake gives way to large masses of floating plants (bog), finally converting into land.
  • Cultural or Accelerated Eutrophication
  1. Pollutants from man’s activities like effluents from the industries and homes can radically accelerate the aging process and the process is called as Cultural or Accelerated Eutrophication.
  2. It causes by
  • Sewage, agricultural and industrial wastes.
  1. Nitrates and phosphates, which act as plant nutrients.
  • Sewage and plant nutrients overstimulate the growth of algae, causing unsightly scum and unpleasant odors, and robbing the dissolved oxygen from water, and other pollutants flowing into a lake may poison whole populations of fish, whose decomposing remains further deplete the water’s dissolved oxygen content and thus a lake can choke to death.
  • Thermal wastewaters flowing out of electricity-generating units are another cause of pollution.
  • Thermal wastewater can eliminate the number of organisms sensitive to high temperature and also can enhance the growth of plants and fish in extremely cold areas but only after causing damage to the indigenous flora and fauna.

Control of water pollution

  • The cleaning of waste water occurs in two stages –
  1. The conventional sedimentation, filtering and chlorine treatments are given.
  2. The biologists developed a series of six connected marshes over 60 hectares of marshland.
  • Plants, algae, fungi and bacteria were seeded into this area, which neutralize, absorb and assimilate the pollutants.
  • The water flows through the marshes, it gets purified naturally.
  • A citizens group called Friends of the Arcata Marsh (FOAM) is responsible for the upkeep and safeguarding of this wonderful project.
  • Ecological sanitation is a sustainable system for handling human excreta, using dry composting toilets.
  • With the help of ecological sanitation human excreta can be recycled into a resource (as natural fertilizer).

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