Environmental Issues - Notes | Class 12 | Part 2: Water Pollution and its Control



Domestic Sewage and Industrial Effluents 

0.1 % impurities make domestic sewage unfit for human use. They include
  • Suspended solids: Sand, silt, clay etc.
  • Colloidal materials: Faecal matter, bacteria, cloth, paper fibres etc.
  • Dissolved materials: Nutrients like nitrate, NH3, phosphate, Na, Ca etc.
Removal of dissolved materials, organic compounds and toxic metal ions are most difficult.

Domestic sewage contains biodegradable organic matter. It is decomposed by microorganisms.

The amount of biodegradable organic matter in sewage water is estimated by measuring Biochemical Oxygen Demand (BOD).

During biodegradation, microbes consume O2. It results in a sharp decline in dissolved O2. This causes death of aquatic organisms. 

Effect of sewage discharge on some important characteristics of a river 

Presence of more nutrients in water causes excess growth of planktonic algae (algal bloom). It imparts a distinct colour to the water bodies and deteriorates the water quality resulting in death of fishes. Some bloom-forming algae are extremely toxic to human beings and animals.

Water hyacinth (Eichhornia crassipes) is the most problematic aquatic weed (‘Terror of Bengal’). They grow abundantly in eutrophic water bodies.

Sewage from homes & hospitals contain pathogens that cause dysentery, typhoid, jaundice, cholera, etc.

Industrial waste water contains toxic substances like DDT, heavy metals (mercury, cadmium, copper, lead, etc.) and organic compounds.

Biological magnification (Biomagnification) 

It is the accumulation of the toxicant (mercury, DDT etc.) at successive trophic levels of a food chain.

Organisms cannot metabolize or excrete the toxicant. So, it is passed on to the next trophic level.

Biomagnification of DDT in an aquatic food chain:

Water (DDT: 0.003 ppb) → zooplankton (0.04 ppm) → small fish (0.5 ppm) → large fish (2 ppm) → birds (25 ppm).

DDT disturbs calcium metabolism in birds, which causes thinning of eggshell and their premature breaking. It causes decline in bird populations.


It is the natural aging of a lake by nutrient enrichment.

In a young lake, water is cold and clear supporting little life. With time, streams draining into the lake introduce nutrients (N2, P etc.). It increases lake’s fertility.

Thus plants & animals grow rapidly, and organic remains are deposited on the lake bottom. So, the lake grows shallower and warmer, with warm-water organisms.

Marsh plants take root in the shallows and fill in the original lake basin. Eventually, the lake becomes land.

Depending on climate, size of the lake and other factors, the eutrophication may span thousands of years. However, pollutants like effluents from industries and homes accelerate eutrophication. This phenomenon is called Cultural or Accelerated Eutrophication.

The prime contaminants are nitrates & phosphates. They overstimulate the growth of algae. It causes unsightly scum and unpleasant odors, and robs the water of dissolved oxygen. It leads to death of other organisms.

Heated (thermal) wastewater from electricity-generating units (e.g. thermal power plants) eliminates organisms sensitive to high temperature. It may enhance the growth of plants and fish in extremely cold areas but, only after causing damage to the indigenous flora and fauna.

Integrated Waste Water Treatment 

It includes artificial and natural processes.

The townspeople of Arcata (northern coast of California) and biologists from the Humboldt State University created an integrated waste water treatment process. The cleaning occurs in 2 stages:
  • Sedimentation, filtering & chlorine treatments. After this, remaining pollutants like dissolved heavy metals were removed using an innovative approach.
  • Biologists developed a series of six connected marshes over 60 hectares of marshland. Appropriate plants, algae, fungi & bacteria were seeded into this area. They neutralize, absorb & assimilate pollutants. Thus, as the water flows through marshes, it gets purified naturally.
Friends of the Arcata Marsh (FOAM) is a citizens group for the upkeep and safeguarding of this project.

Ecological sanitation 

It is a sustainable system for handling human excreta, using dry composting toilets.

This is a practical, hygienic, efficient and cost-effective solution to human waste disposal.

Human excreta can be recycled into a resource (as natural fertilizer). It reduces the need for chemical fertilizers.

There are ‘EcoSan’ toilets in Kerala & Sri Lanka.

Government of India has passed the Water (Prevention & Control of Pollution) Act, 1974 to safeguard water resources.
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