Water Pollution

  Water Pollution Definition


Water pollution means such contamination of water or such alteration of the physical, chemical or biological properties of water or such discharge of any sewage or trade effluent or of any other liquid, gaseous or solid substance into water (whether directly or indirectly) as may, or is likely to, create a nuisance or render such water harmful or injurious to public health or safety, or to domestic, commercial, industrial, agricultural or other legitimate uses, or to the life and health of animals or plants or of aquatic organisers


Any change or modification in the physical, chemical and biological properties of water that will have a detrimental consequence on living things is water pollution.


  Types of Water Pollution

  Surface Water Pollution


If a river get self purification time, it means if till 1-2 km there is on further addition of contaminant, a river can purify itself through aeration and settlement. Also the vegetation and soil takes nutrients from wstewater and purify water.

  Major Water Pollutants

  Major Water Pollutant Sources

  Major Water Borne Diseases

  Oxygen Demanding Waste (ODW)

ODW are the substances that oxidize in the receiving water bodies, as bacteria decomposes this waste they utilize dissolved oxygen of water.

Impact of reduce Dissolved Oxygen (DO)

Maximum Dissolved Oxygen content in water is 8 – 15 ppm. The oxygen dissolving rate in water is mainly governed by temperature, at higher temperation the DO decreases. Fishes need minimum 5 ppm, if it drops fishes suffocates and die. Moreover undesirable taste, odour, and colour of water renders it unfit for domestic and recreational uses.

ODW are generally biodegradable organic substances however the oxidation of certain inorganic compounds can also contribute to oxygen demand. Naturally occurring leaves, animal dropping can also deplete oxygen.

The organic pollution in water is measured by parameter called Biochemical/Biological Oxygen Demand (BOD)

BOD (measure the strength of organic pollution) and Chemical Oxygen Demand (COD)

Chemical Oxygen Demand (COD) measures all organic compounds that can be chemically oxidized.

Biochemical Oxygen Demand (BOD), on the other hand, measures the dissolved oxygen consumed by organisms to oxidize organic compounds.

In other words, BOD measures only the organic compounds that can be consumed by organisms while COD measures [almost] all. Hence, COD should always be higher than BOD


Nutrients  are chemicals essential for growth. N, P, C, S, Ca, K, Fe, Mn, B, Co etc. They are considered pollutant when their concentration are sufficient to allow excessive growth of aquatic plants particularly algae

Nutrient enrichment can cause algal bloom; the excessive growth of algae which blocks sunlight and also reduces DO. Can further reduce DO due to decomposition of dead algae. Decaying organic matter add colour, turbidity, odour and objectionable taste to water

Nutrient enrichment is known as Eutrophication. Water soluble nitrates and phosphates that cause excessive growth of algae and other aquatic plants. The excessive growth of algae and aquatic plants due to added nutrients is called eutrophication. They may interfere with the use of the water by clogging water intake pipes, changing the taste and odour of water and cause a buildup of organic matter. As the organic matter decays, oxygen levels decrease and fish and other aquatic species die.

Aquatic species require nutrient most importantly C, N, P in large quantity, else the growth of plant will be limited. The nutrient that is least available relative to the plant’s needs is called limiting nutrient. Thus, algal growth can be restricted by restricting the supply of particular nutrient.

Carbon is mostly available, usually N, P can become limiting nutrients.

N in sea water and P in fresh water Nitrate (non-toxic) can be converted into nitrite (highly toxic) by bacteria found in intestinal tract of infants (6 months). Haemoglobin has higher affinity for nitrite than oxygen, and when oxygen is replaced by nitrite, condition is called Methemoglobinemia or “Blue baby syndrome” It cause bluish discoloration of the infants, in extreme cases victim may die from suffocation Phosphorus is not much available thus can become a limiting nutrient in lakes and rivers however; Detergents, agricultural runoff and sewage can add excess of phosphorus.


Salts are the dissolved solids present in water. Example cations Na, Ca, Mg, K and anions Chloride, sulphate, bicarbonate etc.
Industries, urban runoff can increase the amount of salts in water. Irrigated agriculture in arid areas is vulnerable to salt accumulation due to high evapotranspiration. It does not happen in rainfed irrigation because the rains are the evaporated water from water bodies which get condensed and thus no salt content. The fresh water has lot of dissolved salts; the salts remain in field when water get evaporated and cause soil salinity.

In rivers, that are heavily used for irrigation the salt concentration increases downstreams as the volume of water to dissolve same amount of salts decreases

Salts can be directly measured by measuring the TDS of water sample

  Thermal pollution

Thermal water pollution is the rise or fall in water temperatures. This changes in the temperature of water can be caused due to industries.

Some industries use water as cooling agent, the heated water is let-off directly into the natural environment at a higher temperature. Cold water pollution happens when cold water is released into the water bodies. Aquatic organisms like fish are vulnerable to slight changes in the temperature.

Heated water decreases oxygen in the water killing fish and other aquatic organisms. Cold water affects eggs and larvae, some invertebrates of the aquatic ecosystem

  • The hot water released from the thermal power plants and some other industrial operations
  • As water temperature increases, aquatic life is affected. Firstly, the metabolic rates increases with temperature (by factor of 2 for every 10 degree rise in temp), this cause increase in oxygen demand. At the same time at higher temperature the available DO goes down.

  Heavy Metals

Metals (total 80) having specific gravity greater than 4 or 5 are called Heavy Metals
Important heavy metals are Hg, Pb, Cd, As, Cd, Co, Fe etc.

Cr, Fe are essential but higher concentration can damage nervous system, kidney etc
Liquid Hg is not very toxic but Hg vapors are highly toxic. As it enter lungs get diffused in blood and cause serious damage to CNS. In contrast, Pb not toxic in vapour form but very dangerous in ionic form Pb+2 can damage kidney and brain and can be passed from mother to foetus. Example aerosol from tetra ethyl lead in gasoline
Kidney eliminate toxins from human body through Nephrons. Cd, Pb, Hg are known as Nephrotoxins (toxic to kidneys)



Pesticides are used by farmers to protect crops from insects and bacterias. Insecticides, herbicides, rodenticides, fungicides etc. Main category of pesticides are:

Organochlorines/ Chlorinated Hydrocarbons

Dichlorodiphenyltrichloroethane (DDT)

DDT was widely used for control of insects (malaria, plague).Its human toxicity is very less but its impact on food chain led to its ban in developed nations

DDT disruptive to food chain

  • Very persistent
  • Soluble in hydrocarbon solvents (easily accumulated in fatty tissues). Concentration keep on increasing at higher trophic levels(Biomagnification). In birds the concentration was very high where DDT/DDE interfere with the enzyme that regulate calcium distribution in birds resulting in egg shells that are too thin to support the weight of the parent, thus difficulty in reproduction
  • Widely used Organochlorines are Aldrin, dieldrin endosulfan, methoxychlor, heptachlor etc.

Organophosphates (parathion, malathion, diazinon etc)

Not persistent but acutely toxic to humans, they are readily absorbed through skin, lungs and gastrointestinal tract

If exposed, can cause tremor, slurred speech, convulsions impaired memory, disorientation, severe depressions, irritability, confusion, headache, speech difficulties etc


A carbamate is an organic compound derived from carbamic acid (NH2COOH). Short lived but high human toxicity. Examples propoxur, carbaryl, aldicarb

Impacts-nausea, vomiting, blurred vision, convulsion and death

Chlorinated HC also used as herbicides, Chlorophenoxy compounds 2,4,5 – T and 2,4 – D (2,4-Dichlorophenoxyacetic acid) most common called as Agent Orange. Kills broad leafed plants without harming grasses. Used to control aquatic plants in lakes

2,4,5 – T ban as it produce a highly toxic side product called Dioxins during its manufacture. It is carcinogenic (cancer causing) in nature.

  Volatile Organic Chemicals (VOCs)

Volatile Organic Chemicals (VOCs) are carbon-containing compounds that evaporate easily from water into air at normal air temperatures

Carcinogenic (cancer causing) and mutagenic (modify genes thus genetic effects)

Special concern VOC in drinking water. Most common are:

  • Vinyl Chloride (carcinogen PVC)
  • Trichloroethylene (solvent cleaning electronics/jet engine/septic tanks)
  • Tetrachloroethylene (heat transfer medium in CFC manufacturing) most common out of 5
  • Carbon tetrachloride (solvent, fire extinguisher, grain fumigant)
  • 1,2 Dichloroethane (metal degreaser in manufacturing of fumigants, varnish remover, soap compounds etc)