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Water Pollutants: Definition, Types, Sources, Effects, Practice Problems and FAQs

Water Pollutants: Definition, Types, Sources, Effects, Practice Problems and FAQs

How relaxing is it to be at a sea shore and walk for hours at the beach? Those who have had this experience can vouch for how gratifying it is. But what if I tell you that no matter how serene and aesthetic the sea or ocean looks from outside, its depths are often filled with unwanted and harmful wastes which have been dumped by humans.

How do you feel when you pass a stinky pool that has been dumped with garbage? The answer remains obvious, “very bad”. Such places are not only filled with foul odour, but are also breeding places for germs and pests. What do you think is the reason behind such poor condition of these water bodies? The prime reason is the high degree of pollution in these water bodies. The pools and ponds that are dumped with garbage, the seas or oceans, our rivers, etc have lots of unwanted elements which result in the degradation of water quality and hence these are designated as polluted water bodies. The substances polluting the water bodies can range from simple household wastes to harmful wastes dumped by factories and industries. Let’s go through the details of water pollution and the pollutants.

Table of contents

  • Definition
  • Types of water pollutants
  • Sources of water pollutants
  • Effects of water pollutants
  • Practice Problems
  • FAQs


The harmful and unnecessary substances like chemicals, organic wastes, pathogenic microorganisms, which when discharged in the water bodies like ponds, rivers, streams, lakes, oceans or any other water body, lead to a degradation in the water quality making it unfit for use are collectively known as water pollutants.

Sources of water pollutants

Pollutants contaminating water can come from either a single point source or from dispersed sources.

A point source refers to a single point which may be a pipe or channel dedicated for discharge of wastes into the water bodies. For example: Pipe discharging treated wastes from an industrial area or a city sewerage system into the rivers.

A dispersed (also called as non-point) source is a huge unconfined area from which a wide variety of pollutants get discharged into the water bodies. Examples include surface runoff from an agricultural plot.

Types of water pollutants

Water pollutants are of several types. These include

  • Putrescible organic wastes and pathogenic microorganisms which are released from domestic households
  • Chemicals like fertilisers and pesticides from agricultural lands
  • Toxic industrial wastes such as carcinogens, poisons, radioactive substances, heavy metals
  • Sediments
  • Petroleum products
  • Hot water and industrial effluents

Please enter alt text
                                                            Fig: Type of water pollutants

The broad categories of impurities based on their states are tabulated as follows:



Suspended solids

Sand, silt and clay

Suspended liquids

Oil and petroleum

Colloidal materials

Cloth, paper fibres, bacteria, faecal matter

Dissolved materials

Nutrients or chemicals containing nitrates, phosphates, ammonia, sodium, calcium etc.

Some of the common types of water pollutants have been discussed below:

Domestic wastes

Domestic wastes can be solid wastes such as paper bags, wrappers, plastic bottles, organic kitchen wastes such as vegetable peels, leftover food, etc.

                                                               Fig: Solid domestic wastes

It can also include wastewater coming from the toilets, washing machines, kitchen, etc which we collectively call as domestic sewage. Domestic sewage mainly contains organic matter which are biodegradable in nature. The sewage comprises faecal matter, animal wastes, dissolved organic compounds like urea, carbohydrates etc. as well as nitrates and phosphates of detergents. These can be naturally broken down by microbial treatment, however, up to a certain limit, beyond which accumulation occurs. Also, sewage wastes contain pathogenic microbes like bacteria and viruses which are a threat to human health.

                                                        Fig: Domestic sewage

Industrial wastes

The wastes produced by several manufacturing units of industries pollute the water bodies when discharged into the same. The industries releasing wastes in the form of effluents include fertiliser factories, petro-chemical industries, oil refineries, textile factories, steel mills, paper mills, plastic manufacturing units etc. The industrial wastes which act as water pollutants include detergents, metals like lead, mercury etc., petroleum, acids, phenols, alkalis, cyanides, arsenic, chlorine, radioactive wastes, etc.

                                             Fig: Industrial effluents

Thermal wastes

Majority of the industries use water as a coolant. The resulting warm water generated is discharged in the water bodies. This is one of the major sources of heat. The water used as a coolant, when discharged in the lakes or streams, may be even 15 °C warmer than the water present in the water body. Coal- or oil-fired generators as well as atomic energy plants generate excess heat which is carried away by water. This hot water is a source of thermal pollution.

                                Fig: Sources of thermal pollution of water bodies


Majority of the agricultural practices, construction activities, downpour with resultant floods, poor soil conservation practices etc. increase the abundance of silt in water bodies like streams and lakes. These are also potential water pollutants.

                                         Fig: Silt deposition in water bodies


Agriculture and crop production involves the use of agrochemicals such as fertilisers, herbicides and pesticides. All these chemicals are water pollutants which come into the water bodies as the overflowing water from the surface of crop fields get discharged. These chemicals even percolate deep inside the soil reaching the groundwater and contaminating the same.

                                    Fig: Applying agrochemicals in agricultural lands


The release of hydrocarbons such as oil and petroleum occurs in the oceans due to mining activities. The ships transporting oil or petroleum products across the ocean also undergo leakage due to inappropriate maintenance. This results in leakage of oil in the oceans. Oil and petroleum don’t mix with water, hence, form a thick sludge there.

                                                             Fig: Oil spill


Different types of water pollutants have different effects on the environment and human health, although the roles of all of them are negative.

Domestic sewage contains a lot of organic matter along with pathogenic microbes. These can cause a wide variety of waterborne diseases like typhoid, dysentery, cholera, etc.

                                                    Fig: Water borne diseases

Additionally, the oxidation of organic wastes by microorganisms requires oxygen. Excess of sewage in water bodies requires a lot of oxygen for its oxidation, causing deficiency of oxygen available to the aquatic organisms. This may lead to the death of aquatic life forms. Dead and decaying plants further produce toxic substances in water which are potentially fatal for consumption.

                     Fig: Concentration of dissolved oxygen reduces in water polluted with sewage

Excessive dissolved nutrients like nitrates and phosphates in water bodies cause havoc growth of planktonic algae, which soon form a blanket over the water bodies. This phenomenon is termed as algal bloom. Bloom of blue-green algae causes unpleasant taste and odour while bloom of dinoflagellates produce toxic metabolites which are fatal for fishes.

                                                                 Fig: Algal bloom

Eutrophication is the natural ageing of lakes which occurs as their nutrient content increases with time and eventually leads to an increase in the number of aquatic organisms that they support. As the organic content of the lakes increase, they become shallower, warmer, get covered with floating plants and eventually turn into lands. Increased addition of agricultural, industrial and domestic wastes into the lakes increases the concentration of nitrates and phosphates in the water and accelerates the process of eutrophication. Nitrates and phosphates are nutrients for plants and lead to an increased growth of algae which eventually results in formation of scum on the surface of the water body, unpleasant odours, reduced dissolved oxygen concentration, and can eventually choke the lake to death.

                                                        Fig: Eutrophicated lake

Industrial pollutants as well as agrochemicals affect the water quality and have detrimental effects. Chlorine used to control bacterial and algal growth in the cooling system of power stations, when discharged in streams, causes the death of planktons. Mercury, lead, and cadmium are extremely toxic if consumed. These don’t get metabolised or excreted out, rather get accumulated across the food chain. The concentration of these elements increases with increase in trophic levels, the phenomenon called biomagnification. One of the prominent effects of DDT biomagnification in the birds is the thinning of their egg shells.

                                        Fig: Biomagnification of DDT in water bodies

Thermal water pollutants increase the average temperature of water bodies. This affects aquatic organisms. The decomposition of wastes occurs faster leading to more oxygen consumption compared to its replenishment. Thus the equilibrium is disturbed, leaving less oxygen for aquatic life. Certain fish eggs hatch early in an immature condition while some clothes fail to hatch. Failure of spawning also occurs. The coral reefs are also affected by temperature fluctuations.

Excessive load of silt in the water bodies result in a decline in primary productivity. This is because the particulate matter forms a layer on the top of the water body, limiting the penetration of sunlight to deeper layers. The eggs laid by aquatic organisms are also destroyed due to smothering of silt. This interferes with reproduction and thus restricts subsequent population growth of the species.

Oil and petroleum released in marine environments remain as a thick sludge suffocating aquatic fauna. The fishes get suffocated as the oxygen supply is cut off. Besides, the birds get their feathers stuck due to immersion into oil and this restricts their flight abilities. Additionally, the aquatic flora are bereft of proper sunlight as the penetration of sunlight is affected. Thus, the photosynthesis gets hampered.

                                                         Fig: Bird affected by oil spill

Practice Problems

  1. Which of the following wastes present in wastewater can most easily be removed during wastewater treatment?

    1. Nitrates
    2. Phosphates
    3. Piece of cloth
    4. Faecal matter

Solution: Piece of cloth is a solid waste. Solid wastes are most easy to remove by using sedimentation and filtration methods during wastewater treatment. Nitrates and phosphates are dissolved wastes while faecal matter is a colloidal waste. Hence, these are difficult to remove. Hence, c is the correct option.

2. Why are point sources of water pollution easier to control when compared to dispersed sources?

Answer: Point sources of water pollution are single points of collection and treatment of contaminated water, after which the treated water is discharged in water bodies. Thus it is easier to have a quality check on the point sources.

On the other hand, the dispersed sources are not at a single point, rather spread across many areas. Although modern sewage-treatment plants have been constructed to control water pollution by dispersed sources, the implementation is still not satisfactory. Also quality checks can’t be done properly. Thus pollution by dispersed sources is difficult to control.

3. Following figure shows the feeding relationship between organisms A, B, C, D in an aquatic ecosystem. Which among them has the highest concentration of Mercury?

Answer: D is at the highest trophic level and hence the number of organisms at this trophic level will be least as we know that the number of organisms decrease as we move from lower to higher trophic levels in aquatic ecosystems. Thus, as the mercury travels through the food chain, it's concentration in the population keeps magnifying. This is why D has the highest concentration of Mercury due to biomagnification.

4. While you visited a nearby river and dipped your feet in the water, you found it considerably warm near the zone which is receiving the pipe water. Cite a possible reason for the same.

Answer: The localised warmness might be due to the discharge of industrial hot water in the river through the pipes.


1. What is cultural eutrophication?
The phenomenon of enrichment of water bodies by supply of nutrients, especially compounds containing high nitrogen and phosphorus content, by human activities is known as cultural eutrophication. It is an artificially-led acceleration of the ageing process of the water bodies due to discharge of excess fertilisers and other nutrient sources into them.

2. Eichhornia crassipes is commonly called the ‘Terror of Bengal’. Why?
Eichhornia crassipes, also known as water hyacinth, is adept in utilising and consuming aquatic nutrients as well as solar energy. It spreads rapidly on the surface of water bodies. This cuts off the light penetration into deeper layers of rivers and ponds, leaving less light available for the dwellers in those regions to perform photosynthesis. Thus the growth of endemic aquatic flora gets affected. This inturn affects the growth and development of fishes which indirectly depend on the flora. Thus the ecosystem dynamics gets hampered due to this aquatic weed.

3. What is BOD? How does the BOD test help to determine the concentration of organic water pollutants?
BOD is also known as Biochemical Oxygen Demand. It refers to the amount of oxygen required by the microorganisms to carry out the oxidation of organic matter dissolved in one litre of sewage water. High BOD indicates greater extent of water pollution or higher concentration of organic water pollutants.

4. What is the cause of Minamata disease?
Severe mercury poisoning leads to Minamata disease. It occurs due to consumption of huge quantities of fishes that were contaminated with the toxic chemicals such as mercury.

YOUTUBE LINK: https://youtu.be/Q4fCYFZ3aF0

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