What is the primary difference between BOD and COD?

BOD (Biochemical Oxygen Demand) measures the amount of oxygen consumed by microorganisms while decomposing biodegradable organic matter in water over a specific period, typically 5 days. It indicates the level of organic pollution that can be biologically broken down. COD (Chemical Oxygen Demand), on the other hand, measures the total oxygen required to chemically oxidize all organic and inorganic substances in water using a strong chemical oxidant. Therefore, COD is generally higher than BOD because it accounts for both biodegradable and non-biodegradable organic matter, providing a more comprehensive measure of total oxidizable pollution.

How does eutrophication occur and what are its main consequences?

Eutrophication is the process where a water body becomes excessively enriched with nutrients, primarily nitrates and phosphates, often from agricultural runoff, sewage, and detergents. This nutrient overload leads to an explosive growth of algae and aquatic plants, known as an algal bloom. The main consequences include: reduced light penetration for submerged plants, their subsequent death, and then the decomposition of this dead organic matter by bacteria. This decomposition consumes vast amounts of dissolved oxygen, leading to anaerobic conditions, which suffocates fish and other aquatic organisms, severely degrading the ecosystem and causing foul odors.

What are heavy metal pollutants and why are they particularly dangerous?

Heavy metal pollutants are toxic metallic elements like lead, mercury, cadmium, arsenic, and chromium, often discharged from industrial activities such as mining, electroplating, and battery manufacturing. They are particularly dangerous because they are non-biodegradable, meaning they do not break down naturally in the environment. Instead, they persist and can accumulate in living tissues (bioaccumulation) and become more concentrated up the food chain (biomagnification). This can lead to severe health problems in humans and animals, including neurological damage, kidney failure, developmental issues, and various cancers.

Explain the concept of biomagnification with an example.

Biomagnification is the increasing concentration of a persistent, non-biodegradable pollutant in the tissues of organisms at successively higher trophic levels in a food chain. For example, consider DDT, a persistent pesticide. If a small amount of DDT is present in water, plankton absorb it. Small fish eat many plankton, accumulating more DDT. Larger fish eat many small fish, accumulating even higher concentrations. Finally, birds of prey or humans who consume these large fish will have the highest concentrations of DDT in their bodies, leading to toxic effects like reproductive failure or neurological damage. This process highlights the long-term danger of persistent pollutants.

What role does Dissolved Oxygen (DO) play in water quality?

Dissolved Oxygen (DO) is the amount of gaseous oxygen dissolved in water, and it is a critical indicator of water quality. It is essential for the survival of most aquatic organisms, including fish, invertebrates, and aerobic bacteria, which use oxygen for respiration. Clean, healthy water typically has high DO levels (8-10 ppm). When water becomes polluted with organic matter, microorganisms decompose this matter, consuming large amounts of DO. A significant drop in DO (below 4-5 ppm) creates hypoxic or anoxic conditions, stressing or killing aquatic life, leading to a severely degraded ecosystem. Therefore, monitoring DO helps assess the health of a water body.

How do pesticides and fertilizers contribute to water pollution?

Pesticides and fertilizers are widely used in agriculture to enhance crop yield and protect against pests. However, when applied excessively or improperly, rainfall and irrigation runoff can wash these chemicals from fields into nearby rivers, lakes, and groundwater. Fertilizers, rich in nitrates and phosphates, lead to eutrophication, causing algal blooms and oxygen depletion. Pesticides, many of which are persistent organic pollutants, can directly poison aquatic life, accumulate in the food chain through biomagnification, and pose long-term health risks to humans and animals, even at low concentrations.

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Water Pollution

Chemistry

NEET UG

Version 1Updated 22 Mar 2026

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Water pollution refers to the contamination of water bodies, such as rivers, lakes, oceans, groundwater, and aquifers, typically as a result of human activities. This contamination renders the water unfit for its intended use, whether for drinking, recreation, agriculture, or supporting aquatic ecosystems. It involves the introduction of physical, chemical, or biological substances that degrade wa…

Quick Summary

Water pollution is the contamination of water bodies by harmful substances, making water unfit for use. It stems primarily from human activities, categorized into point sources (identifiable discharges like factory pipes) and non-point sources (diffuse runoff from agriculture or urban areas).

Key pollutants include pathogens (disease-causing microorganisms from sewage), organic matter (leading to oxygen depletion, measured by BOD), inorganic chemicals (like heavy metals, acids, salts), and excess nutrients (nitrates, phosphates, causing eutrophication).

Water quality is assessed using parameters like Dissolved Oxygen (DO), Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), and pH. Low DO and high BOD indicate severe organic pollution. Eutrophication, driven by nutrient overload, causes algal blooms and subsequent oxygen depletion, devastating aquatic life.

Control strategies involve proper sewage and industrial waste treatment, sustainable agricultural practices, and public awareness to protect this vital resource.

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Key Concepts

Biochemical Oxygen Demand (BOD) Calculation & Significance

BOD is a critical parameter for assessing organic pollution. It quantifies the oxygen required by aerobic…

Eutrophication Mechanism and Impact

Eutrophication is a natural process that is greatly accelerated by human activities, primarily the discharge…

Heavy Metal Toxicity and Bioaccumulation

Heavy metals such as mercury ( $Hg$ ), lead ( $Pb$ ), and cadmium ( $Cd$ ) are significant water pollutants due to…

Water Pollution: — Contamination of water bodies by harmful substances.

Point Sources: — Identifiable, localized discharge points (e.g., factory pipes).

Non-Point Sources: — Diffuse, widespread sources (e.g., agricultural runoff).

Pollutants: — Pathogens, organic waste, heavy metals (

Hg, Pb, Cd

), nutrients (

NO_3^-, PO_4^{3-}

), sediments, thermal.

Dissolved Oxygen (DO): — Oxygen in water, essential for aquatic life. Clean water:

8-10,\text{ppm}

. Polluted water:

<4-5,\text{ppm}

Biochemical Oxygen Demand (BOD): — Oxygen consumed by microbes for organic decomposition. High BOD = high organic pollution. Clean water:

<5,\text{ppm}

. Highly polluted:

>17,\text{ppm}

Chemical Oxygen Demand (COD): — Oxygen for chemical oxidation of all organic/inorganic matter. Usually

>

BOD.

Eutrophication: — Nutrient enrichment (

NO_3^-, PO_4^{3-}

)

ightarrow

algal blooms

ightarrow

DO depletion.

Biomagnification: — Increasing pollutant concentration up the food chain (e.g., DDT, Mercury).

Minamata Disease: — Caused by Mercury (

Hg

) poisoning.

Itai-Itai Disease: — Caused by Cadmium (

Cd

) poisoning.

To remember the key water quality parameters and their relation to pollution: BODy DOesn't like COD.

BOD — (Biochemical Oxygen Demand): High BOD = Bad (polluted water, low DO).

DO — (Dissolved Oxygen): High DO = Good (clean water, low BOD).

COD — (Chemical Oxygen Demand): Comprehensive measure, always higher than BOD, also indicates pollution.

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