Causes and Effects — Core Principles

Core Principles

Water pollution is the contamination of water bodies by harmful substances, rendering them unfit for use and disrupting ecosystems. Key causes include domestic sewage (introducing organic matter, nutrients, pathogens), industrial effluents (heavy metals, toxic chemicals, thermal discharges), and agricultural runoff (fertilizers, pesticides).

Point sources are identifiable (e.g., factory pipes), while non-point sources are diffuse (e.g., farm fields). The effects are severe: oxygen depletion in water due to decomposition of organic waste (measured by BOD), leading to 'dead zones'; eutrophication from excess nutrients causing algal blooms; biomagnification where pollutants like mercury and DDT accumulate up the food chain; and widespread human health issues such as waterborne diseases (cholera, typhoid) and chemical poisoning (Minamata, Itai-Itai diseases).

Understanding these causes and effects is vital for environmental protection and public health.

Important Differences

vs Chemical Oxygen Demand (COD)

Aspect	This Topic	Chemical Oxygen Demand (COD)
Definition	Biochemical Oxygen Demand (BOD): Amount of dissolved oxygen consumed by aerobic microorganisms to decompose biodegradable organic matter.	Chemical Oxygen Demand (COD): Amount of oxygen required to chemically oxidize all organic and inorganic substances using a strong chemical oxidant.
Pollutants Measured	BOD: Primarily measures biodegradable organic matter.	COD: Measures both biodegradable and non-biodegradable organic matter, as well as oxidizable inorganic substances.
Time Required	BOD: Typically takes 5 days (BOD5 test) for completion.	COD: Can be determined in a few hours (e.g., 2-3 hours).
Reagents Used	BOD: No chemical reagents are directly added to oxidize; relies on microbial activity.	COD: Uses strong chemical oxidants like potassium dichromate ($K_2Cr_2O_7$) in acidic medium.
Ecological Relevance	BOD: Directly indicates the potential for oxygen depletion due to microbial decomposition, crucial for aquatic life.	COD: Provides a broader measure of total oxidizable pollution, useful for industrial wastewater characterization and treatment plant design.

While both BOD and COD are crucial indicators of water pollution, they measure different aspects. BOD specifically quantifies the oxygen demand from biodegradable organic matter by microorganisms over a longer period, directly reflecting the impact on aquatic aerobic life. COD, on the other hand, provides a faster, more comprehensive measure of total oxidizable substances, including both biodegradable and non-biodegradable organic compounds, as well as certain inorganic ones, making it valuable for industrial effluent analysis where complex chemicals are present. A high BOD indicates significant biological pollution, whereas a high COD suggests a broader range of chemical contamination.