Biological Consequences of Water Pollution

Sewage

Wastewater containing organic matter, pathogens, and nutrients

Organic matter

Material decomposed by bacteria, increasing oxygen demand

Aerobic bacteria

Microorganisms that use oxygen to break down organic waste

Biochemical Oxygen Demand (BOD)

Amount of oxygen needed to decompose organic matter

Anoxic water

Water with no dissolved oxygen

Pathogens

Disease‑causing microorganisms found in untreated sewage

Eutrophication

Excess nutrients cause algal blooms and oxygen depletion

What happens when sewage enters water?

Aerobic bacteria multiply and use up dissolved oxygen

Why does sewage reduce oxygen levels?

Bacteria respire while decomposing organic matter, increasing BOD

What happens when oxygen levels fall?

Fish and other aquatic animals suffocate and die

What is anoxic water?

Water with no oxygen, where only anaerobic bacteria survive

What gases do anaerobic bacteria release?

Methane, ammonia, hydrogen sulfide

How does sewage cause eutrophication?

Nutrients cause algal blooms

Why do algal blooms reduce biodiversity?

They block light; plants die; decomposition removes more oxygen

How does sewage spread disease?

Pathogens contaminate water and infect humans

Overall effect of sewage pollution

Oxygen depletion, death of aquatic life, reduced biodiversity