Biochemical Oxygen Demand (BOD) in a river water increases when sewage gets mixed with river water. Biochemical oxygen demand or biological oxygen demand (BOD) is a chemical procedure for determining how fast biological organisms use up oxygen in a body of water. It is used in water quality management and assessment, ecology and environmental science. BOD considered as an indication of the quality of a water source.

Correct Option: C

Biochemical Oxygen Demand (BOD) in a river water increases when sewage gets mixed with river water. Biochemical oxygen demand or biological oxygen demand (BOD) is a chemical procedure for determining how fast biological organisms use up oxygen in a body of water. It is used in water quality management and assessment, ecology and environmental science. BOD considered as an indication of the quality of a water source.

Eutrophication is often seen in fresh water lakes. The main causes of eutrophication in these water systems are sediments and thermal stratification, with agricultural runoff from fertilizers and manure, containing nitrates and phosphates and discharge of partially treated or untreated sewages or phosphate containing detergents.

Correct Option: B

Eutrophication is often seen in fresh water lakes. The main causes of eutrophication in these water systems are sediments and thermal stratification, with agricultural runoff from fertilizers and manure, containing nitrates and phosphates and discharge of partially treated or untreated sewages or phosphate containing detergents.

Marsh gas is another term for methane, produced by plants decomposing under water.

Correct Option: D

Marsh gas is another term for methane, produced by plants decomposing under water.

In secondary treatment mainly settled sewage flows to an aerobic biological treatment stage where it comes into contact with micro-organisms which remove and oxidise most of the remaining organic pollutants.

Correct Option: D

In secondary treatment mainly settled sewage flows to an aerobic biological treatment stage where it comes into contact with micro-organisms which remove and oxidise most of the remaining organic pollutants.

BOD is a measure of organic matter present in water. It refers to amount of O₂ consumed by microbes to decompose all the organic matter in 1 L of water at 20°C for 5 days. The greater the BOD of waste water, more is its polluting potential.

Correct Option: A

BOD is a measure of organic matter present in water. It refers to amount of O₂ consumed by microbes to decompose all the organic matter in 1 L of water at 20°C for 5 days. The greater the BOD of waste water, more is its polluting potential.